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Backyard BMP Design Manual - Div. of Soil & Water Conservation

1. SOIL amp WATER MM hri i i Community Conservation Assistance Program BMP Design Manual nRE nl Eiclogieal amp 10000 r 1000 T T eM us Watershed Q Ten EL 1 Size sf 7 a 2 447 0 f i ia i ee ae 5000 8 A eet AS 7500 Pete uu 10000 E 100 LAE 15000 pae 20000 E 30000 40000 REE 50000 10 LL dL qp ud 0 20 40 60 80 100 Impervious Figure 3 5 Estimated runoff volumes from various watersheds for 1 5 inch rainfall 3 4 Peak Flow Determining the peak flow leaving a watershed during a design storm is important when designing many stormwater BMPs The peak flow is simply the largest flow that leaves the watershed through the course of a storm event Figure 3 6 shows a sample flow vs time relationship and the associated peak flow The depth of rain that falls throughout the event can be observed on the right side of the graph M gt m L Rainfall in S S9 Gm y AS RS y xd ge qx e g S V S S d d NN X O qv qv qw qw D Re Rey QU qe qv a gr w wv 9 Date and Time Figure 3 6 Peak Flow Illustration 15 a TA ORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual The peak flow is used to determine the proper size of conveyances such as swales and
2. Water Conservati onservat DI w oera n X d 4 Stormwater Best Management Practice Design Manual BA NCDENR NC STATE UNIVERSITY NORTH WATER hd y A gt CONSERVATION Community Conservation Assistance Program BMP Design Manual Prepared By Biological and Agricultural and Engineering Department North Carolina State University William F Hunt PhD PE Assistant Professor and Extension Specialist Jon M Hathaway El Extension Associate Ryan A Smith PE Extension Associate ORTH CAROLINA SOIL amp WATER Table of Contents Community Conservation Assistance Program BMP Design Manual PR IP E P o 6 1 1 Introduction to the Community Conservation Assistance Program 6 1 2 The CCAP Best Management Practice Design Manual T 2 0 Introduction to Stormwater ooocconconccccconcocnconnoncconcnncnancnnrnnnennronrnanennrnnnennnnnes 7 2 1 Definition of Stormwater ooccocccooccoccccncoccocnconcocnonnonncnnnonncnnnononancnronononnnnnnnnes 7 Zoe eels OF RD aI ZaAMIOM too 8 2 9 LO MW BMP S aaa 10 3 0 General Stormwater BMP Design Considerations 11 SENE FIS A RT 11 3 2 Connected IMpervi0USNESS oocooccoccccncoccccnconcncnconnncononcnnnnnonononronnnnanencnnanans 12 SEM IM PIS lolo Ar E oil E T 12 lo AO No PP ePR Pz A 15 34 SUOMI AI CTISI reed 16 3 4 2 Runoff Coefficient eanan arr cece ccc E E
3. 15 85 0360 17 16 A315 2 80 039 19 18 20 25 75 043 21 20 20 Table A3 Flow calculations and suggested weir length 1 2 acre watershed various watershed scenarios intensity of 6 5 in hr ra Paun qid um Weir Required Weir Suggestion cfs 0 f o L s 99 10 90 032 ao 10 10 2 80 039 13 12 36410 Table A4 Flow calculations and suggested weir length 1 4 acre watershed various watershed scenarios intensity of 6 5 in hr o a M aeni pA Weir Required Weir Suggestion cfs ft Lm a 1 9 x os 05 15 85 036 06 05 10 20 so os os os so 25 75 043 07 07 10 3 70 046 07 207 AJ10 s 5 oso os os 10 40 6 053 09 08 40 Table A5 A8 use a rainfall intensity of 7 2 in hr These calculations should only be used for Charlotte Greensboro Winston Salem Raleigh and Durham Table A5 Flow calculations and suggested weir length 1 acre watershed various watershed scenarios intensity of 7 2 in hr imperious 6 ne ra Weir Required Weir Suggestion Impervious cfs US es 92 2i 10 90 os l 22 20 15 85 036 26 24 25 20 80 039 28 26 23125 Table A6 Flow calculations and suggested weir leng
4. 00L 08 09 OV Oc 0 0000 00007 0000 Y 00002 RE 2 7 VOL 00081 fat T f 00001 wp a i 0087 gt ae a 0008 nidis eer 0001 JS eziS ern E p9USISIBMM NP 0000 yidap Hulpuod 6 ejuiey 1 SNOIAJEd Wy pue js 000S lt 9ZIS P3YSISIEM jo uomnoungj e se eaoJy ooejungs puepoeM zg eJnDiJ JS 9ZIS oonoeJg snoiAJeduu Y 001 08 09 OV Oc 0008 000v 000 0002 0061 000 006 JS eziS D9usJojeAA Ol 001 OOOL yydap Buipuod 6 11eJuley uG b SNOIAJOd Wy pue Js 0008 9ZIS P9YSIDIEM Jo uomnouny e se easly ooejung puepoM qg aJunDi J JS eziG o2noeJg 00004 0000v 0000 0000 O00S 00001 0087 000 JS eziS peusJojeAA snoiAJoedu Y 00L 08 09 OV Oc yydap Buipuod 6 jjejuresl 4S I Snoad pue Js 0005 lt ezis peusJejeM jo uomnoungj e se eaJy ooejung puepoeM yg ainbi Ol OO
5. Chrysongonum virginianum Mouse Ear Coreopsis 2 Coreopsis auriculata Tickseed 1 2 Coreopsis lanceolata Swamp Coreopsis 2 Coreopsis rosea Joe Pye Weed 3 Eupatorium dubium Swamp Sunflower 3 Helianthus angustifolius Swamp Mallow 3 Hibiscus moscheutos Texas Star 3 Hibiscus coccineus Blue Flag Iris 3 Iris virginica Seashore Mallow 3 Kosteletskya virginica Gayfeather 2 Liatris spicata Cardinal Flower 3 Lobelia cardinalis Cinnamon Fern 3 Osmunda cinnamomea Royal Fern 3 Osmunda regalis Garden Phlox 2 PAlox paniculata Moss Pinks 1 2 PAlox subulata Rudbeckia 1 2 Rudbeckia fulgida Green Headed Coneflower 3 Rudbeckia laciniata Goldenrod 3 Solidago rugosa Stoke s Aster 2 Stokesia laevis Ironweed 3 Vernonia novaboracensis Verbena 1 2 Verbena canadensis Ornamental Grasses River Oats 1 3 Chasmanthium latifolium Muhly Grass 1 2 Muhlenbergia capillaris Panic Grass 1 3 Panicum virgatum Indiangrass 1 2 Sorghastrum nutans Sedges and Rushes Lurid Sedge 3 Carex lurida Fringed Sedge 3 Carex crinita Southern Waxy Sedge 3 Carex glaucescens White topped Sedge 3 Rhynchospora latifolia Woolgrass 3 Scirpus cyperinus Non native perennials and ornamental grasses suitable for rain gardens include Liriope 1 2 Liriope muscarii and L spicata Siberian Iris 2 Uris sibirica Daylil
6. Figure 3 2 Estimated runoff volumes from various watersheds for 1 inch rainfall 13 NORTH CAROLINA SOIL amp WATER eae Ri Community Conservation Assistance Program BMP Design Manual Estimated Volume as a function of Watershed Size gt 5000 sf and Impervious 1 Rainfall 10000 4 0 gt Watershed T E bus Size sf SEE Joa4 4 m 2 1000 4 75000 N E 7500 77 S 10000 15000 E 100 ls 20000 30000 40000 f 50000 po Litt iit rit Pe ty 0 20 40 60 80 100 Impervious Figure 3 3 Estimated runoff volumes from various watersheds for 1 inch rainfall Volume cf 1000 Estimated Volume as a function of Watershed Size 5000 sf and Impervious 1 5 Rainfall Watershed Size sf 100 20 40 60 Impervious 80 500 1500 2000 3000 4000 5000 1000 100 Figure 3 4 Estimated runoff volumes from various watersheds for 1 5 inch rainfall NORTH CAROLINA
7. Overflow Frequency Percentage of rainfall events that resulted in overflow from the cistern Dry Cistern Frequency Percentage of days when there was a water demand but not enough water stored in the cistern to meet that demand First Flush Volume Captured Percentage of runoff volume associated with the first flush rainfall depth that the cistern was able to capture without overflow Annual Nitrogen Removed Weight of nitrogen removed from runoff assuming that all nitrogen captured by the cistern is removed via the cistern itself or subsequent water usage Payback Period Number of years of water use required for total water savings to equal the cost of the cistern Model Assumptions When calculating the daily water balance the model first adds rainfall inputs then subtracts the daily water usage Cost estimates are based upon local city water costs from 2005 and do not include sewer costs which are often based upon water usage Calculation of pay return period is a simple calculation that does not account for interest rates or changes in water cost over time Total nitrogen captured by the cistern is assumed to be removed through denitrification within the cistern or subsequent water usage Denitrication within the cistern may be minimal however using the water for toilet flushing or irrigation effectively reduce or eliminate the nitrogen loading to surface waters Troubleshooting Problem o lreceive an erro
8. 10 3 3 Concrete Edging Concrete edging should be used around the perimeter of the paved area whenever permeable interlocking concrete pavers are installed see Figure 10 4 The edging should be approximately 18 inches deep and have a width of 6 inches This edging will reduce the possibility of the pavement blocks on the edge of the paved area sloughing over Permeable concrete and concrete grid pavers will not require this edging provided that the very edge of the paved area is not exposed to vehicular traffic 66 ITA CONSERVATION NORTH CAROLINA SOIL amp WATER xi sii Ra Community Conservation Assistance Program BMP Design Manual Basgan amp Esgisaaring Figure 10 4 Pavers abutting a concrete edge 10 3 4 Gravel Base The gravel base that is laid over the existing soils is used to 1 store captured stormwater and 2 provide structural stability Figure 10 5 The depth of and type of stone in the gravel base will vary depending on whether permeable concrete or permeable pavers are installed If the soil criteria described in sections 10 3 1 and 10 3 2 is met the total gravel base should be approximately 6 inches deep Any stone used as a gravel base should be washed effectively reducing the amount of fines in the gravel If possible the stone should be double washed These fines if not removed will migrate to the soil sub base and clog the soil pores This clogging would result in reduced infiltration and t
9. 6 4 1 Laying out the wetland oooccoccccnconcccnconconnonconnonnnononnnnanoncnnnnnanennnnanos 0 4 7 MOON DNE i re m ee ee ee ee ee eee eee ee ee ee eee ee re ee ee OB NON EA o AAA 6 5 Backyard Wetland Maintenance ocooccocccoccociconconoocnnononconancnnonnnnononcnnnnnos AO on NET T E TE NES a OL acu asas OT 7 2 Selection of this practice over OthNerS oocoocccoccconcocnconcornconnonooncanonnnnanos EROS t PP 1 3 1 Determining Cistern Watershed Characteristics ESPE SA A e A III 1 3 3 Additional Cistern Components ooccoccccncoccccnconconocanonnoconcnnonnncanonnnnnnnnns ESO UE ESTIU A Gittern COST UCO ranita 1 4 1 Preparing Me Soll DASE scisti neaei na aR Or i 4 2 Preparing the gutter COWNSPOUL cccccecc cece eee eeceeeeeeseeteeeaeeseesaeeaees 4 3 Preparing the gutter dOWNSPOUl coocconnccccccncocconnconcononononnonnnoancnnonannnos 1 4 4 Setting the cistern into p lace ooococcniocnnconiconiconocononononnncanonanonanonanos 4 5 Final cistern construction considerations ooccoocconcccnconocnncancnnnnnnnnns 7 5 Cistern Maintenance ooccocccnccoccconcococcnconoocnconcononononnnonnnnnonnonanonrononcnnnnnnnnnos 8 0 Vegeltated OWalOS wisac che scsesecesccerenersieneseneneneeenetensnenstetanenadesenenenesesesetevensstdeneds 6 T Overview Of Pace isaac aaa 9 2 Determining Site o o CO OO 8 3 Depth and Velocity CalculatiOnNS oocccoccconcccnconnc
10. Helianthus angustifolius Swamp Mallow 3 Hibiscus moscheutos Texas Star 3 Hibiscus coccineus Blue Flag Iris 3 Iris virginica Cardinal Flower 3 Lobelia cardinalis Cinnamon Fern 3 Osmunda cinnamomea Royal Fern 3 Osmunda regalis Garden Phlox 2 Phlox paniculata Moss Pinks 1 2 Phlox subulata Rudbeckia 1 2 Rudbeckia fulgida Green Headed Coneflower 3 Rudbeckia laciniata Goldenrod 3 Solidago rugosa Ironweed 3 Vernonia novaboracensis Ornamental Grasses River Oats 1 3 Chasmanthium latifolium Muhly Grass 1 2 Muhlenbergia capillaris Panic Grass 1 3 Panicum virgatum Indiangrass 1 2 Sorghastrum nutans Sedges and Rushes Lurid Sedge 3 Carex lurida Fringed Sedge 3 Carex crinita White topped Sedge 3 Rhynchospora latifolia Woolgrass 3 Scirpus cyperinus Non native perennials and ornamental grasses suitable for rain gardens include Liriope 1 2 Liriope muscarii and L spicata Siberian Iris 2 Uris sibirica Daylily 1 2 Hemerocallis hybrids Rain Lilies 3 Zephyranthes species Crinum Lilies 3 Crinum species Japanese Painted Fern 2 Athyrium nipponicum and Maiden Grass 1 2 Miscanthus cultivars 1 Plants that once established can withstand considerable drought 3 4 weeks without rainfall 2 Plants that grow best in moist to average soils and will only tolerate short periods 1 2 days of flooding 3 Plan
11. LLL 0008 Se ah eee JS eziS E peusJejeM uydep Buipuod Ol OOL OOOL 00001 000001 ejuiey L SNOIAJEd Wy pue js 000S lt 9ZIS peusJejeM Jo uonounj e se eaJy ooejung puepeM OLG e4n61J JS 9ZIS oonoeJg snoiAJeduu Y OOL 08 09 OV Oc 0008 000r 000 0002 x 009 2 me 000 ZI E a l A os Fp eh eo JS 98ZIS r rr ee peusiejeAA oo yidep Buipuod jjesurey uG L SNOIAJad W pue js 0008 9ziS P9YSISIEM Jo uoun e se Buy ooejung puepoeM LLG e4n61iJ Ol 001 0001 00001 JS 9ZIS o2noeJg snoiAJedui Y OOL 08 m M 00004 0000v 0000 0000 ine 000S B 0000 0087 gt E es 0008 02 pe T 1S ZIS O p ys197e M uydep Buipuod ejurey 4S b Snoad wj pue JS 000S lt ezis peusJjejeM Jo uomnoung e se eaJy ooejung puepoeM ZLg SANBIY Ol OOL 0001 00001 000001 JS 9ZIS 391 98 1d APPENDIX E Plant Selection for No
12. The major danger in allowing a backyard wetland to dry is plant mortality Thus selecting hardy drought tolerant plants will result in a well functioning wetland under these drier conditions Table 6 4 shows a set of suitable plants that can grow in these harsh conditions Table 6 4 Plant selection for drought resistant wetland In times of extreme drought even the drought tolerant plants in the wetland may be impacted by the lack of water If the plants begin to dry out and show signs of stress consistent with a lack of water such as wilted foliage the plants should be watered The wetland can be irrigated in the same manner as any adjoining lawn and can obviously tolerate more irrigation than a standard lawn if resources are available 6 4 Backyard Wetland Construction 6 4 1 Laying out the wetland Once the suggested wetland surface area has been determined and a location has been selected marking paint string rope or flags can be used to lay out the perimeter of the wetland Figures 6 5a and 6 5b The wetland dimensions 39 tT SOIL amp WATER Tt Community Conservation Assistance Program BMP Design Manual will vary based on site conditions and landscaping preferences The layout should be measured multiple times before digging to ensure that the suggested surface area of the wetland is achieved with the dimensions that have been marked Belgien amp L E 4inaar ns Figures 6 5a and 6 5b Ma
13. Total Cistern Figure 1 Pump with pressure tank before installation and example of system in use ATE UNIVERSITY A amp T STATE UNIVERSITY COOPERATIVE head is a description of the energy required to move the water and is typically expressed in feet of water or pounds per square inch psi This publication reviews the pump selection process and at the end provides a sample calculation of flow and total head When properly configured a rainwater harvest ing system delivers many benefits including reduced flooding improved water quality reduced demand on a well or city water supply and monetary savings For more information on water harvesting systems see Permeable Pavements Green Roofs and Cisterns Stormwater Practices for Low Impact Development AG 588 06 part of the Urban Waterways series PUMP TYPES Pumps can be divided into two major categories positive displacement pumps and dynamic pumps Positive displacement pumps operate by mechani cally moving water by means of the action of a gear or piston and are able to produce high heads but rela tively low flows They are generally used in hydraulic systems and in a number of specialty applications Typical positive displacement pumps are piston and rotary pumps Dynamic pumps provide a continuous flow by de veloping a high fluid velocity and are able to produce high flows but relatively lower heads They are used to satisfy most general pumping requirements and
14. area that will be draining to the proposed BMP location in square feet 2 the percentage of the drainage area that is impervious and 3 the desired depth of rainfall targeted for capture normally the water quality event The simple method is described in equations 1 and 2 below Equation 3 1 Calculate Runoff Coefficient Rv 0 05 0 009 I 12 STA NORTH CAROLINA SOIL amp WATER Equation 3 2 Calculate Runoff Volume Community Conservation Assistance Program BMP Design Manual V Rv A P 12 Where Rv Runoff Coefficient fraction of rainfall that will produce runoff Connected impervious percentage in watershed 96 V Volume of runoff ft A Area that drains to BMP ft P Depth of storm to be captured normally water quality event in Figures 3 2 3 3 3 4 and 3 5 use the simple method to calculate the volume of water that can be expected from a given watershed these values are calculated using water quality events of either 1 inch or 1 5 inches as defined in figure title Estimated Volume as a function of Watershed Size 5000 sf and Impervious 1 Rainfall 1000 ELEME pL ad 7 Watershed Size sf 500 1000 1500 2000 3000 4000 5000 Volume cf 0 20 40 60 80 100 Impervious
15. undersized they do not provide as much runoff reduction when compared to larger multi hundred or multi thousand gallon cisterns 45 lt ae NORTH CAROLINA Community Conservation Assistance Program BMP Design Manual s aaa CONSERVATION Gutter Figure 7 4 Schematic of cistern system 7 2 Selection of this practice over others The primary advantage of installing a stormwater cistern is that the stormwater can be captured and used for some other objective In other BMPs stormwater is lost to the atmosphere is used by plants or drains into the soil A property owner should be able to describe the intended use of the stormwater to SWCD staff This is important because the cistern is only an effective BMP if it has storage space Storage space is created by using the cistern water between rain events Cisterns can be installed in many cases without excessive digging and soil type and water table depth have little impact on above ground cistern installation The disadvantage of using cisterns is that they will only treat runoff from rooftops in a backyard stormwater BMP setting Thus rain gutters are necessary on the rooftops being treated SWCD will not provide any cost share for gutter installation Any runoff produced by parking lots or other impervious surfaces are typically not captured by the cistern 7 3 Cistern Design 7 3 1 Determining Cistern Watershed Characteristics Determining the characteristics of the
16. 3 0 L Length of Weir ft H Height of Water Over Top of Weir ft Use 0 5 feet The flow that should be passed is the flow associated with the 10 year event as determined using the rational method Section 3 4 The weir coefficient is set to 3 0 and the height of the water over the top of the weir should be no higher than 6 inches 0 5 feet so the water will not flow over the containment berm Thus the only unknown in the equation is the length of the weir This equation can be rearranged to solve for the weir length Equation 3 6 Weir Equation L Q Cy x H gt Sample 10 year peak flow calculations for various watershed scenarios are shown in Appendix A Included is a required weir length for each watershed scenario 3 6 3 Outlet installation Appendix B shows a diagram of a wooden weir structure that could be used for an outlet control structure on large systems For any type of outlet used concentrated flows should only pass over a structure or undisturbed and vegetated soil All wooden weir structures should be tied into the underlying soils Soils filled around structures should be compacted and protected 3 7 Riparian Buffer Considerations Many regulations have been implemented to improve the quality of surface waters in North Carolina including the riparian buffer rules detailed by the North Carolina Division of Water Quality in the Redbook of surface waters and wetlands standards Administrative Code 15A NCAC 0
17. 3 1 Rain garden siting 26 5 3 2 Determine watershed and impervious area draining to BMP 26 EE RUGA GE SIZ o PRECOR 2 5 3 4 Rain garden bottom topography sees 20 5 3 5 Plant selection ee 20 5 3 6 Inlet protection and pretrealMent ocooccccccncocnccnccnconconoonconoononncononnnnnos 29 NE DUNE OCS OM 31 5 4 Rain Garden Maintenance ccccecceecceeceeeeeeceeteeeeeeteeeeueteeteeeeeseeeeeeees 31 6 0 Backyard Wetland Design eese ree rene 32 DT OVERVIEW OF TPEOoHeE sus DUE ud eso INEVLMIDN MIDI IE FINE ERN 32 3 ORTH CAROLINA SOIL amp WATER 6 2 Selection of this practice over OthNerS cooccoccccncoccocnconcocnconcnnoconcnnnnnnnanos 6 3 Backyard Wetland Design cocccocccccccncoccocnconcocnonnnononnnnnncnnonnnnancnnoncnnnnnns 6 3 1 Determining wetland watershed characteristics 019 2 Backyard WEIGAND SIZING stos iii 6 3 3 Backyard wetland inlet consideratiONS coococnoonconconconconionionionnnnos 6 3 4 Backyard wetland outlet considerations eeeseseeuse 05 9 9 Backyard wellan pla S sso ibssa ose Eiaa aE Eae se ai 6 3 6 Drought tolerant Wetland ccccccccccccceccecceecceecueeceecueeeuecuesueeseesaeeseess 6 4 Backyard Wetland Construction occcoccccccconcconccnncocnnocnnnconanonanconnnononononos
18. Community Conservation Assistance Program BMP Design Manual Table 6 2 Sample shallow water wetland plants Scientific Name Elegant arrowhead shaped leaves and Arrow Arum Peltandra virginica interesting green flowers on a clump forming plant Upright plant producing numerous 3 tall spikes Pickerelweed Pontederia cordata topped with blue flowers all summer Tough and attractive Spreading perennial that will grow in shallow Lizard s Tail Saururus cernuus standing water and wet soils Pendant spikes of white flowers in late spring and summer Tough emergent aquatic with arrowhead shaped Duck Potato Sagittaria latifolia leaves and spikes of white flowers produced throughout summer Reproduces rapidly Common rush found throughout NC 2 3 tall Soft Rush dci esas with dark green spiky foliage Green flowers are to brown seed pods throughout summer Near evergreen Large 3 4 tall and wide clump forming bulrush Woolgrass Scirpus cyperinus producing wooly green flower heads in summer that age to an attractive rusty brown as seed mature Table 6 3 Sample upland wetland plants Scientific Name Swamp Milkweed Asclepias incarnata Pink flowers in early summer Larval food of monarch butterflies Masses of rosy mauve flowers in late summer Joe Pye Weed Eupatorium fistulosum fall attract butterflies Helianthus Towers of 3 wide golden sunflowers in fall Swamp Sunflower hes angustifolius attracts butterflies Red Star Hi
19. Criteria esee 10 4 Permeable Pavement Construction ccooccoccocnconconnconconoconcnnnonncanoncnnanons TOU T SOW OXCQVOlOE rason m detnivmIeSqNu A DN OPEN DUE TOM LAVO DASS 10 4 3 Concrete edging cooococcnoccncocnconiconocanonanonononnnnanonanonanonanonanennnnaninas 10 4 4 Permeable pavement installation essere 10 5 Permeable Pavement Maintenance ccocccccocccccccccoccccnccnconcnncnnnoncnncnnnonons LO RSL NCS in 72 Community Conservation Assistance Program BMP Design Manual ORTH CAROLINA SOIL amp WATER 1 0 Introduction Community Conservation Assistance Program BMP Design Manual 1 1 Introduction to the Community Conservation Assistance Program The population distribution of North Carolina is becoming more urban and suburban causing the North Carolina Division of Soil and Water Conservation to address nonpoint source pollution from these non agricultural lands In the summer of 2006 a survey was distributed to all districts to inventory their level of interest in best management practices on urban suburban and rural lands Many districts completed surveys about their needs for this program and they requested over 6 5 million for local projects Division staff used the survey responses to develop two grant applications for program funding both were approved at 100 funding In addition the Soil and Water Conser
20. Figure 5 5 A gravel verge and grass filter strip for use in dissipating energy and removing sediment from sheet flow into a bioretention cell 2i ad Figure 5 6A gravel verge precedes a sod filter strip leading into this bioretention area rain garden 30 ORTH CAROLINA SOIL amp WATER 5 3 Outlet design Community Conservation Assistance Program BMP Design Manual The outlet structure controls the ponding depth in the rain garden and allows excess flows to exit the garden in a controlled manner without damaging any berms used to form the garden or the drainage infrastructure The first step in designing an outlet structure is determining the peak flow of the desired design storm for the given watershed often the 10 year storm Section 3 4 discusses the use of the Rational Method for calculating peak flow rate A weir structure is commonly used for bioretention cells with the weir sized so not to be overtopped during the peak of the 10 year storm During the peak flow of the 10 year storm all higher velocity outflow should be in contact with the weir not contacting soil composing the berm Section 3 5 2 discusses weir sizing Appendix B describes the layout and construction of a wooden weir structure For smaller rain gardens as will often be the case a simpler structure than what is described in the appendix may be used For example a treated 2x12 with a weir notch cut in it installed using some of the guidance in
21. L7 1001 XE F Friction head loss ft L Linear length of pipe ft L Equivalent length for pipe fittings ft F o Friction loss per 100 feet of pipe from Table 2 ft If the calculated friction loss 1s very high 1t may be necessary to consider using pipes of larger diameter to prevent damage to the pump or plumbing system A rule of thumb 1s to limit the water velocity in the pipes to 7 5 feet per second or less If water will be pumped through a filter the head loss through the filter should be included with the friction term Because head loss due to filtration varies depending on the type of filter being used and the flow rate the filter product literature must be consulted for spe cific head losses description of pump performance and can be obtained from a pump manufacturer or vendor While the publication of pump curves varies depending on the source the most common curve 1s the head flow curve This curve illustrates what flow the pump will supply for a given head requirement The pump is designed to operate at a point some where along this curve It is important that the target head and flow lie either on or below this curve If the target head and flow lie above and to the right of this curve the pump will not be able to supply enough water to meet design requirements For best over all efficiency the target discharge should be in the middle third of the curve Head Discharge Curve for 6 Impeller s
22. Location Close attention should be paid to where the water from a stormwater BMP exits a given property During small rain events depending on soil conditions all the stormwater produced in a small watershed may be retained in the BMP However the BMP will also experience larger rain events that overload it with stormwater During these events the stormwater BMP will fill to capacity and spill into the adjoining area Special care should be taken to ensure that the area to which the excess stormwater spills is able to convey the stormwater safely to a nearby drainageway such as a Culvert or swale The easiest way to allow excess stormwater to leave a property is to direct it into the existing drainage pathways For instance if a roof drain leads to a swale which routes the water off the property a BMP could be placed in the path of the swale whereby small amounts of runoff are retained and treated and stormwater from larger storms flows out of the BMP and continues down the swale Other drainage conveyances can be used to safely allow large stormwater volumes to leave a property Yard inlets swales and ditches can all be used for this purpose In some cases a swale must be constructed between the BMP and the closest natural drainageway to allow proper draining of the system More information on swales is available in section 8 0 When earthen berms are used to retain stormwater a common practice in rain garden and backyard wetland desi
23. R AEA ENE EE AAAA 17 FDIS SION D N e o Pr 17 3 6 Determining Outlet Location and Size ccooccccccccncocncocococononocancnncnnnnnnonos 18 3 6 1 Determining Outlet Location eese 18 3 6 2 Determining Outlet Size cesses eese nennen nnne nnn nis 18 OA OM MEER 19 3 7 Riparian Buffer ConsideratiONS oocooccoccconcoccconioncocnconoocncnncnnoonncanoncnnanans 19 3 8 General Soils DISCUSSION ooccooccoccconconiocnconoocnooncoronnnnononcnonncornnnnnanencnnnnnns 20 3 8 1 Information from Soil SUIVeyS ocoooccccccccccccnccnnconnconnonnonononononononnnnnnonans 21 3 8 2 Information from Site Investigation oocooncconiconiconoconocnnconnonnnnnonans 21 3 9 Information on Water Table Depth ooccocccoccociccccoccocnconconconncanoncnonnnos 23 4 0 General Stormwater BMP Considerations 23 4 1 Avoid Concentrated Flow ocoocccccconcoccconcoccocnconconocononconononnonnncanonnonononos 23 AZ Multiple Properly OWNGElS wasdnccshiountaandatwanaanstuansubaawdadasdsdoabebannonneuaneamnalasaeiuts 24 4 3 General Information Matrix for Backyard Stormwater BMPS 20 5 0 Backyard Rain Garden Design ceres seen nennen 25 SGT adii OF adr eut serrana diaria 25 5 2 Selection of this practice over OtNerS cooccocccoonconiccnconcocnconconoonncanonnnnannns 25 Ral Garden DESIO RETE TTE 26 5
24. a piece of wire a pole or your hand Inspect it regularly the drawdown hole can clog at any time Visit the site once a month to make sure water is flowing freely through the orifice and inspect the outlet after every rainfall event exceeding 2 inches CLEAN AWAY FLOATING DEBRIS AND TRASH Stormwater wetlands and wet ponds are located in low elevations of the landscape All water from sev eral acres drains to wetlands and wet ponds With this water comes trash and other debris called floatage It must be removed from wetlands and wet ponds for several reasons e tis unsightly particularly when the wetland or wet pond is designed to be an attractive amenity Floating trash such as cups or plastic bags often store small amounts of water in a sheltered environment Studies have shown that mosquito larvae are more likely to be protected inside floating trash than in the exposed pond Trash and other floating debris can clog the drawdown hole the orifice which is often used to slowly release captured runoff Figure 5 Inspect wet ponds and wetlands for trash regularly and frequently typically once a month but occasion ally once a week On smaller wetlands and wet ponds collect trash by simply wading along the edges With Figure 5 Trash floats to the drawdown hole where it can clog the small hole restricting flow Removing the trash is often very simple but essential larger facilities a small boat or vac
25. a sit i m en ts kS Lc dm TOP a e a z s mi m d Cg E a 1 p y A E e 1 A Us emi s r 1 Teki lie i E ta e un t s Amt 2 L ee as t T gt de di ae 7 Ti i m ko Fa im i NL QUEE pa j im b a i E m7 e bns E Mix iom en iy b je es mos m a a 2 A ds m au rip E zs EM D bul a z a bs Ts H a n M rt a Ww x h a ih 4 i i Me dd d 1 Pa E ris m EE Je by apa E KL gh a E ba 1 7 Qu me s Figure 7 3 300 gallon cistern in Greenville N C The cistern is part of a larger system consisting of gutter diversions outlet lines and often a pump Figure 7 4 This system is referred to as a rainwater harvesting system Rainwater captured in cisterns is used or harvested for uses such as irrigation toilet flushing vehicle washing and clothes washing Irrigation and car washing are expected to be the primary use for stormwater collected in backyard cisterns These uses are non potable non drinkable only It is important to alert users that this water is not for drinking such as was done in Figure 7 3 Figure 7 3 Warning signs on a cistern to prevent human intake of the stormwater The demand for cistern water and the size of the cistern relative to the contributing watershed rooftop tends to govern how much runoff reduction the water harvesting system provides Because rain barrels tend to be very
26. as the underlying soil for rain gardens and permeable pavement As discussed in section 10 2 the need for infiltration in the underlying soils of permeable pavement prohibits its use in locations other than the Coastal Plain or Sandhills Conversely BMPs that require saturated soils to maintain BMP function and plant vitality backyard wetland require that water be easily accessible to plant roots via the water table or that poorly drained soils at the site will hold the captured stormwater long after a rain event Section 6 0 When a site is 20 STA ORTH CAROLINA N SOIL amp WATER sii Ri Community Conservation Assistance Program BMP Design Manual CONSERVATION pe Enginering evaluated for BMP implementation determining the soil type at the site can be performed by collecting information from soil surveys and most importantly from field reconnaissance 3 8 1 Information from Soil Surveys Division of Soil and Water Conservation staff should be familiar with the soil survey of their particular county This soil survey provides some understanding of the type s of soils present at the site being considered for BMP installation Once the soil that is present at the site in situ soil is identified the staff member should observe the detailed description of the soil series in the soil survey This description will give some detail as to the common location of a given soil in the county landscape and will give some
27. be required each at least 4 feet away from the other again this should be done in multiple locations around the proposed site Each of the three holes will have a different depth 1 foot 2 foot and 3 foot Each hole should be filled with water The 1 foot hole should be empty in 6 hours the 2 foot hole should be empty in 12 hours and the 3 foot hole should empty in 18 hours This test is performed to ensure no impermeable layer is present in the top 3 feet of soil 65 STA ORTH CAROLINA SOIL amp WATER In general the soil classification should be no finer than a Loamy Very Fine Sand as defined by the United States Department of Agriculture Natural Resources Conservation Service More detail on permeable pavement is available in section 3 10 of the North Carolina Stormwater BMP Manual 2005 If these infiltration criteria are met the soils at the site meet the screening criteria for permeable pavement installation however detailed soil analysis should be performed to ensure that the site is suitable It is recommended that a soil scientist or a professional competent in evaluating soils be consulted to determine a given soil s permeability if it is questionable Community Conservation Assistance Program BMP Design Manual LT Engineering 10 3 2 Soil Analysis Lab Test If the initial field soil test described in section 10 3 1 indicates that the soils may be suitable for further analysis soil samples should b
28. effective stormwater practices while efficiently distributing funds throughout participating districts As part of the CCAP North Carolina State University Biological and Agricultural Engineering Department BAE has also prepared a document entitled Stormwater BMP Costs for use by employees of the Soil and Water Conservation Division Hathaway and Hunt 2006 This resource was produced SO proper cost estimates can be prepared by district staff Proper BMP cost estimates will lead to efficient allocation of cost share funds This document should be referenced in conjunction with the CCAP Stormwater Best Management Practice Design Manual to fulfill the intentions of the program 2 0 Introduction to Stormwater 2 1 Definition of Stormwater Stormwater is produced immediately following a rainfall event or as a result of snowmelt When a rainfall event occurs there are four potential fates for the precipitation 1 Some infiltrates into the soil 2 a portion is taken up by plants and released to the atmosphere called transpiration 3 a fraction evaporates to the atmosphere and 4 the remainder of the rainfall runs off land surfaces and E lt NORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual CONSERVATION impervious areas This final fraction is stormwater North Carolina Division of Water Quality Stormwater Unit 2 2 Effects of Urbanization North Carolina contain
29. lex opaca Red Cedar 1 2 Juniperus virginiana Sweet Bay 3 Magnolia virginiana Devilwood 1 2 Osmanthus americanus Red Bay 1 2 Persea borbonia Evergreen shrubs that can be grown as small trees include Yaupon Wax Myrtle and Anise Shrub Shrubs Deciduous Chokeberry 1 3 Aronia arbutifolia Beautyberry 2 Callicarpa americana Sweet Shrub 2 Calycanthus floridus Buttonbush 3 Cephalanthus occidentalis Pepperbush 2 Clethra alnifolia Strawberry Bush 2 Euonymous americanus Fothergilla 2 Fothergilla gardenii Winterberry 3 Ilex verticillata Virginia Willow 3 tea virginica Spicebush 2 Lindera benzion Possumhaw 3 Viburnum nudum Dusty Zenobia 2 Zenobia pulverulenta Shrubs continued Evergreen Florida Leucothoe 2 Agarista populifolia Inkberry 2 Ilex glabra Yaupon 1 2 Ilex vomitoria Florida Anise Shrub 3 llicium floridanum Anise Shrub 1 2 Illicium parviflorum Coastal Leucothoe 2 Leucothoe axillaris Wax Myrtle 1 2 Myrica cerifera Dwarf Palmetto 3 Sabal minor Perennials Blue Star 3 Amsonia tabernaemontana Lady Fern 2 Athyrium felix femina Butterflyweed 1 Asclepias tuberosa Swamp Milkweed 3 Asclepias incarnata Climbing Aster 3 Aster carolinianus False Indigo 1 2 Baptisia species Boltonia 3 Boltonia asteriodes Turtlehead 3 Chelone glabra Green and Gold 2
30. no sensitive upstream areas are flooded because of the wetland during any rain event 6 3 5 Backyard wetland plants Wetland plants like any other plant species have specific environments that they thrive in Most notably for backyard wetlands water depth plays an important role in plant selection For the purposes of the CCAP these water depths will be divided into three groups upland plants not normally in contact with surface water planted around rim of wetland where roots can access moist soil shallow water plants water depth consistently between 0 and 4 inches and deep water plants depths consistently greater than 6 inches 36 lt ORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual A short list of plants which thrive in each environment is shown in tables 6 1 6 2 and 6 3 This information was gleaned from A Citizen s Guide to Protecting Wilmington s Waterways Butler 2005 Additional plant choices are available especially with regard to perennials A local cooperative extension horticulture agent should be contacted for additional plant choices In times of extreme drought the land owner is encouraged to water the wetland as needed based on the condition of the plants if they show signs of stress due to lack of rain Most backyard wetlands will NOT retain water deep enough for a long enough period to support deep water plants However if after the wetlan
31. overflow whenever a berm is necessary Community Conservation Assistance Program BMP Design Manual LT Engineering Whether the BMP is constructed on a flat landscape or one that is sloped a strip of erosion control fabric the width of one roll of fabric which is approximately 6 feet should be added immediately downslope of the backyard wetland This is shown in both Figures 6 6 and 6 8 This erosion control fabric will help prevent erosion as the water leaves the wetland After the berm and outlet structure have been constructed the internal cavity or bowl of the wetland can be prepared If the wetland is not sited where the water table will be readily accessed by plant roots high water table Section 3 8 the wetland plants will rely on ponded stormwater remaining in the wetland due to poor infiltration In locations where poor infiltration is relied upon a hand tamper or other mechanical compactor can be used to compact the underlying soils This compaction will result in reduced soil infiltration leading to improved water retention within the wetland After the underlying soils have been prepared the topsoil that was removed should be reapplied on top of the underlying soils at a depth of 2 3 inches This topsoil can be raked into the underlying soils to provide a suitable media for plant growth this topsoil should not be compacted Some 10 10 10 slow release fertilizer may also be added to the soils in small qua
32. short periods 1 2 days of flooding Plants that will tolerate longer periods of flooding 3 5 days but will also grow in moist to average soils Establishment usually takes 1 2 years for trees and shrubs and 1 year for perennials For more detailed information and images of each plant visit the Plant Fact Sheets available on NCSU s Urban Horticulture website www ncstate plants net NC TATE UNIVERSITY A amp T STATE UNIVERSITY COOPERATIVE MA EXTENSION Helping People Put Knowledge to Work APPENDIX F Plant Selection for Zone Rain Garden A List of adaptable plants that can be used in a zoned rain garden Large Trees River Birch Betula nigra Bald Cypress Taxodium distichum Small Trees 10 20 Green Hawthorn Crataegus viridus Possumhaw llex decidua Sweet Bay Magnolia virginiana Shrubs Buttonbush Cephalanthus occidentalis not as readily available others but very tough Virginia Willow Itea virginica Possumhaw Viburnum nudum Winterberry llex verticillata Dwarf Palmetto Sabal minor Coastal Plains Only Perennials Blue Star Amsonia tabernaemontana Joe Pye Weed Eupatorium dubium Blue Flag Iris Iris virginica Ironweed Vernonia novaboracensis Swamp Mallow Hibiscus moscheutos Common Rush Juncus effusus APPENDIX G Pond and Wetland Maintenance A ES Distributed in furtherance of the acts of Congress of May 8 and June 30 1914 North Carolina State Uni versi
33. stormwater practice is sized to capture 90 of all rain events it will theoretically be able to treat at least 90 of the pollutants from a given watershed Prior to the study the water quality storm was considered to be 1 inch in all areas of North Carolina excluding CAMA counties In CAMA counties the water quality storm was considered 1 5 inches Figure 3 1 shows a map of the CAMA counties in North Carolina oO Lee SEI Ta L LE TR Sd HT KS CT ae gt P Figure 3 1 Map of CAMA counties in North Carolina 11 tT ORTH CAROLINA SOIL amp WATER Thirty years of rainfall data were analyzed from 9 locations in North Carolina For most of North Carolina capturing approximately 1 inch of rainfall would only result in capturing 80 of all the rainfall that fell on a given watershed Likewise in CAMA counties 1 5 inches would result in capturing approximately 80 of all the rainfall Because of the premise that the majority of pollution is carried in the earlier portions of the storm it was assumed that at least 90 of pollution would be transported by the runoff produced by 80 of the rainfall NCDENR considered these values to be acceptable based on this assumption so the stormwater design standards remained the same There are a few supporting studies in literature for this Flint and Davis 2007 but the theory has not been proven completely Sansalone and Cristina 2004 Community Conservation Assistance Program BM
34. types of vegetation that survive in the practice Usually this change is undesirable Muskrat infestation is a difficult maintenance problem that usually must be addressed only when a pond or wetland has suffered from neglect If the practice is infested muskrats can be trapped under water where they drown Muskrats frequently escape traps however which makes live trapping difficult Hire a licensed experienced trapper who takes care to place traps where pets cannot be trapped by mistake Once muskrats have been removed from the pond their dwelling holes should be destroyed KEEP GEESE AWAY FROM WET PONDS AND STORMWATER WETLANDS Canada geese are attracted to an open body of water with good visibility around the pe rimeter and they enjoy eating grass This describes many old design wet ponds that are mowed to the edge allowing geese easy access in and out of the pond Designers can include features in a wet pond to prevent Canada geese from taking up residence Build a visual barrier along the pond pe rimeter the aquatic shelf By taking away good visibility geese will not feel as safe Most newly designed ponds include some aquatic shelf Place shiny objects such as silver tape around the perimeter of the pond if build ing an aquatic shelf is not feasible e Place a grid of string across the wet pond to prevent easy waterfowl water landing This string can also have shiny tape at tached to it Figure 10
35. typical flows for common fixtures Specific flows for many devices can be found in the product literature and should be used in calculations when available HEAD The head required of the pump is deter mined by calculating the total dynamic head of the system Equation 1 Total Dynamic Head TDH TDH h h h TDH z total dynamic head ft h operating head pressure required by fixture ft h h h elevation difference between pump and fixture ft h friction loss in system ft When the cistern feeds multiple fixtures the total dynamic head should be calculated independently for each fixture Because total dynamic head 1s not cumu lative the specifications for the fixture that demands the most head should be used in pump selection Table 1 Typical minimum requirements of common fixtures in water harvesting we Pressures Pressuretps Fow Clothes washer 5 gpm 4 gpm Cistern Figure 3 Graphic representation of the terms in Equation 1 PRESSURE H The term pressure corresponds to the pressure required at the outlet to operate whatever device 1s being used Typical minimum operating pres sures for a number of devices are found in Table 1 For specific pressure requirements consult the product lit erature Equation 2 can be used to convert pressures in psi to feet of head If the pressure selected is too low the pump may not provide an adequate flow to oper ate the d
36. typically require less maintenance than displacement pumps Typical dynamic pumps include centrifugal and jet pumps Figure 2 Centrifugal pump 2 A centrifugal pump is suitable for most water har vesting applications because of its relative simplicity range of head and flows and relative low cost A cen trifugal pump consists of a motor pump casing and impeller Figure 2 The pump casing holds the impel ler and channels water to and from the impeller The impeller is a rotating vane structure that draws water into the pump casing transfers energy to the water and discharges it from the pump casing The motor supplies the energy needed to rotate the impeller For a specific pump casing the motor and impeller often can be sized to meet head and flow requirements When a pump is installed above a cistern prob lems with priming the pump and maintaining the prime arise For this reason either a submersible pump should be installed toward the bottom of the cistern or a plumber should be consulted for specific pump requirements CALCULATION OF PUMP DEMAND FLOW The flow for a pump system is determined by adding the flows of all fixtures such as toilets wash ing machines sprinklers etc that may draw water from the cistern at the same time For example if a lawn needs to be watered with an impact sprinkler and a toilet is flushed while the sprinkler 1s running the flows should be added for both uses Table 1 contains
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38. will wither and die Figure 7 The herbicide must be applied by hand rather than by broadcast spray because it will kill every herba ceous plant it touches Use only aquatic formulations of glyphosate because they do not harm fish and other aquatic species The frequency of cattail removal can vary Several factors influence the need to apply herbicide to cat tails the density at which the wetland is planted with desirable species the time of year the wetland is planted and the maturity of the wetland During the first year or two after wetland construction remove cattails twice a year As the wetland matures 6 and desirable species begin to dominate reduce the maintenance frequency to once a year The amount of time needed to remove unwanted vegetation via the glyphosate wipe varies but a well maintained mature wetland requires visits of about 2 hours per acre of wetland Other unwanted plant species include common reed Phragmites species Figure 6 various noxious floating aquatics such as parrot feather Myriophyl lum aquaticum and giant salvinia Salvina spp and Asiatic dayflower Murdannia keisak Phragmites species can be removed in a manner similar to that described for cattails Noxious floating aquatics may require careful chemical or physical removal If you observe these exotic invasive species contact your county Extension center MOW THE PERIMETER OF WET PONDS Stormwater wetlands are not mowed to the
39. 0 e 032 31 29 3o 20 80 039 38 36 0 35 Table A14 Flow calculations and suggested weir length 3 4 acre watershed various watershed scenarios intensity of 9 7 in hr dd lans m iem Weir Required Weir Suggestion cfs im fi 8 95 9 Eze 10 e os 23 22 25 2 80 039 28 427 330 J Table A15 Flow calculations and suggested weir length 1 2 acre watershed various watershed scenarios intensity of 9 7 in hr adr anon cem Weir Required Weir Suggestion cfs MM tf _ s 3 3 14 mw e 032 16 18 15 2 80 039 19 4318 Z3 20 Table A16 Flow calculations and suggested weir length 1 4 acre watershed various watershed scenarios intensity of 9 7 in hr Edu Unt eem Weir Required Weir Suggestion cfs E E 1 9 93 1 E 31 328 181 424 2 80 039 09 09 10 J 40 60 053 13 12 J 15 APPENDIX B Outlet Construction Wooden Weir Control Structure A Figure B1 is a diagram of a wooden weir control structure This type of structure can be used to control ponding depth drawdown rate and depth of outflow from a variety of devices including wetlands and rain gardens Only lumber treated for at least in ground use should be selected Do not use CCA treated lumber Stainless steel fasteners are recommended The str
40. 1 5 days after a rain event it is likely at least somewhat poorly drained Additional information can be gathered by digging a hole in the location of the proposed BMP that is approximately 2 foot deep or to the depth of the bottom of the proposed BMP whichever is deeper As the hole is being dug the soil should be observed for signs that it is a wetland soil Wetland soils are commonly grey with ribbons of brown Figure 3 7 and 3 8 If wetland soils are identified within 1 foot of the surface at a given site the site is likely poorly drained A detailed description of wetland soils is available in Vepraskas 1999 21 STA Community Conservation Assistance Program BMP Design Manual i EM ww Figure 3 7 Brown ribbons in Figure 3 8 Brown ribbons in wetland soil wetland soil close The last evaluation that should be used during the site investigation is a simplified soil infiltration test This is a test to check the permeability of the soils being evaluated for BMP suitability A hole should be dug using an auger or spade approximately 1 foot below the expected bottom of BMP The newly dug hole should be filled with water Figure 3 9 Monitor how quickly the hole drains and use this information to select the appropriate BMP Table 3 3 shows a range of outcomes from the simplified soil infiltration test and the resulting BMP that is suggested based on the results More information will be presented on these BMPs in section
41. 2B 0100 0200 amp 0300 These rules state that a 50 foot riparian buffer should be maintained around surface waters in the Tar Pamlico Basin Neuse River Basin the main stem of the Catawba River and in the Randleman Lake watershed all these water bodies being nutrient sensitive The purpose of these rules is to make use of the 19 STA ORTH CAROLINA N SOIL amp WATER l sii Ri Community Conservation Assistance Program BMP Design Manual CONSERVATION preston Esgisaaring inherent ability of riparian buffers to remove nutrients and other pollutants from stormwater runoff These riparian buffer rules can impact the function and siting of a backyard stormwater BMP When siting a backyard BMP in one of the nutrient sensitive watersheds described in the NCDWQ Redbook a measurement should be made perpendicular to the stream from the stream bank towards the area being considered for a BMP No part of the BMP should be located within this 50 foot boundary running along the stream bank Additionally backyard swales can not be constructed in the riparian buffer Only existing swales can be utilized to route water through the riparian buffer The riparian buffer rules not only include a requirement to maintain a 50 foot buffer along the stream bank they also require that stormwater released into the buffer be done so in a diffuse manner This means that in systems where water will be collected and dispersed through an outl
42. 5 and 4 days a drought tolerant wetland should be constructed at the site More information on drought tolerant wetlands is given in Section 6 3 6 Community Conservation Assistance Program BMP Design Manual SWCD staff should evaluate sites with poorly drained soils extremely carefully The overwhelming majority of CCAP projects will be too small to impact jurisdictional wetlands if wetland soils are detected on a large project gt 1 4 acre then the property should be evaluated by a member of the Army Corps of Engineers and or the North Carolina Division of Water Quality 401 404 Wetlands Unit 6 3 Backyard Wetland Design 6 3 1 Determining wetland watershed characteristics The first step in backyard wetland design is determining the watershed area that drains to the proposed site The percentage of impervious area in this watershed should also be determined This impervious area will most likely consist of driveways and rooftops In order to capture stormwater runoff from driveways and other associated parking areas the drainage must naturally flow to the proposed location Rooftops normally are drained via gutters which may or may not drain to the proposed location In this case either a 4 to 6 inch plastic flexible pipe or a diversion berm Section 3 5 can be used to direct this water to the proposed rain garden location After any modifications are considered routing rooftop drainage to the proposed area the tot
43. 6 24 25 20 80 039 31 29 3530 J Table A10 Flow calculations and suggested weir length 3 4 acre watershed various watershed scenarios intensity of 8 0 in hr bis ven KE Weir Required Weir Suggestion mpervious cfs Pr COS e 32 4 w 90 032 19 18 20 20 80 039 23 22 25 Table A11 Flow calculations and suggested weir length 1 2 acre watershed various watershed scenarios intensity of 8 0 in hr pendons zc uat Kr Weir Required Weir Suggestion cfs COS ICI 95 i 10 90 032 13 12 185 y 20 80 039 16 A315 A315 Table A12 Flow calculations and suggested weir length 1 4 acre watershed various watershed scenarios intensity of 8 0 in hr importas IR Edu ent DUET em Weir Required Weir Suggestion Impervious cfs ST O TOR 2 80 039 08 207 X 10 20 25 75 043 oo 08 10 30 70 046 _09 09 A410 35 65 oso 10 09 4140 40 60 053 11 10 j 510 Table A13 A16 use a rainfall intensity of 9 7 in hr These calculations should only be used for Wilmington Table A13 Flow calculations and suggested weir length 1 acre watershed various watershed scenarios intensity of 9 7 in hr Eur uen eem Weir Required Weir Suggestion cfs 8 95 9 28 1
44. Appendix B may be sufficient All structures should extend at least 6 inches below final grade at the base and 12 inches into the sides of the berm Soil filled around the structure should be well compacted in 4 to 6 inch lifts The rain garden should be designed so that no sensitive upstream areas are flooded because of the garden during any rain event 5 4 Rain Garden Maintenance The first step in maintenance of a rain garden is designing the system to be low maintenance If sediment can be removed before entering the body of the rain garden using the inlet protection methods discussed in section 5 3 6 maintenance will be less intensive Any sediment buildup in the garden or in an inlet protection area should be cleared as necessary to prevent clogging of the garden Plant health should be monitored during establishment and during droughts this can help ensure their survival It may be necessary to water the rain garden in such times depending on the garden s condition It is also necessary to check the outlet structure to make sure it is clear and isn t preventing the garden from draining properly Fertilizers are not recommended as sufficient nutrients for the plants are delivered from the watershed Mulch will need to be replaced 1 or 2 times per year for aesthetics to shade the soil and to help keep weeds out Weeds will arrive in the garden and must be occasionally removed Pesticides are not recommended for weed control in rain gar
45. Carolina State University and Bill Lord Area Environmental Agent North Carolina Cooperative Extension Published by NORTH CAROLINA COOPERATIVE EXTENSION SERVICE COLLEGE O F AGRICULTURE y LIFE SCIENCES ACADEMICS RESEARCH EXTENSION NC STATE UNIVERSITY AGW 588 07 E07 45831 08 2006 bs APPENDIX H Cistern Additional Information Water Budget Model E EE ML A A A A A A A Rainwater Harvester 1 0 User Manual Developed by Matthew Jones BAE Stormwater Engineering Group Biological and Agricultural Engineering Department North Carolina State University NCSUWaterHarvesting gmail com NI IC The Rainwater Harvester model simulates the performance of a rainwater harvesting system using historical rainfall data to evaluate a daily water balance The model is intended to assist designers in sizing a rainwater harvesting cistern based upon rainfall inouts and anticipated usage General Operating Instructions Load rainfall data file Input system design values Change tab to Water Usage and input usage values Click Simulate button at lower right View system performance summary data Optional 6 Click Animate button to view a visual representation of the cistern level over time 7 Save the simulation results to a comma separated variable csv file Inputs System Configuration Rainfall Input File Rainfall input files for several North Carolina cities are av
46. E Head Discharge Curve or 4 5 Impeller Cea g 10 12 14 Flow gpm Figure 4 Example of a pump curve with 2 impeller options PUMP ACCESSORIES Depending on the water harvesting application some pump accessories may be needed Consult a pump vendor to determine which accessories are necessary and compatible with a specific pump PRESSURE TANK The tank stores pressurized water to prevent the pump from cycling on and off to meet small demands it also supplies a constant pressure PRESSURE SWITCH This device engages the pump when a pressure drop is observed and disengages it when there is no demand For example the pump will engage to supply water when a faucet is opened and disengage when no water is being used CHECK VALVE The internal check valve prevents water from flowing back through the pump when it is not running Some pumps do not have these valves Consult a pump vendor or plumber to determine 1f ad ditional check valves are needed FLOAT LEVEL SWITCH This switch when installed in the cistern can prevent damage to the pump by disen gaging the pump when the cistern runs dry or cistern water falls below a predetermined level THROTTLING VALVE This device is intended to control the flow and pressure of water exiting the pump and is typically in the form of a gate valve Special control systems can be installed to maintain a constant output despite changing water levels within a cistern EXAMP
47. Guide to Protecting Wilmington s Waterways City of Wilmington NC Storm Water Services Donaldson Susan 2007 The Effects of Urbanization on the Water Cycle Nevada Cooperative Extension Fact Sheet 04 43 Ferguson B K ed 2005 Porous Pavements Boca Raton FL CRC Press LLC Flint K R and A P Davis 2007 Pollutant Mass Flushing Characterization of Highway Stormwater Runoff from an Ultra Urban Area Journal of Environmental Engineering 133 6 616 626 Hathaway J M and W F Hunt 2006 Stormwater BMP Costs Division of Soil and Water Conservation Community Conservation Assistance Program Raleigh NC Biological and Agricultural Engineering Department North Carolina State University Leopold L B 1968 Hydrology for Urban Land Planning U S Geological Survey Circular 544 North Carolina Division of Water Quality Stormwater Unit What is Stormwater http h2o enr state nc us su what is stormwater htm Accessed April 2 2007 Richardson J L and M J Vepraskas eds 2001 Wetland Soils Genesis Hydrology Landscapes and Classification Boca Raton FL CRC Press LLC Sansalone J J and C M Cristina 2004 First Flush Concepts for Suspended and Dissolved Solids in Small Impervious Watersheds Journal of Environmental Engineering 130 11 1301 1314 Schueler T R and H K Holland eds 2000 The Practice of Watershed Protection Ellicott City MD Center for Watershed Protection 12 tT NORTH WA
48. LE CHOOSING A PUMP FOR A WATER HARVESTING SYSTEM An aboveground water harvesting cistern is installed at a home in coastal North Carolina The cistern will be used to flush a toilet on the second floor of the 3 gt home and to run an impact sprinkler for the lawn All pipes in this example are inches in diameter Select a pump that meets the needs shown in Figure 5 90 Elbow Sprinkler Figure 5 Example of a simple water harvesting system REQUIRED FLow From Table 1 the required flow is 6 gpm for the toilet and 4 5 gpm for the impact sprinkler It may be necessary to operate both fixtures at the same time therefore Total Peak Flow 6 gpm 4 5 gom 10 5 gpm REQUIRED TOTAL Dynamic HEAD Because there are 2 fixtures for this system the required total dynamic head for each fixture must be calculated independently TOILET h h h 12 ft from Figure 5 h 46 ft Table 1 Friction L 5 0 3 4 Tee Branch Flow Table 3 2 0 3 4 90 Elbow Table 3 7 0 ft L 30 12 3 45 ft Adding lengths of pipe segments h 45 7 0 100 x 8 85 Friction factor for 34 pipe at 6 gpm Table 2 4 6 Equation 3 Total Head Requirement 12 46 4 6 62 6 ft Equation 2 Sources for numbers are noted by blue italics IMPACT SPRINKLER h h h 0 ft from Figure 5 h 93 ft Table 1 Friction L 1 4 ft 3 4 Tee Straight Flow Table 3 L 30 60 90 ft A
49. NA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual Where Qp Peak Flow ft s section 8 2 n Manning s n value Table 8 2 D Swale depth ft S swale slope ft ft Equation 8 4 Manning s Equation for triangular swales sideslope 4 to 1 Qp 1 49 n x 1 26 x D 9 x S Or D Qp x n 1 88 x S9 Equation 8 5 Manning s Equation for triangular swales sideslope 5 to 1 Qp 1 49 n x 1 46 x D x SO Or D Qp x n 2 18 x S p 7 8 4 2 Simplified triangular swale design process With the simplifications made to the Manning s equation for triangular swales the design process for these practices becomes fairly straightforward In a basic design scenario the slope S of the swale will likely be set Section 8 2 Likewise the peak flow Qp from the supplying watershed will be calculated using the rational method Therefore the depth of swale D and Manning s n will be the only unknowns once a given swale geometry is chosen If space is available sideslopes of 4 to 1 or 5 to 1 can be used most often 4 to 1 This will allow easy access to the swale for mowing equipment If limited space is available 3 to 1 sideslopes may be used The design process may require a few iterations to achieve an acceptable design In general the steps of the design process are as follows Calculate peak flow for the 10 year storm from the watershed Determine slope from swale be
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51. P Design Manual LT Engineering 3 2 Connected Imperviousness The idea of impervious areas was discussed above in section 2 2 however not all impervious areas substantially contribute to stormwater runoff in a given watershed Impervious areas that immediately drain to a drainage system such as a pipe or swale are considered to be connected impervious areas and more readily produce runoff within urban environments For example a rooftop that drains to a gutter which drains directly unto a nearby street as can be found in many cities and into the street drainage would be considered connected impervious Conversely if a rooftop drained onto a lawn where runoff would sheet flow across the grass the rooftop would not be considered connected impervious as it is does not immediately drain into any stormwater conveyance system Runoff from non connected impervious areas is routed to some pervious area where it has a chance to infiltrate As a rule of thumb if a rooftop or pavement is allowed to sheet flow for at least 30 feet before it re concentrates e g forming a swale or spilling into a drop inlet it may be considered a non connected impermeable surface 3 3 The Simple Method The water quality storm can easily be used to calculate the anticipated volume of runoff that will leave a given residence or small business through the application of the simple method Schueler 1987 The required information is as follows 1
52. TER 3 Community Conservation Assistance Program BMP Design Manual Schueler T 1987 Controlling Urban Runoff A practical Manual for Planning and Designing Urban Best Management Practices Metropolitan Washington Council of Governments Washington DC 240 pp Eagen A Aci al Engineering omith D R 2000 Permeable Interlocking Concrete Pavements Selection Design Construction Maintenance 2 edition Washington DC Interlocking Concrete Pavement Institute APPENDIX A Flow Calculations and Associated Weir Sizing for Sample Watersheds mp Assumptions 1 Assume C value for impervious areas Is 0 95 Assume C value for pervious areas is 0 25 Assume 6 inch driving head Use 10 year 5 minute intensity Based on Equations 3 4 3 5 and 3 6 SA O N Table A1 A4 use a rainfall intensity of 6 5 in hr These calculations should only be used for Asheville Table A1 Flow calculations and suggested weir length 1 acre watershed various watershed scenarios intensity of 6 5 in hr ic uu vt Dem Weir Required Weir Suggestion cfs ft 8 s 99 9m 49 10 90 03 21 20 2 2 80 039 25 24 025 Table A2 Flow calculations and suggested weir length 3 4 acre watershed various watershed scenarios intensity of 6 5 in hr i add mx unda Weir Required Weir Suggestion cfs ft ft L8 pe 38 Pap 13 Y 10 90 032 16 015 15
53. You can also bring a dog to the pond regu larly to scare the geese Geese do not like certain species of dogs particularly border collies If they often encounter a frightening dog the Canada geese will eventually move elsewhere Figure 10 A grid of fishing line adorned with silver tape has been strung across this wet pond to prevent geese from making it their home The string grid makes water landing more difficult and geese do not like shiny or flashy objects like wind blown silver tape If the stormwater practice has been regularly main tained muskrat populations can more easily be con trolled Encourage muskrats to move away from the wetland or wet pond by making it an uncomfortable place to live If muskrat holes are observed around the perimeter of a wet pond or stormwater wetland destroy them or fill them with soil Figure 9 Identify and destroy muskrat holes during any regular main tenance activity whenever the wetland or pond 1s being inspected to verify that the drawdown is freely flowing and during mowing and trash removal Removing beavers is more difficult that removing muskrats If a beaver is observed living in or around a stormwater wetland or wet pond contact a profes sional trapper who specializes in beaver removal SUMMARY Well designed stormwater wetlands and wet ponds remove pollutants and mitigate floods To accomplish these goals and remain safe aesthetically pleasing and free of mosquito
54. a A EE Y al gt 1 PS m ans Li al a a a TE p Y LAM qe be used for flood control as well For more informa tion on stormwater practices see Urban Stormwater Structural Best Management Practices BMPs in the Urban Waterways series AG 588 01 MAINTENANCE GOALS Maintenance of stormwater wetlands and wet ponds is performed to achieve four goals efficient hydraulic flow and pollutant removal aesthetic appeal safety and mosquito control Most of the maintenance activi ties associated with wetlands and wet ponds pertain to two or more of these goals The following activities should be performed regularly to maintain stormwater wetland and wet pond efficiency Remove sediment and gross solids from forebays Keep the orifice the drawdown hole free flowing Clean away floating trash and debris Remove vegetation along the dam face Remove invasive plant species Mow the perimeter of wet ponds Control pests such as muskrats and beavers Figure 1 Stormwa ter wetlands across North Carolina A Brevard College in the mountains B Hillandale Golf Course in Durham C Carteret Craven Electric Cooperative near Morehead City and D Smithfield Selma Senior High School in Johnston County REMOVE SEDIMENT AND GROSS SOLIDS FROM FOREBAYS Forebays are located at the inlets to stormwater wet lands and wet ponds They are designed to slow in coming water dissipating the water
55. a A amp T State University U S Department of Agriculture and local governments cooperating NC MA EXTENSION imm Helping People Put Knowledge to Work evwa S Choosing a Pump for Rainwater Harvesting INTRODUCTION Water harvesting is the practice of cap turing rainwater runoff normally from a rooftop and storing it in a tank or cistern for use around a home or busi ness While gravity flow from a cistern is generally enough to fill a bucket or to water a few nearby plants most wa ter harvesting systems require a pump to convey the water from the holding tank to the spot where it will be used Figure 1 Typical uses include water ing plants washing vehicles flushing toilets and washing clothes A pump can increase the uses of the water har vesting system and is required when consistent pressure is needed Knowledge of how pumps operate and how they are selected will ensure that an efficient and appropriate pump is included in the wa ter harvesting system design Because pumps and plumbing sys tems are diverse people installing wa ter harvesting systems should consult an experienced plumber to make sure all ap plicable codes are met and that there will be no damage to the pump or plumbing Pump selection should be based on the flow and total head the pump is required to supply Flow is the rate at which water travels through a pipe and often 1s mea sured in gallons per minute gpm
56. aid down for systems 1500 to 3000 gallons Figure 7 8b The area of excavation should be a couple feet larger than the footprint of the cistern so the cistern can be shifted into place to receive the stormwater flow placed underneath the gutter downspout Larger cisterns such as those made of metal exceeding 5000 gallons of storage should be placed above a 4 to 6 inch concrete pad Figure 7 9 a hole similar to that described above for the gravel base can be excavated and the cement can be poured in to the proper thickness 49 NORTH CAROLINA QR ES Community Conservation Assistance Program e preparatio Figure 7 9 a Concrete pad constructed to support b a 5000 gallon metal cistern 4 2 Preparing the gutter downspout As mentioned previously gutters should already be installed when a cistern is to be implemented The cistern can temporarily be sat in the excavated area near its final location Since the downspout should still be in place it will not be possible to place the cistern exactly in its final location The height of the top of the debris filter should be marked on the gutter downspout Since the screen that actually filters the water lies just below the top of the debris filter See Figure 7 5 the location of that screen should be marked on the downspout as well The cistern can then be moved back out of the way The gutter downspout should be cut approximately 2 inches up from the location of the s
57. ailable from the NCSU rainwater harvesting website The file can have a rai or csv extension with the data formatted as comma separated variables with the following data columns e Index Day Beginning with 1 1 1900 as day 1 Year Month Day Rainfall in 1 100ths of an inch Roof Area Horizontal footprint of the roof area contributing runoff to the cistern Capture Factor This factor represents the ratio of rainfall depth that is captured by cistern Research from USARS suggests a ratio of 0 9 the model s default value Research at NCSU has shown that this factor can be dramatically reduced if gutters aren t kept free of leaves or an inlet filter on the cistern restricts flow City selecting a city from the list loads the default values for water cost and nitrogen concentration Water Cost The cost of a gallon of water from municipal sources Nitrogen Concentration The typical concentration of nitrate and ammonia in rooftop effluent First Flush Depth Depth of rainfall containing the most polluted runoff Values vary by region but typically range from 0 5 to 1 5 Cistern Size Cistern volume in gallons Cistern Cost The cost of the rainwater harvesting system and installation Table 1 Example cost estimates for a variety of cisterns Cistern Volume Plastic 1O0galons 300 O Water Usage Importance of Water Usage Information In order for the model to closely
58. ak 3 Quercus laurifolia Small Trees under 30 tall Deciduous Red Buckeye 2 Aesculus pavia Ironwood 1 3 Carpinus caroliniana Redbud 1 2 Cercis canadensis Fringe Tree 2 Chionanthus virginicus Washington Hawthorn 3 Crataegus phaenopyrum Possumhaw 1 3 Ilex decidua Evergreen American Holly 1 2 lex opaca Red Cedar 1 2 Juniperus virginiana Shrubs Deciduous Chokeberry 1 3 Aronia arbutifolia Beautyberry 2 Callicarpa americana Sweet Shrub 2 Calycanthus floridus Buttonbush 3 Cephalanthus occidentalis Pepperbush 2 Clethra alnifolia Strawberry Bush 2 Euonymous americanus Winterberry 3 Ilex verticillata Virginia Willow 3 tea virginica Spicebush 2 Lindera benzion Possumhaw 3 Viburnum nudum Evergreen Inkberry 2 Ilex glabra Wax Myrtle 1 2 Myrica cerifera Perennials Blue Star 3 Amsonia tabernaemontana Lady Fern 2 Athyrium felix femina Butterflyweed 1 Asclepias tuberosa Swamp Milkweed 3 Asclepias incarnata Climbing Aster 3 Aster carolinianus False Indigo 1 2 Baptisia species Boltonia 3 Boltonia asteriodes Turtlehead 3 Chelone glabra Green and Gold 2 Chrysogonum virginianum Mouse Ear Coreopsis 2 Coreopsis auriculata Tickseed 1 2 Coreopsis lanceolata Swamp Coreopsis 2 Coreopsis rosea Joe Pye Weed 3 Eupatorium dubium Swamp Sunflower 3
59. al area draining to the proposed location and the percentage of impervious area in the watershed can be calculated Manual measurements taken with a tape measure can help determine the watershed area as well as the area draining from driveways patios and parking areas The rooftop area can be measured manually or housing plans can be obtained If the total rooftop area is known a judgment must simply be made on how much of the roof drains to a given downspout that will be routed to the backyard wetland location 6 3 2 Backyard wetland sizing Backyard wetlands should be sized such that the ponding depth of the captured runoff is no more than 9 inches In some cases a property owner may desire a larger wetland if so the ponding depth can be as little as 3 to 6 inches Using the volume of runoff produced in the contributing watershed and this maximum ponding depth the surface area of the wetland can easily be calculated The 34 tT NORTH CAROLINA SOIL amp WATER CONSERVATION shape of the backyard wetlands that are installed as part of the CCAP will vary upon topography obstructions e g trees and utilities and landowner desires The shape will largely be determined by the available space at the stormwater collection point aesthetic appeal should also be considered In general neither the length nor the width should be less than 3 feet Community Conservation Assistance Program BMP Design Manual Once the water
60. an Buffer Width ft Riparian Buffer Vegetation Thick ground cover For any width Forested Forested Forested FOREBAY INCLUSION Forebays should be utilized in lev el spreader systems to dissipate energy and reduce the sediment that accumulates behind the level spreader lip Figure 5 The forebay 1s essentially a bowl shaped depression lined on the bottom and sides with Class B rip rap The forebay should be sized so that it is at least 0 2 percent of the contributing catchment s impervious or paved surface area The catchment is the land area draining to the system The depth of the forebay where the stormwater 1n1 tially enters should be 3 feet The forebay should then slope upward to a depth of 1 foot prior to discharging into the level spreader Figure 5 FLOW BYPASS If runoff from high intensity storms those that deliver more than 1 inch of rain per hour is allowed to flow through a level spreader and ripar ian buffer system that 1s not designed to handle such storms erosion can occur within the buffer Thus dur ing heavy rain storms that produce more runoff than can be infiltrated by the buffer excess stormwater should bypass the buffer and be sent through a pro tected channel to a predetermined protected stream entry point This 1s achieved by allowing the runoff produced by a rainfall intensity of 1 inch per hour to enter the level spreader while diverting runoff from heavier rainfalls to the stream The
61. ants that tolerant extended drought once established All of these plants are native to the southeastern United States in wetland habitats and most are readily available at local nurseries Wetland plants can generally grow well in moist or well drained soils whereas plants adapted to dry soils rarely survive in soggy conditions How wet a rain garden stays will vary considerably depending on the site where it is installed Rain gardens created on sandy soils will rarely hold water for more than a few hours On these sites it 1s most important to choose plants for their drought tolerance Rain gardens created on loamy or silty soils could pond water for 1 2 days 1f your site ponds water for more than 3 days you should consider creating a wetland On these sites choosing plants tolerant of extended flooding 1s critical to success Remember you are not limited to planting just within the excavated area Extending plantings around this area will help the rain garden to blend in with the overall landscape Any plants adapted to the site conditions can be used outside of the excavated area Large Trees over 30 tall Deciduous Red Maple 2 Acer rubrum River Birch 1 3 Betula nigra Green Ash 3 Fraxinux pennsylvanica Black Gum 2 Nyssa sylvatica Willow Oak 1 2 Quercus phellos Willows 3 Salix species Nutall Oak 1 2 Quercus nuttalii Evergreen southern Magnolia 1 2 Magnolia grandiflora Swamp Laurel O
62. arden should be located at least 10 feet away from any foundations The rain garden should be at least 25 feet away from any septic system drain fields and 25 feet away from any well heads It should not be upslope from wells and should be lateral to septic systems not upslope or down slope Utility lines should be located and marked prior to any excavation For the best plant growth the BMP should be located in an area that receives partial to full sun exposure The rain garden should be located where stormwater naturally drains or water should be routed to it Avoid routing large areas to a new rain garden that would overload the existing drainage ways This will ensure that the garden will intercept runoff during storm events and that overflows can be directed to existing drainage paths 5 3 2 Determine watershed and impervious area draining to BMP The first step in rain garden design is determining the watershed area that drains to the proposed rain garden site which will contain some impervious area typically between 10 and 40 This impervious area will most likely consist of driveways and rooftops In order to capture stormwater runoff from driveways and other associated parking areas the drainage must flow to the proposed location Rooftops normally are drained via gutters which may or may not drain to the proposed location In this case however a 4 or 6 inch pipe can be used to route this water to the proposed location After any mo
63. ater quality in urban environments by re ducing stormwater peak flow reducing runoff volume through infiltration and removing nutrients and sediment through physical and biological pro cesses They are most effective how ever when stormwater flows through them at a shallow uniform depth the diffuse flow that well designed level spreaders can provide LEVEL SPREADER SYSTEMS AN OVERVIEW Level spreader systems consist of three parts the forebay the channel and the riparian buffer Figure 2 Figure 1 A level spreader providing diffuse flow Frank Hahne Mecklenburg County FOREBAY The first part of the system is the forebay which is used for the preliminary treatment of storm water It is an excavated bowl shaped feature that slows the influent stormwater and allows heavy sedi ment and debris to settle The forebay may be lined with riprap to reduce erosion within the excavated area The uneven riprap surfaces function as small sediment traps When a level spreader is used to dis perse outflow from a detention pond a forebay may be necessary to reduce runoff velocity before the outflow reaches the level spreader CHANNEL After the stormwater passes though the forebay it enters a concrete rock or grassed chan nel the main body of the level spreader This 1s a dead end channel because it does not directly connect the watershed to the stream Instead the channel is a long shallow impoundment that fills t
64. be cause they routinely fail Another stable material 1s a metal gutter Like concrete level spreaders pre fabri cated metal level spreaders can be expected to remain level with minimal maintenance Ideally the lip of the concrete level spreader should Figure 3 Failure at level spreader constructed with ABC stone be higher than the existing ground by 3 to 6 inches This allows water to pass over the lip without interfer ence from buffer vegetation To limit any erosion that could occur as water falls from the top of the level spreader to the existing soil a layer of filter fabric should be extended a distance of 3 feet from the level spreader lip towards the buffer Stone such as No 57 ageregate should be placed on top of the filter fabric 3 to 4 inches deep to reduce erosion just downslope of the level spreader Figure 4 A 3 foot wide strip of erosion control matting can be used in place of the filter fabric and No 57 stone combination However such an area must be stable and have adequate vegeta tion before receiving stormwater LEVEL SPREADER DIMENSIONS Level spreader dimen sions have a broad range and no combination seems to be superior The width of a level spreader however should be at least three times wider than the diameter of the inlet culvert The design depth or depth be tween the invert of the level spreader channel and the level spreader lip is currently recommended to be no less than either 9 inch
65. be noted however that downstream property owners usually benefit from their upslope neighbor s 24 HORTH CAROLINA SOL amp WATER sii Ri Community Conservation Assistance Program BMP Design Manual CONSERVATION rota installation of backyard BMPs which bring about a potential reduction in flooding and erosion on the downstream owner s property 4 3 General Information Matrix for Backyard Stormwater BMPs 5 0 Backyard Rain Garden Design 5 1 Overview of Practice A rain garden or bioretention area is essentially a vegetated sand filter and is designed so it quickly dries after rain events These BMPs are depressions in the landscape to which runoff is directed to be stored and infiltrate into the soil A typical rain garden will not have ponded water for more than 12 hours although a modified rain garden design can be used if water remains ponded between 12 and 36 hrs 1 5 days Rain gardens are typically planted with a combination of trees shrubs and perennials and then mulched Grass can also be used as cover without other vegetation or along with trees shrubs or perennials Cover and layout of rain gardens can be very flexible to suit a given site DW odit Figures 5 1a Picture of rain garden installations in Caldwell County left and b Brunswick County right NC 5 2 Selection of this practice over others Rain Gardens offer an attractive versatile and multi functional option in stormwater tr
66. biscus Hibiscus coccineus Tougn cump forming SUIS PANEN shaped red flowers in summer Rose Natio Hibiscus moshes Tough durable plants with huge white pink or rose flowers in summer Tall spikes of crimson red flowers in late summer Sana rover pone Herel alias and fall attracts hummingbirds and butterflies Common rush found throughout NC 2 3 tall ooft Rush Juncus effusus with dark green spiky foliage Green flowers are to brown seed pods throughout summer 6 3 6 Drought tolerant wetland As noted in Sections 6 2 and Table 3 4 the results of the simplified soil infiltration test will sometimes lead to the decision that the soil in question is neither poorly drained nor well drained Thus it is not suitable for a standard rain garden or a standard wetland A modification of these practices is needed for the range of 38 ITA NORTH CAROLINA SOIL amp WATER soils between poorly and well drained When the test hole at a given location drains in 1 5 to 4 days and the soils at the site have a significant clay content the site will likely support a wetland However the wetland that is constructed must the drought resistant Community Conservation Assistance Program BMP Design Manual Eagen A Aci al Engineering In general drought resistant wetlands can be designed and constructed in the same way that regular wetlands are designed and constructed The main difference in these wetlands is the plant selection
67. bypass channel or swale should employ turf reinforcement matting or rip rap Level Spreader Lip Underdrain Sized P4 For Site Soll Conditions Drain Into Bypass Swale Flow Path me alone Riparian Channel P E 3 Layer of Butter 3 tt Wide Strip of Filter Fabric Appropriately Sized Concrete Footer Figure 4 Sample cross section of a concrete level spreader Pipe From Diversion Influent To Level Stormwater Spreader A Figure 5 A Forebay schematic B Image of level spreader with forebay MAXIMUM SLOPE The first 10 feet of riparian buffer downslope of the level spreader should have a slope less than or equal to 4 percent The overall slope of the buffer should not exceed 6 percent for wooded buffers and 8 percent for buffers containing thick ground cover such as grass When slopes are greater than this other practices such as bioretention stormwater wetlands and ponds can be used to reduce peak flows and provide water quality improvements However on a case by case basis the DWQ may approve a series of level spreaders for ri parian buffer slopes of 12 to 15 percent This approval is contingent on a site visit and the professional judg ment of the DWQ permitter MAINTENANCE Level spreaders require at least yearly maintenance to remove trees and shrubs that begin to grow on the level spreader lip or that impede flow just downslope of the level spreader in the s
68. cer negundo Evergreen Small Trees under 30 tall Deciduous Ironwood 1 3 Carpinus caroliniana Redbud 1 2 Cercis canadensis Fringe Tree 2 Chionanthus virginicus Washington Hawthorn 3 Crataegus phaenopyrum Umbrella Tree 2 Magnolia tripetala PawPaw 2 Asimina triloba Tag Alder 3 Alnus serrulata Serviceberry 2 Amelanchier arborea Pagoda Dogwood 1 Cornus alternifolia Witch hazel 1 Hamamelis virginiana Silky Dogwood 3 Cornus amomum Evergreen American Holly 1 2 lex opaca Sweetbay Magnolia 3 Magnolia virginiana Shrubs Deciduous Chokeberry 1 3 Aronia arbutifolia Sweet Shrub 2 Calycanthus floridus Buttonbush 3 Cephalanthus occidentalis American Hazelnut 1 Corylus americana Strawberry Bush 2 Euonymous americanus Winterberry 3 Iex verticillata Virginia Sweetspire 3 Itea virginica Spicebush 2 Lindera benzion Ninebark 2 Physocarpus opulifolius Common Elderberry 2 Sambucus canadensis Meadowsweet 3 Spiraea latifolia Hardhack 3 Spiraea tomentosa Swamp Azalea 3 Rhododendron viscosum Swamp Rose 3 Rosa palustris Blueberry 1 Vaccinium ashei Arrow wood 2 Viburnum dentatum Possumhaw 3 Viburnum nudum Evergreen Native Brake Cane 3 Arundinaria gigantea Carolina Rose 1 Rosa carolina Perennials Blue Star 3 Amsonia tabernaemontana Lady Fern 2 Athyri
69. creen that will filter the stormwater This will keep the downspout within the confines of the debris filter walls but allow the cistern to be positioned properly The downspout can be cut with a hack saw or by another method that will produce a smooth straight edge 4 3 Preparing the gutter downspout As mentioned previously gutters should be present where a cistern is to be installed The cistern can temporarily be placed in the excavated area near its final location Since the gutter downspout should still be in place it will not be 50 e NORTH CAROLIN A SOL amp WATER E a Community Conservation Assistance Program BMP Design Manual CONSERVATION prea possible to fix the cistern in its final location since the cistern will sit under the gutter once the gutter has been cut The height of the top of the debris filter should be marked on the gutter downspout Since the screen that actually filters the water lies just below the top of the debris filter See Figures 7 10 and 7 11 the location of that screen should be marked on the downspout as well The cistern can then be moved back out of the way Figure 7 10 Gutter Figure 7 11 Cistern empties into cistern screen close up screen The gutter downspout should be cut approximately 2 inches above the location of the filter screen This will keep the downspout within the confines of the debris filter sides but allow the cistern to be positioned properly Th
70. d Community Conservation Assistance Program BMP Design Manual Balpen A Aprica Erqisaari g 8 5 Construction Guidance owales can be constructed using a variety of heavy equipment The equipment used will depend on what is already on site if anything and the size of the swale A small excavator would work well on smaller jobs and may be on site for construction of other BMPs If turf reinforcement matting is used it should be installed according to the manufactures recommendations Use plenty of staples to stabilize the mat and maintain good contact between the mat and the ground surface in all locations It is helpful to use short lengths of matting on curves to avoid bunching Seed and lime should be applied under the matting or as suggested by the manufacturer Native grass will perform the best however the grass should only be applied at the proper time of year proper time varies depending on grass type The property owner may desire that the grass planted match what is already in their yard County extension offices can help with grass selection and planting Temporary seed can be used if construction must take place during a time of year when the onsite grass will not germinate Annual Rye grass can be used in cool seasons or Millet in warm seasons for example If temporary seed is used the swale must be re seeded with perennial grass at the proper time of year For grass covered swales the surface should be pr
71. d Sandhill and Coastal Plain soils are often sandy and well draining It is imperative that in situ soils are analyzed in depth before permeable pavements are installed A three foot hole should be dug in multiple locations in the proximity of the proposed permeable pavement site The soils should be well drained See Section 3 7 with no signs of wetland characteristics within the soil section the entire 3 feet See Section 3 8 Additionally the infiltration rate at 64 ORTH CAROLINA SOIL amp WATER the site should exceed 2 in hr throughout the top 3 feet of soil Infiltration rates at the top of the soil column within the top 6 inches can actually be quite low and the pavement will still function well This is because the pavement is expected to drain from the bottom which is at least 6 inches from the pavement surface and usually several inches deeper This is an important consideration because often where impermeable surfaces were once located the soil immediately beneath them has been compacted and has become relatively impermeable In short the bottom of the permeable pavement is below the compacted area created by the existing pavement which is to be removed Community Conservation Assistance Program BMP Design Manual LT Engisaaring The site where the BMP is to be installed must currently consist of impervious area that will be removed and replaced with the permeable pavement The area must have a very s
72. d as a guide to plant selection Grass can also be used in this case as an alternative to other vegetation or in combination with other vegetation Warm season grass e g Bermuda Centipede Zoysia is recommended in much of the State but cool season grass Fescue Kentucky 31 can be used in cooler climates or to match existing lawn It may be necessary to re seed the cell in the fall When the garden drains between 12 and 36 hours and the zoned rain garden is built contains areas of high and low elevation the plants in Appendix F should 28 ORTH CAROLINA SOIL amp WA TER Community Conservation Assistance Program BMP Design Manual be used In each case it is good to use a variety of plants for aesthetic value and to improve the chances of plant survival In the zoned system place some plants in the base of the garden and others in higher areas and on the banks 5 3 6 Inlet protection and pretreatment It is a good practice to dissipate the energy of the inflow and to remove sediment from runoff before it enters the main body of the rain garden A few methods for accomplishing this include Vegetated filter strip Directing flow into the garden as sheet flow over a vegetated surface like a lawn This spreads the stormwater to limit concentrated flow and the associated erosive velocities and to allow filtration by the grass This can also increase the volume of infiltration at the site owale A swale can als
73. d be performed by a professional familiar with their installation The methods used to install permeable concrete vary from those used to install standard concrete Because permeable pavement contains void spaces which allow stormwater to infiltrate the integrity of these pores must be maintained Vibratory screens such as those used in some cases to compact standard concrete should not be used during permeable concrete installation Such instruments can smear the surface of the concrete resulting in reduced or restricted infiltration Additionally concrete vibrators employed to reduce air pockets are not to be used A heavy steel roller is used in place to provide adequate compaction without smearing the concrete Finishing exercises such as floating and brooming are not necessary and could result in surface sealing on the lot Permeable concrete does not contain the same curing compounds as traditional concrete thus plastic is placed over the concrete for a number of days to prevent drying Ferguson 2005 The plastic should never be drug across the pavement as smearing could result These are a few of the differences between traditional and permeable concrete installation While seemingly minimal the system will be ineffective if these differences are not addressed Installing permeable block pavers is similar to installing regular block pavers the difference in these systems is that voids are maintained between the pavers allowing wat
74. d from the channel behind the level lip All trees and vegetation that grow in the section of no 57 stone should be removed The grass in all swales should be maintained and the level spreader and buffer should be checked for signs of erosion Erosion that is discovered in the buffer should be addressed through the application of erosion control mat and through re grading if necessary NCDENR must be notified prior to any work performed in a protected riparian buffer RESOURCES RELATED WEB SITES RELATED FACT SHEETS in the Urban Waterways series BAE Stormwater Group North Carolina Cooperative Extension and North www bae ncsu edu stormwater Carolina Agricultural Research Service Bulletins Highlights of stormwater research projects and N C State University Extension programs provided across North Carolina by N C State University s Biological and Hunt W F 2000 Urban Stormwater Structural Best Agricultural Engineering Department Management Practices BMPs AG 588 01 Online http www bae ncsu edu stormwater state of North Carolina PublicationFiles UrbanBMPS199 pdf www stormwater org NCDENR s stormwater site Hunt W F and L L Szpir 2006 Permeable Pave ments Green Roofs and Cisterns Practices for http h20 enr state nc us su Manuals Factsheets Low Impact Development AGW 588 06 Online htm http www bae ncsu edu stormwater DWQ manuals fact sheets and a link to the revised PublicationFiles BMPs4LID pdf
75. d has been constructed the property owner observes 6 to 9 inch deep water reliably in the spring summer and fall the plants listed in table 6 1 should be incorporated into the planting scheme Some rules of thumb for backyard wetland plant selection include 1 Choose native species Nearly all the species provided in tables 5 3 through 5 5 are native to most of North Carolina 2 Include evergreen vegetation to provide some color in the backyard wetland during the winter months 3 Select a few species of plants with showy flowers which will attract butterflies and dragonflies This will add aesthetic appeal butterflies and an important mosquito predator dragonflies to the BMP Table 6 1 Sample deep water wetland plants Scientific Name Comments Bold plant with foliage and flower stems standing American Lotus Nelumbo lutea 4 6 above water s surface Large showy yellow flowers produced throughout summer Spadderdock Cow Heart shaped leaves float on water s surface 1 j Lil Nuphar luteum 2 wide globe shaped yellow flowers are born throughout summer Rounded heart shaped leaves float on water s Fragrant Water Lily Nymphaea odorata surface Large white sweetly fragrant flowers open throughout summer It should be noted that if the water level in the wetland is not consistent and normally drains in between rain events the deep water plants should not be used 37 NORTH CAROLINA SOIL amp WATER
76. dding lengths of pipe segments h 90 1 4 100 x 8 85 8 1 Equation 3 Total Head Requirement 0 93 8 1 101 ft Equation 2 Because the impact sprinkler has a higher head re quirement than the toilet the impact sprinkler head value will be used for the pump design Two pump curves are shown in Figure 6 Find the design point by locating the intersection of the re quired head and flow denoted by the x in the graph Notice that the design point lies above the curve for the 4 5 impeller therefore that impeller size will not meet the system demand The design point lies below the curve for the 6 impeller indicating the 6 impel ler will provide an adequate water supply Because the 6 impeller can supply the required head and flow this pump will satisfy the demands of this water har vesting system 8 10 12 14 Flow gpm Figure 6 The pump s design point is at x the intersection of the required head and flow INTERNET RESOURCES 2002 North Carolina State Building Code http www ncdoi comOSFM Engineering engineering_home asp Engineering Tool Box www engineeringtoolbox com North Carolina State University Biological and Agricultural Engineering Stormwater Group www bae ncsu edu stormwater State of North Carolina Stormwater Home Page www ncstormwater org Texas A amp M Rainwater Harvesting http rainwaterharvesting tamu edu Urban Waterways series Permeable Pavement Gr
77. dens NORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual Table 5 1 Maintenance Activities for Backyard Rain Gardens Activity Remove any sediment that has accumulated in garden or inlet Once per year or more Check contributing watershed for frequently as needed signs of instability Remove trash from rain garden As needed Replace mulch in rain garden Every 6 months Remove debris from around l As needed overflow weir Water garden plants During drought Remove weeds prune garden Re needed plants 6 0 Backyard Wetland Design 6 1 Overview of Practice A backyard wetland also referred to as a pocket wetland or a wetland garden is built in an area that is perennially moist The wetland is designed such that it will usually be wet even several days after a rain event A properly functioning backyard wetland will be substantially more wet than a rain garden and is usually constructed as an alternative to rain gardens A typical backyard wetland will be a shallow excavated bowl ranging from 3 to 12 inches deep Inside this bowl a variety of vegetation is planted It is essential that a variety of plants are grown in the wetland to avoid a monoculture and potential mosquito hazard Wetland vegetation tends to grow densely Stormwater wetlands are most frequently used to treat large watersheds such as shopping centers and residential communities however in locations with
78. description of what type of soil is present in each soil horizon in the soil series This information should only be used to gather preliminary data on a particular site A site investigation will be needed to verify the information gathered in the soil survey In general the soils will be classified as either well draining or poorly draining The soil survey will also provide an indication of the expected seasonal high and low water table Generally the soil types that are present from the soil surface to 36 inches below the soil surface are the most crucial for backyard BMP siting If the soils in this range 0 36 inches deep mostly consist of sand or loam they may be suitable for backyard bioretention areas If the soils in this range consist primarily of sandy soils they may also be suitable for permeable pavement areas only install in Coastal Plain or Sandhills Lastly if the soils in this range consist primarily of clay they are potentially suitable for backyard wetlands 3 8 2 Information from Site Investigation Soil classifications gathered from a soil survey should never be used exclusively to determine which type of soil is present at a given site A site investigation will be needed to verify that the soil on site is suitable for a given BMP The first step in this investigation is to talk to the property owner about the condition of the proposed location immediately following a rain event If the area remains wet or saturated
79. difications are considered like routing rooftop drainage to the proposed area the total watershed area and total impervious area draining to the proposed location can be calculated Manual measurements taken with a tape measure will work well in determining the area draining to the BMP 26 ITA 255210 Community Conservation Assistance Program BMP Design Manual Sef amm CONSERVATION Arcum Esgisaaring Figure 5 2 A rain garden s perimeter is laid out and a soil test is taken 5 3 3 Rain garden sizing Rain gardens should be sized to capture the first flush event Once the amount of watershed and impervious area has been calculated the Simple Method Section 3 3 can be used to determine the volume of water that will enter the BMP during the first flush event The depth of ponding in the bioretention cell can be selected based upon topography available area and home owner preference The approximate area needed for the rain garden is the runoff volume divided by the depth of ponding Appendix D shows the results of such calculations and can be used to determine the BMP size based upon the percent of impervious area watershed size and depth of ponding Figure 5 3 First flush events completely fill a rain garden to a ponding depth of 6 inches 27 zm HORTH CAROLINA SOIL amp WATER AAA 5 3 4 Rain garden bottom topography Community Conservation Assistance Program BMP Design Manual The t
80. drainage pipes and can also be used to determine the proper size of high flow overflow devices Peak flow can be calculated easily using the rational method Bedient and Huber 1992 The rational method is a simple model used to estimate the peak flow from a given watershed For applications such as backyard stormwater BMPs where small highly impervious watersheds will be treated the rational method offers a somewhat coarse but adequate estimate of peak flow The rational method is as follows Equation 3 3 Rational Method Q CxIxA Where Q Peak flow ft s C Runoff Coefficient dimensionless varies based on land use A Watershed area being treated acres Intensity of storm event to be captured in hr Note There are 43 560 ft in 1 acre 3 4 1 Storm Intensity The rational method is highly adaptable to various land uses storm events and watershed characteristics For the purpose of this manual some generalizations can be made to simplify the calculation Rainfall intensity varies throughout the course of a storm For small watersheds those being treated by the backyard BMPs constructed as part of this program stormwater from the far end of the connected impervious area does not have a long distance typically 100 s but not 1000 s of feet to travel to the BMP thus the time it takes stormwater to reach the BMP from all parts of the watershed is short With these watershed characteristics the peak flow from th
81. e A clogged drawdown hole poses several problems including the loss of storage to capture later storms and flooding of desirable plant species When water levels remain too deep for the desirable plants to survive stronger usually invasive plant species take WHY SUCH A SMALL HOLE A large opening would release the water too quick ly and not provide adequate time for treatment Stormwater wetlands and wet ponds are designed to capture the first flush or water quality volume from their upstream drainage areas The first flush is runoff generated by a 1 to 1 5 inch storm The total volume of water can range up to several acre feet Once the first flush is captured in the wetland or wet pond it must be slowly released to allow time for sediment and other gross solids to settle Design standards require that the first flush volume be kept for at least 2 days with a recommended 3 to 4 day retention time To release this water slowly a small hole is often necessary When the hole is only 2 3 or 4 inches in diameter clogging is a significant con cern over The aesthetics and performance of the practice can suffer when the plant community changes The wetland or wet pond needs to store water between storms to perform its intended function It cannot store water from the next storm effectively if the orifice is clogged and the wetland is continually full Unclogging the orifice 1s relatively simple Clean the hole with a stick
82. e conveyed to the contractor and a SWCD staff member should be on site during construction 10 4 1 Soil excavation For permeable pavement to be used as part of the CCAP some amount of impervious removal must be performed After the impervious area has been removed likely an impervious driveway sidewalk parking pad or patio the site excavation can commence Excavation depth should be measured from the soil surface down No fill should be used to bring the surrounding soil to the top of pavement level Whenever possible the excavated area should not be driven on by any construction equipment This is preferable as the soils can be compacted when exposed to heavy machinery reducing their infiltration capability During the construction a SWCD staff member should be on site to verify that the excavated area is brought to the correct depth A survey instrument Such as a level would be beneficial for this process Additionally survey measurements can be made within the excavated area to ensure that it is relatively flat lt 0 5 10 4 2 Gravel base The gravel base should be applied after the area has been excavated to the correct depth Again washed or double washed stone should be used for any permeable pavement installation The gravel base installation will vary slightly depending on whether permeable concrete or permeable pavers are being installed If permeable concrete is being installed a 6 inch layer of washed 57 st
83. e downspout can be cut with a hack saw or by another method that will produce a relatively smooth straight edge 7 4 4 Setting the cistern into place Once the site has been properly prepared the cistern can be set into place The cistern should be situated such that the gutter downspout can feed into the debris screen that is attached to the top of the cistern If the gutter downspout and the base have been prepared properly this should be easy to do By preparing a support base surface area slightly larger than the cistern the cistern can be shifted into the proper place Figure 7 12a and 7 12b 51 NORTH CAROLINA Community Conservation Assistance Program BMP Design Manual CONSERVATION 7 4 5 Final cistern construction considerations The last step in the construction process is to attach the overflow apparatus and any spigots or other distribution apparatus Teflon tape should be used on any joints where a water tight seal is desired Pumps should be installed per manufacturer s recommendation 7 5 Cistern Maintenance Maintenance is crucial to cistern performance Most notably the gutters and the debris screen should be checked for leaves and other debris Since these two components are essential in routing stormwater to the cistern they should be checked frequently particularly when a tree canopy is near a roof top Any hose or pipe connections associated with the cistern should be checked for leaks The c
84. e infiltrated stormwater and any building foundations the wetland should be located at least 10 feet away from any foundations The wetland should be at least 25 feet away from any septic system drain fields and 25 feet away from any well heads Because stormwater wetlands are located in areas that are perennially wet they tend to be placed down slope of wells and septic tanks and well away from home foundations Utility lines should be located and marked prior to any soil excavation For the best plant growth the stormwater wetland should be located in an area that receives partial to full sun exposure to allow plant growth In general backyard wetlands should be selected over other BMPs when site conditions involve poorly drained soils and or high water tables When an initial soil investigation is performed if the seasonal high water table Section 3 8 is within 1 foot of the soil surface the site should be considered for a backyard wetland Additionally if the soil investigation shows the soils to be poorly drained Section 3 7 the site will be more suitable for a backyard wetland than a rain 33 aa T ORTH CAROLINA SOIL amp WATER garden Another way to verify that a site has poorly drained soils is to identify the spot at a given property where standing water is present for multiple days after a rain event If the initial simplified soil infiltration test described in Section 3 7 2 results in the test hole draining in between 1
85. e taken from the area where the BMP is to be implemented These samples should be taken to a laboratory for analysis to ensure that they will be structurally sufficient to support the permeable pavement and will be suitable for infiltrating the captured stormwater One sample should be taken for every 200 square feet of area to be constructed Since the area that is to be used for the BMP should be taken up by impervious area prior to the permeable pavement installation samples may be taken along the periphery of the current impermeable area At each sampling location a hole should be dug to a depth of 3 feet The topsoil can be discarded top 6 inches however all other soil excavated from the hole should be placed in a bucket and mixed until all soil lenses are combined homogeneously A sample should be taken from this bucket and placed into a vessel that is approved by the laboratory that will be analyzing the sample The laboratory selection should be approved by SWCD staff Again this procedure should be repeated at each sampling location The laboratory should perform a sieve analysis to determine what percentage of the soil sample consists of fines clay and silt particles not prone to high infiltration or structural stability If the percentage of fines is less than 8 or the amount passing the 270 sieve which means the sample consists primarily of sand and loamy soils the soils are suitable as a base for the permeable pavement
86. e thickness of the permeable pavers is determined by the manufacturer normally approximately 3 25 inches Plastic grid pavers tend to be 2 inches thick The permeable concrete should be poured to a thickness of 6 inches 10 4 Permeable Pavement Construction Due to the scale of many permeable pavement installations it is recommended that a certified permeable pavement contractor install permeable pavements Installing permeable concrete or permeable block pavers requires some experience and expertise and should not be attempted by the general public A list of professionals who are certified to install permeable interlocking concrete pavers PICP and concrete grid pavers CGP is available from the Interlocking Concrete Pavement Institute ICPI at http www icpi org Carolinas Ready Mixed Concrete Association CRMCA can be contacted for a list of professionals with experience in installing permeable concrete www crmca com The only exception to this would be small backyard patios that will be installed using permeable block pavers In such an installation traffic load will be minimal and the result of a structure failure will not be dangerous to human health and well being 68 ORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual The construction steps below can be completed by a responsible experienced contractor with minimal instruction The key points from each section should b
87. e watershed is likely to be produced by the most intense part of the storm A short duration of high intensity rainfall is the most intense part of the storm thus a the amount of rainfall that falls during the most intense 5 minutes of the storm will be chosen as the intensity used in the rational method calculation Table 3 1 shows the 5 minute peak intensity for various North Carolina cities for the 2 5 10 and 25 year storm events Data from the city closest to the proposed BMP location can be used Table 3 1 or further information can be gathered for a specific city by visiting the National Weather Service s Hydrometeorological Design Studies Center http ndsc nws noaa gov hdsc pfds orb nc_pfds html NORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual Table 3 1 Peak Intensity in hr for Various Storm Events 5 minute duration Location 2 year 5 year 10 year 25 year Charlotte 56 66 72 80 Greensboro 54 62 68 74 Winston Salem 53 62 68 76 89 Wilmington 74 86 97 109 Elizabeth city 61 70 79 90 L Gmewie 62 71 890 91 Taken from the National Weather Service 3 4 2 Runoff Coefficient Although it is assumed that the BMPs implemented as part of the CCAP will treat watersheds that are primarily impervious rooftops driveways parking lots there will likely be other land uses associated with eac
88. e wetland with respect to water depths encourages plant diversity If the water table is encountered while digging the wetland this is a good sign that wetland vegetation will survive in this location The topsoil will be added back to the wetland after it has been dug thus the wetland may have to be a little deeper to account for the reapplication of the topsoil A small deeper section forebay can be included in the wetland design were the stormwater initially enters This forebay will cause some influent sediment to fall out and will dissipate the energy of the stormwater reducing erosion inside the BMP Often if the watershed is small 0 5 acres and stable a forebay is not needed When a backyard wetland is constructed on a slope the berm height will vary based on the severity of the slope In general the berm should not be more than 1 foot higher than the surrounding ground surface This will minimize the erosive 41 ORTH CAROLINA SOIL amp WATER potential of any stormwater that spills over the back side of the berm The berm should be stabilized with grass seed and straw after the construction is complete During the period before the grass has been established the berm will be susceptible to erosion and should be checked after every rain storm If the property owner desires sod strips can be used to stabilize the outlet berm An outlet should be constructed as described in Section 6 3 4 to provide safe passage of stormwater
89. eatment They can sometimes be installed without the help of heavy equipment although a small excavator would be beneficial in many cases This will likely be one of the more commonly selected BMPs The proposed BMP location should not have a seasonal high water table closer than about 2 5 feet from the existing ground surface A high water table will limit drainage from the system between rain events increasing the likelihood of plant 25 ORTH CAROLINA SOIL amp WATER loss due to wet conditions The seasonal high water table can be evaluated using the method described in Section 3 8 The percolation rate of the soils should be tested as described in Section 3 7 2 When performing a simplified soil infiltration test it is best to use several holes Holes should extend approximately 1 foot below the anticipated depth of the proposed rain garden Water should drain within 12 hours from each hole in order for the native soil to be acceptable for a traditional rain garden If the water drains between 12 hours and 36 hours a modified rain garden design called a zoned rain garden could be used Community Conservation Assistance Program BMP Design Manual LT Engineering 5 3 Rain Garden Design 9 3 1 Rain garden siting There are a number of siting concerns regarding rain gardens The soils must be suitable as described in section 5 2 To avoid contact between the infiltrated stormwater and any building foundations the rain g
90. eavily overgrown a contrac tor who specializes in removal should be consulted Because dams of larger ponds and wetlands are re sponsible for retaining large volumes of water dam failure can be catastrophic if homes businesses or roads are downstream REMOVE INVASIVE PLANT SPECIES Stormwater wetlands and wet ponds with aquatic shelves can become overgrown with invasive plants The most common invasive plant is the cattail Typha species Figure 6 Cattails while native to North Carolina crowd out other more desirable plants Cat tails tolerate a variety of conditions and do a good job of pollutant removal From this functional standpoint cattails can be considered good plants to have in a wetland However cattail monocultures fail to meet two very important design goals aesthetics and mos quito control A wetland or wet pond that is overgrown with cat tails is not a diverse ecosystem Ecosystem diversity is critical for mosquito control Cattails provide a safe environment for mosquito larvae to mature to adulthood When cattails go dormant in the fall some of the fronds will form a protective thicket for mos quitoes For more information on mosquito control in wetlands and wet ponds see Mosquito Control for Stormwater Practice Designers and Managers AGW 588 04 in the Urban Waterways series Figure 6 A Cattails Typha sp and B common reeds Phragmites sp are very ag gressive invasive species Once establish
91. eck dam constructed of stone Source USDA Natural Resource Conservation Service Illinois 6 3 4 Backyard wetland outlet considerations The outlet structure controls the ponding depth in the backyard wetlands and allows excess flows to exit in a controlled manner without damaging any berms used to form the wetland or the drainage infrastructure The first step in designing an outlet structure is determining the peak flow of the desired design storm for the given watershed often the 10 year storm Section 3 4 discusses the use of the Rational Method for calculating peak flow rate A weir structure is commonly used for wetlands with the weir sized so not to be overtopped during the peak of the 10 year storm During the peak flow of the 10 year storm all higher velocity outflow should be in contact with the weir not contacting soil composing the berm Section 3 5 2 discusses weir sizing Appendix B describes the layout and construction of a wooden weir structure For smaller rain gardens as will often be the case a simpler structure than what is described in Appendix B may be used For example a treated 2x12 with a weir opening installed using some of the guidance in Appendix B may be sufficient All structures should extend at least 6 inches below final grade at the base and 12 inches into the sides of the berm Soil filled around the structure should be well compacted in 4 to 6 inch lifts The backyard wetland should be designed so that
92. ection of No 57 stone from Figure 4 Any debris and sediment that build up in the level spreader forebay and channel should be removed annually and after storms greater than or equal to a two year storm the precipitation associated with a 24 hour storm event that occurs on average once every two years Figures 6 and 7 In North Carolina the two year storm ranges from 3 5 to 4 25 inches over a 24 hour period Additionally the level spreader channel and the riparian buffer should be examined annually and after storms greater than or equal to the two year storm for possible erosion and gulley formation If possible the buffer vegetation immediately downslope of the level spreader lip should be mowed Figure 7 Sediment and debris build up after ap proximately 4 5 years of operation regularly to encourage low dense growth and to facilitate inspection The use of perennial dense low growing ground covers such as common bermu dagrass downslope of the lip may help to maintain diffuse flow These issues should be addressed immediately to restore proper function If erosion is apparent correc tive action must be taken such as installing erosion control matting and possibly regrading NCDENR must be notified before any work is performed in the protected buffer RIPARIAN BUFFER TOPOGRAPHY Riparian buffers are highly variable Depending on internal buffer topography water will tend to re concentrate almost immediately in
93. ed each plant will crowd out more desirable plant species Figure 7 Aquatic glyphosate herbicide can be applied via direct contact a protected hand or via a small weed wipe pole A Stroking individual cattail fronds sends the herbicide down the shoot and reaches tubers located in the soil B Once the herbicide reaches the tubers the cattails will die within 2 3 days C The herbicide can also be applied with a small brush Franklin County Cooperative Extension Center Removing cattails can be challenging It is almost impossible to remove a mass of cattails by hand Cattails grow from tubers that spread and they also spread by seed If a piece of cattail is left in the wetland or wet pond after removal the stand will probably re establish Use a backhoe for mass cattail removal when a wet pond or wetland is completely overgrown by cattails If a wetland or wet pond has a variety of vegetation but cattails are beginning to colonize it use an alter native form of cattail removal such as applying an aquatic formulation of the herbicide glyphosate one trade name for this is Rodeo Wear a chemical resis tant glove underneath a cloth glove Soak the cloth glove in 2 percent glyphosate and stroke the cattail leaves Or brush the herbicide onto the leaves with a small weed wiper Not every leaf needs to be touched by the herbicide because many of the cattails are con nected by tubers Within 10 to 12 days the cattails fronds
94. ed as often as once a year Like the forebay the final deep pool of the pond or wetland near the outlet also must be inspected and maintained The major difference between the two is that the final deep pool takes longer to fill with soil The drawdown hole located at the outlet and described in the next section is where cap tured stormwater slowly drains from the wetland or wet pond It must be free of accumulated debris and sediment to work properly Remove sediment and gross solids from the deep pool near the outlet whenever the material is within 1 vertical foot of the drawdown hole Figure 3 A wide path is provided for heavy equipment to access the forebay located to the right of the path Figure 4 A small orifice allows slow release of captured stormwater A but can easily clog due to its size A clogged orifice can affect plant communities inside the wetland or wet pond B KEEP THE ORIFICE DRAWDOWN HOLE FREE FLOWING Wetlands and wet ponds are designed to capture and detain stormwater from 2 to 5 days On smaller ponds and wetlands a relatively small hole or orifice is used to detain water for this period The diameter of the orifice can be as small as 2 inches which makes it susceptible to clogging Figure 4 Because many ponds and all wetlands contain vegetation dead plants can float to and clog the orifice Moreover floating trash and debris see the next section will potentially clog the orific
95. een Roofs and Cisterns Stormwater Practices for Low Impact Development AG 588 06 http www bae ncsu edu stormwater Publication Files BMPs4LID pdf Water Harvesting at North Carolina State University www bae ncsu edu topic waterharvesting Prepared by Matthew P Jones E l and William F Hunt P E Ph D Biological and Agricultural Engineering North Carolina State University Copyright 2006 by North Carolina State University Published by NORTH CAROLINA COOPERATIVE EXTENSION SERVICE COLLEGE OF AGRICULTURE LIFE SCIENCES ACADEMICS RESEARCH 4 EXTENSION CS A E UNIVERSITY AG 588 08 E07 45836 10 06 JL APPENDIX J Nomograph For Use In Solving The Manning Equation For Trapezoidal Channels From the Draft lowa Stormwater Management Manual www ctre iastate edu P UBS stormwater mj Trapezoidal solution nomograph The trapezoidal channel nomograph solution to Manning s equation in Figure 10 can be used to find the depth of flow if the design discharge is known or the design discharge 1f the depth of flow 1s known a Determine input data including slope in feet per feet Manning s n value bottom width in feet and side slope in feet per feet b Given the design discharge do the following 1 2 3 4 Find the product of Q times n connect a line from the slope scale to the Qn scale and find the point of intersection on the turning line Connect a line from the turning point from Ste
96. er to infiltrate The blocks are placed on top of the compacted 78 stone Once set into place the blocks are compacted before additional 78 stone is added to the top of the pavement area and allowed to fall into the voids in between the permeable paver blocks Excess 78 stone is swept off of the area and the permeable paver blocks are compacted one final time Tight placement of the blocks is essential to maintaining the structural integrity of the system Smith 2000 10 5 Permeable Pavement Maintenance Like any other BMP permeable pavements require maintenance to be effective over time The greatest concern related to these systems is clogging of the surface void spaces Once these surface voids become clogged the system loses its function and effectively becomes an impervious area 70 STA NORTH CAROLINA SOIL amp WATER One means of removing sediment and debris from the permeable pavement area is to vacuum and sweep the area On most commercial sites this can be done easily by running a vacuum truck over the area This may not be possible in a backyard BMP setting however the site should be swept regularly and cleared of all debris As mentioned in section 10 4 5 the area around the permeable pavement lot should continually be evaluated to verify that sediment from adjoining areas is not washing onto the permeable pavement areas This sediment can clog the permeable pavement over time Additionally any weeds or mosses that
97. ers The model will show the optimal size for a cistern based upon runoff reduction nutrient removal and payback period cost More information on this model is provided in Appendix H 3 3 Additional Cistern Components A few additional components may be desirable to ensure proper cistern function and to increase the usability of the collected rainwater Most notably a filter should be used to remove leaves and large debris that have made their way down the gutter with the stormwater This filter can be placed just below the gutter downspout where the stormwater enters the cistern Figure 7 5 Figure 7 5 Debris filter just below gutter downspout An overflow assembly is part of the cistern as well This assembly generally consists of a system of pipes that attach to the overflow orifice that is built into the cistern If the cistern does not have enough storage space to capture an entire rain event this system of pipes allows any overflow stormwater to leave the cistern and discharge onto the ground below without cascading down the side of the cistern Figure 7 6 and 7 7 48 e ORTH CAROLINA SOIL amp WATER Additional equipment may be needed to access the cistern water such as spigots and other pipes The cistern vendor should be contacted to determine exactly what outlet configuration will be required to access the cistern water for a given water usage In some cases a pump will be required to send the stormwater to t
98. es they must be maintained properly to meet their design goals Most stormwa ter wetland and wet pond maintenance activities are simple and inexpensive But without them the effec tiveness of these stormwater management practices will decline TABLE 1 STORMWATER WETLAND AND WET POND MAINTENANCE TASKS AND FREQUENCIES Tak Frequency Notes Remove sediment from forebay and deep pool dredging dipping 5 to10 years is typical Varies In stable watersheds once every In unstable watersheds those with active construction the frequency increases to once a year assuming the forebay is correctly sized Monitor sediment depth in Once a year In a large pond or wetland a small boat forebay and deep pools may be needed Maintaining free flowing orifice drawdown hole Remove floating trash and debris week to once a month Once per month and after every storm Perform inspection regularly Unclog exceeding 2 inches ging the hole when needed is simple Depends on design aesthetics once a Remove trash whenever the drawdown hole is being inspected Inspect for trash more often if necessary and remove as needed Remove vegetation from Once a year Dam top and faces should consist of dam top and faces mowed grass if vegetated Remove invasive species particularly cattails year spring Mow the wet pond perim Remove muskrats and beavers once a month RESOURCES Fact shee
99. es or half of the inlet culvert di ameter whichever is greater When discharging into a buffer with thick ground cover there must be 13 feet of level spreader for every cubic foot per second cfs of flow This design specification is based on maximum flow ve locities and is intended to limit erosion in the buffer Grass for example is more resistant to erosion than mulch and detritus Therefore a shorter length of level spreader 1s needed upslope of grass than upslope 3 of mature woods In forested buffers this number varies based on the width of the riparian buffer The wider the riparian buffer particularly wooded buffers the more stormwater will infiltrate the buffer When infiltration within the buffer is taken into account the length of level spreader per unit of flow can be reduced e A level spreader discharging onto a 50 foot wide wooded riparian buffer should be sized at 65 feet per cfs of flow e Discharging onto a 100 foot wide wooded buffer requires 50 feet of level spreader per 1 cfs of flow e Discharging onto a 150 foot wide wooded buffer requires 40 feet of level spreader per 1 cfs of flow The minimum length of any level spreader should be 13 feet and the maximum allowable length by DWQ standards is 130 feet A summary of the sizing guide lines for level spreader lip length is shown in Table 1 Table 1 Level Spreader Lip Sizing Guidelines Length of Level Spreader ft per 1 cfs of flow Ripari
100. et wetlands bioretention areas etc the outlet should be designed to spread the water out as it enters the riparian buffer No newly constructed channel should be used to convey the water into the buffer instead a large weir or berm can be used The North Carolina Division of Water Quality 401 404 Wetlands Unit should be contacted for additional information should a newly constructed drainage way be necessary General level spreader guidance is found in Hathaway and Hunt 2006 Appendix C and on line via the North Carolina Division of Water Quality Wetlands and Stormwater Branch http h20 enr state nc us ncwetlands Avoiding the installation of CCAP projects in areas adjacent to riparian buffers is generally recommended If a project is located in a watershed with protected riparian buffers the BMP discharge should be routed into any existing drainage ways if at all possible 3 8 General Soils Discussion otormwater BMPs are impacted significantly by the soil in which they are constructed Therefore it is important to know which soil types are present at a given location before designing or constructing a stormwater BMP Backyard Stormwater BMPs that rely heavily on infiltration rain gardens and permeable pavement must contain soils that will drain at most within 24 hours of a rain event further details are provided later in this document In North Carolina the presence of clay precludes using soils from many parts of the state
101. evice on the other hand if an extremely high pressure is selected it could damage seals and other plumbing equipment The 2002 North Carolina state building code specifies that a water distribution system should not leak when tested at 100 psi Equation 2 Pressure Conversion Feet of Head PSI x 2 31 ELEVATION H The term elevation corresponds to the vertical distance water must be lifted to reach the location where it will be used This distance 1s mea sured from the pump intake h to the water outlet h Figure 3 FRICTION H Friction accounts for energy losses associated with moving water through a plumbing network The primary components are energy loss due to friction with the pipe walls and losses through el bows valves and other fittings Manual calculation of friction 1s a complex process therefore information on typical friction losses for common pipe diameters is included in Tables 2 and 3 Friction loss due to pipe fittings can be calcu lated by using what is known as the equivalent length method Look up the equivalent pipe length for the appropriate diameter of each fitting 1n Table 3 Add the equivalent lengths for all fitings encountered between the pump and the intended fixture for use in Equation 3 To calculate the friction loss within the pipe first find the friction factor F that corresponds to the flow and pipe diameter of the plumbing in Table 2 Next find the total linear distance of p
102. ginning to end Select swale geometry typically 4 to 1 or 3 to 1 sideslopes Assume n value from Table 8 1 use n value for grass make selection based on vegetation and straightness of swale Use equation to determine D depth of swale if completely full of stormwater during 10 year storm use equations 8 3 8 4 or 8 5 PINE e 59 STA ORTH CAROLINA SOIL amp WATER 6 Use D to calculate swale area as define in Section 8 4 1 also calculate channel width as defined in Section 8 4 1 make sure space is available if not steeper sideslopes may be used 7 Use area calculated in step 6 and Qp to determine the velocity of the stormwater in the swale during the 10 year storm use equation 8 1 the continuity equation 8 Check velocity against table 8 2 to determine if grass alone can withstand the velocity produced in the channel if so the design is complete 9 f grass can not withstand the velocity produced in the channel turf reinforcement matting can be used If turf reinforcement matting is not desired rock can be used however the design process must be repeated using a different Manning s n value Always perform a check to determine if the velocity produced in the channel can be passed by the channel lining without eroding 10 If a safe velocity can not be achieved with a given swale design the slope of the system can be adjusted by changing the course of the swale or a trapezoidal channel can be use
103. gn there is opportunity for the berm to erode if the BMP fills to capacity and spills over the retention berm In these cases it is good practice to design and protect a spillway for the additional stormwater An Opening in the berm can be constructed where the stormwater will be conveyed out of the system A piece of pretreated lumber with a rectangular notch cut in it can be installed in the berm in this opening so the stormwater passes over a stable edge instead of over the easily eroded earthen berm This lumber acts as a weir to spread the flow thus reducing velocities More information on BMP outlet design and construction is given in Sections 3 6 2 and 3 6 3 3 6 2 Determining Outlet Size The outlet weir rectangular notch described above in Section 3 6 1 should be sized such that the notch is not overtopped during a 10 year storm event This means that the weir notch must be long enough to allow the peak flow associated with the 10 year storm to pass without the water rising high enough that the top of the board is reached To determine the appropriate length of weir notch to pass the 10 year storm the weir equation can be applied The weir equation is as follows 18 tT ORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual Equation 3 5 Weir Equation Q CyXxLxH Where Q Flow ft s Use peak flow from 10 year event Cy Weir Coefficient dimensionless Use
104. grow in the pavement voids can be removed using herbicide or be flamed Community Conservation Assistance Program BMP Design Manual Lastly over time the stone that is used to fill the voids in between the permeable pavements can be lost as tires track it off of the lot and as the gravel settles This rock should be replaced periodically to restore the proper amount of gravel in the void spaces Table 10 1 shows the maintenance activities associated with permeable pavements and the frequency with which these activities should take place Table 10 1 Permeable Pavement Maintenance Activities Frequency Semi annually initially less Shavers Piscean frequently with time Avoidance of landscape debris E Mh During landscape maintenance leaves grass clippings opray weeds and mosses with Observe adjoining area for l Continually sediment deposition 71 ORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual 11 0 References Bean E Z 2005 A Field Study to Evaluate Permeable Pavement Surface Infiltration Rates Runoff Quantity Runoff Quality and Exfiltrate Quality M S thesis Raleigh NC Biological and Agricultural Engineering Department North Carolina State University Bedient P B and W C Huber 1992 Hydrology and Floodplain Analysis 2 Edition Reading Massachusetts Addison Wesley Publishing Company Butler J D and T Caudill 2005 Citizen s
105. h watershed Landscaped areas lawns and other pervious areas are likely in each watershed The rational method is versatile enough to account for both the impervious and pervious areas of each watershed Runoff coefficients C vary per type of land use In an effort to simplify the process of determining a composite C value that can be applied to a given watershed a C value of 0 95 should be applied to all impervious areas and a C value of 0 25 should be applied to all pervious areas Equation 3 4 can be used to determine a C value that can be applied to the whole watershed Equation 3 4 Determine the Composite Runoff Coefficient C Impervious Area x 0 95 Pervious Area x 0 25 Watershed Area 3 5 Diversion Berms Diversion berms are earthen structures that are designed to intercept stormwater and direct it to the BMP They can increase the effectiveness of the BMP by allowing it to treat a larger watershed The sideslopes of the berm should not be graded steeper than 2 1 The height of the berm does not need to exceed 1 foot The diversions can be covered with grass or mulch depending on the landscape and the expected runoff velocities in the diversion Erosion control blanket should be used for cover while vegetation is being established 17 STA ORTH CAROLINA SOIL amp WATER 3 6 Determining Outlet Location and Size Community Conservation Assistance Program BMP Design Manual 3 6 1 Determining Outlet
106. he cistern manufacturer should be contacted to determine what size cisterns are available Plastic cisterns are more desirable for the purpose of the CCAP and a range of colors are available to suit aesthetic needs Table 7 1 Estimated Runoff Volumes and Cistern Suggestions for Various Rooftop Areas 0 75 inch Runoff Runoff Suggested Runoff Runoff Suggested Volume Volume Cistern Size Volume Volume Cistern Size cf gal gal cf gal gal 65 rain barrel 133 rain barrel 133 rain barrel Because cistern sizing is based upon water supply and demand the chart provided above will often undersize but occasionally oversize the cistern The preferred way to determine the size of a cistern is to run a simple water budget model such as those available on the following website http www bae ncsu edu topic waterharvesting This model simulates 30 years of actual rainfall data from several cities in North Carolina and couples this supply of water with user defined demands such as the number of gallons of water used per week to irrigate and wash vehicles Before a cistern is installed it is highly recommended that the eventual operator 47 tT Community Conservation Assistance Program BMP Design Manual A i CONSERVATION of the cistern know how much water he she is using on a daily basis This information can be obtained using a simple hose attached water meter available for purchase at most garden cent
107. he drawdown hole CONTROL PESTS Rodents such as muskrats and beavers are attracted to stormwater wetlands and wet ponds Figure 8 Once Figure 8 A Muskrat Ondatra zibethicus B Beaver Castor canadensis U S Fish and Wildlife Service Figure 9 Muskrat holes along the perimeter of the wetland or wet pond are a sign of infestation Destroying the holes is a simple way of forcing muskrats to move if the population is limited there they can damage the stormwater management practice Muskrats eat aquatic vegetation and burrow holes in the deeper pools When muskrats actively burrow near the outlet of a wetland or wet pond they will add sediment and increase turbidity to the outflow in creasing the release of pollutants from the wetland or wet pond Moreover muskrats will sometimes burrow holes around and through dams These muskrat holes artificially lower the water level inside the pond or wetland causing some plant species to die At worst the holes can lead to dam failure Beavers are attracted to the sound of running water Once a beaver colonizes a wet pond or stormwater wetland it will remove trees and shrubs surrounding the stormwater practice to build its lodge and dam Beaver activity will clog or block the drawdown structure thus raising the height of water inside the pond or wetland This change in the depth of water inside wet ponds with aquatic shelves and stormwater wetlands will alter the
108. he point of usage Vendors can give some guidance on pump sizing A North Carolina Cooperative Extension publication has been produced that will also aid in pump selection This document has been attached as Appendix I Community Conservation Assistance Program BMP Design Manual Eagen A Aci al Engineering 7 3 4 Outlet structure Cisterns have an internal overflow device that is used when they do not have enough storage space to capture a given rain event Figure 7 6 and 7 7 The quantity of stormwater that overflows or bypasses the cistern will vary depending on event size Thus some effort should be made to route the overflow stormwater to a nearby drainageway This may be done via a swale or plastic drainage line Failure to do so may result in the formation of an erosion channel where the stormwater leaves the cistern 7 4 Cistern Construction 4 1 Preparing the soil base Cisterns are extremely heavy when full of water A stable level soil base is needed to ensure the stability of the cistern If the site is not level the cistern could tip and fall The area to be occupied by the cistern should be cleared of any vegetation or debris that could make the cistern unstable over time Figure 7 8a Cisterns that are 1500 gallons or greater in size may require some additional stability underneath A level depression 12 18 inches can be excavated in the shape of the cistern before a gravel base 4 5 inches is l
109. high water tables pocket wetlands can capture runoff from small watersheds such as an individual property Figures 6 1 and 6 2 show a wet spot in a yard an ideal location for a pocket wetland and types of vegetation found in a backyard wetland respectively 32 HORTH CAROLINA SOIL amp WATER mE e sisi Ra Community Conservation Assistance Program BMP Design Manual I o a TE Engisaaring CONSERVATION Figure 6 1 A perennially wet spot in a Figure 6 2 This pocket wetland in yard is a good location for a pocket Tarboro NC features vegetation such wetland as lily pads pickerelweed and joe pye weed 6 2 Selection of this practice over others Backyard wetlands are used in similar environments as rain gardens As with rain gardens backyard wetlands should be located in a low area where stormwater naturally drains This will ensure that the wetland will operate properly during storm events The wetland should also be located near the current surface drainage system the drop inlet or swale that currently drains the watershed This will allow the designer to direct stormwater that overflows the wetland to the current surface drainage system via a swale or overland flow By directing the stormwater overflow to the current surface drainage system no additional drainageways will be required to direct stormwater off the property There are a number of other siting concerns for backyard wetlands To avoid contact between th
110. hould be used in all level spreaders The lip should be tied into the soil with an appropriately sized concrete footer or similar footer Level spreader lip dimensions The concrete lip should extend 3 to 6 inches above the existing grade on the buffer side Just after the lip a 3 foot wide 3 to 4 inch thick layer of No 57 stone should be used to minimize erosion due to the water spilling over the level lip This gravel should be laid on top of filter fabric that has been tied into the soil Buffer slope Riparian buffer slopes should not exceed 896 when discharging into a densely vegetated buffer and 696 when discharging into a forested buffer A series of level spreaders may be approved by DWOQ for buffer slopes up to 12 to 15 Flow bypass Only the amount of flow associated with a rainfall intensity of 1 inch hour should be routed through the level spreader All additional flow should be routed to the stream via a properly designed and maintained swale or pipe Stream banks should be protected at the point where the additional flow will be discharged Forebay A forebay or some other form of pretreatment should be a part of any level spreader design The forebay surface area should be no less than 0 296 of the contributing catchment s impervious surface area Maintenance Level spreaders should be maintained yearly and after storms greater than or equal to a two year 24 hour event Sediment and debris should be removed from the forebay an
111. hus reduced permeable pavement performance When permeable concrete is used for a given location the gravel base will consist of a washed 57 stone that is approximately 6 inches thick The permeable concrete can be poured on top of this gravel base after the base has been properly compacted If washed 57 stone is unavailable a semi angular to angular stone alternative can be used Round support stone will not interlock allowing shifts in pavement to occur such as rocking or splitting 67 zm fa ee ee When permeable block pavers or plastic grid pavements are used for the BMP 4 inches of washed 57 stone can act as the gravel base that lays on top of the in situ soil however the pavers must sit on a bedding layer of smaller stone This bedding layer should be made of washed 78 stone and be approximately 2 inches thick This 78 stone should also be used to fill any voids present in between the permeable paver blocks after they are installed Figure 10 6 Los a ug E EPA as a hh a aed F TRA e Figure 10 Filling permeable paver voids with gravel 10 3 5 Permeable Pavement Sizing Criteria The permeable pavement BMPs used as part of the CCAP will treat no more than 2 times their surface area No real sizing criteria is required for these systems as part of the CCAP program The square footage of the permeable pavement lot will generally be the same as the square footage of impervious area that was removed Th
112. icated that none of the level spreader riparian buffer systems was able to provide diffuse flow through the riparian buffer from the level spreader to the stream Common causes for failure to maintain diffuse flow included the follow ing e Lack of maintenance e Poor design e Riparian topography vegetation or both e Poor construction methods level spreader lip not level channel built with easily eroded materials as in Figure 3 e Human interference after the level spreader has been constructed This field evaluation indicated that level spreader sys tems in North Carolina would benefit from design re visions construction guidance and maintenance As a result the design guidelines developed by NCDENR in October 2001 were revisited and revised in the summer of 2006 DESIGN RECOMMENDATIONS Based on this research the following recommenda tions should be considered when designing level spreaders LEVEL SPREADER LIP A level spreader system obviously needs a stable lip that cannot be eroded Concrete level spreaders can be built with minimal slope along the length of the channel s downslope side Concrete level spreaders resist erosion better than level spread ers made of earth gravel or both If a flow greater than the design flow is routed over a level spreader made of concrete the level spreader lip will not be damaged Level spreaders made of earth gravel or both should not be used in any urban applications
113. ing areas and driveways can all be removed and converted to pervious areas Gardens lawn and permeable pavements all can be used in place of the impervious area For permeable pavement costs to be offset by CCAP it must be accompanied by impervious surface removal 61 ORTH CAROLINA SOIL amp WATER 9 2 Selection of this practice over others Community Conservation Assistance Program BMP Design Manual Impervious surface removal should only take place when there is a patio walkway parking area or driveway that is no longer being used or is going to be converted to a permeable surface Impervious areas that drain directly into the stormwater sewer system should be targeted Impervious areas that are directly connected connected imperviousness Section 3 2 to the larger stormwater drainage system via a swale or pipe have the greatest impact on stormwater runoff Impervious areas that drain onto pervious areas forested areas lawns etc before collecting in the storm drainage network present less of an impact as runoff from these impermeable surfaces has an opportunity to infiltrate into the nearby pervious area Impervious surface removal can provide a water quality benefit when there is limited space to construct a rain garden pocket wetland or other BMP in the path of the stormwater at it routes to the stormwater sewer system Impervious removal should be performed in combination with the creation of perviou
114. ipe that water must travel from the pump to the intended fixture Use the calculated values in Equation 3 to determine the total friction loss TABLES FOR USE IN FRICTION LOSS CALCULATIONS Table 2 Friction loss for every 100 linear feet of schedule 40 PVC pipe units feet of head ri tamara Que wm s 3 am va 2 a 3 a su ism oa om cum oos exe 0002 e mar sme zer ons oss exo oon oos EN 14 697 4 355 1 222 0 567 0 165 0 068 0 025 21 811 6 455 1 808 0 838 0 244 0 100 0 037 13 234 3 697 1 710 0 496 0 203 0 075 6 156 2 845 0 823 0 337 0 124 9 154 4 227 1 222 0 500 0 183 Excessive Velocities Table 3 Equivalent length for pipe fittings units feet of pipe length tame mes ve a 3 a am 2 am 3 W i5 20 as sa do 57 69 75 smo os m n ve m a5 an a0 GpeGeww o3 oa as os i 1s 20 30 Tw hexahwe 4o so 6o ro so vo iso 160 Tables 2 and 3 adapted from www engineeringtoolbox com HOW TO SELECT A SPECIFIC PUMP Once the required flow and total dynamic head are determined a pump vendor can be consulted for information on specific pumps that are best suited for system demand If a designer wishes to select his her own pump or to understand why a vendor recom mends a specific pump knowledge of pump curves is required Figure 4 A pump curve provides a Equation 3 Friction loss in system h Lr
115. istern should also be checked for stability If the cistern is consistently low on stormwater it may become light and require some sort of anchoring system to keep it in pace If a windy storm event is possible such as before a tropical storm the owner of the cistern may want to fill it part way with potable water to prevent the wind from tipping the cistern A list of maintenance activities and their associated frequency is shown in Table 7 2 52 NORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual Table 7 2 Cistern Maintenance Activities Activity Clean gutters of debris that have Bi annual accumulated check for leaks Clean debris screen to allow unobstructed Monthly stormwater flow into the cistern Check cistern for stability anchor system Bi annual Spring Fall if necessary Check accuracy of water level indicator if Annual present Check pipe and valve connection for leaks Make sure cistern manhole is accessible Annual operational and secure Before any major wind related 1 storms such as tropical Ban vou storms or hurricanes Fill tank 72 full with water if tank is less 8 0 Vegetated Swales 8 1 Overview of Practice owales often serve to convey water around and away from businesses and residences They are vegetated open channels most often lined with grass From a water quality perspective they are preferable to pipes because they allow more s
116. ity This concentration can lead to erosion of berms riparian buffers lawns or other ground cover If possible backyard stormwater BMPs should be sited near or in the path of the current drainage way leading from the watershed being treated Figure 3 6 The current drainage path can be utilized as a way to convey water away from the BMP should a storm occur that is larger than what the BMP can hold If the drainage path is currently eroding see section 8 0 for information on swale protection otential Site o X Drainage Paths Figure 3 6 A low area in the drainage path provides a suitable location for a backyard BMP 4 2 Multiple Property Owners Before a site is chosen for a backyard stormwater BMP the property boundaries should be clearly defined by the SWCD staff member This is to ensure that no part of the stormwater BMP is sited on property belonging to an individual not participating in the program If a stormwater BMP is to be sited in such a way that multiple property owners will be impacted all property owners should be contacted and should agree upon the project There is an additional concern in impacting adjoining property owners not participating in the program Stormwater BMPs are designed to slow and capture stormwater as it leaves a given property thus a pool of water can form as water slows and enters the stormwater BMP This pool of water should not extend to a neighbor s property without consent It should
117. l it receives to evapotranspiration 25 of the annual rainfall goes into the shallow groundwater 25 of the rainfall goes into deep infiltration and 10 runs off When impervious areas cover 35 8 aa T NORTH CAROLINA SOIL amp WATER XE sii Ri Community Conservation Assistance Program BMP Design Manual zu to 50 of a similar watershed only 35 of the annual rainfall is lost to evapotranspiration 20 of the annual rainfall goes into the shallow groundwater 15 goes into deep infiltration and 30 runs off Leopold 1968 Figures 2 2 and 2 3 illustrate the impact of urbanization Donaldson 2007 Water Cycle Before Urbanization ly ZI solar ene Water vapor transport bd precipitation a NE ra evapotranspiration evaporation Figure 2 2 Watershed hydrology prior to urbanization Water Cycle After Urbanization precipitation evapotranspiration evaporation increased pollution evaporation Figure 2 2 Watershed hydrology after urbanization ORTH CAROLINA SOIL amp WATER Using traditional analyses such as the Natural Resources Conservation Services stormwater model TR 55 or the U S Army Corps of Engineers many versions of HEC it can be shown that peak flows alone can increase by up to five fold from pre to post development conditions The public knows this effect of urbanization as flooding While an increase in stormwater runoff is an easily seen result of urba
118. level spreader design guidelines Osmond D L J W Gilliam and R O Evans 2002 ACKNOWLEDGEMENTS Riparian Buffers and Controlled Drainage to Re All photographs provided by the authors except for duce Agricultural Nonpoint Source Pollution N C Figure 1 courtesy of Frank Hahne Mecklenburg Agricultural Research Service Technical Bulletin County Engineering Department 318 Online http www soil ncsu edu lockers Osmond D web RiparianBuffers pdf Prepared by Jon M Hathaway Extension Associate William F Hunt Assistant Professor and Extension Specialist Biological and Agricultural Engineering North Carolina State University Published by NORTH CAROLINA COOPERATIVE EXTENSION SERVICE COLLEGE OF AGRICULTURE y LIFE SCIENCES ACADEMICS RESEARCH EXTENSION AGW 588 09 E07 45839 11 2006 bs APPENDIX D Sizing Charts for Rain Gardens and Backyard Wetlands SS SS L l SNOIAJOdu Y OOL 08 09 OV Oc 0 0008 000r 000 Ol 000 pt 0081 et P 000 pee Em P 00S a E a a t i a P e OOL D pa gd E es um S eziS r a r DeusJojeAA OOOL yidap Bulpuod 6 IIeJuley L snorntaduu pue js 0008 9ZIS p3yS I9 eM JO UOI J9UNF e se eoJy IBNS puej jeM Lg 3ANBIJ js 9ZIS 921 9B 1d snoiAJeduu Y
119. light slope less than 0 596 to allow maximum stormwater storage This practice should only be used for backyard patios sidewalks and residential parking pads The sites that are selected for permeable pavement installation should not experience heavy vehicular traffic such as garbage trucks The traffic load should be consistent with normal residential use Any parking lot associated with a business or larger than 2 stalls should be designed by an engineer to ensure structural stability Permeable pavement should not be used to treat runoff from any adjoining areas larger than the square footage of the permeable pavement itself It is imperative that these pavements are surrounded by a stable area No disturbed soil should be present in an area draining to the permeable pavement Permeable pavement is used primarily to infiltrate the rain that falls onto it Typically if grassed and rooftop areas are to be treated at a given residence a BMP other than permeable pavement may be more desirable The collected rainwater will infiltrate into the soil below thus if water capture and reuse is desirable permeable pavement is not a good BMP choice These systems are meant for low traffic areas and can not handle loads heavier than standard passenger vehicles 10 3 Permeable Pavement Design 10 3 1 Initial Soil Analysis Field Test The infiltration rate at the site should be 2 in hr throughout the top 3 feet of soil To test this three holes will
120. ll three of these locations may be prone to erosion If erosion is observed the area can be graded with a rake and shovel and reseeded Erosion control matting and check dams described above can be installed if additional protection is required Once the vegetation has established and if there have been no signs of erosion inspection frequency can be decreased to only following storms exceeding 2 inches Invasive species can result in reduced plant diversity within the wetland Cattails are one species that often becomes established in stormwater wetlands The herbicide aquatic formulations of glyphosate example trade name of Rodeo can be applied to the cattail stalks resulting in plant mortality This should be performed the first time a cattail is identified in the wetland typically in mid spring Care should be taken when using these chemicals A chemical resistant glove should be worn whenever handling herbicides An easy way to apply the chemical is to wear a cloth glove over the chemical resistant glove and to let some of the chemical soak into the cloth glove The stalks can simply be wiped with the cloth glove For a detailed description of this method of removing cattails please see Stormwater Wetland and Wet Pond Maintenance Hunt and Lord available in Appendix G Additional maintenance activities include such things as cleaning out the sediment that has accumulated in the forebay of the BMP using a shovel and picking up any
121. n The economic incentive 75 of average installation costs will encourage voluntary conservation oince this is a grant funded program only districts participating in the surveys will receive an allocation The maximum amount of assistance per practice is limited to 50 000 To see which districts will be implementing CCAP BMPs through these grants refer to the map below Figure 1 1 Other Districts are implementing these BMPs through funding sources they have secured on their own The goal of the DSWC is to seek additional funding sources including recurring state appropriations to offer this program statewide in the future The 6 tT NORTH CAROLINA Community Conservation Assistance Program BMP Design Manual CONSERVATION Community Conservation Assistance Program is expected to be an asset in addressing current stormwater pollution issues within North Carolina Districts Participating in the Community Conservation Assistance Program Grants awarded based on 2005 district surveys Figure 1 1 Districts Participating in the Community Conservation Assistance Program 1 2 The CCAP Best Management Practice Design Manual The purpose of the CCAP Stormwater Best Management Practice BMP Design Manual is to be a resource for soil and water conservation district staff in siting selecting designing installing and maintaining stormwater BMPs This manual is intended to educate staff members in order to implement
122. nization there are many less visible water quality impacts associated with development Erosion and sedimentation have long been recognized as water quality concerns Although the North Carolina legislature passed laws to curb sediment pollution in 1973 sediment remains the number one pollutant of N C waters In addition metals and chemicals from vehicles and industries pollute stormwater runoff in increasing amounts Under forested conditions these pollutants would only be found in trace amounts Likewise nutrients are found in the urban environment in a variety of forms one such form being fertilizer Fertilizer contains nutrients for plants to grow but excess fertilizer or fertilizer that is inadvertently applied to pavement harms water quality Even if proper amounts of fertilizer are applied nutrients can enter our streams in other ways including atmospheric deposition wildlife and pet waste and septic system malfunction There are numerous ways to reduce pollutant loadings including source reduction such as proper application of fertilizer and correctly maintaining septic systems Structural devices can also help curb this problem This manual will provide design guidance for several structural Best Management Practices BMPs that can be constructed to treat runoff and thereby reduce the amount of pollution entering streams Community Conservation Assistance Program BMP Design Manual 2 3 Stormwater BMPs An urban storm
123. ntities to initiate plant growth After initial fertilization the wetland should not be fertilized again In many cases the wetland soil should also be limed To verify pH soil samples can be sent to the NCDA amp CS laboratory in Raleigh for testing 6 4 3 Planting the wetland oome wetland plants can be purchased at home improvement stores others will only be available at specialty native plant nurseries Plant nurseries can also provide valuable insight on plant selection Planting wetland vegetation is similar to planting any other type of vegetation A trowel or shovel can be used to create a small hole for planting Wetland plants should be placed in the location around the wetland in which they are best suited the zones discussed in Section 6 3 5 are a good guide in planting the various species in the correct environment The depth of zones is found by measuring from the top of the overflow weir or lowest elevation along the wetland perimeter if no weir is used to the underlying soil 42 STA ORTH CAROLINA SOIL amp WATER 6 5 Backyard Wetland Maintenance Community Conservation Assistance Program BMP Design Manual Backyard wetlands must be maintained to ensure their long term functionality As mentioned earlier the BMP should initially be checked frequently every month for signs of erosion around the containment berm if one is present where the stormwater enters the system and just downslope of the wetland A
124. nts that have collected on impervious surfaces will wash off during the first part of a storm event Essentially the first portion of a given rain event will flush the impervious surface of its pollutants resulting in stormwater runoff that contains more pollutants than runoff produced later in the storm This concept was adopted by many in the stormwater management field who theorized that if the first half inch of runoff the first flush could be captured and treated by a stormwater practice that 90 of the pollutants leaving the site would be treated by the stormwater practice Schueler 2000 Of course one half of an inch of runoff is not produced by one half of an inch of rainfall Some water is stored in depressions in roadways lawns and other places and some infiltrates into the watershed s grassed and forested areas Thus capturing the runoff associated with 1 inch of rainfall was previously considered the standard for capturing the first flush However because rainfall patterns vary throughout North Carolina it was questionable that 1 inch of rainfall corresponded to the first flush in all areas of North Carolina A study was performed by BAE for NCDENR to evaluate the first flush or water quality event in various locations throughout North Carolina Bean 2005 For the basis of this study the water quality storm event was defined as the event size in depth which 90 of all storms are equal to or less than Essentially if a
125. o increase stormwater infiltration and sediment filtration See Section 8 0 for more information on designing swales Stone or rip rap can be used at the entrance to the rain garden if velocities exiting the swale are too high Forebay A forebay can be used at the entrance of the garden to pool water settle sediment and dissipate energy before flow enters the main body of the rain garden These features should be used particularly in larger watersheds Gravel verge This method should frequently be used when space is limited or when stormwater enters as sheet flow from an impervious surface directly adjacent to the garden such as from a driveway Figure 5 5 shows a gravel verge and vegetated filter strip combination on the slope of a rain garden Sheet flow leaving the impervious surface should first contact a strip of washed 57 stone over geotextile fabric and then a grass strip Each section should be at least 1 foot wide NORTH CAROLINA SOIL WATER CONSERVATION Community Conservation Assistance Program BMP Design Manual Bloretention Inlet Protection Under sheet Flow ANM moll drop from pavement to store 1 2 Inches ta allow free Flow Sheet flow Woshed 37 stone fat least 27 thick l wldes Geotextile Fabric Continued grass or other vegetation and mulch Grass Flten strip Addie TTTT TTTTPT Nate Thls system ls for Inlet protectlon under sheet flaw It Is not sultable for concentrated flow
126. o the level of its lower side The lower side the downslope side of the channel is constructed so that it is level along its full length This lower side or level spreader lip 1s often constructed of concrete or rock so that it resists erosion As stormwater enters the channel it rises until it fills the channel and exits evenly over the lip The downslope side of the system functions as a long broad crested weir RIPARIAN BUFFER After the stormwater passes over the level spreader lip it enters the riparian buffer often simply called the buffer As the stormwater passes through the buffer vegetation some of the water 1n filtrates Ideally the buffer will remove sediment and nutrients from runoff before it reaches the stream Flow Bypass Structure Watershed Remtorced Channel Level Spreader Lip Figure 2 Level spreader system Zone 2 Butter Zone Butter Y y v v Ny v L cer t LU Ld Y v v y v y y cere ee y y v y y y Nom mtt ttr m v 9985 9 t9 497 Stone coe P Lo i Reinforced Vegetated Swale RECENT LEVEL SPREADER RESEARCH The Biological and Agricultural Engineering Depart ment at North Carolina State University received a grant from NCDENR in December 2005 to evaluate level spreaders as stormwater best management prac tices BMPs The researchers visited 24 locations where level spreaders were in use and performed vari ous qualitative and quantitative analyses The results of the study ind
127. oil water contact and more opportunity for infiltration Filtration of sediment and debris can also occur within swales especially if the grass is kept relatively long When swales are designed to be flat and wide stormwater velocity is decreased leading to increased soil water contact and thus promoting infiltration and filtration Figures 8 1a and 8 1b show a grassed swale 53 CONSERVATION Figures 8 1a and 8 1b show two swales in the Raleigh Durham area Both have turf reinforcement mats The former in Durham b is mowed while the latter in Raleigh a contains a taller grass stand 8 2 Determining Site Constraints The first step in designing a vegetated swale is to determine the constraints of the site Slope The swale must lead from an upland area where it ties into an existing grade to a lower area where it again ties into an existing grade Often swales will lead from a point in the watershed to a BMP or from a BMP to the existing drainage infrastructure This information can be used to determine the allowable slope of the swale The swale should have a relatively constant slope between it s inception and destination the slope should gradually change if necessary Slope S elevation change from start to end ft length of swale ft Top Width The maximum width of the swale may be dictated by site constraints Determine the maximum width before beginning calculations Side Slopes It may be desirable at some
128. on sandy soils will rarely hold water for more than a few hours On these sites it 15 most important to choose plants for their drought tolerance Rain gardens created on loamy or silty soils could pond water for 1 2 days 1f your site ponds water for more than 3 days you should consider creating a wetland On these sites choosing plants tolerant of extended flooding is critical to success Remember you are not limited to planting just within the excavated area Extending plantings around this area will help the rain garden to blend in with the overall landscape Any plants adapted to the site conditions can be used outside of the excavated area For more information on designing rain gardens and bioretention areas refer to the following NCSU publication Designing Rain Gardens Bioretention Areas available from your local NC Cooperative Extension office or online at http legacy ncsu edu classes a bae cont ed bioretention lecture design rain pdf Large Trees over 30 tall Deciduous Red Maple 2 Acer rubrum River Birch 1 3 Betula nigra Cherry birch 2 Betula lenta Willows 3 Salix species Bald cypress 1 3 Taxodium distichum Sycamore 2 Platanus occidentalis Yellow buckeye 2 Aesculus octandra Persimmon 2 Diospyros virginiana Silverbell 1 Halesia carolina Blackgum 3 Nyssa sylvatica Black cherry 2 Prunus serotina White basswood 1 Tilia heterophylla Box Elder 3 A
129. one can be placed in the excavated area The stone should be spread and leveled to the appropriate slope The stone should then be compacted with a minimum 10 ton static roller at least 4 passes The stone should be compacted until there is no visible movement within the gravel base Smith 2000 If permeable pavers are being installed the same procedure that is described for the permeable concrete gravel base would be performed with a 4 inch thick layer of washed 57 stone Next a 2 inch layer of 78 stone should be added to the top of the 57 stone layer This layer should be compacted using the same process that was used for the first layer The gravel base will now be structurally ready to support permeable pavement 69 STA ORTH CAROLINA SOIL amp WATER 10 4 3 Concrete edging When permeable interlocking concrete pavers are installed some edging will be required to keep the pavers stable along the edges of the new permeable area As stated in section 10 3 3 a 6 inch wide 18 inch deep concrete curb should be sufficient The framing for the curb should be constructed before the compaction of the gravel takes place The curbing should simply run along the perimeter of the permeable area to provide support to the pavers on the edge of the lot Community Conservation Assistance Program BMP Design Manual 10 4 4 Permeable pavement installation As stated above installing permeable concrete or permeable pavers shoul
130. onncocicanocanconnnononononos 8 4 Simplified Triangular Swale Design occoococcccocncocncococococonoconconnconononnnos 8 4 1 Adjusting Manning s equation for simplified triangular swale design 57 8 4 2 Simplified triangular Swale design process seen 59 8 5 Construction GUIdANCEO ooccoccccccoccoccconcoroccncnnconncononnnnnnnnncnnnnnnonnnnnnanoncnnnnnns 60 OVI Ue e ro 0 OO E 61 9 0 Impervious Remo Valiasansiaindadadaradctadasadaia arado 61 RO o O LOG E oaro rrr ereere E e ESE EUU CUEN 61 9 2 Selection of this practice over OtNerS cooccocccoonconiocnconcocnoonconconncancnnnnanons 62 9 3 Carrying Out Impervious Removal occooccccccccccocncocncocococonanonanonnncnnnnncnnos 62 10 0 Permeable Pavement eeeseeeeeeere eren enne nnn nnn nnn nns 62 10 1 OVETVICW OF Practice anses rar 62 10 2 Selection of this practice over OtherS ocooccocccoccocnconconoconcnnnonocanoncnnannns 64 10 3 Permeable Pavement DesiQN occocccoccccnccncococcncoccncnnancononanonconnnnanonnonanons 65 Community Conservation Assistance Program BMP Design Manual NORTH CAROLINA JOIL amp WATER A e CONSERVATION 10 3 1 Initial Soil Analysis Field Test oooocccociconiconccnnconnconncononanononononcncnnos 10 3 2 Soil Analysis Lab Test cessisse nennen nnne EEEE 10 33 Bo elo C AOI o Om IAE Cl Se AA o o Ann 10 3 5 Permeable Pavement Sizing
131. opography of the bottom of the rain garden will largely be a function of the infiltration rate of the soil The infiltration rate can be tested as described in Section 3 7 2 If the soil drains in 12 hours or less the bottom of the rain garden can be essentially flat If the soil drains between 12 and 36 hours grade the bottom of the rain garden so that zones of varying bottom elevation exist This will create areas where the flood pool depth is shallow and ponding time is shorter which can help the survival of some bioretention plants some of which cannot tolerate wet conditions for extended periods Creating higher and lower elevation zones along with planting some plants on the banks of the bioretention cell can help ensure the survival of plants when drainage conditions are somewhat uncertain Keep in mind that higher elevation zones take away from the storage volume of the garden and the area should be increased accordingly This can be done by dividing the runoff volume by an average depth for the cell For example if 50 of the cell will be ponded 9 inches deep and 50 will be ponded 6 inches deep the average depth is 7 5 inches Shallow rain gardens take up more surface area than their deeper counter parts Figure 5 4 Mounding of soil allows certain rain garden plants to survive better in a zoned rain garden 5 3 5 Plant selection When the rain garden drains in less than 12 hours the bioretention plant list in Appendix E can be use
132. orem Figure 8 4 we can determine the length of the unknown side x In triangular swales with 3 to 1 4 to 1 or gentler sideslopes the length of x is found to be approximately the same as the length y The y value is equal to half of the swale width thus the unknown distance x is also approximately equal to half the swale width Because the wetted perimeter section 8 3 of a triangular swale is calculated as the sum of the two x distances seen in the figure above the wetted perimeter is approximately equal to the swale width When constructing a swale with known sideslopes the width of the swale can be defined in terms of the depth since they have a consistent relationship with one another For example 3 to 1 sideslopes on a swale indicates that for every 1 foot of depth each sideslope will be 3 feet wide for a total swale width of 6 feet In general terms Swale width swale depth x 6 Also we can define the area of the swale as Swale area swale depth x 3 With these geometrical assumptions the equations used to size swales section 8 3 can be simplified as the area and wetted perimeter can both be written in terms of depth in the hydraulic radius calculation section 8 3 Thus our final equations are as follows Equation 8 3 Manning s Equation for triangular swales sideslope 3 to 1 Qp 1 49 n x 1 04 x D x S Or D Qp x n 1 55 x S p 7 58 tT ORTH CAROLI
133. otected using a suitable erosion control product while seed is being established to avoid erosion Sod may also be used in many cases 60 ITA NORTH CAROLINA SOIL amp WATER 8 6 Maintenance Community Conservation Assistance Program BMP Design Manual owale maintenance should consist mostly of mowing Adjusting the mower to keep the grass long is preferable for filtration If erosion occurs the swale may be re shaped and stabilized If substantial sediment deposition occurs it must be removed to allow unobstructed flow in the channel Table 8 3 Maintenance Activities for Swales Activity Frequency Mow grassed channel keep i As needed relatively long As frequently as once per month Check swale for signs of erosion Remove deposited sediment which could obstruct flow As needed If needed late spring late Reseed summer depending on grass type As needed rarely a problem Repair Turf Reinforcement Matting if mower does not catch the mat 9 0 Impervious Removal 9 1 Overview of Practice As discussed in section 2 2 urbanization characterized by increases in impervious area can cause a substantial increase in stormwater runoff One basic stormwater management practice is to reduce the amount of impervious area in a given urbanized area If an area has already been urbanized this can be accomplished by removing impervious areas that are no longer needed Patios walkways park
134. p b1 to the b scale and find the intersection with the z 0 scale Project horizontally from the point located in Step b1 to the appropriate z value and find the value of d b Multiply the value of d b obtained in Step b3 by the bottom width b to find the depth of uniform flow d c Given the depth of flow do the following 1 2 3 4 Find the ratio d divided by b and project a horizontal line from the d b ratio at the appropriate side slope z to the z 0 scale Connect a line from the point located in Step cl to the b scale and find the intersection with the turning line Connect a line from the point located in Step c2 to the slope scale and find the intersection with the On scale Divide the value of Qn obtained in Step c3 by the n value to find the design discharge Q 0 mja NOTE Project horizontally from Z 0 scale y to obtain values for Z 1 to a EJ k ASA CE CEN TREF DED Gare PR Ge ec B a C c Cad n2 o 0 04 0 05 0 06 0 08 Liu 0 1 a 5 a aa gt 0 2 0 3 3 0 4 Fi 0 5 if 0 6 jf f A 0 8 EXAMPLE 1 0 z GIVEN FIND SOLUTION 20 01 d Qn 0 3 Qa10FT 5 d B 0 14 n 0 03 d 0 14 4 0 56 FT B 4FT Z 4
135. percent of all phosphorus entering the wetland Figure depicts N C STATE UNIVERSITY A amp T STATE UNIVERSITY COOPERATIVE n ION RU Hering People Put Knowledge to Work some wetlands located across North Carolina For more information on stormwater wetlands see Designing Stormwater Wetlands for Small Water sheds AG 588 02 1n the Urban Wa terways fact sheet series Wet ponds are typically much deeper than stormwater wetlands their average depth ranges from 4 to 8 feet They are designed so that most of the pond is open water Wet ponds are the most common stormwater management practice in North Caro lina and have been constructed since the 1970s in some parts of the state More recent pond configurations 1n corporate wetland features such as an aquatic shelf or wetland bench and a forebay An aquatic shelf 1s a shallow water zone of a pond usually along the bank edges planted with wetland veg etation These shelves flood during storms A forebay 1s a pool where inflow first enters the pond and heavier pollutants such as sediment initially settle there Research con ducted across the United States shows that wet ponds effectively remove sediment and the pollutants associ ated with it from stormwater Both wet ponds and stormwater wetlands can T a MEE f AA O A e E FL WP i m A e eT EE de e li aie id r a a SIR LC RD a CUTEM ELA TY mt Ig mn T LES o MIR EN re b uet e
136. r Wp see Figure 8 2 or for trapezoidal channels R bd xd b 2dx all dimensions in ft For convenience the top width of the trapezoidal channel can be calculated as Swale Top Width b 2Dx all dimensions in ft Freeboard Figure 8 2 Trapezoidal swale cross section The correct cross section must be determined by trial and error A computer spreadsheet or other software e g TR 55 would be helpful in this process and has been provided to SWCD personnel in the past Once a channel geometry is assumed then the Continuity Equation Equation 8 1 and Manning Equation Equation 8 2 are used to determine the average velocity of flow for the design storm A nomograph has been provided in Appendix J that can be used to solve the Manning Equation Using the maximum permissible velocity method it can be determined whether the channel will erode Table 8 2 shows maximum permissible velocities for a variety of channel linings Manufactured products should have maximum permissible velocity specifications available If the channel velocity exceeds the maximum permissible velocity for a given surface lining erosion will occur It is good practice to design conservatively Multiply the calculated velocity by a safety factor of 1 3 when comparing with the maximum 56 IT NORTH CAROLINA CONSERVATION permissible velocities in Table 8 2 Assume that grass will not be mowed regularly when selecting a Manning n Leave a
137. r about the NET Framework or the program won t open Solution o Check in the add or remove programs area of the windows control panel to see if you have the NET Framework installed The NET Framework is often installed with windows updates but can be manually installed from http www microsoft com downloads details aspx familyid 0856eacb 4362 4b0d 8edd aab15c5e04f5 amp displaylang en Problem o laccidentally changed or deleted an input file Solution o The input data files can be extracted and manually copied from the zip file located on the NCSU rainwater harvesting website Problem o receive an error when trying to save the output to a network folder Solution o Currently output files can only be saved to local drives and then copied to network folders Problem o My output values don t seem reasonable Solution o If using custom rainfall data ensure that the input data file is in the correct format APPENDIX Pump Sizing Information for Cisterns A Wat Distributed in furtherance of the Acts of Congress of May 8 and June 30 1914 North Carolina State Uni versity and North Carolina A amp T State University com mit themselves to positive action to secure equal opportunity regardless of race color creed national origin religion sex age or disability In addition the two Universities welcome all persons without regard to sexual orientation North Carolina State University North Carolin
138. r power equipment the top of a sledge hammer works well or the end of a shovel handle for even smaller spaces The surface of the compacted soil can be further protected using filter fabric and stone or rip rap as necessary wooden wele Control Structure CJ T 22 Profile Ad jurinke pate anfice cational recomendo stiles steel eS a e E SEEDEN PY s l6 io SO oT RTS TEE nm s Tp E er Lo Faction T Jorrts i a ud Lom A A O r S _ _ ar ar 1 i Y d i dx4 nr xb Homia vo fe Bie a eae Lu x Mate Stabllze around wel as necessary Figure B1 Diagram of a wooden weir control structure APPENDIX C Level Spreader Design Construction and Maintenance mje Distributed in furtherance of the acts of Congress of May 8 and June 30 1914 North Carolina State Uni versity and North Carolina A amp T State University commit themselves to positive action to secure equal opportu nity regardless of race color creed national origin reli gion sex age or disability In addition the two Universities welcome all persons without regard to sexual orientation North Carolina State Univer sity North Carolina AST State University U S Department of Agriculture and local gov ernments cooperating Level Spreaders Overview Design and Maintenance Level spreaders are stormwater structures that can support the filter ing action of riparian buffers if designed and installed properl
139. rk wetland area prior to excavating the area 6 4 2 Digging the wetland After the wetland area has been laid out digging can commence Topsoil should be removed and stored by the side of the wetland this topsoil will be put back into the wetland later If the BMP is built on a slope the remaining soil removed from the wetland area should be placed around the wetland perimeter to create a berm that will retain the stormwater Figures 6 6 and 6 7 If it is not built on a slope the entire BMP can be dug into the ground and no berm is required Figure 6 8 In this case the soil is not needed for a berm and can be discarded Erosion Control Mat Earthen Bem Chack Dam Overflow Wair Built into Berm Forebay Figure 6 6 Illustration of backyard wetland constructed on 4 slope 40 NORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual d Figure 6 7 Berm Construction at downslope edge of BMP Original Ground Surface Erosion Control Mat Captured Stormwater Figure 6 8 Illustration of backyard wetland constructed on 0 5 slope Since an average depth of 9 inches is desired some areas in the wetland can be as deep as 1 foot while other areas will be 3 inches or less The depth is found by measuring down from the top of the overflow weir or lowest elevation along the wetland perimeter if no weir is used to the excavated soil surface Building variability into th
140. rmal Rain Gardens e a Plants for Rain Gardens Recommended for Southeastern North Carolina Charlotte Glen Urban Horticulture Agent North Carolina Cooperative Extension New Hanover County Center Soil conditions in rain gardens alternate between wet and dry making them tough places for many plants to grow The following plants are adapted to these conditions though some plants will tolerate more moisture than others Each plant is marked according to its flooding tolerance with 3 s being tolerant of longer flooding 2 s only tolerating brief flooding and 1 s indicate plants that tolerant extended drought once established All of these plants are native to the southeastern United States in wetland habitats and most are readily available at local nurseries Wetland plants can generally grow well in moist or well drained soils whereas plants adapted to dry soils rarely survive in soggy conditions How wet a rain garden stays will vary considerably depending on the site where it 1s installed Rain gardens created on sandy soils will rarely hold water for more than a few hours On these sites 1t 15 most important to choose plants for their drought tolerance Rain gardens created on loamy or silty soils could pond water for 1 2 days 1f your site ponds water for more than 3 days you should consider creating a wetland On these sites choosing plants tolerant of extended flooding 1s critical to success Remember you are not limi
141. rt layer ranging in thickness from 4 to 12 inches Permeable pavements work best when 62 STA NORTH CAROLINA WATE Community Conservation Assistance Program BMP Design Manual sited on sandy soils such as those found on Barrier Islands the Coastal Plain and the Sandhills Permeable pavement should not be placed near active construction zones as they are prone to clog An example of a patio constructed of permeable pavement is shown in Figure 10 2 and a residential driveway application is shown in Figure 10 3 Figure 10 2 Four types of permeable pavements starting top left corner and moving clockwise permeable concrete permeable interlocking concrete pavement grass pavers and concrete grid pavers 63 i NORTH CAROLIN SOIL amp WATER eae Ri Community Conservation Assistance Program BMP Design Manual TFI 411111114 i 10 2 Selection of this practice over others In parts of North Carolina NC DENR awards a runoff reduction credit if permeable pavement is used in place of traditionally impermeable pavement This blanket credit for permeable pavement as a stormwater BMP is only available in the Sandhills and Coastal Plain Barrier Islands thus these backyard practices should only be installed in these two locations The reason for this geographic limitation is that the stormwater infiltration associated with permeable pavements Is limited by the soils in which they are installe
142. s 5 and 6 This drainage rate is particularly important for plant selection and bottom grading of the practice Figure 3 9 Simplified soil infiltration test Table 3 2 Selection Chart for Soils with Moderate Infiltration Time For 2 Foot Hole to Drain BMP Selection Y2 day or less Standard Rain Garden Y day to 175 days Zoned Rain Garden 1 days to 4 days Drought tolerant wetland Standard wetland More information on the Zoned Rain Garden is available in Section 5 0 More information on the Drought tolerant wetland is available in Section 6 0 22 NORTH CAROLINA SOIL amp WATER Community Conservation Assistance Program BMP Design Manual 3 9 Information on Water Table Depth While the soil analysis is being performed at the proposed BMP location the location of the seasonal high water table can be approximated This is an evaluation of the highest consistent level that the water table reaches with respect to the ground surface during the year As the soil is removed from the 2 foot hole that is dug at the site the soils should be observed for signs of the seasonally high water table After the topsoil is excavated from the hole the soil layers underneath provide some information regarding the location of the seasonally high water table When soils are observed as gray or contain gray mottles spots of gray color this soil is likely in contact with the water table for extended periods of time Thus as the soil i
143. s areas such as lawn gardens or permeable pavement Permeable pavement conversion can only be performed in the Sandhills and Coastal Plain as part of the CCAP 9 3 Carrying Out Impervious Removal While removing impervious area is a simple way to limit stormwater runoff removing asphalt concrete or brick can be challenging Breaking the impervious area into small pieces and removing these pieces should be performed by a contractor to avoid injury The impervious area once broken up will consist of very heavy sharp edged debris A large truck will be required to transport the debris from the location to a disposal site after heavy equipment such as a backhoe breaks the pavement apart and carries it to the truck 10 0 Permeable Pavement 10 1 Overview of Practice Traditionally paved surfaces are impermeable converting nearly all rainfall to runoff These pavements were designed to be strong that is able to withstand heavy traffic Permeable pavement allows water to pass through it reducing runoff Figure 10 1 These pavements are best used in low traffic situations similar to those found in patios parking pads and driveways making them a good backyard BMP Permeable pavements may be constructed of permeable asphalt pervious concrete permeable interlocking concrete pavers concrete grid pavers and grassy pavers Close up views of many of these surfaces are found as Figure 10 2 They are typically underlain by a gravel suppo
144. s being extracted from the hole the seasonal high water table is likely to be at the depth at which these mottled soils are encountered More information on determining the seasonal high water table is available in Richardson and Vepraskas 2001 Obviously if groundwater is encountered as the hole is being dug the site very likely has a high water table Figure 3 10 Figure 3 10 Obvious wet conditions in this location lead to selecting a backyard wetland as a stormwater BMP If the seasonal high water table is found within 2 feet of the ground surface the site is likely not a good candidate for certain stormwater BMPs rain gardens permeable pavement If such conditions exist at a site a backyard wetland is a more appropriate choice as a stormwater BMP although a seasonal high water table within 1 foot of the ground surface is more desirable for this BMP 4 0 General Stormwater BMP Considerations 4 1 Avoid Concentrated Flow Stormwater collects and concentrates very easily into depressions swales and natural runoff conveyances Stormwater concentration can also occur as water 23 sa TD NORTH CAROLINA SOIL amp WATER mE gt a Community Conservation Assistance Program BMP Design Manual EEE _E_____________ _ _________________ _____ __ ______ Bislagieal Agru uter al CONSERVATION overflows from a backyard rain garden or wetland or as water passes through a cistern that is already filled to capac
145. s communities that are considered among the best places to live in America as a result the state s growth rate is consistently one of the highest in the country The resulting urban influx affects many facets of the state s infrastructure more cars drive our roads more children enroll in our schools more people create higher wastewater discharges and more development necessitates stormwater runoff controls How does urbanization affect stormwater runoff Roads parking lots sidewalks homes and offices replace the natural and permeable landscape Rainfall that once soaked into vegetated ground is now available for stormwater runoff As more surfaces become impermeable water simply moves across them Impermeable surfaces connect to form a stormwater superhighway that allows runoff to reach streams more quickly Figure 2 1 c Tar 7 v P Po Figure 2 1 Stormwater runoff ina highly urbanized watershed There are many affects of this increase in impermeable area 1 more stormwater reaches streams because there is less opportunity for it to infiltrate into the ground 2 peak flows increase because the stormwater superhighway transports runoff from large areas rapidly 3 velocities in the stream increase causing a larger erosion potential and 4 baseflow is lower during dry weather due to a lack of infiltration into the underlying groundwater An area with natural ground cover loses 40 of the annual rainfal
146. s energy and to provide a location for sediment and other gross solids such as leaves other tree debris cigarette butts and trash to settle and accumulate A forebay is typically 2 feet deep in a stormwater wetland and sometimes deeper in a wet pond If the forebay fills with sediment and gross solids these ma terials will bypass the forebay and begin to accumulate in other portions of the wetland or wet pond that may be more ecologically sensitive To check sediment levels inside the forebay record the depth of the forebay at the same time each year Depending on the size of the forebay a fish finder can be used from a small boat or someone can survey the depth along a grid of the forebay with a rod Figure 2 If the forebay water is clear the depth can often be determined visually Once the forebay is half full of sediment or the average sediment level is within 1 foot of the water surface remove the sediment and gross solids This task is typically accomplished by a track hoe or backhoe Figure 2 The water level inside the wet pond or stormwater wetland can be lowered if needed to aid excavation of the forebay Depending upon the size of the forebay cleaning it can require anywhere from a day to a week Once the excavated soil or spoils from the dredg ing has begun to dry either spread it in the watershed away from the banks of the wetland or wet pond and seed it or take it to a landfill Consider the location
147. shed size and the percentage of impervious area have been calculated the Simple Method Section 3 3 can be used to determine the volume of water that will enter the BMP during the first flush event The wetland surface area will simply be this volume divided by the ponding depth desired for the site The figures in Appendix D use the simple method to determine the BMP surface area that is required for various watershed scenarios various sizes and impervious percentages These figures can be used to size a stormwater wetland for 3 6 and 9 inch average ponding depths 6 3 3 Backyard wetland inlet considerations As stormwater enters the wetland from a pipe or swale the energy of the stormwater can cause erosion Placing a check dam in the path of the stormwater just before it enters the wetland will reduce the velocity of the water This check dam can be constructed of rolled erosion control matting or with 6 8 nominal diameter rip rap This may not be required in all situations however if the slope of the swale or pipe entering the backyard wetland is steep 769 erosion is likely to occur Examples of check dams constructed of erosion control matting and rock are shown below in Figures 6 3 and 6 4 Figure 6 3 Swale leading to BMP with check dams constructed of rolled erosion control matting 35 NORTH CAROLIN SOIL amp WATER Community Conservation Assistance Program BMP Design Manual Figure 6 4 Swale with ch
148. simulate the performance of a rainwater harvesting system accurate water usage data is essential Ideally water usage measurements should be collected at the location where the rainwater harvesting system will be installed These usage estimations can be collected with a variety of inexpensive flow metering devices Table 2 Typical water usage that could be replaced with harvested rainwater Usage Frequency Volume Irrigating Established Lawn 62 gal 100 ft Home Car Wash 50 100 gal Professional Car Wash 20 gal Daily Usage Gallons per day that are used consistently on a day to day basis Seasonal Usage Gallons per day evaluated each week during the specified months Toilet Usage Estimated toilet usage based upon 5 uses per individual per day Weekend Usage If checked the model will include toilet usage during all 7 days of a week Outputs Total Volume Captured Percentage of total runoff volume the cistern was able to capture without overflow Usage Replaced Percentage of total water demand that the cistern was able to supply Note The total volume captured and usage replaced serve as quick indicators of how well the cistern performs with regards to water quality and water supply Annual Water Usage Average amount of water used from the cistern in a given year Annual Water Savings Average amount of monetary savings from using the cistern to replace municipal water supply
149. sites to leave the side slopes of a swale mild enough that they can be mowed with a riding lawn mower If this is the case a slope of 4 1 H V or flatter is recommended no steeper than 3 1 is recommended in most cases and never more than 2 1 unless the designer has extensive experience with swales Triangular swales can be used on small watersheds less than 2 acres They can be designed as trapezoidal swales with very small bottom widths Shape Both trapezoidal and triangular cross sections will be commonly used for water quality swales The depth of flow within these channels will be shallow during 54 ORTH CAROLINA SOIL amp WATER small storms allowing increased soil and grass contact with the stormwater and thus increased infiltration and filtration Triangular shaped swales are more appropriate for small drainage areas Community Conservation Assistance Program BMP Design Manual Peak Flow Swales should be designed to convey the peak flow of the 10 year storm without overtopping as calculated in section 3 4 using the Rational Method During extreme events e g 25 or 100 year storms portions of the swale may be breached and should be repaired by the property owner as necessary 8 3 Depth and Velocity Calculations Swales should be able to carry their design flow without overtopping or eroding If the velocity is too high for grass cover in a given swale design and the slope and cross section cannot be adjusted
150. some cases Certain level spreader and riparian buffer systems may only partially disperse influent concentrated flow The systems still can improve water quality and reduce flow peaks However level spreaders should be situated in areas away from natural swales depressions and mounds where diffuse flow 1s more attainable If the riparian buffer is not conducive to diffuse flow other BMPs should be considered SUMMARY Table 2 shows suggested site selection design and maintenance criteria for level spreader systems Level spreader design is not as technically challenging as other stormwater BMP designs Nevertheless siting and installing a level spreader that functions properly is challenging The design criteria described in this publication represent elements of good level spreader design but do not ensure overall system effectiveness For the system to function as intended it must be maintained and the internal topography of the riparian buffer must be conducive to keeping flow diffuse Before designing a level spreader system the designer should visit the site This site visit is highly recommended If conditions are not suitable for the installation of the level spreader for example if the slope 1s too steep or adequate space is not available other stormwater BMPs should be used Table 2 Level Spreader Site Selection Design and Maintenance Recommendations Level spreader lip material A concrete or sturdy metal lip s
151. t least 0 3 ft of freeboard at the top of the swale during the 10 yr storm The channel can be reinforced if velocities cannot be lowered sufficiently by enlarging the swale Community Conservation Assistance Program BMP Design Manual E Table 8 1 Manning s n values for various channels TypeofChammellining Designan e at Umstead State Park 4 A uga h NAWA A Figure 8 3 Turf reinforcement mats used along a s wal Table 8 2 Maximum permissible velocities for various channel linings Type of Channel Lining uis perianal Graded Rock D50 3 in Graded Rock D50 6 in 90 Turf reinforcement mat products 9 5 25 8 4 Simplified Triangular Swale Design 8 4 1 Adjusting Manning s equation for simplified triangular swale design In many cases a given watershed will be small enough to justify the use of a triangular swale Some assumptions and generalizations can be made when designing triangular swales these assumptions will simplify the design process When the triangular swale cross section is split in half two right angles are formed figure 8 4 27 NORTH CAROLINA SOIL amp WATER assi i Ri Community Conservation Assistance Program BMP Design Manual CONSERVATION Figure 8 4 Triangular swale cross section broken into two right angles Breaking the swale into two right angles allows some simplification of the triangular swale design method By the Pythagorean The
152. tate University Extension publication no W 3 2001 Online http ohioline osu edu w fact 003 html BAE Stormwater Group Web site www bae ncsu edu stormwater Obtain information on upcoming workshops including BMP Inspection and Maintenance Certification Courses publications PowerPoint presentations images to download and design and construction specifications State of North Carolina Stormwater Web site www ncstormwater org ACKNOWLEDGEMENTS All photographs were provided by the Department of Biological amp Agricultural Engineering at N C State University except as noted for Figures 7 and 8 Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label Be sure to obtain current information about usage reulations and examine a current product label before applying any chemical For assistance contact your county Cooperative Extension agent Prepared by William F Hunt Ph D P E Assistant Professor and Extension Specialist Biological and Agricultural Engineering North
153. ted to planting just within the excavated area Extending plantings around this area will help the rain garden to blend in with the overall landscape Any plants adapted to the site conditions can be used outside of the excavated area For more information on designing rain gardens and bioretention areas refer to the following NCSU publication Designing Rain Gardens Bioretention Areas available from your local NC Cooperative Extension office or online at http legacy ncsu edu classes a bae cont ed bioretention lecture design rain pdf Large Trees over 30 tall Deciduous Red Maple 2 Acer rubrum River Birch 1 3 Betula nigra Green Ash 3 Fraxinux pennsylvanica Black Gum 2 Nyssa sylvatica Willow Oak 1 2 Quercus phellos Willows 3 Salix species Bald Cypress 1 3 Taxodium distichum Pond Cypress 1 3 Taxodium ascendens Nutall Oak 1 2 Quercus nuttalii Evergreen Atlantic White Cedar 1 3 Chamaecyparis thyoides Southern Magnolia 1 2 Magnolia grandiflora Longleaf Pine 1 2 Pinus palustris Swamp Laurel Oak 3 Quercus laurifolia Small Trees under 30 tall Deciduous Red Buckeye 2 Aesculus pavia Ironwood 1 3 Carpinus caroliniana Redbud 1 2 Cercis canadensis Fringe Tree 2 Chionanthus virginicus Washington Hawthorn 3 Crataegus phaenopyrum Possumhaw 1 3 Ilex decidua Evergreen Dahoon Holly 1 2 lex cassine American Holly 1 2
154. th 3 4 acre watershed various watershed scenarios intensity of 7 2 in hr vn cem Weir Required Weir Suggestion cfs a ee e 92 15 340 90 os ar 16 ms 2 80 039 21 20 20 Table A7 Flow calculations and suggested weir length 1 2 acre watershed various watershed scenarios intensity of 7 2 in hr imperious 6 Kr Weir Required Weir Suggestion Impervious cfs IE Y es es 92 10 o e og 12 11 10 15 85 036 13 12 f 410 L 0 03 14 ta f 18 25 75 043 15 14 015 30 70 046 17 16 J 015 Table A8 Flow calculations and suggested weir length 1 4 acre watershed various watershed scenarios intensity of 7 2 in hr monii a uns cem Weir Required Weir Suggestion Impervious cfs en pao n S 99 93 L 38 e CEI es o o 19 2 80 039 07 07 J 10 25 75 os os Q0 moo 30 70 o os os 10 35 65 0580 09 08 f 40 40 60 053 10 09 36 40 Table A9 A12 use a rainfall intensity of 8 0 in hr These calculations should only be used for Fayetteville Elizabeth City and Greenville Table A9 Flow calculations and suggested weir length 1 acre watershed various watershed scenarios intensity of 8 0 in hr d um Paur aleni s Weir Required Weir Suggestion mpervious cfs nm COS e 32 23 10 90 032 2
155. the swale can be reinforced with rip rap or turf reinforcement matting which can withstand a higher velocity There are several approaches to designing a swale A given site may have constraints which dictate the channel slope and cross section in which case the designer must simply ensure that the channel lining will be strong enough to handle the velocities produced Conversely there may be flexibility with respect to the channel cross section and the designer may choose to size it such that a simple grass lining will be sufficient to prevent erosion Figure 8 2 is a diagram of a swale with a trapezoidal cross section and illustrates some of the parameters discussed in this section The equations used in trapezoidal channel design are as follows Equation 8 1 The Continuity Equation Q VA Where Q flow rate cfs V average velocity of the cross section ft s A cross sectional area of the flow ft And A bd xd refer to figure 8 2 Equation 8 2 The Manning Equation can be used to calculate the velocity of the stormwater in a given channel 55 STA NORTH CAROLINA JOIL amp WATER CONSERVATION Community Conservation Assistance Program BMP Design Manual V 2149 1 n R S Where V velocity ft s n Manning roughness Table 8 1 R hydraulic radius ft o channel bed slope ft ft The hydraulic radius R is equal to the cross sectional area of flow A divided by the wetted perimete
156. trash that has entered the wetland Table 6 5 Maintenance Activities for Backyard Wetlands Activity Frequency Initially every month until Monitor wetland for signs of erosion stabilization and after any large storm gt 2 inches Remove sediment from forebay As frequently as once per year if ever Romae rei fiomefiend spring Remove debris from around 3 As needed overflow weir Community Conservation Assistance Program BMP Design Manual fa ee eee 7 0 Cistern Design 7 1 Overview of Practice Cisterns are perhaps the oldest rainwater treatment practices They are used to capture runoff primarily from roof tops Sometimes runoff from pavement is also temporarily held in cisterns A cistern is simply a tank that stores runoff These tanks range in size from thousands of gallons to as small as 50 gallons The small version of a cistern is known as a rain barrel Cisterns can be employed above or below ground with the former type of cistern typically being cheaper to purchase and install Above ground cisterns will be the focus of this document These systems vary in size color and shape Figures 7 1 7 2 and 7 3 show examples of cisterns installed in various locations in North Carolina iil Jl Figure 7 2 5 600 gallon cistern in Kinston N C 44 NORTH CAROLINA Community Conservation Assistance Program BMP Design Manual E CONSERVATION PS y 2 E A 4 e
157. truck may be re quired Because most trash follows the movement of water it tends to collect near the outlet of the wetland or wet pond This makes trash easier to collect but it increases the risk of clogging the drawdown orifice REMOVE VEGETATION ALONG THE DAM FACE Dam inspection officials require earthen dams to be free of large shrubs and trees Roots can conduct water through the dam from the open pond to the downstream side of the embankment The movement of water along the roots 1s called piping which can eventually lead to soil erosion and if unchecked dam failure Piping tends to be a problem for large ponds and wetlands that have a large dam face Some small wetlands and wet ponds and those with concrete dams do not have this problem If a dam face is vegetated it should be grassed exclusively Inspect the dam once a year and remove all shrubs and trees from the dam top and both faces If the wetland or wet pond has been regularly maintained and any shrubs and trees growing are juvenile simply mowing the bank is sufficient Otherwise a weed wiper which applies herbicides to plants more than 12 inches tall can be used along the bank The weed wiper will kill any plant it touches or scrapes If a bank is severely overgrown trees and shrubs should be cut down and removed A systemic herbi cide can be applied to the freshly cut stumps which will kill the root systems This is a laborious process If the dam face is h
158. ts in the Urban Waterways series North Carolina Cooperative Extension N C State Univer sity Hunt W F Urban Stormwater Structural Best Man agement Practices BMPs AG 588 01 Online http www bae ncsu edu stormwater PublicationFiles UrbanBMPs1999 pdf Hunt W F and B A Doll Design of Stormwater Wetlands for Small Watersheds AG 588 02 Online http www bae ncsu edu stormwater PublicationFiles SWwetlands2000 pdf Hunt W F C A Apperson and W G Lord Mosqui to Control for Stormwater Facilities AG 588 04 Online http www bae ncsu edu stormwater PublicationFiles Mosquitoes2005 pdf Hunt W F and W G Lord Bioretention Perfor mance Design Construction and Maintenance AGW 588 05 Online http www bae ncsu edu stormwater PublicationFiles Bioretention2006 pdf In years 1 and 2 twice a year spring and fall From year 2 onward once a Depends on design aesthetics Ranges eter from every 1 to 3 weeks to once a year Muskrat hole inspection and destruc tion should occur every time the wetland or wet pond is visited at least If spread of cattails is somewhat limited use the glyphosate wipe method Wet ponds that are a design amenity will require more frequent mowing every 1 to 3 weeks Contact a professional beaver trapper to remove beavers Use muskrat traps to remove muskrats or contact a profes sional trapper Rodewald A D Nuisance Canada Geese How to Deal with the Problem Ohio S
159. ts that will tolerate longer periods of flooding 3 5 days but will also grow in moist to average soils Establishment usually STATE UNIVERSITY takes 1 2 years for trees and A amp T STATE UNIVERSITY shrubs and 1 year for A IVE perennials Helping People Put Knowledge to Work For iii A more detailed information and images of each plant visit the Plant Fact Sheets available on NCSU s Urban Horticulture website www ncstate plants net Plants for Rain Gardens Recommended for Western North Carolina Charlotte Glen Urban Horticulture Agent North Carolina Cooperative Extension New Hanover County Center Soil conditions in rain gardens alternate between wet and dry making them tough places for many plants to grow The following plants are adapted to these conditions though some plants will tolerate more moisture than others Each plant is marked according to its flooding tolerance with 3 s being tolerant of longer flooding 2 s only tolerating brief flooding and 1 s indicate plants that tolerant extended drought once established All of these plants are native to the southeastern United States in wetland habitats and most are readily available at local nurseries Wetland plants can generally grow well in moist or well drained soils whereas plants adapted to dry soils rarely survive in soggy conditions How wet a rain garden stays will vary considerably depending on the site where it is installed Rain gardens created
160. ty and North Carolina A amp T State University commit themselves to positive action to secure equal opportu nity regardless of race color creed national origin reli gion sex age or disability In addition the two Universities welcome all persons without regard to sexual orientation North Carolina State Univer sity North Carolina A amp T State University U S Department of Agriculture and local gov ernments cooperating erways Maintenance of Stormwater Wetlands and Wet Ponds Stormwater management practices must be kept in proper working order to maintain their intended functions and aesthetic appeal This publication presents maintenance guidelines for stormwater wetlands and wet ponds two stormwater prac tices that are being constructed across North Carolina OVERVIEW As its name implies a stormwater wetland 1s a wetland system designed to treat stormwater runoff Wetlands typically have shallow water ex cept for intermittent deep pools and dense vegetation A well functioning stormwater wetland will be a diverse ecosystem that includes many plant and animal species It will also do an excellent job of removing pollution from stormwater runoff its intended function Stormwater wetlands are very efficient at nutrient removal Recent studies conducted by North Carolina State University researchers indicate that a stormwater wetland re moves 40 to 80 percent of all nitrogen and 50 to 70
161. ucture can be installed as follows 1 Dig a trench 2 to 3 feet wide and long and deep enough for the weir Each side of the weir should extend at least 18 inches into the adjacent bank and 18 inches below final grade at the base of the weir 2 Two or 4 posts can be used to stabilize the structure depending on the length of the weir The posts can be 4x4 or 4x6 depending on the BMP size although 4x4 will be sufficient in most cases Points can be cut on the tips of the posts and they can be driven into the ground at the bottom of the trench If they can not be driven sufficiently post holes can be dug and backfilled with concrete Use a string line to make sure the posts are aligned 3 o Flashboards can then be nailed or screwed to the posts starting from the bottom and working up Check the level on the boards Caulk between each board Leave a weir opening near the top of the flashboard wall as necessary 4 An additional board or metal plate can be nailed or screwed to the flashboards and used to adjust ponding depth rather than having to modify the flashboard structure and or as an adjustable orifice plate for wetlands If a metal plate is used consider any safety issues with sharp edges 5 N Proper backfill around the structure is very important to keep it from washing out during the first large storm event Soil should be added in small lifts 4 6 inches and compacted with each lift If the space is too small fo
162. um felix femina Swamp Milkweed 3 Asclepias incarnata Butterflyweed 1 Asclepias tuberosa False Indigo 1 2 Baptisia species Turtlehead 3 Chelone glabra Green and Gold 2 Chrysongonum virginianum Mouse Ear Coreopsis or Lobed Tickseed 2 Coreopsis auriculata Lanceleaf Tickseed 1 2 Coreopsis lanceolata Rose Mallow 3 Hibiscus moscheutos Texas Star 3 Hibiscus coccineus Jewelweed 3 mpatiens capensis Swamp Iris 3 ris versicolor Blue Flag Iris 3 Iris virginica Gayfeather 2 Liatris spicata Cardinal Flower 3 Lobelia cardinalis Cinnamon Fern 3 Osmunda cinnamomea Royal Fern 3 Osmunda regalis Garden Phlox 2 Phlox paniculata Moss Pinks 1 2 Phlox subulata Pickerelweed 3 Pontedara cordata Rudbeckia 1 2 Rudbeckia fulgida Cutleaf Coneflower 3 Rudbeckia laciniata Goldenrod 3 Solidago rugosa Stoke s Aster 2 Stokesia laevis Ironweed 3 Vernonia novaboracensis Ornamental Grasses River Oats 1 3 Chasmanthium latifolium Panic Grass 1 3 Panicum virgatum Indiangrass 1 2 Sorghastrum nutans Sedges and Rushes Lurid Sedge 3 Carex lurida Fringed Sedge 3 Carex crinita Woolgrass 3 Scirpus cyperinus Soft Rush 3 Juncus effuses Plants that once established can withstand considerable drought 3 4 weeks without rainfall Plants that grow best in moist to average soils and will only tolerate
163. vation Commission received authorizing legislation to establish the Community Conservation Assistance Program through Session Law 2006 78 partially due to the strong support of District Supervisors The Community Conservation Assistance Program CCAP supports the installation of water quality best management practices on non agricultural lands This program is an innovative approach to controlling water quality impacts caused by stormwater runoff Through locally led conservation the Division of Soil and Water Conservation DSWC and Soil and Water Conservation Districts SWCD have been successful in implementing voluntary agricultural BMPs which have addressed many water quality parameters The CCAP is intended to operate under the same guidance and accountability as the NC Agriculture Cost Share Program and achieve the same successes CCAP will focus its efforts on retrofitting stormwater BMPs on existing land uses it will not be used to assist new development sites to meet state and federal stormwater mandates Soil and Water Conservation Districts have the technical expertise to install stormwater BMPs and a successful history of promoting voluntary conservation practices The program will give the Districts the structure and financial assistance to carry out this mission CCAP will encourage local governments individual landowners and businesses to incorporate stormwater BMPs within their landscape where BMPs were not required by regulatio
164. water BMP is believed to be a best way of treating or limiting pollutants in stormwater runoff It can be as simple as applying the proper amount of fertilizer to a home lawn or as complex as building an engineered structure such as a stormwater wetland Each BMP has certain conditions under which it will function properly The pollutants to be treated the size of watershed the imperviousness of the watershed the local water table and the amount of available land for the practice all influence the selection of a BMP Some of the BMPs are relatively well known and researched while others are in their infancy This document will focus on BMPs that can be installed in small scale settings such as individual residences and small businesses The stormwater BMPs that will be discussed in this document are backyard rain gardens backyard wetlands cisterns grassed swales impervious surface removal and permeable pavements Each of these BMPs can be valuable in treating the stormwater leaving a given watershed Guidance on BMP selection based on site conditions will be discussed in detail in this document ORTH CAROLINA SOIL amp WATER 3 0 General Stormwater BMP Design Considerations Community Conservation Assistance Program BMP Design Manual Eagen A Aci al Engineering 3 1 The First Flush The term first flush has become common nomenclature in the stormwater management field The concept behind this term is that polluta
165. water s edge and tend to be surrounded by mature grasses As a result mowing the perimeter of wetlands is not a typical stormwater wetland maintenance activity Many wet ponds however do have a grassed perime ter that needs to be maintained Mowing maintenance is almost purely aesthetic The type of grass used its growing season and pond aesthetics dictate the height and frequency of mowing Mow cool season grasses to a recommended height of 4 inches and no lower than 2 5 inches Cool season grasses such as fescue tend to be used west of Interstate 95 in North Carolina Mow warm season grasses to a recommended height of 2 5 inches and no lower than 1 5 inches Warm season grasses include centipede Bermuda and zoysia and are principally found in eastern North Carolina Mow every one to three weeks during the growing season when the wet pond is part of an accessible landscape or treated as an amenity Mow wet ponds that are located out of sight once or twice a year The size and severity of slopes along the wet pond determine the type of mower to use For small ponds a standard push mower is often adequate Larger ponds or ponds with steep banks will probably require a specialized pond mower Grass clippings can be left adjacent to the pond to provide organic matter that encourages grass to grow Do not discharge grass clippings into the water as this will encourage the growth of algae and could poten tially clog t
166. watershed draining to the cistern is relatively easy Since the cistern will be receiving drainage from a rooftop the rooftop area draining to the gutters feeding the cistern should be determined Design plans internet tax maps and site measurements can be used to determine the drainage area 46 tT HORTH CAROLINA SOIL amp WATER 3 2 Cistern sizing Community Conservation Assistance Program BMP Design Manual The first step in cistern sizing is to determine the volume of stormwater that is to be captured These systems capture runoff from areas that are in most cases 100 impervious The simple method described in equations 3 1 and 3 2 can be applied to these rooftop watersheds to estimate the volume of runoff that can be expected for a given storm event It is unreasonable to capture the entire first flush 1 or 1 5 inches depending on location due to the size of cistern that would be required to do so Thus a reasonable water quality and volume reduction can be achieved by capturing the first 0 5 inches of rainfall 0 75 inches in CAMA counties An appropriately sized cistern can be selected based on the capture volume that was calculated Cisterns come in standard sizes and shapes so the cistern may be slightly over or under sized based on the desired volume and the available cistern sizes Table 7 1 shows the estimated runoff associated with various rooftop areas and a suggested cistern size for each rooftop T
167. when disposing of the soil Spoils from wet ponds downstream of industrial facilities may contain pol lutants that need to be disposed of in a landfill while those from a residential wetland or wet pond may not If there is any concern as to proper disposal samples of the excavated soil should be sent to a laboratory for chemical analysis This can be costly WHAT DESIGNERS CAN DO TO MAKE FOREBAY CLEANOUT EASIER Access to older wet ponds and stormwater wetlands is often a problem New design recommendations can make forebay cleanout easier by improving ac cessibility Include reinforced paths that give heavy equip ment easy access to the forebay Figure 3 Some times the path doubles as a separation between the forebay and the remainder of the pond e Make forebays relatively long and narrow A narrow forebay makes it easier for a trackhoe or backhoe arm to reach at least to the middle of the forebay from either side Figure 2 Inspection A of sedi ment depth and cleaning or dipping B of forebays A long boom on the excavator is sometimes essential to ac cess sediment collected in the middle of the forebay A recent study by N C State researchers indi cates that sediment and gross solids from forebays typically need to be removed also known as dipped or dredged once every 5 to 10 years If wet ponds and stormwater wetlands are located in watersheds with active construction however spoils may need to be remov
168. y This publication presents the latest research findings on level spread ers in North Carolina and describes recommended practices for design ing installing and maintaining these structures Since 1998 North Carolina has implemented rules to protect riparian buffers 1n several major river basins These rules require that concentrated stormwater runoff be diffused or spread prior to discharge into a ripar ian area To accomplish this the Divi sion of Water Quality DWQ in the N C Department of Environment and Natural Resources NCDENR recom mended the use of level spreaders and developed initial design standards in October 2001 An overview of level spreaders and riparian buffers can be found in Urban Stormwater Structural Best Management Practices AG 588 01 of the Urban Waterways series This fact sheet provides more detailed information on designing installing and maintaining level spreaders SEAT 8 SPSREIESITS URN STATE GS eS a Mr Pee Pe heer pr de Hn DIFFUSE FLOW WHAT IS IT Diffuse flow sometimes called sheet flow occurs when water spreads out evenly across an area Figure 1 In contrast when stormwater collects in a drainage system and flows to a stream via a pipe swale or ditch it does not make enough contact with or bypasses the riparian buffer a vegetated area along streams riv ers and other water bodies that helps to filter runoff and prevent erosion Riparian buffers can improve w
169. y 1 2 Hemerocallis hybrids Rain Lilies 3 Zephyranthes species Crinum Lilies 3 Crinum species Japanese Painted Fern 2 Athyrium nipponicum and Maiden Grass 1 2 Miscanthus cultivars Plants that once established can withstand considerable drought 3 4 weeks without rainfall 2 Plants that grow best in moist to average soils and will only tolerate short periods 1 2 days of flooding 3 Plants that will tolerate longer periods of flooding 3 5 days but will also grow in moist to average soils Establishment usually takes 1 2 years for trees and shrubs and 1 year for perennials For more detailed information and images of each plant visit the Plant Fact Sheets available on NCSU s Urban Horticulture website www ncstate plants net NC TATE UNIVERSITY A amp T STATE UNIVERSITY COOPERATIVE MA EXTENSION Helping People Put Knowledge to Work Plants for Rain Gardens Recommended for central North Carolina Charlotte Glen Urban Horticulture Agent North Carolina Cooperative Extension New Hanover County Center Soil conditions in rain gardens alternate between wet and dry making them tough places for many plants to grow The following plants are adapted to these conditions though some plants will tolerate more moisture than others Each plant is marked according to its flooding tolerance with 3 s being tolerant of longer flooding 2 s only tolerating brief flooding and 1 s indicate pl

Backyard BMP Design Manual - Div. of Soil & Water Conservation

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Backyard BMP Design Manual - Div. of Soil &amp; Water Conservation

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Backyard BMP Design Manual - Div. of Soil & Water Conservation