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FINAL APPENDIX A WITH PICTURES C REVISED

1. The shape factor Kt is defined as the ratio of X1 X2 Equations 1 29 and 1 30 compute the value of Kt Kt is used in Equation 1 28 to compute the reduction in scour due to any streamlining of the abutment shape APRIL 2011 Page 18 Vertical Wall Abutment Wing Wall Abutment xy m Spill through Abutment p xp pee NOT TO SCALE Figure 2 12 Abutment Shape Factor Selection of X and X Measurements APRIL 2011 Page 19 F Embankment skew angle The angle measured from the flow direction to the centerline of the left or right approach roadway embankment in degrees Refer to sketch Figure 2 13 The embankment angle is used to account for the effect of the orientation of the embankment on the contracting approach flow For an embankment angled downstream the scour depth is decreased for an embankment angled upstream the scour depth 1s increased Please note that the embankment skew angle may be different from the abutment skew angle dal X Left embank angle LE b P X Aij X 7 N BY T CS a a f 7 L i d FH Right embank 4 Pi angle Figure 2 13 Embankment Skew Angle Future lateral movement of the channel This input is a yes or no answer It serves as a reminder to take lateral movement into account Lateral movement needs to be considered for both bridges and culverts The structure is fixed but the channel is free to modify its bed and banks ov
2. and the overbank one at 0 ft setback and another at 50 ft setback In this example only the live bed condition 1s used The contraction scour for a setback at 30 ft 1s calculated as y2 7 144 9 89 7 14 50 30 50 0 42 5 7 14 0 278 7 42 ft B Intermediate Setback of 70 Feet Wide Overbank Section CASE B in Figure A1 1 The Intermediate Setback zone exist only for an overbank wider than 6 67 yo For this example the channel flow depth is 10 ft and the right overbank at bridge is 100 ft The intermediate zone exists The computation of contraction scour depth for the right setback of 70 ft is as follows Step 1 Compute flow velocity For an intermediate setback the flow is neither mixed nor separate It will gradually change from mixed flow to separate flow ABSCOUR first computes the mixed flow velocities at Syo 50 ft setback and separate flow velocity at 0 75W 75 ft setback Then the velocity at 70 ft setback will be computed by linear interpolation For 50 ft setback V2 Q A 600 1600 1200 20 5 20 10 50 4 6 8 ft s For 75 ft setback V2 Q A 1200 75 4 4 ft s For 70 ft setback by linear interpolation APRIL 2011 Page 60 V2 4 6 8 4 75 70 75 50 4 56 TUS Step 2 Compute unit discharge q2 V y0 4 56 4 18 27 cfs ft Step 3 Compute contraction scour depth y2 y1 18 27 12 90 638 1 31 y2 1 31 3 8 4 98 ft C Long Setback CASE C in Figure Al 1 For a long setback the flow in the overbank is cons
3. the channel lateral movement zone D50 soil particle sizes 1n the channel flood plain and whether the type of scour to be expected 1s clear water or live bed 4 Surface and subsurface information on channel bed load flood plain soils borings etc geometric information about the bridge and approach roads HEC RAS runs for the given hydraulic conditions including stream channel cross sections hydraulic data tables reliable bridge tailwater elevations 0 selection of appropriate approach section and flow distribution and 1 appropriate flow distribution at bridge with regard to channel flood plain and overtopping flows ll TEM II DEVELOPMENT OF THE INPUT DATA FOR THE ABSCOUR ABUTMENT SCOUR MODEL SHA has been conducting and reviewing ABSCOUR analyses for a number of years It is our experience that one of the biggest sources of error in scour computations 19 an incorrect hydraulic model It is not an easy task to model a 3 D flow pattern with a 1 D model such as HEC RAS In particular special care needs to be given to the following three primary sources of error in developing the input data e Water surface elevation under bridge The hydraulic model should include a APRIL 2011 Page 3 sufficient reach downstream of the bridge to establish a reliable tailwater elevation at the downstream side of the bridge Guidance on the required length of the downstream reach 19 provided in Chapter 4 of the H amp H Manual e Fl
4. Agqgr Degr Abutment scour EL 55 114 144 66 70 Et 17 631 18 676 lt FIGURE 2 12 CONTINUED The ABSCOUR output file contains the scour calculations necessary for inclusion in the scour report Each line of the output file has an accompanying line number for easy identification Many of the formulas and the adjustment parameters are shown in the output file reference The output sheets are labeled in the same manner as the input menu cards The following is a summary of the sample output sheets included below Please note that the line numbers and descriptions may vary slightly from run to run depending on the input data INPUT DATA Project information Lines 1 27 Approach Section Data Lines 29 39 ABSCOUR Over rides Lines 40 51 Downstream Bridge Data Lines 53 72 Upstream Bridge Data Lines 73 90 Mm BW WN OUTPUT COMPUTATIONS AND RESULTS 1 Approach Section Lines 93 106 APRIL 2011 Page 26 2 Downstream Bridge Computations Lines 107 117 3 Downstream Contraction Scour Computations Lines 119 130 4 Total Bridge Scour at Abutments Lines 132 145 ABSCOUR can also generate plots of the approach section bridge section and the bridge scour cross section Figures 2 15 2 16 and 2 17 show the plots created for the approach section bridge section and bridge scour section respectively The plots may be printed directly from the program to a specified scale or the user may export dxf files for inclusion in
5. Neill s Guide to Bridge Hydraulics Second Edition June 2001 Please note that there are two lines drawn close together for the top two curves representing two different soil types The top line is comprised of straight lines drawn through the data points in Neill s table The lower line is a curve mathematically fitted to the data points 2 For more refined estimates of the critical velocity of cohesive soil layers at a bridge site take Shelby Tube samples of the various soil layers and test them in the EFA Apparatus in the SHA Soils Lab 8 Bios Resistance very stiff 7 to hard soils 5 Avg Resistance medium stiff to stiff soils S Low Hesistance very soft or soft soils 2 XM COMPETENT MEAN VELOCITY fps 0 5 10 15 20 FLOW DEPTH ft APRIL 2011 Page 70 25 ATTACHMENT 5 ESTIMATING CONTRACTION AND ABUTMENT SCOUR AT BRIDGES CROSSING LARGE SWAMPS AND WETLANDS NON TIDAL COASTAL PLAIN OF SOUTH CAROLINA We were unable to get the ABSCOUR program to provide reasonable answers for bridge abutments in the wide swamps and wetlands in the non tidal coastal plain in South Carolina Accordingly an alternative approach to estimating scour for such sites based on the U S Geological Survey s studies Reference 13 is proposed below We anticipate that such crossing sites will not be common in Maryland The characteristics of the South Carolina Streams excerpted
6. Over Ride Options The ABSCOUR Program contains various over ride features to allow the user flexibility in making the scour evaluation The user is cautioned to use the over ride features only after giving full consideration to the consequences of this approach Problems with the program output or with unrealistic scour estimates can often be traced to improper use of the over ride functions We recommend that none of these features be used on the initial run They are provided primarily to assist in the evaluation of a bridge with special problems or flow conditions We suggest that users contact the Office of Structures for guidance on using the over ride functions The common overrides include e Critical and Boundary Shear Stress For use where these values have been measured and determined to be reasonable e Live Bed Clear Water Use to change the determination made by ABSCOUR regarding the scour condition live bed scour or clear water scour A common use for the override is made for the condition on flood plains where there are low flow velocities and depths coupled with heavy vegetative cover and a clear water scour condition is considered reasonable Note that the stream morphology report 19 typically the best source of information regarding the type of scour to be expected e Bridge Section Unit Flow Values This over ride can be useful in conducting sensitivity analyses of complex flow patterns For example consideration of higher unit
7. bridge section el Program calculates critical velocity at bridge section aes Program calculates sediment transport parameter Ez da Program calculate the Clow velocity at abutment face ad Program calculates spiral flow coefficient Kf Be Clear water scour uses a modified Meill s method for Piedmont Zone 26 English Units ae Section orientation is looking downstream ao as 29 Approach Section Data DUS eene eene 33323 aaa SSS a a a a a ee 3l Left Channel Right BES SS eae sate 33 Approach section discharge cfz 33 964 1660 3d Approach section top width fti 37 160 aab 35 Approach flow depth hydraulic depth li fti 1 94 13 16 4 16 36 Approach median particle size DSO ft 0 003 0 003 0 003 37 Bank slope iZ in the vicinity of the bridge i z H V Z 36 Energy slope 3 at approach section 0 0004 39 40 ABSCOUR Overrides Al dz Reserved for override approach critical shear stress 43 Reserved for override approach boundary shear stress 44 Reserved for override scour type 45 Reserved for override sediment transport parameter 46 Reserved for override location header 17 Reserved for override unit width discharge 45 Reserved for override critical velocity 49 Reserved for override Z D velocity at abutment 50 Reserved for override average velocity in portion of bridge Sl Reserved for override spiral flow coefficient APRIL 2011 Page 24 Iai 53 Downstream Bridge Data SN e S V A
8. factor is suggested to take into account the effect of complex flow patterns which can be expected to occur at bridges abutments However the ABSCOUR calibration safety factors have been reassessed on the basis of the USGS comparison study of ABSCOUR computed scour values vs measured abutment scour at South Carolina Streams The current recommended factors based on both the flume and field studies are presented below SELECTION OF BASE CALIBRATION SAFETY FACTORS 100 YEAR FLOOD PLAIN CHANNELS AND FLOOD CHANNELS AND FLOOD WIDTH PLAINS WITH FINER PLAINS WITH COARSER BEDLOADS BEDLOADS D50 2 MM D50 gt 2MM LESS THAN s00 FEET OB GREATER THAN 800 FEET SELECTION OF INCREMENTAL CALIBRATION SAFETY FACTORS BASED ON SITE CONDITONS Channel Description at Bridge Site Incremental Safety Factor Straight channel with uniform flow Add 0 0 Moderately meandering upstream channel Add 0 0 Severely meandering upstream channel Add 0 1 Channel with complex approach flow conditions Sharp upstream Add 0 2 bend in channel confluence unstable reach lateral migration etc available Tidal river with wide tidal flats or wetlands and complex two Add 0 1 dimensional river and flood plain flow patterns that may change with river stage where a 2 D analysis is appropriate but not available Non tidal river with wide flood plains and complex two Add 0 1 dimensional river and flood plain flow patterns that may change with river stage where a
9. flow values on the outside of a bend e Bridge Section Critical Velocity This over ride should be helpful in evaluating the characteristics of the critical velocity for cohesive soils e Sediment transport parameter Not recommended for use unless the engineer has specialized knowledge of the sediment transport characteristics of the stream e Two Dimensional Flow For studies utilizing 2 D flow models the user can input directly the velocity of flow measured at the abutment face e Spiral Flow Coefficient Ke ABSCOUR 9 has been calibrated using the kf values computed by the program A higher kf value override may be justified in certain cases such as an abutment located in a wide wetland where flood plain velocities are low Clear water scour method The SHA has experienced reasonable results in the use of the modified Neill s equation for evaluating clear water scour In general Laursen s equations result in much deeper scour estimates for very fine grained non cohesive bed material in channels The user may wish to compare both methods A non tidal Coastal Zone method is included because of a number of bridges located in the wide wetlands of APRIL 2011 Page 6 South Carolina that were a part of the U S G S calibration study This method is not recommended at this time Unit Option The user can choose between Metric and English units Calibration Safety Factor Information from the USGS study of scour at South Carolina bridges
10. for the exposed footing pile cap T Pile cap is not in the water no contribution to the scour component be Scour component for the exposed pile group 107 108 Only one pile column the pile spacing in width direction is set to 7 times pile size 109 Adjusted depth of flow upstream of pier y3 19 46 ft 110 Adjusted velocity for the flow approaching the pier v3 4 368 fps 111 Sum of overlapping projected width of piles 1 5 ft 112 Coefficient of pile spacing Ksp 1 113 Coefficient of number of aligned pile rows Km 1 114 Effective width of the pile group 1 5 ft 115 Correction factor for armoring K4 for pile group 1 116 Height of pile group aboved lowered stream bed h3 24 ft 117 Pile group height factor Kh pg 1 118 Froude Number Fr3 for pile group 0 1745 119 Scour component for the exposed pile group 3 82 ft 120 Total pier scour depth with respect to revised flow depth 3 82 ft 121 Total pier scour depth with respect to initial flow depth 6 28 ft 122 123 Summary of results 124 125 Control Method Assume contraction scour does occur 126 Control option Option 3 Pier with pile cap and pile group exposed 127 Contraction scour depth at pier 2 46 ft 128 Local scour depth at pier 3 82 ft 129 Total scour depth at pier 6 28 ft 130 Total pier scour elevation 16 2 ft 131 Aggr Degr total Pier Scour Elevation 16 2 ft BACKGROUND ON THE MARYLAND SHA PIER SCOUR COMPUTATIONS The following informat
11. from the USGS Report are depicted below TABLE 1 Range of Selected Stream Characteristics for Measurements of Clear Water Abutment Scour Collected at 129 Bridges in the Piedmont and Coastal Plain of South Carolina Drainage area miles Minimum Median Maximum Minimum Median Maximum Parameter was estimated with the 100 year flow Properties for Full Cross Section Upstream of Bridge Channel Average Average Cross i i Observed slope cross cross section i i abutment ft ft section section i scour velocity Piedmont 90 abutment and 66 contraction scour measurements 0 00037 0 49 3 4 213 6 7 lt 0 062 0 0012 1 80 7 3 711 29 7 0 091 0 0024 4 38 15 8 2663 72 9 1 19 Coastal Plain 104 abutment and 42 contraction scour measurements 0 00007 0 25 Zl 463 3 8 lt 0 062 0 0006 0 47 4 7 2154 17 7 0 19 0 0024 0 94 16 3 28952 35 0 78 The significant factor in this table for the Coastal Plain 1s that for the most part contraction and abutment scour at bridges crossing these wetlands and swamps is small with some notable exceptions Observed contraction scour depth P Determined by ABSCOUR program APRIL 2011 Page 71 Procedures for Estimating Contraction and Abutment Scour in swamp wetland areas with characteristic similar to that of the non tidal Coastal Plain of South Carolina T1 Design Procedure No 1 USGS Envelope Curve Applicability This procedure is recommended only f
12. ft 18 87 16 87 16 387 Ha Flow velocity at upstream face of bridge fpa 0 66 du di 1 09 83 Low chord height 79 50 ft 25 79 25 79 25 79 dd Vertical blockage of flow by superstructure ft 0 DU 0 00 0 00 85 Pressure flow Yes or NO Yes if 8l a3 No No No BG xl Ft 5 15 Br xz Et 130 160 88 Ratio Xl XZ 0 06 0 06 69 Embankment skew angle degrees 55 125 90 Is future lateral migration of channel likely to occur No FIGURE 2 12 CONTINUED 91 Output Computation 4nd Results ga 93 Approach Section 24 95 Total approach discharge cfs 11540 96 Left Channel Right in nnee ene nn ene nnen nn nei 96 Approach average flow velocity fps 0 46 4 157 0 449 99 Approach unit width discharge cfs Eti 0 892 54 706 1 376 l D Approach section depth ft 1 94 13 16 4 18 101 Approach section Froude Number 0 0582 0 2019 0 0387 l z Approach section critical shear stress psft 012 n01lz 0 012 103 Approach boundary shear stressipsf 0 0484 0 3285 0 1043 l d Approach sediment transport parameter kij 0 665 0 641 0 649 LUS Scour type Live Bed Live Bed Live Bed 106 LO Downstream Bridge Computations 108 109 Total discharge under Bridge cfsi 11540 110 Left Channel Right a ACC CEDE llz Method of computing flow velocity adjustment short Sethack short Setback ll3 Flow velocity pa 5 724 5 724 5 724 114 Adjustment to hydraulic depth yOladj ift 15 441 15 441 15 441 115 Unit width d
13. in the output report Figure 2 14 shows the screen that appears after running the ABSCOUR program The user can scroll down through the output to look at input data output data and program notes The output can be sent directly to a printer or it can be saved as a text file so that it can be inserted into an electronic report A Figure 2 14 ABSCOUR 9 Output Report MD 313 over Marshy Hope Creek APRIL 2011 Page 23 Abutment H XOOSVOBDBDDWMHEHVS TAN srd3md313_100 asc PEE File Run Draw Help Project Info Approach Section Downstream Bridge Data Upstream Bridge Data Pier Data Actual Sections Output Graphic l XK nin nir lr cr rn nr ok or or or ook oa ook ok ol or ol ok ola ook a oo ok ol ar or ok ol a ook oo ok a ao ok ol ao ok oo or ok ol ar a en Maryland State Highway Administration N d m Office of Structures i 4 N Maryland Scour Program Abutment Scour T 55 k Version 9 Build 2 1 January 2010 N b raro Ti Time stamp 11 04 2010 9 37 13 AM E a ga Input Data l ll Project information o AE LR DL l3 Project name md 313 over Marshyhope Creek ld Project number 15 Description l lyr flood l5 Bridge cross section is skewed 35 degree l7 Project options 18 Program calculates critical and boundary shear stresses at approach section 19 Program decides the scour type as either live bed or clear water scour 20 Program calculates the unit width discharge at the
14. is necessary to account for the high local flow velocities and turbulence near the APRIL 2011 Page 33 abutments caused by the contracting flow in the overbank areas upstream of the bridge Findings from recent laboratory studies of compound channels indicate that the velocity of flow under a bridge tends to be highest at the abutments due to rapid acceleration and turbulence of the overbank flow entering the bridge contraction and in the thalweg section of the channel This phenomenon has been observed in field surveys conducted by the U S Geological Survey and 19 consistent with the theory of potential flow at a contraction The procedure used by the ABSCOUR Program to determine the flow distribution under the bridge is explained in Part of this guideline C EVALUATION OF THE PROGRAM OUTPUT C l Overrides A special message indicating that OVERRIDE IS ACTIVE is printed when the user over rides the computer values Any over ride function should be used with caution and the logic of the over ride carefully checked in this evaluation phase Please be aware that the sediment transport functions and the hydraulic flow conditions must be compatible If the user imposes unrealistic conditions on the program the resulting scour estimates will be in error C 2 Bridge Section Data Based on the user s input data the program determines the discharge unit discharge and velocity of flow for each cross section sub element under the br
15. is set back a distance from the edge of the channel The program takes the input information for the main channel flow and the abutment characteristics and the moves the main channel to the abutment to compute the abutment scour for this condition APRIL 2011 Page 53 APRIL 2011 Abutment scour with future channel movement Project MD 287 over Choptank River CHANNEL PIER Calibrationisafety factor SF3 h Unit option t Metric SI Main Channel Data From ABSCOUR output Adjustment to hydraulic depth WOad drm Interpolated scour flow depth i7 dr Downstream water surface elevation tint Sediment transportation parameter ki Tk Of Kah l l Sjaj a Tl w tat of of oo ral RI co I m m Aggradation or degradation i ttm Data of the LEFT abutment from ABSCOUR output Abutment local velocity factor Ei 1 028 Abutment spiral flaw factor Kf Pressure flow coefficient py Coefficient for abutment shape factor Ki Coeficient of embankment angle Ker TTF Correction factor for low chord submergence tim Estimated Abutment Scour Considering Future Movement of Channet Abutment scour flow depth za im Initial abutment scour depth isa Urmi Final abutment scour flaw depth ysajadj frm 13644 Cancel Abutment scour elevation flim Ham O i Print Import Data From Recent ABSCOUR Run Page 54 ATTACHMENT 1 COMPUTATION OF THE VELOCITY OF F
16. obtain the abutment scour For a contraction scour value of 5 feet the corresponding abutment scour value is 5ft x 1 4 2 7 feet This value will serve as the minimum abutment scour value 4 USGS Envelope Curve of All Abutment Scour Measurements in the Coastal Plain 25 Envelope curve Observed abutment scour for all 20 Coastal Plain sites Observed abutment scour at sites with known maximum historic flow Envelope curve OBSERVED ABUTMENT SCOUR DEPTH IN FEET 0 0 0 2 0 4 0 6 0 8 1 0 GEOMETRIC CONTRACTION RATIO Figure 76 Relation of observed clear water abutment scour depth and the 100 year flow geometric contraction ratio identifying sites with known maximum historic flows in the Coastal Plain of South Carolina The USGS Envelope Curve Figure 76 plots all of the measured abutment scour depths in the Coastal Plain Vs the geometric contraction ratio associated with the bridge site where the measurements were taken CONTRACTION SCOUR Use a value of 5 feet ABUTMENT SCOUR 1 Measure the geometric contraction ratio m for the bridge site APRIL 2011 Page 73 m 1 b B Where b bridge opening width and B approach flow width Note for overtopping flows use only that portion of the approach flow width that actually goes through the bridge 2 Read the Observed Abutment Scour Depth from the Envelope Curve in Figure 76 e Use a minimum abutment scour depth of 7 feet e Use a maximum a
17. rock along with the computed safety factor D BRIDGE UPSTREAM SECTION This Utility can be used to import the cross section of the upstream face of the bridge from HEC RAS in order to provide a check on the values that are used to estimate the ground elevation high chord elevation and low chord elevation E ABUTMENT SCOUR CONSIDERING THE FUTURE MOVEMENT OF THE STREAM CHANNEL INTO THE ABUTMENT This Utility is a valuable addition to ABSCOUR 9 It is common to find a conclusion in the Stream Morphology Report that one or more of the abutments of a bridge are within the Lateral Channel Movement Zone of the stream being crossed For this case it 1s necessary to estimate the scour at the abutment in the event that the channel does move into the abutment Up to now such computations have been required to be done manually This Utility is used in the following manner e Run the ABSCOUR program for the existing conditions e Open the utility and click on Import Data from Recent ABSCOUR run In the window which opens up indicate which abutment left or right that you wish to evaluate and then click OK e The program computes the scour which is expected to occur for main channel flow next to the abutment The MD 313 Bridge over Marshy Hope Creek could not be used as an example for this condition since both abutments are in the channel Instead an example was taken from the MD 287 bridge over the Choptank River since the abutment for this bridge
18. setbacks set at the channel edge and at five times the channel flow depth 5yo Step 1 Compute flow velocity As the setback of the left abutment is short as well as the right abutment total flow will be mixed For right setback of 30 ft V2 Q A 3400 20 5 20 10 30 4 8 1 ft s Step 2 Compute Unit discharges q2 V yo For setback of O ft q2 8 1 10 81 cfs ft For setback of 50 ft q2 8 1 4 32 4 cfs ft Step 3 Compute contraction scour depth The ABSCOUR program will compute two scour depths for each setback for two sediment transport modes live bed and clear water All together four values will be included on the output sheet For this example only the live bed contraction scour computations for the two setbacks will be presented The sediment transport coefficient k2 is computed as 0 638 For setback U ft Approach section yl 9 8 ft ql 1600 20 80 cfs ft APRIL 2011 Page 59 Bridge section y2 to be computed q2 8 1 10 81 cfs ft Computation by Lausen s Equation for a setback equal to zero at the channel bank y2 y1 81 80 40 638 1 01 y2 1 01 9 8 9 89 ft For setback 50 ft Approach section yl 3 8 ft ql 12 cfs ft Bridge section y2 value to be computed q2 8 1 4 32 4 cfs ft y2 y1 32 4 12 0 638 1 88 y2 1 88 3 8 7 14 ft Step 4 The contraction scour for the setback of 30 ft requires interpolation ABSCOUR will use two appropriate values based on the modes of sediment transport in the channel
19. to check for a reasonable representation of the actual HEC RAS sections and to view a depiction of the scour cross section This exercise is well worthwhile to assure that there are no obvious errors in the input data Please note that the user can input the results of the pier scour modules into the ABSCOUR bridge cross section Scour Results to prepare a complete scour cross section at the bridge However the pier scour elevations apply to the upstream side of the bridge whereas the abutment scour elevations are computed at the downstream side of the bridge Combining these results provides a simplified and conservative means of evaluating the scour The user is encouraged to redraw the scour cross section on the bridge plans to develop a more readable sketch and to account for the issues discussed below 1 Perhaps the most common problem encountered with the ABSCOUR 9 program output is reconciling the rectangular ABSCOUR bridge section with the irregular HEC RAS section In most cases the two sections should be reasonably congruent However there are situations where adjustments are needed to refine the scour cross section e PROBLEM The area of bridge piers is subtracted from the ABSCOUR waterway area under the bridge consequently in some cases the ABSCOUR 9 cross section area may be smaller than the HEC RAS section Consequently the ABSCOUR channel bottom may plot above the HEC RAS channel bottom EXAMPLE SOLUTION Compute the ABSCOU
20. 13 100pier psf Date 10 15 2010 Page 1 1 KOR KKK KK KK KK KR KR KK RK KK KK KKK KERR KKK KKK RK RR KKK KKK ko k ko k ko k ko kk s 2 N Maryland State Hightway Administration T 3 Office of Structures 4 Maryland Scour Program Pier Scour St Version 9 Build 2 December 2009 6 eck deck keke RK KER KEE KK EKER RE KEK KEKE KEK KERR EK ERK ck ko EK ko ko ko ko ko oko kok ko k kk M 7 3 8 Time stamp 10 15 2010 1 02 17 PM 9 10 Input data 11 12 Project information L3 SoS oS Soe See eo Soe Soe eee et Se eS 14 Project name md313 over Marshyhope Creek 4 15 Project number 16 Description 100 vear flood 17 Bridge opening is skewed 35 degree 19 Pier scour condition Option 4 Pier with pile group auto solve options 1 thru 3 and contraction conditions 20 Units used English Units 4 21 Flow depth upstream face of the pier ft m 17 ft s 22 Flow velocity upstream face of the pier right at nose fps mps 5 fps 23 Width of pier stem ft m 1 5 ft 24 Length of pier stem ft m 1 5 ft 25 Flow attack angle degree 0 degree 26 Contraction scour depth at pier ys f ft m 2 46 ft 27 Water surface elevation upstream face of the pier ft m 7 08 ft 28 Aggradation or degradation ft m 0 ft E 29 Median grain size D50 ft m 0 5 0 00259 0 5 0 00105 4 0 000089 ft 30 84 finer grain size D84 ft m 0 5 0 06440 5 0 005944 0 0038 ft 31 95 finer grain size D95 ft m 0 5 0 064 0 5 0 0059
21. 2 D analysis is appropriate but not APRIL 2011 Page 68 This table is used in the following manner The user reviews the site conditions or descriptors which are present at the bridge site under consideration and selects the factor in the table that best describes the crossing site under consideration The engineer may select a higher safety factor if it is considered necessary to reflect a high risk crossing site Please note that the current scour evaluation procedure described in Chapter 11 of the Manual directly calculates the potential effects of both channel migration and degradation This calculation serves to decrease the need for reliance on a safety factor to account for lateral channel movement and degradation APRIL 2011 Page 69 ATTACHMENT 4 CRITICAL VELOCITIES IN COHESIVE SOILS There are no definitive data available for determining critical velocities in cohesive soils In an unpublished paper Permissible Shear Stresses Critical Velocities 2005 Sterling Jones Research Engineer FHWA has collected and commented on various methods available in the literature regarding this subject The Office of Structures has conducted limited tests of critical velocities in cohesive soils using the EFA Apparatus in the SHA Soils Lab On the basis of this existing information OBD recommends the following 1 For preliminary guidance on estimates of critical velocities in cohesive soils use the figure below developed from information in
22. 4 0 0038 ft 32 Pier stem nose shape correction factor K1 1 33 K2 calculated by the program 34 Streambed condition correction factor K3 1 1 35 K4 calculated by the program 36 Footing Pile cap width ft m 3 ft 37 Footing Pile cap length ft m 36 ft 38 Pile cap Thickness ft m 3 ft 39 Footing Pile cap shape factor Klf 1 1 40 Distance from streambed to top of footing pile cap Negative if downward ft m 27 ft 41 Distance between front edge of footing pile cap and pier stem ft m 1 5 ft 42 Number of pile columns in pier width direction 1 hi 43 Number of pile rows in pier length direction 8 44 Pile center to center spacing in the pier width direction ft m O ft 45 Pile center to center spacing in pier length direction ft m 4 5 ft 46 Pile size in pier width direction ft m 1 5 ft 47 Pile size in pier length direction ft m 1 5 ft 48 Pile shape factor Klp 1 50 Output Results 53 Method 1 Option 3 55 Revised flow depth 17 ft 56 Revised flow velocity 5 fps 57 Revised distance from streambed to top of footing pile cap 27 ft 58 Revised soil layer 1 thick 0 5 ft 59 Revised soil layer 2 thick 0 5 ft EM Control soil is layer no 3 with D50 0 000089 ft D95 0 0038 ft S Scour component for the pier stem in the flow Pes Pier stem is not in the water no contribution to the scour component S Scour component for the exposed footing pile cap S Pile cap is not in t
23. AutoCAD or Microstation The cursor can be used to determine various elevations and distances depicted on the plots If the HEC RAS Approach Section and Bridge Section have been imported into ABSCOUR they will be included in the above noted Figures Comparison of these cross sections will be helpful in evaluating the answers obtained from the program Figure 2 15 Approach Section Plot MD 313 over Marshy Hope Creek APRIL 2011 Page 27 4 L me rd ra nn peg RA BJ Figure 2 17 Sample Scour Cross Section Under Bridge ABUTMENTS SET BACK FROM THE EDGE OF THE CHANNEL Excerpts from the Office of Structures scour report for the MD Route 313 bridge crossing over Marshy Hope have been presented above MD 313 a six span steel structure with abutments on spill through slopes All foundation elements are on piles The APRIL 2011 Page 28 ABSCOUR 9 abutment module computes the scour cross section at the bridge across the channel up to the toe of the spill through slope which in this case happens to be a bulkhead The Pier Scour Module computes the total pier scour taking into account the effect of contraction scour The ABSCOUR 9 program prints out the scour cross section for the bridge The procedure for evaluating worst case scour at the abutment piles set back from the channel is illustrated below in the sketch of the elevation view of the bridge The contraction scour elevation is p
24. ILITY INDEX FOR ROCK Please note that the erodibility index can be expected to vary with the depth of the rock below the channel Typically it will increase but this is not necessarily true in all cases In conducting studies of scour in rock it is necessary to compute the erodibility index for the same elevation at which the rock scour will occur Normally this will involve a trial and error approach using the computer program The references below pertain to appropriate tables and pages in Dr Annandale s manual Calculation of Pier Scour Using the Erodibility Index Method The Erodibility Index is APRIL 2011 Page 51 computed from the following equation K Ms Kb Kd Js 2 2 where K erodibility index Ms mass strength number Kb block size factor Kd inter particle bond shear strength number Js relative ground structure number C 5 DESIGN PROCEDURE STEP 1 DETERMINE Ms THE MASS STRENGTH NUMBER This value is selected from Table 5 Intact Material Strength Number Ms for Rock Page 18 STEP2 COMPUTE Kb THE BLOCK SIZE FACTOR Kb RQD Jn RQD Rock quality designation where RQD gt 5 This is obtained by qualified engineers and geologists through an inspection of rock cores taken at the bridge site e Obtain Jn the joint set number from Table 7 page 21 STEP3 COMPUTE KD THE INTER PARTICLE BOND SHEAR STRENGTH NUMBER Kd Jr Ja Obtain the joint roughness number Jr from Table 8 page 26 Ob
25. L a ELLE 55 Downstream water surface elevation under bridge 86 84 ft 56 Left Channel Right HS SSS SS E SSS SaaS nn ne 56 HEC RAS discharge under Bridge cfsi ab 10890 400 58 Waterway area measured normal to flow sf al lazz 113 60 Top width iT measured normal to flow ft 30 lla 34 6l Hydraulic depth AST ft 2 70 15 44 dad 62 ABSCOUR X Section elevation 55 61 ft 4 14 amp 60 3 42 63 Abutment type spill through apill through 64 Setback for an abutment in channel ift 0 65 Low chord elevation downstream side of bridge ftl 16 16 16 66 Correction factor for low chord submergence 55 65 gt 0 ft 0 00 0 00 0 00 67 Median particle size under bridge D5 ft Layer 1 0 5 0 0025 0 5 0 00259 0 5 0 00259 68 Median particle size under bridge D50 ft Layer Z 0 5 0 0010 0 5 0 00105 0 5 0 00105 69 Median particle size under bridge D50 ft Layer 3 4 0 00089 4 0 00059 4 0 00089 TO Estimated long term aggradation or degradation ift U 7l Calibration satety factor See F li 1 en 73 Upstream Bridge Data D MEL locnm CE 75 Water surface elevation upstream side of bridge 7 08 ft TO Left Channel Fight ennen nn one nmr nnen ns nnn 78 High chord elevation upstream side of bridge ift al al al 79 Low chord elevation upstream side of bridge ft l8 16 16 80 Bed elevation at upstream side of bridge ift 8 79 9 79 9 79 gl Water depth at upstream side of bridge 75 00
26. LOW USED IN THE ABUTMENT SCOUR COMPUTATIONS I COMPUTATION OF VELOCITY AND SCOUR Field observations of flows at bridge crossings in wide streams revealed that the flow in the overbank sections is contracted by the abutment and moves toward the main channel where it mixes with the main channel flow When the abutment setback from the main channel was less than five times the flow depth in the channel the flows were well mixed and the flow velocity in the channel and overbank became uniform If the abutment setback was large being located near the edge of the flood plain the flows in the main channel and in the overbank section remained separated as they passed under the bridge These findings are utilized in computing flow velocity in ABSCOUR program Abutment setbacks are classified into three categories short intermediate and long setbacks The term short setback is used to define the condition where the setback is equal to or less than five times the channel flow depth 5yo The term long setback is used to define the condition where the setback being is equal or greater than 75 of the overbank width 0 75W A setback between these two limits 1s defined as an intermediate setback For short setbacks the velocity V is computed as a uniform velocity V Q A in the waterway area under the bridge A where Q 1s the discharge through the bridge For long setbacks the velocity in the overbank 1s computed independently from the channel fl
27. OFFICE OF STRUCTURES BRIDGE SCOUR PROGRAM CHAPTER 11 APPENDIX A ABSCOUR USERS MANUAL PART 2 GUIDELINES FOR APPLYING THE ABSCOUR PROGRAM Maryland SHA Scour Program Welcome to Maryland SHA Bridge Scour Program 9 EZ EB Utilities dc should r nderstai die aera ded r 11 of the Office of Structur Mai al for r Hydr TS SAT ulic Design and HEC 18 by FWHA Version 9 Build 2 3 January 2011 APRIL 2011 PART 2 GUIDELINES FOR APPLYING THE ABSCOUR PROGRAM See Part 1 for a Table of Contents I INTRODUCTION Available technology has not developed sufficiently to provide reliable scour estimates at abutments for all possible site conditions The policies and guidance in the abutment scour program ABSCOUR and in Chapter 11 of the SHA Manual of Hydraulic and Hydrologic Design have been developed with this consideration in mind The ABSCOUR program provides for considerable flexibility in the input format and the computations to permit the user to model field conditions However the user should make a critical review of all scour computations using ABSCOUR for sensitivity analyses of input factors to evaluate whether the answers obtained are reasonable Part 2 guide has been written to assist the user in this evaluation The user assumes all responsibility for any decisions or actions taken as a result of the use of this program Please note that definitions used and references cited refer back to the text 1n Part 1 The di
28. R 9 WORKSHOP NOVEMBER 201013md313 100 asc Project Info Approach Section Downstream Bridge Data Upstream Bridge Data Fier Data Actual Sections Output Graphic Project Mame ma 313 over Marshyhope Creek Mna Descriptian DOyr flood Bridge cross section is skewed 35 degree ser override options Clear water scaur method f SHA modified Meill s method for Piedmont Zane Coastal Zone Critical amp boundary shear stress Live bed ar clear water scour C Laursen s method Bridge section unit discharge Unit option Bridge section critical velocity English units C Metric SI units Sedimenttranspart parameter KZ l Section Orientation 2 D flow computations E D m Spiral Flow Cefficient kf Looking downstream Looking upstream Calibration safety factor See F1 Help h Mote Additional help is available for each input cell by pressing the F1 key while the cursor is at the cell Figure 2 1 ABSCOUR Project Information Screen APRIL 2011 Page 5 Project Name and Description Use this input to provide information on the project bridge number magnitude and frequency of the flood being evaluated special conditions used in the analysis etc Since the user may make several ABSCOUR runs this section can be used to detail the flood frequency and magnitude and any special conditions or modifications used in the analysis This approach will help to clearly delineate and identify each run User
29. R contraction scour area and distribute it along the length of the HEC RAS channel at the elevation of the HEC RAS channel e PROBLEM For small one span bridges crossing V shaped channels the ABSCOUR contraction scour elevation may plot above the channel thalweg EXAMPLE SOLUTION It is likely that the channel thalweg may move within the limits of the abutments over the life of the bridge Subtract the contraction scour depth from the thalweg elevation to compute the elevation of contraction scour for the scour cross section e PROBLEM A narrow flood plain under bridge ABSCOUR cross section divided between the channel and the flood plain does not fit well with the HEC RAS cross section As a basis for comparison this section will be referred to as Model A EXAMPLE SOLUTION Assume area under bridge 1s all channel and APRIL 2011 Page 32 compute the scour cross section on this basis This section will be referred to as Model B compare the scour cross sections for Model A and Model B select the most reasonable answer e PROBLEM For a bridge location on a sharp bend contraction bend scour may be unequally distributed with most of the scour occurring on the outside of the bend EXAMPLE SOLUTION 1 use the ABSCOUR program to compute the area of contraction scour 2 pro rate more of the scour on the outside of the bend keeping the scour area constant Other guidance on plotting the scour cross section on the bridge plans
30. Select the corresponding value of the critical velocity from the Vanoni upper limit curve in the plot below 4 Use the over ride feature in ABSCOUR 9 to enter the critical velocity of the soil at the abutment and compute the abutment scour for the selected condition APRIL 2011 Page 74 Discussion There may be a significant difference between the abutment scour estimates determined from Design Procedures 1 and 2 Use engineering judgment to select the most appropriate scour depth for the given conditions Id ET d mnm AL LLL 6 0 on ARRAS shinee on HUM T SIS ata ZH 200 Brn Ss B E ET TRS T HT y ot Velocity ft s 0 6 0 1 0 001 0 004 0 01 0 02 0 04 0 2 04 0 6 1 4 Sonn sediment size mm Design Procedure No 2 Using the Vanoni Upper Limit Curve for Estimating Threshold Critical Velocities for Clear Water Abutment Scour APRIL 2011 Page 75 Velocity cm s
31. abutment piles The length of the exposed piles are determined to provide a basis for evaluating the stability of the abutment DISCUSSION OF THE ABSCOUR REPORT A ABSCOUR PROJECT INFORMATION e Project Information Use this section to outline the primary factors of interest in the scour evaluation flood flow project description and any special conditions to be evaluated discharge trial selections for soils types of scour e Project Options This section prints the options used by the program B INPUT DATA e Approach Section Data These numbers reflect the information provided by the User for the Approach Section An important item to check here Line 101 is whether the flow is live bed or clear water e ABSCOUR Overrides This summary should always be reviewed to make sure that the User 1s aware of any overrides input into the program e Downstream Bridge Data This summarizes the information used to construct the ABSCOUR cross section under the bridge It computes a correction factor for the case where the downstream water surface 19 higher than the elevation of the low bridge chord e Upstream Bridge Data This is a summary of the information needed to compute APRIL 2011 Page 30 the shape factor for the bridge and to determine if pressure flow will occur OUTPUT COMPUTATIONS AND RESULTS Approach Section This is a summary of the data used to compute the unit discharges at Section 1 and to develop the computations to d
32. btained from HEC RAS It is also used in the pressure flow computations See Figure 1 6 Abutment shape factor Left and Right Overbanks Abutment scour is reduced by a streamlined shape that facilitates a smooth transition of the flow and a corresponding reduction in turbulence Two common examples of streamlined abutment shapes are vertical wall abutments with flared wing walls and abutments placed on spillthrough slopes The effectiveness of the abutment shape in reducing scour depends on two factors 1 the horizontal length X1 of the streamlined portion of the abutment or spillthrough slope and 2 the total horizontal abutment and approach road length X2 that is within the effective flow width of the approach flow Please refer to Figure 2 12 for an illustration of the X1 and X2 values As indicated in the Figure measure X1 and X2 on the ABSCOUR cross section not on the actual cross section The XI value for a flared wing wall is the horizontal distance perpendicular to the flow from the abutment face to the end of the wing wall 2 The XI value for a spillthrough slope is the horizontal distance perpendicular to the flow between the abutment toe on the ABSCOUR cross section and the location of the water surface line on the spillthrough slope In some cases the water surface may extend back to the abutment 3 A vertical wall abutment without wing walls or with a 90 degree wing wall is not a streamlined shape and has an X1 value of zero
33. butment scour depth of 15 feet Design Procedure No 2 Using the Vanoni Upper Limit Curve for Estimating Threshold Critical Velocities for Clear Water Abutment Scour The following guidance is excerpted from the studies by Stephen Benedict of clear water abutment scour at bridges in the non tidal Coastal Plain of South Carolina Ref 12 For the low gradient streams and sandy soils of the Coastal Plain the Fortier and Scobey 8 Laursen 9 and Neill 10 methods have a significant number of under predictions particularly with respect to abutment scour This trend is undesirable for design and assessment purposes making them a poor method for application at such streams In contrast the Vanoni 7 upper limit curve has a significantly lower number of under predictions but with over predictions that are at times excessive None of these methods perform in an ideal way for the lower gradient streams and sandy soils of the Coastal Plain but the Vanoni 7 upper curve performs the best with regard to limiting significant under prediction Application 1 This procedure is recommended only for bridges crossing wetlands and swamps with characteristics similar to those presented 1n Table 1 for a non tidal Coastal Plain 2 For the abutment under consideration estimate the D50 particle size of the soil at the expected depth of scour This may involve several attempts to correlate the scour depth with the appropriate layer of soil 3
34. ce and the scour type live bed or clear water at overbank and channel For example When the channel 1s live bed and the overbank is clear water then the overbank contraction scour for the actual setback between 0 and 5 times channel flow depth will be interpolated between case 3 live bed scour with no setback and case 2 clear water scour with setback 5yo The interpolation depends on the distance that the abutment 1s set back from the channel bank and the scour type at the overbank and channel sections A parabolic interpolation is used for the contraction scour flow depth calculation y2 since this method provides for a smooth transition that approximates the scour depths computed through the application of Laursen s contraction scour equations The following parabolic equation is used for interpolation y2 y2 bank y2 channel y2 bank 1 setback 5 yo p APRIL 2011 Page 56 Where p 4 5 Z and p is limited to the values of 1 lt p lt 4 Z 1s the approach section bank slope H V y2 bank is the scour flow depth at setback Sy0 y2 channel is the scour flow depth with no setback Please note that the bank slope determines the shape of the parabola and therefore the relative effect of the channel scour on scour at the abutment Steeper bank slopes such as 1 1 will reduce the effect of channel scour whereas flatter slopes such as 4 1 will increase the effect of channel scour The bank slope can be used as a variable in
35. cing the flows regarding the Upstream Bridge Data Card Project Info Approach Section Downstream Bridge Data Upstream Bridge Data Pier Data Actual Sections Output Graphic Downstream water surface elevation under bridge flim See F1 Show Scour Definition Sketches section Looking Downstream Left Channel Right 10890 400 Es HEC RAS discharge under bridge cfs crms Override discharge under bridge icfsicms Blank if na override Waterway area A measured normal ta the flow Stis mj 34 Low chord elevation at downstream side of bridge flim 5 16 Abutment type Spill t hrough Spill through lt Setback Measure fram ABSCOUR s Sectian Urmi Refer to F1 for help Median particle size under bridge O50 frm 0 5 0 00259 0 5 0 001 OF 0 5 0 00259 0 5 0 001 OF 0 5 0 00259 0 5 0 001 OF Referto F1 for help on layered sail Estimated long term aggradation or degradation i f r 0 n 0 Tap width T measured normal ta the flow rni Ili DN Figure 2 6 Downstream Bridge Input Screen APRIL 2011 Page 12 ABSCOUR Elevation hl hl Mall V No Figure 2 7 Definition sketch for Bridge Section Please note that W and T are sometimes used interchangeably in figures and equations to designate a channel or floodplain width Downstream water surface elevation under bridge Enter the information from the hydraulic model Check that there are enough downstream cro
36. d Contraction Scour Envelope Curves in the Coastal Plain and Piedmont Provinces of South Carolina U S Geological Survey SIR 2005 5289 112 p http pubs usgs gov sir 2005 5289 selection of Scour Parameters The USGS study will be used to identify those sites where measurements of abutment scour values were high They key factors in identifying locations with potentially large abutment scour depths are discussed below 1 Geometric Contraction Ratio m is defined as m l b B Where b bridge opening width and B approach flow width As an example if a bridge opening b is 150 feet and the approach flow width is 1500 APRIL 2011 Page 72 feet m 1 150 1500 0 9 Conversely if the bridge opening is 1200 feet and the approach flow width is 1500 feet m 1 1200 1500 0 2 Therefore if the value of m 1s large this is an indication of contracted flow with resulting high velocities and scour If the value of m is small this 19 an indication of little change to velocities at the bridge and resulting low values of scour 2 Contraction Scour The maximum contraction scour observed at the 42 measured sites was 3 9 feet For design purposes a contraction scour value of 5 feet will be used in this assessment process 3 ABSCOUR Abutment scour For streams with low approach velocities as occurs in wetlands the ABSCOUR amplification factor is typically 1 4 The amplification factor is multiplied by the contraction scour to
37. e highest velocity in the channel e Soils information can be obtained from the Stream Geomorphology Report and borings taken at the pier Degradation and Contraction Scour values should be consistent with the input used in the ABSCOUR Program When the input data for this card is complete click on the Footing Pile group data tab APRIL 2011 Page 40 Pier N OOS OBDBDDAHEHNS TAN IMD313_100pier psf File Run Help Footing Pile cap data Footing Pile cap width frm 3 Footing Pile cap length cin 3 Pile cap Thickness fim Footing Pile cap shape factor KAT h d Pick Distance from streambed to tap of faatindg pile cap Negative if downward flim 27 Refer F1 Distance between front edge affaating pile cap and pier stem thm h 5 ma Pile group data Number of pile columns in pier width direction Number of pile rows in pier length direction Pile center to center spacing in the nier width direction Urmi Pile center to center spacing in pier length direction urm Pile size in pier width direction ftr Pile size in pier length direction tim File shape factor C1 p AL Fick Footing Pile Group Data Menu The information for the Footing Pile Group Data should be available from the bridge plans When this information is completed click on Run to obtain the program scour calculations The output results for scour at the MD 313 bridge are presented below APRIL 2011 Page 41 A File C scour 1MD3
38. e scour coefficient factor is computed See Figure 1 6 e e High chord elevation at upstream side of bridge The average elevation of the high chord or highest part of the superstructure on the upstream side of the bridge over the channel and left and right overbank sections The elevation of the high chord is used by the program to determine whether the bridge will be subject to pressure flow If pressure flow exists the program adjusts the predicted scour value to account for pressure flow See Figure 1 6 e Low chord elevation at upstream side of bridge The average elevation of the low chord or lowest part of the superstructure on the upstream side of the bridge over the channel and left and right overbank sections The elevation of the low chord 1s used by the program to determine whether the bridge will be subject to pressure flow If pressure flow exists the program adjusts the predicted scour value to account for pressure flow See Figure 1 6 APRIL 2011 Page 17 e Bed elevation at upstream side of bridge This value can be obtained from HEC RAS It is also used in the pressure flow computations See Figure 1 6 Measure average low chord elevation at red dots near the middie of the section Right Overbank Existing Cross Section Left Overbank P in Seer Figure 2 11 Input Values for Low Chord Elevations e Flow Velocity at Upstream Side of Bridge Face This value can be o
39. ect of the abutment shape on the predicted scour e A Safety factor input by the user is applied to increase the calculated scour depth This safety factor permits the user to apply judgment to the design considerations based on the site conditions reliability of available data and the risks to the bridge the transportation system and the traveling public C 5 Scour Depth Elevation The scour depth elevation is used for plotting the scour cross section and for evaluating the scour APRIL 2011 Page 35 C 6 Occurrence of Rock Where rock of varying elevations and resistance to scour is encountered the user needs to take this into account in the scour cross section C 7 Evaluation of the Computed Scour Values Use the computed values of scour from the ABSCOUR program as a guide 1n the design of the bridge abutment keeping the following considerations 1n mind e the SHA policies and procedures set forth in Chapter 11 Bridge Scour e the guidance in the FHWA HEC 18 Manual regarding abutment scour Reference 1 e the need to provide some form of scour countermeasure to protect the bridge abutment and inhibit the formation of a scour hole Base the design of the riprap on the anticipated contraction scour depths near the abutment Use the utility section of the program to compute the minimum D50 size of the riprap for each abutment These calculations are based on the procedures set forth 1n the 2001 edition of HEC 23 Use this infor
40. er recommended Wide piers in shallow water Apply option APRIL 2011 Page 39 Project Information Data Pier N OOS OBDBDDAHEHNS TANYI MD 31 3_100pier psf File Run Help Flow depth upstream face of the pier Urmi HT Flow velocity upstream face ofthe pier right at nose dfpsmpsy S Width of pier stem um hs Length of pier stem ffr HS Flow attack angle degree m Contraction scour depth at pier sf trm 2 AB Water surface elevation Upstream face of the pier ftr T 08 Aggradation ar degradation f r 0 Grain size data for streambed at pier Median grain size D50 Urmi 0 5 0 0025 64 finer grain size D84 Urmi 0 5 0 064 t 95 finer grain size O95 ftm 0 5 0 084 C Reer to F1 for help an layered soil Pier stem nose shape correction factor KT i Pick KT Override angle of attack correction factor Leave blank far deafultik zy streambed condition correction factor KC3 i 4 Pick K3 Override armoring correction factor Leave blank far default KA Pier Scour Data The information for the Pier Scour Data Menu can be obtained from the HEC RAS run the stream morphology report and the bridge plans e Use the initial flow velocity immediately upstream of the bridge as determined from HEC RAS For small channels compute the velocity as V q y where q is the unit flow in the channel For larger channels use the velocity distribution flow tube option in HEC RAS to select th
41. er closest to the left abutment looking downstream already listed Column 2 The Pier ID number depicted on the plans Column 3 The elevation of the bottom of the scour hole at the pier This needs to be the elevation of the total scour depth the sum of local scour plus contraction scour degradation Column 4 Distance from the left abutment face to the centerline of the pier Forthe special case of a spill through slope at which the water edge is at the spill through slope instead of the abutment face one more piece of information needs to be input into the cell at the top of the card Distance from the water s edge to the left abutment face This step locates the left abutment with regard to the edge of water All measurements are made from the left abutment face File Run Draw Help Distance fram left edge of water to left abutment face spill through slopes only tim Location amp Scour Data Looking Downstream Computed From Pier Module 16 30 1B bU 16 40 16 120 16 150 Figure 2 14 Pier Data Card G STEP 7 ACTUAL SECTIONS The Actual Sections menu allows the user to import HEC RAS cross sections into the ABSCOUR program and to superimpose the HEC RAS Actual Sections on the ABSCOUR Computed Sections This option can be exercised for both the APRIL 2011 Page 21 APPROACH SECTION 1 and the BRIDGE SECTION 2 The user can view and compare the fit between the Actual and ABSCOUR sections by accessing t
42. er time Design considerations for piers and abutments relative to channel movement are presented in the SHA Chapter 11 Scour Manual and in the FHWA publications HEC 18 and HEC 20 The stream morphology study including the evaluation of the stream location over time typically provides insight as to future trends of the stream channel and guidance on providing for an adequate abutment setback and scour protection Please note that the design approach should be made for every bridge foundation element within the channel lateral movement zone to use the thalweg velocity and depth to compute the scour at the bridge foundation element The Utility Module in ABSCOUR 9 provides a convenient method for computing the effect of channel movement on abutment scour STEP SIX PIER DATA Figure 2 14 depicts the Pier Data Card It is used to input information on the bridge piers into the ABSCOUR Program so that a complete scour cross section under the bridge can be generated for the scour report The User needs to calculate the elevation of total pier scour contraction scour elevation local pier scour before entering information on the Pier Data Card Use the Pier Local Scour module Option 4 to calculate total pier scour Obtain the contraction scour at the pier from the ABSCOUR output Once this is done the following information needs to be supplied on the Pier Data Screen APRIL 2011 Page 20 Column 1 A listing of pier numbers beginning with the pi
43. es May 2001 Edition The FHWA method and scour equations account for complex pier geometry as well as bed load conditions The User is encouraged to review HEC 18 for a discussion on the research used to develop the pier scour equations and the implementation method developed for computing pier scour The Maryland program facilitates the computations required to obtain pier scour depths To simplify the computations for Pier Scour included in previous ABSCOUR versions ABSCOUR 9 incorporates Option 4 which automatically makes the pier scour computations and provides a complete output file for the pier USING OPTION 4 TO COMPUTE PIER SCOUR The following example is taken from the MD 313 bridge over Marshy Hope Creek Since all piers are in the channel the conditions of highest velocity and deepest depth were used to design all of the piers Open the pier scour module and select OPTION 4 on the Project information Menu Click on the Apply Option button Then click on the Pier Scour Data Tab Pier N OOS OBDBDDIHEH N TAN IMD313_100pier psf File Run Help Project Name md313 over Marshyhope Creek Mo Description 100 year flood Bridge opening is skewed 35 degree HEC 18 Pier Scour Type Option Units Option t Option 1 Pier foundation not exposed English units t Option 2 Pier with exposed footing slab or pile cap t Metric SI units t Option 3 Pier with pile cap and pile group exposed f Option 4 SHA procedures for complex pi
44. etermine if the flow condition 1s for live bed or clear water scour Downstream Bridge Computations Based on the abutment setback and channel flow depth the program computes the flow distribution and velocities as described in Part 1 for short setback intermediate setback or long setback There are 16 possible combinations of flood plain geometry and abutment setback distances that are utilized in the ABSCOUR Program to compute the appropriate velocity used in the scour equations These combinations are presented in Attachment 1 For clear water scour the user has the option to compute the critical velocity from Laursen s equations or the SHA modification of Neill s curves The ABSCOUR program computes contraction scour depth by setting the average flow velocity equal to the critical velocity Neill s competent velocity of the D50 stone size An adjustment is made for the hydraulic depth at the abutment if the abutment is within the limits of the bank slopes line 110 Downstream Contraction Scour Equations Line 118 and 119 reflect computed contraction scour for clear water and live bed respectively and Line 120 provides for an interpolated scour depth depending on the scour conditions In the Case C example presented above there 1s live bed scour on the overbank and in the channel The live bed scour flow depth in the channel line 119 1s 12 4 feet and on the left overbank at a distance of 5yo 34 feet it is 5 9 ft The abutment setback f
45. g ground to define the ground line If there 1s a pier on the over bank section the pier width should not be included in the top width value T This may result in a condition that the top width as measured from the channel edge will not extend to the abutment and abutment scour will be computed as zero For this case the setback distance needs to be adjusted to equal the top width T If the abutment projects into the channel beyond the channel bank enter the setback as a negative number APRIL 2011 Page 14 Setback Main Channel Ver tic al wall Spill Through Abutment Abutment Figure 2 9 Illustration of Setback Median particle size This value is important for clear water scour and should represent the particle size at the bottom of the scour hole The D50 particle size can be entered for up to three soil layers and the program will compute the extent of the scour into each layer See the F 1 help card Input the median D50 particle size in feet meters for the material under the bridge culvert using following format For single soil layer input the D50 in feet meter For two soil layers input top layer thick top layer D50 bottom layer D50 For example 2 5 0 05 0 25 For three soil layers input top layer thick top layer D50 2nd layer thick 2nd layer D50 3rd layer D50 For example 2 5 0 05 5 0 254 2 5 The first layer should be the stream channel in which D50 is obtained by sampli
46. he DRAW option on the top MENU bar for the Approach Section Bridge Section and Scour Section Abutment N OOS OBDBDD HEH S TAN srd3md313_100 asc MEE ES Run Draw Help Project Info Approach Section Downstream Bridge Data Upstream Bridge Data PierData Actual Sections Output Graphic Approach Section EUM Downstream Import HecRas Bridge Section Looking Downstream Import HecRas Right bank Sta 886 58 Left bank Sta BB5 44 Right bank Sta 833 21 Right 0 06 Manning n Left or Channel 0 04 Right 08 Stream Section mm Deck Data pe insert Row ALLL 7 475 00 16 00 E Delete Row Delete Row 577 50 14 00 595 00 12 00 Tools 610 30 10 00 Tools 619 50 0 00 645 00 G T 659 58 6 00 BB5 44 1 40 665 98 0 38 KEO Br 1 12 h74 11 1 85 12 67827 3 79 13 683 30 4 92 Figure 2 15 Actual Sections The user can use this information to advantage in making an evaluation of the ABSCOUR scour computations 1 Identify errors in the input data for the ABSCOUR cross sections 2 Compare how well the ABSCOUR Section fits the Actual Section 3 Determine if fine tuning adjustments in scour elevations should be made in order to match the actual cross section more closely Application Clicking on the Actual Sections Menu brings up two tables the Approach Section Looking Downstream and the Bridge Section Looking Downstream The top of each table provides cells to inpu
47. he right abutment INITIAL CONDITION MODIFIED CONDITION FROM HECRAS BY USER APPROACH FLOW APPROACH FLOW 400 1200 300 400 1200 300 OVERTOPPING FLOW OVERTOPPING FLOW 190 0 400 100 0 409 FLOW UNDER BRIDGE 100 1200 100 All numbers gre in cts Figure A2 2 Flow re distribution examples By inspection some of the overtopping flow on the left is coming from the main channel and the right overbank section A rapid shift of the flow from left to right occurs in order to meet the HEC RAS distribution based on conveyance This redistribution of flow may not actually occur Accordingly the user may wish to consider the consequences of a greater flow on the right overbank section A trial flow distribution as depicted on the right sketch can be selected for a worst case type of analysis These values may be input APRIL 2011 Page 66 instead of the HEC RAS values to assess worst case scour at the right abutment The total flow through the bridge remains the same in both cases as does the total overtopping flow The difference 1s that the user can modify the program to provide a different flow distribution under the bridge III Example 3 Bend in the River For a bridge located on a bend in the river particularly a sharp bend momentum forces may affect the flow distribution under the bridge More flow may move to the outside of the bend than 1s indicated by the HEC RAS conveyance calculations This condition can be investiga
48. he water no contribution to the scour component n Scour component for the exposed pile group 72 73 Only one pile column the pile spacing in width direction is set to 7 times pile size 74 Adjusted depth of flow upstream of pier y3 17 ft 75 Adjusted velocity for the flow approaching the pier v3 5 fps 76 Sum of overlapping projected width of piles 1 5 ft 77 Coefficient of pile spacing Ksp 1 78 Coefficient of number of aligned pile rows Km 1 79 Effective width of the pile group 1 5 ft 80 Correction factor for armoring K4 for pile group 1 81 Height of pile group aboved lowered stream bed h3 24 ft 82 Pile group height factor Kh pg 1 83 Froude Number Fr3 for pile group 0 2137 84 Scour component for the exposed pile group 3 975 ft 85 Total pier scour depth with respect to revised flow depth 3 975 ft 86 Total pier scour depth with respect to initial flow depth 3 975 ft 88 Method 2 Option 3 90 Revised flow depth 19 46 ft 91 Revised flow velocity 4 3679 fps APRIL 2011 Page 42 File C scour 1MD313 100pier psf Date 10 15 2010 Page 2 92 Revised distance from streambed to top of footing pile cap 27 ft 93 Revised soil layer 1 thick 0 ft 94 Revised soil layer 2 thick 0 ft io Control soil is layer no 3 with D50z0 000089 ft D95z0 0038 ft ou Scour component for the pier stem in the flow lo Pier stem is not in the water no contribution to the scour component 102 Scour component
49. his value using the Equations in Appendix A The scour depth y is the depth of contraction scour Ys Y2 Yo Where y depth of contraction scour y2 vertical distance from the water surface to the stream bed after contraction scour has occurred and yo depth of flow under bridge before scour occurs Bridge Section Data Please note that the output table will indicate whether or not pressure scour is computed in accordance with the procedure in Part 1 C 4 Abutment Scour Table The abutment scour depth ysa aa represents the total scour including contraction scour and local scour which is predicted to occur at the abutment It does not include long term degradation which the user must account for in the final scour evaluation The scour depth elevation is the elevation the Engineer should use to evaluate scour It reflects all of the adjustments made by the program to account for the various factors affecting abutment scour These adjustments include the following e For a skewed embankment crossing the ABSCOUR program will adjust the computed scour by a skew coefficient in accordance with the procedure set forth in FHWA Hydraulic Engineering Circular 18 2001 Edition The user must enter the theta angle of the orientation of roadway with respect to the direction of the flow e The program increases scour depths where necessary to account for the effects of pressure flow e An abutment shape factor is used to evaluate the eff
50. idered independent and not affected by the channel flow For the setback of 80 ft the contraction scour will be Step 1 Compute unit discharge q2 1200 80 15 cfs ft Step 2 Compute contraction scour y2 y1 15 12 40 638 1 15 y2 1 15 3 8 4 37 ft D Special Case Intermediate Setback Narrower Overbank CASE D in Figure A1 1 When the setback gt 5yo in a narrow overbank section width lt 6 67yo there is no intermediate flow consequently the normal interpolation does not apply For this case Figure 1c ABSCOUR will compute contraction scour assuming that the setback 1s equal to 5yofor a conservative approximation For example the contraction scour for a setback of 60 ft in a 65ft wide overbank in Figure 1c will be computed the same as that for a setback of 50 ft Step 1 Compute flow velocity assuming the setback is at 5y0 50ft V2 600 1600 1200 20 5 20 10 50 4 6 8 ft s Step 2 Compute unit discharge q121200 65z18 46 cfs ft q2 6 8 4 27 2 cfs ft Step 3 Compute scour depth y2 3 82 27 2 18 46 0 638z1 28 y2 1 28 3 8 4 87 ft Figure AT 1 illustrates the four contraction scour examples presented above for varying setback distances Figure A1 2 illustrates the resulting contraction scour for these cases although the details of the abutment scour calculations are not presented APRIL 2011 Page 61 The general procedure to compute the abutment scour flow depth is van kf k contraction scour The final abutment
51. idge As noted earlier the widths input by the user and the abutment setbacks should be measured normal to the direction of the approach flow The method of analysis Method A Short Setback B Long Setback or Method C Intermediate or Transition Setback is determined on the basis of a comparison of the abutment setback with the depth of flow in the main channel at the bridge Section 2 as previously described The unit discharges q and velocity V 2 are computed from the equations set forth in Part 1 Attachment 1 provides detailed examples of how the computations are made for various combinations of channel and overbank geometry and abutment setback The critical velocity required for the incipient motion of the Dso particle size for flow under the bridge for clear water scour is computed from the particle size of the channel bed or flood plain material and the flow depth using Neill s competent velocity curves as modified by the Office of Structures An over ride table is provided to allow the user to change this value to account for cohesive soils or other factors This over ride process is the same as that for the scour parameter table The user is also given the option of using Laursen s relationship for clear water scour APRIL 2011 Page 34 C 3 Contraction Scour Table The value of y in this table is the vertical distance between the water surface and the stream bed after contraction scour has occurred The program calculates t
52. ile cap use the following revised input values for flow depth and velocity 2 Set a revised flow depth at an elevation of 1 foot below the top of the footing pier cap The total flow depth to this point y2 y ys where y1 is the depth of the contracted scour bed and ys is the pier scour depth between the contracted channel bottom and the selected elevation one foot below the elevation of the top of the footing pier cap Note If the contracted channel elevation is already below the bottom of the footing pile cap proceed to Option 3 2 Compute a new approach flow velocity as Vo2 Vi yi yi ys 2 4 Run the program and note the computed scour depth Subtract this computed scour depth from the revised flow depth set in Step 2 APRIL 2011 Page 46 D above This determines the scour elevation for Option 2 If the scour elevation from Step 4 is within the limits of the footing pile cap use this value for the pier scour If the scour elevation from Step 4 is below the limits of the footing pile cap go to Option 3 for Method 2 Option 3 for Method 2 Fill in the information regarding the pile group Use revised input values for flow depth and velocity as described below 1 Set a revised flow depth y3 at an elevation of one foot below the bottom of the footing y3 y ys where ys is the scour depth measured from the channel bottom to the point one foot below the bottom of the footing Compute a new approach flow
53. ing Pier Scour Using Method 1 Assume contraction scour does not occur Compute pier scour following the procedure outlined below using the flow depths and velocities obtained from the water surface model typically HEC RAS and the existing channel bed elevation Set the initial channel bed elevation equal to the existing channel bed elevation Set the initial flow depth y equal to the distance between the water surface and the existing bed elevation Select the initial flow velocity immediately upstream of the bridge as determined from HEC RAS For small channels compute the velocity as Vi qg yi where q is the unit flow in the channel For larger channels use the velocity distribution flow tube option in HEC RAS to select the highest velocity in the channel Proceed to Option 1 Option 1 Option 1 computes local scour for the pier stem only Fill in the required information Page 44 APRIL 2011 including the initial flow depth y and the flow velocity V as discussed above Click the run button e If the scour computed by Option 1 is less than the elevation of the top of the footing pile cap use this value for the pier scour depth Then y2 y ys e If the scour computed by Option 1 is deeper than the top of the footing pile cap continue on to Option 2 below Note that ys pier Y2 Y1 Option 2 l Fill in the information for the footing pile cap use the following revised input values for flow depth and vel
54. ing data for the ABSCOUR Model the user will need to obtain hydraulic data as discussed below A 1 Water Surface Profile Prepare a water surface profile using HEC RAS or other program to model flow conditions upstream of through and downstream of the bridge Discharges selected for evaluation of scour should include the overtopping flow Q 00 Qaesign and Qsoo in order to develop the anticipated worst case scour conditions at the bridge Field check Manning s n values to obtain the proper flow distribution between the channel and flood plains Use sufficient downstream cross sections to establish reliable bridge tailwater elevations APRIL 2011 Page 4 A 2 Development of ABSCOUR Model Cross sections Two cross sections are required to run the ABSCOUR program e Section 1 Upstream approach section This section should be upstream of the area of influence of the bridge contraction and should be representative of the approach flow conditions In some cases the user may need to modify the actual approach section so that it 1s representative of actual approach flow conditions See Step 3 e Section 2 Downstream Bridge Section This section is located under the bridge at the downstream end b STEP TWO INPUTTING INFORMATION INTO THE ABSCOUR PROGRAM PROJECT INFORMATION MENU Figure 2 1 shows the ABSCOUR Project Information Menu screen The following section explains each of the input parameters Abutment M 2010 ABSCOU
55. ion is offered only to provide insight into the approach used in ABSCOUR 9 to compute pier scour As noted earlier Option 4 automatically solves the pier scour equations for all the cases discussed below This is the recommended option to use 1 Two alternative methods for evaluating pier scour are described below The recommended procedure is to compare the scour computed from both Method 1 and Method 2 Select the method which results in the deepest scour elevation Use this value as the total pier scour value 2 Method 1 Assume contraction scour does not occur Compute pier scour following the procedure outlined below using the flow depths and velocities obtained from the water surface model typically HEC RAS and the existing channel bed elevation 3 Method 2 Assume contraction scour does occur Compute pier scour following the procedure outlined below using the revised elevation of the channel to account for contraction scour Also modify the flow depth and velocity to account for the effect of the contraction scour APRIL 2011 Page 43 Computing Pier Scour using the ABSCOUR Pier Scour Module For both Methods 1 and 2 three options are evaluated See sketch below e Option 1 only the pier stem is contributing to scour e Option 2 the pier stem and pile cap footing is contributing to the scour e Option 3 the pier stem pile cap and piles contribute to the scour OPTION 1 Se ee ee ee OPTION 2 OPTION 3 Comput
56. ion just upstream of the bridge to compute the stream power e For abutments select the downstream section under the bridge Section 2 as defined in the ABSCOUR Program to compute the stream power Selecting the pier type along with the angle of attack of the flow 3 Calculating the maximum scour in sand for the selected foundation geometry and flow conditions e For piers select a section just upstream of the bridge to obtain the hydraulic values in the pier scour equation Use the Pier Scour Module in the ABSCOUR Program to calculate the scour depth in sand e For abutments select the downstream section under the bridge Section 2 as defined in the ABSCOUR Program to compute the maximum scour in sand Use the ABSCOUR Program to make this computation C 3 ERODIBILITY INDEX CALCULATIONS The recommended approach for computing the Erodibility Index 1s to use the Spread Sheet developed by the SHA See SHA Software Module in the Manual Computations of the Erodibility Index of the rock should be made only by engineers or geologists with knowledge and experience in evaluating the properties of rock It is the practice of the Office of Structures to meet with the SHA geologists for the purpose of 1 inspection of the rock cores and 2 selection of appropriate rock characteristics for purposes of computing the erodibility index of the rock The steps for computing the Erodibility index are outlined below C 4 COMPUTING THE ERODIB
57. ischarge 114 113 cts tt 88 386 88 386 85 356 116 Control soil layer No 3 3 ll7 Critical velocity tps d 317 4 417 4 417 1152 APRIL 2011 Page 25 gt gt gt gt gt 119 Downstream Contraction Scour Computations 120 lal Left Channel Right SSS Se enen 123 Clear water scour flow depth yei ft 20 008 20 008 20 006 124 Live bed scour flow depth Zi ET gt 41 269 17 8965 50 856 125 Interpolated scour flow depth y2 ft 117 895 117 895 17 896 lz6 Pressure flow coefficient Ep l l l lz7 Adjusted scour flow depth yzjadj 126 125 gt yOjadj fc 17 8596 17 896 17 856 126 Contraction scour depth ys 1l27 8114 T SFi Et 2 455 2 455 2 455 129 Final contraction scour depth ys E H128 H11 ft 2 455 2 455 2 455 130 Aggr Degr Contraction scour EL 55 114 8129 66 70 Et 11 056 11 056 11 056 131 l32 Total Bridge Scour At Abutment 133 154 Left Channel Right De a UE 136 Abutment local velocity factor Kw 1 001 1 002 137 Abutment spiral flow factor Ef 1 4 1 4 l38 Pressure flow coefficient Ep l l 139 Abut scour flow depth y2a adj H125 H137 H136 H104 4138 EE 25 068 25 095 140 Initial abutment scour depth ysal 1359 8114 0 Et 9 627 9 654 l41 Coefficient for abutment shape factor Et 5 l l42 Coefficient for embankment angle Ke 0 936 1 044 143 144 Final abutment scour depth ysa adj H110 H14311 H112 H11 ft 9 03 10 075 145
58. istances from the channel bank II EXAMPLE PROBLEM 1 GIVEN LL EEFFOVERBANK CHANNEL RIGHTOVERBANK FAPPROACHSECHON S DISCHARGE cs 49 o 139 L TOPFLOWWIDTHR Tat o o 10 HYDRAULICDEPTHR 48 98 o 38 oOo PONIT DISCHARGE Dew 75 80 12 T RIEN O BRIDGE SECTION DISCHARGE cfs 600 1600 200 TOP FLOW WIDTH 80 20 10 APRIL 2011 Page 58 HI COMPUTATION OF CONTRACTION SCOUR Computations for the contraction scour flow depths y2 for the right overbank section are presented for different abutment setbacks The left abutment is kept at a fixed location with its setback at a distance of 20 ft from the channel edge The methods of computation are demonstrated only for the right overbank Contraction scour of the left overbank for different setbacks can be computed in the same way by keeping the right abutment at the actual fixed location A Short Setback CASE A in Figure Al 1 Since the channel depth is 10 feet any setback less than 5 X 10 50 feet is a short setback Let the setback of the right abutment be 30 ft Since the left abutment setback is also short being 20 feet the velocity is computed as if all flows are mixed The contraction scour depth then shall be computed by interpolating the contraction scours at the
59. l For vertical wall abutments plot values of y2 and y2a under the bridge measuring down from the water surface at the downstream side of the bridge 2 Where the abutment scour is deeper than the channel scour use an angle of 30 degrees to define the sides of the scour hole Use a nominal value of 5 feet to determine the width of the bottom of scour hole 3 Where the abutment scour depths are at a higher elevation than the channel contraction scour use a smooth curve to define the transition area 4 The user will need to determine the total scour at each foundation element taking into account the following factors Contraction scour Abutment scour Local pier scour Lateral channel movement Degradation O OO 0 OO The current policy of the Office of Structures is to make a judgment on how to best consider the total effect of these different aspects of scour on a case by case basis as discussed in Chapter 11 b ABSCOUR PROGRAM LOGIC The following discussion is provided for insight into the logic used by the program in computing flow distribution and velocity distribution at the bridge A current limitation of the HEC RAS program used to model flow through a bridge is that it provides for the distribution of flow under the bridge based on conveyance calculations This approach does not reflect the three dimensional flow patterns actually Observed in the field at bridge contractions To obtain reasonable estimates of scour depth it
60. lic flow conditions a slight increase in the D50 particle size will result in an increase in the scour depth This result of course 1s the opposite of APRIL 2011 Page 11 what we would expect The anomaly is typically small and can be modified by the user to obtain a reasonable answer The user has the option of overriding the calculated values and substituting other values for the critical shear stress and boundary shear stress A first step in the evaluation of these parameters would be to refine the boundary shear stress as calculated by ABSCOUR 1 y RSaye at the approach section by obtaining more detailed information about the flow in the channel reach between Section 1 and Section 2 D STEP FOUR DOWNSTREAM BRIDGE DATA D l Enter the Downstream Bridge Data Figure 2 6 shows the ABSCOUR input screen for the downstream bridge data Figure 2 7 shows the definition sketch for the downstream bridge data Please note that the program is set up to input the flow estimates under the bridge as computed by the HEC RAS model However the user has the option of using the over ride cells to select a different flow distribution where there is a question regarding the HEC RAS distribution which is based on conveyance calculations Examples include a bridge on a bend where the user may expect a larger portion of the flow to move to the outside of the bend a complex overtopping situation or an upstream confluence See also the discussion on balan
61. lotted at the toe of the spill through slope then the scour profile is continued up the spill through slope along the estimated angle of repose of the abutment material as illustrated in the blow up for the bridge sketch for the left abutment The intersection of the scour line with the piles can be used to evaluate the potential loss of support and the resulting stability of the piles i D lowor 8 Sta 9 ur el Dich INANIS ABUT I to Federoisburg we id t ume Bitten zira sonde RRAN vermeer ete per WS i ptas i id IH HH w guo a TET il i il it it H me mB s uo ii H H 1 H i i PROJECTED SCOUR AT ANGLE OF REPOSE EXISTING BULKHEAD TYP 16 0 I00YR PIER SCOUR ELEV SCOUR EL 11 0 MD 313 OVER MARSHYHOPE CREEK ELEVATION CADD Plot of Scour Cross Section for Marshy Creek Bridge APRIL 2011 Page 29 BRG ABUT to Federalsburg PROJECTED SCOUR AT ANGLE OF REPOSE EXISTING BULKHEAD TYP IOOYR CONTRACTION SCOUR EL H 0 16 0 QOO0YR PIER SCOUR ELEV MD 313 OVER MARSHYHOPE CREEK ELEVATION Blow up of the CADD Plot for the Scour Cross Section for the Marshy Hope Bridge The existing bulkhead is at the toe of the spill through slope The elevation of the contraction scour 1s computed at this point Then the scour cross section is continued at the angle of repose of the spill through slope material back to the
62. low velocity HEC RAS discharge under bridge icfsicms Override discharge under bridge t ctsicmsi Top flow width under the bridge normal to flaw Urmi normally at downstream end ofthe bridge Abutment setback from edge of channel flim negative if projected inta the channel Alternate Input characetristic flow velocity Characteristic average flaw velocity ifpsimps Adjusted flaw depth tin Froude Number Required Riprap O50 tr Close New Import Help After running the ABSCOUR Program The utility program can be used in to import the output data from the ABSCOUR run to compute the riprap size required for an abutment or pier This option is illustrated below m di NN APRIL 2011 Page 48 Abutment Riprap Design MIE Unit option English t Metric SI specific gravity of the riprap rock 2 55 Bridge Section Looking Downstream Left Overbank Channel Right Overbank Average flaw depth DIS face of bridge trm 0 i 1 0239 7 0943 Abutment type Vertical Vertical Optional Input and program determine the characteristic flow velocity HEC RAS discharge under bridge icfsicms 0 5784 2482 Override discharge under bridge icfsicms Top flow width under the bridge normal to flow Urmi 0 79 21 h 33 79 fnormally at downstream end of the bridge Abutment setback from edge of channel Urmi 0 123 78 negative if projected into the channel Alternate Input characetristic flaw velocity Characteris
63. luation of this condition by use of the over ride functions The engineer also needs to keep in mind the limitations of the ABSCOUR model used to estimate the depth of clear water scour The concept is that the area under the bridge will scour and thereby increase the flow area while decreasing the flow velocity This process will continue until the flow velocity is below the critical velocity needed to move the selected D50 particle size under the bridge The model application 1s likely to result in high clear water scour depths for high flow velocities in fine grained non cohesive soils The following factors need to be evaluated in this regard Please note that the user can now input the thickness and D50 value of up to three layers of bed material under the bridge on the downstream bridge data card The particle size should be representative of the soil at the elevation of the bottom of the scour hole Armoring of the stream bed may inhibit the depth of the scour SHA s experience on Maryland streams is that critical velocities for fine particle sizes are best modeled by the Office of Structures modification to Neill s curves as discussed in the calibration of ABSCOUR 9 The user has the option of using Laursen s method for clear water scour The hydrograph for the worst case scour conditions should be considered For flashy streams on small watersheds the time period during which scouring velocities actually occur may be relativel
64. m Bridge Data for ideas on complex flow patterns 5 In many cases there is no ideal approach section For a complex flow pattern it may be of help to evaluate scour by comparing the results obtained from two alternative approach sections F PE LP E PETLYPEESTIJPJFJEFJFIFR m TT ree F Approach section water surface elavatian fim T 43 section Looking Downstream Left Overbank Channel Right Overbank Discharge ictsicms 3 8847 1860 Flow tap width f r T 180 985 4 18 Average flaw depth hydraulic depth tim g4 13 16 Median bed grain size 050 frm 0 003 0 003 0 003 Mote see H amp H Manual Chapter 14 Appendix B for D50 TL Average bank slope iA in the vicinity of the bridge 3 LZ harizantalivertical Average Energy Slope between Approach Section and Bridge Section 0 0004 Show Scour parameters Figure 2 2 ABSCOUR Approach Section input sheet APRIL 2011 Page 8 Water Surface l la E ee re ee l lt Existing y y Cross Main y J Section j Channel Wi N Wi ae Left Overbank DU Right Overbank Looking D S Wi Looking D S Figure 2 3 Definition Sketch for ABSCOUR Approach Section Looking Downstream Please note that W and T may be used interchangeably in figures and equations to designate a channel or floodplain width C 1 Enter Approach Section Data The water surface profile models compute flow velocities depths and discharges for the approach
65. mation to select the appropriate riprap size typically Class 2 or 3 There are factors which can affect the extent of contraction scour and abutment scour at a bridge that are not directly computed by the ABSCOUR model However various procedures have been suggested in this manual to permit the user to take some of the factors into consideration in the scour evaluation the possible effect of nearby adjacent piers in modifying flow patterns and resultant abutment scour engineering judgment model studies effect of bends and upstream tributaries in the distribution of contraction scour bendway scour and the effect of a severe angle of attack causing flow to impinge directly on the abutment These conditions may increase scour at abutments located on the outside of bends See Attachment 2 and Reference Numbers 1 2 and 8 effect of ice or debris in clogging a waterway opening deflecting channel currents and increasing flow velocities and resulting scour See HEC 18 effect of two dimensional flow patterns especially for wide flood plains in modifying the flow conditions at a bridge See Attachment 2 use a 2 D model effect of confluences or other geomorphological features affecting the lateral migration of stream channels See Attachment 2 the method does not directly address critical shear stress or critical velocity for cohesive soils or rock The user is provided a means of partial APRIL 2011 Page 36 eva
66. n the above noted cases it 1s likely that the distribution of flow determined by HEC RAS using a 1 D approach based on flow conveyance may not be truly representative of the actual site conditions The ABSCOUR program provides for input boxes for both the HEC RAS analysis and a special analysis provided by the user to explore a worst case type of condition The use of flow distributions other than that provided by HEC RAS is recommended for use only by modelers who have a thorough understanding of the HEC RAS program Further the HEC RAS distribution should always be tested first in the ABSCOUR program so that there 1s a basis for comparison for the flow distribution selected by the user The accuracy of the modeling for such cases will depend on the skill and experience of the user in evaluating flood flows It requires the user to be able to visualize the flow condition so as to select a reasonable flow distribution at the bridge In some cases the momentum equation or other computational methods can be employed to assist with this visualization The ABSCOUR computations are illustrated in the table below with all numbers representing flood flows in cfs ps LEFT OVERBANK CHANNEL RIGHT OVERBANK APPROACH SECTION 2000 OVERTOPPING 300 BRIDGE SECTION 500 300 200 2000 0 2000 250 0 2 250 The user inputs the discharges for the approach section flows and the bridge flows based on the results obtained from the HEC RAS runs As discu
67. nd This information can serve as one factor in making an engineering judgment regarding scour at abutments founded in rock Using the empirical relationships presented in the Erodibility Index Method described above a comparison can be made between stream power and the ability of the rock to resist the hydraulic forces If the rock at the surface of the stream cannot withstand the hydraulic forces of the water it will scour and a scour hole will form at the base of the pier or abutment As the scour hole deepens the stream power at the bottom of the scour hole diminishes in accordance with the relationships determined by the FHWA studies At some point the hydraulic power of the water and the resistance of the rock will achieve a balance and the scour will end A safety factor should be applied to the above scour evaluation to take into account the limited understanding of and experience with evaluating the resistance of rock to scour This safety factor should be determined on a case by case basis however the current SHA thinking is to use a safety factor in the range of 2 to 5 with a range of 2 to 3 being used for most bridges APRIL 2011 Page 50 C 2 STREAM POWER CALCULATIONS The hydraulic calculations are relatively straight forward and consist of the following 1 Inputting the velocity hydraulic radius and energy slope of the flow so that the program can calculate the stream power Pa Pa yVRS e For piers select a sect
68. ng fine grained or by pebble count coarse grained materials Subsurface estimates for the D 50 are often available from borings or possibly the stream morphology report This selection of particle size is often a judgment call due to the lack of good soils data at a distance of 5 10 or 15 feet below the channel bed A conservative approach is recommended where there is limited data for selecting a particle size Cohesive Soils A D50 particle size should not be selected for cohesive soils If the soils are clearly cohesive the clear water scour condition should be evaluated by using an over ride feature and estimating the critical velocity of the soil For particle sizes of about 0 1 mm or less soils may behave more like a cohesive material and the assumption of a cohesionless bed material used in the ABSCOUR computations becomes less valid For silt and clay soils the User is referred to the discussion in Attachment 4 When a critical velocity of such soils can be estimated select the Bridge Section Critical Velocity override function on the Project Information Screen This will activate additional cells on the Downstream Bridge Data Screen so that the appropriate critical velocity values can be entered Armoring A complicating factor in selecting a representative particle size for clear water scour is the potential for armoring of the channel bed A discussion of this APRIL 2011 Page 15 E Ed consideration is presented in Part 1
69. nnel If there is a pier within the limits of the ABSCOUR cross section the top width and flow area should be adjusted to subtract the pier width pier area The program will compute the hydraulic depth for each downstream sub area left APRIL 2011 Page 13 overbank channel and right overbank as y A T e Low Chord Elevation Enter the average low chord lowest superstructure element elevation at the downstream side of the bridge for the left overbank section right overbank section and channel section Refer to Figure 2 8 Measure average low chord elevation at red dots near the middle of the section Existing Cross Section Figure 2 8 Average Low Chord Elevation e Abutment Type Select the abutment type Vertical Wall Wing wall or Spill through Slope e Setback Setback is the horizontal distance measured from the channel bank or edge of channel to the abutment For a vertical wall or a wing wall abutment measure the setback from the channel bank to the face of the abutment For a bottomless arch culvert measure the setback from the channel bank to the culvert wall For an abutment on a spill through slope measure the setback from the channel bank to the point where the ground line intersects the spill through slope If the ABSCOUR cross section is above the existing ground use the ABSCOUR cross section to define the ground line If the ABSCOUR cross section is below the existing ground use the existin
70. nsition 1s needed between the no setback case and the case where the abutment is set well back on the flood plain The limit of the transition zone is defined as five times the flow depth in the downstream channel When there is no setback the channel scour flow depth y2 1s used for the contraction scour When the abutment setback on the flood plain exceeds the limit of the transition zone separate flow is assumed between the channel and the flood plain and no interpolation is required When the setback is within this transition zone of from zero to Syo the following scheme is used to compute contraction scour ABSCOUR separately calculates both clear water scour flow depth and live bed scour flow depth for 1 the channel section and 2 the overbank section The channel contraction scour flow depth y2 is the scour when the setback 1s equal to or less than zero that 1s no setback case The overbank contraction scour flow depth y2 is the overbank scour when the setback 19 located on the flood plain beyond the channel banks a distance equal to 5 times the flow depth in the downstream channel SB 5yo There are four combination of overbank scour in the transition zone clear water scour with no setback 2 clear water scour with setback 5yo 3 live bed scour with no setback 4 live bed scour with setback 5yo The computed overbank contraction scour will be interpolated between these four cases depending on the setback distan
71. ocity A Set a revised flow depth at an elevation of 1 foot below the top of the footing pier cap The total flow depth to this point y2 y ys where ys is the pier scour depth between the channel bottom and the selected elevation one foot below the elevation of the top of the footing pier cap 3 Compute a new approach flow velocity as Vo Vi yj yi ys 2 4 Run the program and note the computed scour depth Subtract this computed scour depth from the revised flow depth set in Step 2 above This determines the scour elevation for Option 2 5 If the scour elevation from Step 4 is within the limits of the footing pile cap use this value for the pier scour If the scour elevation from Step 4 1s below the bottom of the footing pile cap go to Option 3 Option 3 Fill in the information regarding the pile group Use revised input values for flow depth and velocity as described below l Set a revised flow depth y3 at an elevation of one foot below the bottom of the footing y3 y ys where ys is the scour depth measured from the existing channel bottom to the point one foot below the bottom of the footing 2 Compute a new approach flow velocity as V3 V4 y1 yi ys 2 3 Run the program for Option 3 and obtain the scour depth S Compute the scour elevation as the elevation of the selected point one foot below the bottom of the footing pile cap step 1 above scour depth Step 3 6 Compare this scour elevati
72. of Appendix A however a comprehensive treatment of the armoring of channel beds is beyond the scope of this guide and the user is referenced to the FHWA publication HDS 6 River Engineering for Highway Encroachments or similar texts on river mechanics to evaluate this condition Zn general great reliance should not be placed on the expectation that armoring of the bed will limit the extent of contraction scour Estimated long term bed degradation aggradation The stream morphology report typically addresses the potential for long term changes in bed elevation at the bridge If it does not the Engineer will need to make an evaluation of the stream morphology and utilize available information to determine a best estimate of future conditions When a value is provided in the input cell ABSCOUR will include this value in the elevation of the bottom of the scour hole Safety Factor Please refer to the table in Attachment 3 and the accompanying examples for guidance in selecting a safety factor for the abutment scour estimations Over rides Please note that one of the over ride options on the Project Information Card permits the user to select a unit discharge under the bridge that 1s different from that computed by the program An example of the use of this option would be a bridge crossing located in a bend with higher unit discharges on the outside of the bend If the override is selected then the input cells are displayed on the Downstream Bridge Da
73. oint station For the bridge section the program will search through the geometry file of the current active plan of HECRAS project and find the available bridges If more than one bridge exists a list of bridges will be generated and the user can select the appropriate bridge If there is only one bridge the program will import the bridge data without asking The bridge data includes the downstream section or upstream section for the upstream tool and the bridge deck high chord and low chord elevations The left bank and right bank point stations are also obtained If the left bank and right bank stations do not match the ABSCOUR stations used in the scour analysis the user can make the following adjustment Change the HEC RAS stations to match the ABSCOUR section III COMPUTATIONS AND PROGRAM OUTPUT INFORMATION Please note that the ABSCOUR program presents computations with up to three decimal points However final scour values used for design should be rounded off to the nearest foot since the assumption of accuracy of scour estimates to a tenth of a foot 1s not valid After entering the data on the input menus as described in Steps 1 through 5 click on the RUN button to compute the scour If the program inputs are correctly entered the output file appears If there are any of the input items are not filled in an error message will appear prompting the user to correct the input files All input data and output computations are summarized
74. ojectinfo Approach Section nmwnstrea idae Date p2 idae Data PierData Actual Sections Output Scour Parameter table MIE 5cour parameters calculated by the program User may override these parameters stream The override options are located in the project info paze EIU Vegas Lett Channel Right i Sal Approach section flow velocity pS 805 Approach section Fraude Number 418 Approach section critical shear stress pst 0 003 Approach section boundary shear stress psi Scour pe determined by program Calculated sediment transport parameter k2 Average bank slope iA in the vicinity of the bridge 3 horizontalvertical l Average Energy Slope between Approach Section and Bridge Section 0 0004 D 14 Figure 2 5 Scour Parameter Table Please be aware that the sediment transport parameter ko represents a complex function The Level 2 analyses provided by HEC RAS and ABSCOUR offer a reasonable approach for estimating this function However the water surface profile and hydraulic variables are assumed to be fixed for the HEC RAS ABSCOUR analysis remaining constant for changes in the particle size of the bed load This limitation can be minimized by making small changes to the HEC RAS runs to account for varying n values but such refinement is normally unnecessary However we have observed an unusual and special condition for live bed scour while running sensitivity checks For certain combinations of hydrau
75. omputing the boundary shear stress Enter the average energy slope of the flow in the stream reach between the approach section 1 and the downstream bridge section 2 Refer to Figure 2 4 for details The average energy slope 1s computed as Save Energy Line Elevation Section Energy Line Elevation Section 2 L where L distance between Sections 1 and 2 Please note that alternative methods may be more appropriate for some flow conditions especially for backwater conditions The computed value should be compared with information obtained from the HEC RAS runs m Energy Grade Line EGL Elevation Water Surface Elevation m L Approach Bridge Section Section 1 2 Average Energy Slope AEGL L Figure 2 4 Average Energy Slope APRIL 2011 Page 10 e Scour Parameter Button Click on the scour parameter button to view ABSCOUR scour parameters computed from the approach flow conditions Refer to Figure 2 5 As noted earlier over riding any of these values should be undertaken with caution and an understanding of the flow and sediment transport conditions For example if the computations indicate live bed scour on the flood plain and the flood plain is covered with heavy vegetation with attendant low velocities it is likely that clear water scour will actually occur on the flood plain The scour parameter can be over ridden to indicate clear water scour for the flood plain approach flow Pr
76. on with the scour elevation determined from Method 2 Use the lower scour elevation as the total pier scour elevation APRIL 2011 Page 45 Computing Pier Scour Using Method 2 Assume contraction scour does occur Compute pier scour following the procedure outlined below Set the initial bed elevation equal to the contracted channel bed elevation Set the initial flow depth y equal to the distance between the water surface and the contracted channel bed elevation Select the initial flow velocity V for Method 2 taking into account the effect of the contracted scour Vl method 2 V1 method 1 y1 yl ys where ys contracted scour depth Proceed to Option 1 Option for Method 2 Option 1 computes local scour for the pier stem only Fill in the required information including the initial flow depth y and the flow velocity V as discussed above Use the contracted scour bed elevation as the initial bed elevation Click the run button and note the scour depth computed by Option 1 Subtract this depth from the initial contraction scour bed elevation to obtain the pier scour elevation e If the pier scour elevation is less than the elevation of the top of the footing pile cap use this value for the pier scour e If the scour computed by Option 1 is deeper than the top of the footing pile cap continue on to Option 2 below Note that ys pier Y2 Y1 Option 2 for Method 2 l Fill in the information for the footing p
77. or bridges crossing wetlands and swamps with characteristics similar to those presented in Table 1 for a non tidal Coastal Plain The USGS envelope curve depicted above is an empirical method which reports the results of their field investigation of the wetland areas 1n the South Carolina Non tidal Coastal Zone The method should be viewed as a tool to assist the engineer in applying engineering judgment There is a prescribed method for applying the clear water abutment scour envelope curves See the report section Guidance for assessing abutment scour depth using the envelop curves on page 91 of Benedict 2003 In order to properly apply the curves it 1s important that the engineer develop some understanding of the data and its limitations To do this the engineer should become familiar with the content of the USGS reports For the application of clear water abutment scour envelope curves the engineer should refer to Benedict 2003 and for the clear water contraction scour envelope curves he should refer both Benedict 2003 and Benedict and Caldwell 2006 Both are available on line at the links below Benedict S T 2003 Clear water abutment and contraction scour in the Coastal Plain and Piedmont Provinces of South Carolina 1996 99 U S Geological Survey Water Resources Investigation Report 03 4064 137p http pubs usgs gov wri wn0340064 Benedict S T and Caldwell A W 2006 Development and Evaluation of Clear Water Pier an
78. or the left overbank is 169 feet The program makes a parabolic interpolates between the two scour values to compute the contraction scour flow depth in the left overbank as 12 0 ft In some cases the flow width under the bridge for one or more abutments may be less than the abutment setback When this occurs the program assumes that there 1s no water behind the abutment and the abutment scour is calculated as zero Consequently the extent of scour at the abutment is limited to the value of the contraction scour n general this case is more likely to be based on user error than on an actual field condition Total Bridge Scour at Abutment The abutment scour flow depth y2a at the abutment line 134 is computed by multiplying the adjusted contraction scour flow depth determined in line 122 by the kv and kf factors using the procedure explained in line 134 see also Equation 1 23 or 1 24 The computations for final abutment scour depth Line 139 is explained in Equation 1 28 and also by the accompanying notes on Line 139 Please note that SHA uses a minimum default abutment scour depth of 5 feet APRIL 2011 Page 31 COMMENTS ON THE ABSCOUR PROGRAM SCOUR CROSS SECTION e Program Sketches After running the program the user can click on the DRAW button on the Menu Bar at the top of the screen Three options are presented Approach Section Bridge Section and Scour Results We recommend careful inspection of each of these sketches
79. ow It is based only on the discharge and flow area of the overbank section For intermediate setbacks the velocity 1s computed by interpolating the velocity of the mixed flow at a setback distance of Sy from the channel bank with the velocity of separate flow at a setback distance of 0 75W In each case above the unit flow discharge under the bridge is computed by multiplying the velocity and flow depth q V yo For short setbacks very close to the channel banks and within the limits of the bank slope the flow depth is adjusted to reflect the actual location within the bank area Finally the scoured flow depth y2 used to define contraction scour is computed by using the appropriate scour equation Laursen s equations for live bed contraction scour or The user s choice of Laursen s equation or Neill s competent velocity equation to compute clear water contraction scour When the abutment has no setback is at the channel bank the scour at the overbank will be equal to that for channel When the setback is small the scour at the overbank will be very close to the scour in the channel However due to the idealization of channel and overbank flow into the rectangular shapes for the ABSCOUR cross section the calculated overbank scour may be based on clear water scour as determined from the APRIL 2011 Page 55 Approach Section calculations whereas it may be subject to live bed scour from the main channel Some tra
80. ow distribution for overtopping flow The Engineer needs to develop a rational flow distribution to account for the flow through the bridge and the flow over the bridge and approach roads A trial and error approach to the HEC RAS runs is often used to obtain a balanced flow condition e Approach section Selection of a cross section and of hydraulic flow parameters that are representative of the flow distribution in the approach section 1s essential to the scour evaluation See Step 3 below The guidance below provided in a step by step format is offered to assist the user in applying the ABSCOUR Program to a specific bridge site The user is referred to Part 1 for a discussion of definitions and the derivation of equations used for scour calculations Help Options There are two sources of help Short help is available for most input cells by placing the cursor on the cell and pressing the F 1 key More detailed help 1s available from the HELP tab on the Menu Bar at the top of the ABSCOUR screen It is a good 1dea to use the short help F 1 key to check the text and sketches for clarification of the information to be provided in the cell The following guidance provides for a step by step explanation of how to input information into the ABSCOUR 9 model An actual scour evaluation MD 313 over Marshy Hope Creek has been used to illustrate the process and to comment on the parameters selected A STEP ONE HYDRAULIC MODEL Prior to enter
81. rcise judgment to arrive at a practical solution to this problem A big advantage of the ABSCOUR program is the ease of checking the sensitivity of the scour estimate to the different input parameters Where there is a question about the value of the input parameter the recommended procedure is to input the best estimate of the value and then check the sensitivity of the scour depths for reasonable maximum and minimum values of the parameter IV UESTIONS TO ASK AND FACTORS TO CONSIDER IN REVIEWING THE ABSCOUR OUTPUT l Is the ABSCOUR model being used the most up to date version ABSCOUR 9 BUILD 2 1 Check for updates on the web at www gishydro umd edu Are the contraction scour and abutment scour values reasonable If not what are the likely sources of error in the input data that are creating what appears to be high or low scour values Have you checked the performance history of the original structure being replaced or of other nearby bridges What historical information 1s available on scour or on bridge failures during previous floods Does the hydrology study provide for reasonable estimates of flood magnitudes Follow the latest Maryland Hydrology Panel Recommendations Use of TR 20 by itself may overestimate the magnitude of flood discharges and corresponding scour depths Does the HEC RAS analysis provide reasonable values for flow distribution and energy slopes Are the approach section and bridge section reasonable
82. reas e Median Bed Grain Size D50 Determine the D50 median grain size for material on the overbank areas and in the channel from field samples taken at the approach APRIL 2011 Page 9 section Guidance on collecting samples and measuring D50 is provided in Appendix E of Chapter 11 Average Bank Slope Z Enter the average bank slope of the stream in the vicinity of the bridge The program uses this information in evaluating scour when the abutment is close to the channel bank The average bank slope Z of the left side of the channel is the horizontal projection of the slope when vertical is 1 The slope is used to adjust the ground line between the channel and the flood plain The adjustment modifies the idealized ABSCOUR rectangular sections in order to model a more reasonable geometry for the bank condition This adjustment provides for a better prediction of the abutment scour depth for abutments with short setbacks as explained in Attachment 1 The bank slope also determines the relative effect of the channel scour on scour at the abutment for abutments with short setbacks Steeper slopes such as 1 1 will reduce the effect of channel scour whereas flatter slopes such as 4 1 will increase the effect of channel scour The bank slope can be used as a variable in sensitivity analyses of factors affecting abutment scour See Contraction Scour Adjustment for Short Setback Abutment Case A e Average Energy Slope This value is used in c
83. representations of actual effective flow conditions during a major flood Do you need to modify the Approach Section or select a different section How reliable is your estimate of the tailwater elevation at the bridge Do you have a reasonable flow distribution model for overtopping flow at the bridge How accurate and complete are the soils data This 1s particularly important for clear water scour conditions Was the appropriate information obtained from the geomorphology report Do borings and subsurface investigations indicate the presence of rock Have you consulted a geologist if RQD values are less than 75 Is the rock erodible or scour resistant How does the rock affect the scour cross section under the bridge If the rock 1s erodible have you used Annandale s Erodibility index method or other methods to assess the extent to which it will scour If the bed conditions indicate cohesive soils have you selected a critical velocity for cohesive soils to compute clear water scour APRIL 2011 Page 38 7 Have you made sensitivity analyses to evaluate the field conditions you are modeling For example a live bed vs clear water scour b Maryland SHA modifications to Neill s curves vs Laursen s curves for clear water scour etc V COMPUTATION OF PIER SCOUR A Pier Scour Introduction The computational method in the Pier Local Scour Module of ABSCOUR 9 is based on the research reported by the FHWA in HEC 18 Evaluating Scour at Bridg
84. scour depth is computed using the equations presented in Part 1 Right Bank ft K etr RON Q 600 cfs Q 1600 cfs Q3 1200 cfs 20 a Approach Section 0 75 W 15 B Yo 50 7 Loi LE L11 20 L 20 b Bridge Cross Section E 60 1 Case D BET c Bridge Cross Section for Narrow Overbank Figure A1 1 Cross Sections of Approach Flow and Under Bridge APRIL 2011 Page 62 Right Overbank Case A Case D Narrow Overbank Right Overbank for Different Abutment Setbacks APRIL 2011 Page 63 ATTACHMENT 2 COMPLEX APPROACH FLOW CONDITIONS The ABSCOUR Program computations are based on rectangular sections for the channel and overbank areas in the approach section and the bridge section with a straight channel reach between the sections However the user has considerable flexibility in assigning input values on the ABSCOUR menu cards so that the program can be used to model much more complex flow patterns Examples of these flow patterns might include abridge on a bend in the channel 2 large overtopping flows on one or both approach roads 3 the confluence of a tributary stream just upstream of the bridge and 4 combinations of the above conditions 5 Please note that any changes to a HEC RAS model should be made solely for the purpose of sensitivity analysis in assessing scour A deeper scour elevation may be approved based on the sensitivity analysis where justified I
85. scussion on abutment scour in the FHWA HEC 18 Manual Reference 1 explains why the early abutment scour equations developed from laboratory flume studies are generally not reliable for predicting scour at abutments The essence of this discussion is that a rectangular flume with a constant depth and velocity of flow across the width of the flume does not accurately model the field conditions of a channel and its flood plain consequently the equations developed from these lab studies generally predict conservative estimates of scour In the last several years various researchers have begun to model compound channels to reproduce more accurately the field conditions of a channel and its flood plain Information from these studies has been used to develop the ABSCOUR software program The background on the development of the logic and the equations used in the ABSCOUR analysis is presented in Part 1 of this Appendix The Engineer is encouraged to read and understand this information as well as the information in Part 2 Users Guide before using the ABSCOUR computer program In addition to calibrating the ABSCOUR 9 methodology with information obtained from flume studies conducted by the FHWA ABSCOUR 9 was calibrated using information from the USGS database of abutment scour measurements of bridges in South Carolina See the discussion in Part 1 of this Appendix The ABSCOUR program is an expanded application of Dr Emmett Laursen s live bed con
86. section on the basis of conveyance calculations Modify these values as necessary to fit the ABSCOUR cross sections as discussed above Verify that values used for y depth V velocity T top width q discharge per foot of width and Q discharge are consistent q V y Q q T As a general rule information on each channel and overbank subsection is readily available from the output tables of the water surface profile model For example HEC RAS computes the area of each subsection as the top width times the hydraulic depth With a known area hydraulic depth and discharge provided for each subsection of the approach cross section the user can readily obtain the velocity and unit discharge values needed for the program 3 Approach Section Water Surface Elevation This elevation is used as a datum for importing the HEC RAS cross section for the approach section It is a good idea to compare the ABSCOUR and HEC RAS cross sections e Discharge Q Enter the approach section discharge for the left overbank channel and right overbank in cfs or cms e Flow Top Width W From HEC RAS obtain the flow top width for the left overbank channel and right overbank Be careful not to include ineffective areas in the top width computations e Average Flow Depth Hydraulic Depth From HEC RAS obtain the hydraulic depth for the left overbank channel and right overbank Be careful to adjust the hydraulic depth to account for any ineffective flow a
87. sensitivity analyses of factors affecting abutment scour The contraction scour flow depth is modified as necessary to take into account the effect of any pressure scour and to apply a safety factor to the design Next the abutment scour flow depth y2a is computed directly from the interpolated contraction scour value y2a kf kv k2 contraction scour Abutment scour ysa y2a yo adj where yo adj flow depth before scour occurs The final or adjusted abutment scour value ysa ad 1s determined as ysa adj Kt Ke FS ysa Where Kt modification for abutment shape Ke modification for embankment skew FS factor of safety ysa initial abutment scour estimate noted above ysa y2 yo adj The logic presented above is based on the assumption that the overbank area is wide and that 0 75W gt 5yo A special case may exist for a narrow flood plain where 0 75W 5yo In this instance no intermediate zone exists and the interpolation scheme for the intermediate setback cannot be applied If the setback is equal or larger than 5yo the velocity and resulting contraction scour depth is computed assuming that the setback 1s equal to 5yo If the setback is smaller than 5yo the velocity and scour depth are computed the same as it would be for the short setback case Here are some example problems to illustrate the computation of flow velocity and APRIL 2011 Page 57 contraction scour for various setback d
88. ss sections to provide for a reliable estimate of the tailwater elevation Please note that the measurement is to be made at the downstream side of the bridge and on the inside of the bridge For pressure flow conditions enter the water surface elevation immediately downstream from the bridge The downstream water surface elevation serves as the datum for all ABSCOUR computations Show Scour Parameters Button This button provides a quick reference to scour terms when that are used in the program Waterway Area Measured normal to the flow Measure the waterway area bounded by the water surface elevation and the channel cross section for the right overbank section channel section and left overbank section Typically this information cannot be directly obtained from the HEC RAS Tables The bridge plans or the HEC RAS cross sections provide good information for use in measuring the waterway area Please note that for pressure flow conditions where the water elevation 1s above the low chord the top of the waterway area will be defined by the low chord Top Width W or T Measured normal to the flow Measure the top width for the channel and the right and left overbank areas under the bridge Judgment needs to be applied in obtaining this information In some cases the left and right overbank top widths may be very small and it may be more reasonable to model the channel so as to incorporate these small overbank areas as a part of the main cha
89. ssed earlier the HEC RAS program computes flow on the basis of conveyance For complex rapidly changing conditions upstream of the bridge conveyance calculations may not represent the worst case scour conditions APRIL 2011 Page 64 Four examples are presented below to discuss the evaluation of the HEC RAS flow distribution and to suggest approaches to use in arriving at the worst case scour condition as a part of the sensitivity assessment of the scour calculations I Example 1 Typical Flow Distribution FROM HECRAS APPROACH FLOW OVERTOPPINC FLOW FLOW UNDER BRIDGE 250 all numbers are in cfs Figure A2 1 Flow re distribution example Example presents information obtained from HEC RAS for a straight reach depicting the flow distribution at the approach and bridge sections In the HEC RAS model overtopping flow is subtracted from the approach flow to compute the flow through the bridge This appears to be a reasonable flow distribution at the bridge to use in the ABSCOUR computations APRIL 2011 Page 65 II Example 2 Unbalanced Flow Condition The sketch on the left depicts discharge values obtained from HEC RAS for the approach and bridge sections Note that there is 300 cfs at the approach on the left overbank section looking downstream and 400 cfs of overtopping flow at the left bridge section HEC RAS distributes the flow under the bridge according to conveyance and may underestimate the flow at t
90. straight channel The occurrence of a bend would affect the flow distribution in the reach of the stream under study Refer to the discussion included under Upstream Bridge Data for ideas on how to modify flow distributions to account for 2 D flow patterns in the reach of the stream upstream of the bridge The ABSCOUR program uses Laursen s live bed contraction scour equation to determine scour This equation serves to compare the unit discharges and scour in the approach section and in the contracted bridge section assuming similar bed materials and hydraulic conditions The best results will be obtained by selecting an approach section where the flow patterns and bed conditions in the channel are similar to the bridge section keeping the following considerations in mind 1 The approach section should be in a relatively straight reach and be representative of the upstream channel and flood plain If the bridge is in a bend the approach section may be selected in an upstream bend with a similar configuration 2 The cross section should be perpendicular to the stream tube lines 3 The approach section should be near the bridge but far enough upstream when APRIL 2011 Page 7 practicable to be out of the influence of the bridge contraction 4 If upstream conditions are complex select the approach section one bridge length upstream and reevaluate the ineffective flow areas in the analysis Refer also to the discussion under Upstrea
91. t the beginning cross section station Left Bank Station and the ending cross section station Right Bank Station Additional cells are provided to input Manning n values for the channel and left and right flood plains The body of each table consists of 3 columns the designated point number its station and elevation The information in this table can be filled in manually or imported directly APRIL 2011 Page 22 from the appropriate HEC RAS model It is useful to run the example problem included with the ABSCOUR program to view the format for the data in a typical table Manual Input Input the data in the same manner as 1s depicted by the table for the example problem Import Cross section Data Use of the import function is recommended since it is much easier to do This function imports the actual cross section of the stream at the approach and at the bridge At the bridge the program will also import the bridge deck data from HECRAS Note only the geometry file of the last selected plan in HECRAS project will be used To import the approach section select the HECRAS project file in the open file dialog The program will read the current active plan of HECRAS project and generate a list of available cross sections The User can then choose the cross section of the desired approach section on the list The imported data includes the station and elevation of the ground point in the cross section and the left bank and right bank p
92. ta Card Typically such over ride uses might be considered as a part of the sensitivity analyses of the scour evaluation Use all over ride features with caution STEP FIVE UPSTREAM BRIDGE DATA Enter the Upstream Bridge Data Figure 2 10 shows the input screen for the upstream bridge data APRIL 2011 Page 16 Abutment N OOS OBDBDD HEH NS TAN srd3md313_100 asc File Run Draw Help Water surface elevation upstream side of bridge Urmi 7 00 section Looking Downstream Left Overbank Channel Right Cwverbank High chord elevation at upstream side of bridge frm Low chord elevation at upstream side af bridge tim Bed elevation at upstream side of bridge flim 94 79 2 IE Flow velocity at upstream face af bridge fasimpsi 0 88 5 35 04 E 130 55 From HEC RAS Diii Abutment shape factor trm wp Measure from ABSCOUR Section i AZ 5 180 Embankment skew angle degrees 125 Is future lateral movement af the channel expected to occur atthe bridge eresio Ma See F 1 Help Import HEC RAS Upstream Bridge Section Figure 2 10 Upstream Bridge Data Input Screen e Water surface elevation upstream of the structure The water surface elevation just upstream of the structure is determined from the water surface profile HEC RAS model The ABSCOUR program compares this elevation with the upstream bridge low chord or culvert crown elevations to determine whether pressure flow occurs If so a pressur
93. tain the joint alteration number Ja from Table 9 page 27 STEP 4 COMPUTE Js THE RELATIVE GROUND STRUCTURE NUMBER The information required to obtain Js is obtained from Table 10 the Relative Ground Structure Number Table page 29 The value of Js depends upon the appropriate selection of the following rock properties 6 Dip direction in direction of stream flow or dip direction against direction of stream flow degrees e Dip angle of closer spaced joint set degrees e Ratio of joint spacing r The SHA spread sheet provides the user with a convenient method to compute and compare the erodibility index and the stream power and to determine the extent to which the rock will scour for the given conditions The method allows the user to select an appropriate safety factor to be considered in applying the results of the evaluation The following guidance is provided for use in applying the computational method included in the Utility Module Use the following input menu cards APRIL 2011 Page 52 PROJECT DATA CARD Project description 2 Pier or Abutment Data HYDRAULIC DATA 1 Input the data described above in Stream Power Calculations 2 Input the desired safety factor ROCK DATA Input the data as described in the above section on computing the erodibility index for rock After inputting the above noted data click the run tab and then the output tab to obtain the scour report The program will compute the depth of scour in
94. ted 1n the ABSCOUR model by changing the HEC RAS flow distribution IV Example 4 Confluence Upstream of the Bridge There can be a great deal of uncertainty about the flow distribution at a bridge located just below the confluence of two streams The location of the confluence 1s likely to shift over time Further the time of concentration of the two streams is likely to vary affecting the quantity and distribution of flood flows A worst case type of scour analysis is recommended for this type of situation Consider using two or more flow distributions assuming 1 a worst case condition for the left abutment and then 2 a worst case condition for the right abutment APRIL 2011 Page 67 ATTACHMENT 3 SAFETY CALIBRATION FACTORS In developing the ABSCOUR equations for estimating abutment scour available information from laboratory studies collected by the consultant firm of GKY and Associates was used as a means of evaluating the model These laboratory tests were conducted in simple rectangular straight channels laboratory flumes with uniform flow A total of 126 data points were used to develop the envelop equation describing the value of the coefficient for the spiral flow adjustment factor kr These initial studies were augmented by a second set of flume studies conducted by the FHWA in 2004 Natural rivers are not accurately represented by the simple flow conditions modeled in a laboratory flume For practical design use of a safety
95. tic average flow velocity fps mps 8 Adjusted flaw depth tm i Froude Number 04458 Required Riprap D50 tim 4228 Compute Close New 7 Help B CRITICAL VELOCITY This 1s a handy tool for approximating the critical velocity of the soils in a channel bed given the D50 particle size and the flow depth Calculations are based on Neill s competent velocity curves Reference 11 Short Help F 1 key and Regular Help are available for this module A more accurate estimate can be made by using the modified Neill s curves presented later in this appendix Critical velocity Unit option English t Metric Sl Flow depth tfm I Median particle size D50 Urmi oot Critical velocity fpsimpst ERE C SCOUR IN ROCK The Utility Module provides a methodology for the computation of scour in rock entitled ROCK SCOUR However we currently recommend the use of the SHA Spread sheet in the Software Package of this manual for making the erodibility index computations The evaluation of the resistance of rock to scour requires the services of an engineer or geologist who has the specialized training to make such judgments The Rock Scour Module and the Erodibility Index Spreadsheet are based on the Erodibility APRIL 2011 Page 49 Index Method The Erodibility Index Method was developed by Dr George Annandale currently the President of Engineering and Hydrosystems Inc of Littleton Colorado The Office of Str
96. traction scour equation as presented in the FHWA HEC 18 Manual with certain APRIL 2011 Page 2 modifications developed to account for the distribution of flow under the bridge the bridge geometry and the computation of velocity at the bridge abutments The ABSCOUR program computes both clear water and live bed scour and selects the appropriate scour type based on the input information Careful application of the ABSCOUR Program will provide the user with insight into the factors affecting contraction and abutment scour at the bridge site under evaluation Judgment is needed to modify input information and the ABSCOUR cross sections so as to best represent actual site conditions during a flood event Computed scour depths provided by the ABSCOUR Program require evaluation to determine if the results are reasonable Abutment scour can be viewed as a combination of contraction scour and local scour The ABSCOUR Program computes the total scour at the abutment therefore the user should not add contraction scour to this value as is done in the HEC 18 scour evaluation procedure The following information 1s needed to provide the input information for the ABSCOUR program mA Hydr ologic estimates of Q100 Q500 Oovertopping and Odesign 2 topographic map of the stream and its flood plain the location of the bridge crossing and stream channel cross sections 3 information from the geomorphology report regarding estimated channel degradation
97. uctures recommends that the Erodibility Index Method be used as an additional resource by specialists who have the knowledge to apply the method Currently the Rock Scour Module in ABSCOUR 9 is not recommended for use The following overview provides background information on the Erodibility Index Method C 1 Application of the Erodibility Index Method The Erodibility Index Method involves the following steps 1 Calculation of the Erodibility Index of the rock based on its physical characteristics and orientation with respect to the flow direction of the water 2 Calculation of the stream power of the flow in the stream or river for the hydraulic conditions under investigation 3 Calculation of the modified stream power at a pier or abutment due to the effect of the obstruction on the flow These modified values are calculated by a series of equations developed in the FHWA Hydraulic Laboratory for different types of piers under different flow conditions The piers scour equations are recommended for design when used with caution and the application of engineering judgment The abutment scour equations should not be used for design The SHA has derived the abutment scour equations from the rectangular pier equations developed by the FHWA lab studies and there are no data at this time to assure that this approach is valid However these equations can be useful of in comparing the estimated scour in rock with the equivalent scour in sa
98. velocity as V3 V4 y1 y ys 7 2 Run the program for Option 3 and obtain the scour depth Compute the scour elevation as the elevation of the selected point one foot below the bottom of the footing pile cap step 1 above scour depth Step 3 Compare this scour elevation with the scour elevation determined from Method 1 Use the lower scour elevation as the total pier scour elevation APRIL 2011 Page 47 VI UTILITY MODULE A RIPRAP The Utility module provides a means of selecting the D50 size of riprap for abutments culverts and piers The computations for the riprap D50 size for piers and abutments use the procedures set forth in the 2001 edition of HEC 23 Use this information to select the appropriate riprap size typically Class 2 or 3 The computations for the D50 size for bottomless culverts are based on a cooperative FHW A Maryland SHA research study conducted in the FHWA Hydraulic Laboratory The process for using this module 19 the same as for the other modules previously discussed The various input cells are to be filled in then the COMPUTE button is clicked to make the calculation Culvert Riprap Design Unit option f English t Metric SI Specific gravity of the riprap rack 2 55 Bridge Section Looking Downstream Left Overbank Channal Right Overbank Average flow depth DIS face of bridge tim 10 10 Abutment type vertical Vertical Optional Input and program determine the characteristic f
99. was used to modify the recommended calibration factors in earlier versions of ABSCOUR In general lower factors are now recommended Please note that the current scour evaluation process described in Chapter 11 of the Manual recommends the calculation of the potential effect of channel movement and degradation This calculation serves to decrease the need for reliance on a safety factor to account for lateral channel movement and degradation Factors higher than the recommended values should be considered for complex flow conditions C STEP THREE APPROACH SECTION Figure 2 2 shows the input screen for the Approach Section In order to enter the data for this sheet the actual cross section must be converted to the ABSCOUR model cross section for the sub areas of the left overbank main channel and right overbank Refer to Figure 2 3 for a definition sketch of the conversion from the actual cross section to the ABSCOUR cross section The User has the option of superimposing the actual cross section on the ABSCOUR cross section for comparison purposes by using the importing function of ABSCOUR Represent each sub area as a rectangle having a width and average depth Obtain the top width T and flow area A of each sub area from the HEC RAS Program Be careful not to include ineffective flow areas Compute the average depth of flow or hydraulic depth for each sub area as Yave A T The model assumes an ideal one dimensional flow pattern with a
100. y short especially for overbank areas The conditions for clear water or live bed scour are not always clear cut and it 19 possible that both types of scour may occur during different stages of a flood hydrograph The user is encouraged to evaluate both cases As indicated above a limited flexibility has been built into the ABSCOUR program to allow the engineer to account for some of the above factors The engineer 19 encouraged to consider all information obtained from field and office studies the limitations of the scour model and to apply judgment in the selection of the appropriate foundation elements The user should consider the need for a calibration safety factor on the Bridge Data card consistent with the guidance in Attachment 3 of this Appendix which reflects the uncertainties of the scour parameters at the site and the importance of the bridge under design The ABSCOUR program requires accurate hydrologic hydraulic and soils data in order to compute accurate contraction and abutment scour depths The extent to which the Engineer can obtain accurate data will vary from site to site In some cases for example subsurface soils data it may not be practical to obtain a complete and accurate description of all the input parameters However the use of incomplete or inaccurate APRIL 2011 Page 37 input data may significantly affect the accuracy of the ABSCOUR output results of predicted scour depths The Engineer needs to exe

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