Prepared in cooperation with the South Carolina Department of Transportation
The South Carolina Bridge-Scour Envelope Curves
Scientific Investigations Report 2016–5121 Version 1.2, April 2021
U.S. Department of the Interior U.S. Geological Survey Cover. Little Pee Dee River at S.C. 917 in Horry County, South Carolina, April 20, 2007. Photograph by U.S. Geological Survey. The South Carolina Bridge-Scour Envelope Curves
By Stephen T. Benedict, Toby D. Feaster, and Andral W. Caldwell
Prepared in cooperation with the South Carolina Department of Transportation
Scientific Investigations Report 2016–5121 Version 1.2, April 2021
U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior RYAN K. ZINKE, Secretary U.S. Geological Survey William H. Werkheiser, Deputy Director exercising the authority of the Director
U.S. Geological Survey, Reston, Virginia First release: 2016 Revised: January 2018 (ver. 1.1) Revised: April 2021 (ver. 1.2)
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Suggested citation: Benedict, S.T., Feaster, T.D., and Caldwell, A.W., 2016, The South Carolina bridge-scour envelope curves (ver. 1.2, April 2021): U.S. Geological Survey Scientific Investigations Report 2016– 5121, 96 p., https://doi.org/10.3133/sir20165121.
ISSN 2328-0328 (online) iii
Contents
Abstract...... 1 Introduction...... 1 Purpose and Scope...... 2 Description of the Study Area...... 2 Previous Investigations...... 3 The South Carolina Pier-Scour Envelope Curves...... 4 Clear-Water Pier-Scour Envelope Curve...... 5 South Carolina Clear-Water Pier-Scour Field Data...... 5 Clear-Water Pier-Scour Envelope Curve...... 8 Live-Bed Pier-Scour Envelope Curve...... 9 South Carolina Live-Bed Pier-Scour Field Data ...... 9 Live-Bed Pier-Scour Envelope Curve...... 11 Comparison of the South Carolina Pier-Scour Envelope Curves With Other Data...... 13 The PSDb-2014 Database...... 13 South Carolina Clear-Water and Live-Bed Pier-Scour Field Data...... 13 Screened Laboratory and Field Data...... 16 Verification Data...... 16 Skewed Piers...... 16 The PSDb-2014 Pier-Scour Envelope Curve...... 17 Envelope Curve for the PSDb-2014 Screened Laboratory Pier-Scour Data...... 17 Extending the Envelope Curve With the PSDb-2014 Screened Field Data...... 17 Verifying the Envelope Curve With the PSDb-2014 Unscreened Field Data...... 19 Review of Field Data That Exceed the Envelope Curve...... 19 Flow Recurrence Intervals for Data Used in the PSDb-2014 Pier-Scour Envelope Curve...... 22 Comparison of the South Carolina and PSDb-2014 Pier-Scour Envelope Curves...... 22 Relative Increase in Theoretical Pier Scour Associated With the 100- to 500-Year Flows...... 24 Theoretical Pier-Scour Data...... 25 The 500-Year Flow Adjustment Coefficient...... 25 Application and Limitations of the PSDb-2014 and South Carolina Clear-Water and Live-Bed Pier-Scour Envelope Curves...... 26 Evaluating Scour Depth at Pier Widths Less Than or Equal to 6 Feet...... 26 Clear-Water Pier Scour on the Flood Plain...... 27 Live-Bed Pier Scour in the Main Channel...... 27 The PSDb-2014 Pier-Scour Envelope Curve...... 27 Evaluating Scour Depth at Pier Widths Greater Than 6 Feet...... 28 iv
The South Carolina Clear-Water Abutment-Scour Envelope Curves...... 28 South Carolina Clear-Water Abutment-Scour Field Data...... 30 Field Conditions That Influence Clear-Water Abutment-Scour Potential...... 31 Clear-Water Abutment-Scour Envelope Curves With Respect to Embankment Length...... 33 Piedmont Clear-Water Abutment-Scour Envelope Curve...... 33 Coastal Plain Clear-Water Abutment-Scour Envelope Curve...... 35 Clear-Water Abutment-Scour Envelope Curves With Respect to the Geometric-Contraction Ratio...... 35 Piedmont Clear-Water Abutment-Scour Envelope Curve...... 37 Coastal Plain Clear-Water Abutment-Scour Envelope Curve...... 37 The Modified Clear-Water Abutment-Scour Envelope Curves...... 37 Comparison of the South Carolina Clear-Water Abutment-Scour Envelope Curves With Other Data...... 41 Comparison of the Clear-Water Abutment-Scour Field Data to Laboratory Data...... 43 Discrepancies in the Upper Bound of the Clear-Water Abutment-Scour Laboratory and Field Data...... 43 Comparison of the South Carolina Abutment-Scour Envelope Curves to Other Abutment-Scour Field Data...... 45 Relative Increase in Theoretical Abutment Scour Associated With the 100- to 500-Year Flows...... 48 Theoretical Abutment-Scour Data...... 48 The 500-Year Flow Adjustment Coefficient...... 48 Application and Limitations of the South Carolina Clear-Water Abutment-Scour Envelope Curves...... 49 Selecting a Reference Surface...... 50 Estimating the Embankment Length and the Geometric-Contraction Ratio...... 50 Selecting the Appropriate Clear-Water Abutment-Scour Envelope Curve...... 50 Original Clear-Water Abutment-Scour Envelope Curves...... 50 Modified Clear-Water Abutment-Scour Envelope Curves...... 51 Single-Bridge Openings...... 51 Multiple-Bridge Crossings...... 51 Contraction and Pier Scour Within the Clear-Water Abutment-Scour Region...... 52 Top Width of Abutment-Scour Hole ...... 53 Limitations of the South Carolina Clear-Water Abutment-Scour Envelope Curves...... 53 The South Carolina Clear-Water Contraction-Scour Envelope Curve...... 55 South Carolina Clear-Water Contraction-Scour Field Data...... 55 Clear-Water Contraction-Scour Envelope Curve...... 56 Comparison of the South Carolina Clear-Water Contraction-Scour Envelope Curve With Other Data...... 58 Relative Increase in Theoretical Clear-Water Contraction Scour Associated With the 100- to 500-Year Flows...... 59 Theoretical Clear-Water Contraction-Scour Data...... 59 The 500-Year Flow Adjustment Coefficient...... 59 Application and Limitations of the South Carolina Clear-Water Contraction-Scour Envelope Curve...... 61 v
Selecting a Reference Surface for Clear-Water Contraction Scour...... 61 Pier Scour Within Clear-Water Contraction-Scour Areas...... 61 Estimate of Clear-Water Contraction-Scour Hole Location...... 62 The South Carolina Live-Bed Contraction-Scour Envelope Curves...... 62 South Carolina Live-Bed Contraction-Scour Field Data...... 62 Other Field Data...... 64 Live-Bed Contraction-Scour Envelope Curves...... 64 Dimensionless Live-Bed Contraction-Scour Envelope Curves...... 65 Field Envelope Curve for Live-Bed Contraction Scour...... 67 The Modified Live-Bed Contraction-Scour Envelope Curve...... 67 Relative Increase in Theoretical Live-Bed Contraction Scour Associated With the 100- to 500-Year Flows...... 72 Theoretical Live-Bed Contraction-Scour Data...... 72 The 500-Year Flow Adjustment Coefficient...... 73 Application and Limitations of the South Carolina Live-Bed Contraction-Scour Envelope Curves...... 74 Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases...... 75 Using the Bridge-Scour Databases for Site Comparisons ...... 75 Guidance for Applying the South Carolina Bridge-Scour Envelope Curves...... 77 Assessing Clear-Water Scour in the Bridge Overbank...... 77 Assessing Clear-Water Abutment-Scour Potential ...... 78 Assessing Clear-Water Contraction-Scour Potential ...... 80 Assessing Live-Bed Contraction-Scour Potential ...... 81 Assessing Pier-Scour Potential ...... 81 Limitations of the South Carolina Bridge-Scour Envelope Curves...... 82 Example Problems...... 82 Example 1: Coosawhatchie River at Road S-87 in Jasper County...... 83 Clear-Water Abutment-Scour Potential...... 83 Abutment Scour-Hole Top Width...... 84 Clear-Water Contraction-Scour Potential...... 84 Pier-Scour Potential...... 84 Clear-Water Abutment Region...... 84 Clear-Water Overbank Region...... 86 Example 2: Big Swamp at S.C. 51 in Florence County...... 86 Clear-Water Abutment-Scour Potential...... 86 Example 3: Enoree River at Road S-87 in Newberry County...... 89 Clear-Water Abutment-Scour Potential...... 89 Clear-Water Contraction-Scour Potential...... 91 Clear-Water Pier Scour for the Right Overbank...... 91 Live-Bed Contraction-Scour Potential...... 91 Live-Bed Pier-Scour Potential...... 91 Summary...... 92 Acknowledgments...... 93 Selected References...... 93 vi
Figures 1. Map showing location of physiographic provinces and bridge-scour study sites in South Carolina ...... 2 2. Illustration of scour at a cylindrical pier...... 4 3. Graph showing distribution of the ratio of the average flood plain velocity to the critical velocity of the median grain size for selected bridges in the Coastal Plain and Piedmont of South Carolina...... 5 4. Typical bridge cross section with well-defined main channel, showing areas of clear-water and live-bed scour...... 6 5. Typical bridge cross section for a swampy channel or flood plain relief bridge, showing area of clear-water scour ...... 6 6. Diagram showing definition of nominal and effective pier width ...... 7 7–20. Graphs showing— 7. The South Carolina clear-water pier-scour envelope curve ...... 8 8. Distribution of the ratio of the average main-channel velocity to the critical velocity of the median grain size for selected bridges in the Coastal Plain and Piedmont of South Carolina ...... 10 9. Example of ground-penetrating radar pier-scour profile at South Carolina Department of Transportation structure 364012100101 on S.C. Route 121 crossing the Saluda River in Newberry County, South Carolina ...... 11 10. The South Carolina live-bed pier-scour envelope curve...... 12 11. Relation of relative scour to the dimensionless variable for laboratory data used to develop the original HEC-18 pier-scour equation and clear-water and live-bed pier-scour data from selected sites in South Carolina ...... 14 12. Comparison of the South Carolina live-bed pier-scour envelope curve with the envelope curve developed from selected field data from the National Bridge Scour Database ...... 15 13. Relation of scour depth to pier width for the screened laboratory data from the PSDb-2014 database ...... 18 14. Relation of scour depth to pier width for the screened laboratory and field data from the PSDb-2014 database...... 18 15. Relation of scour depth to pier width for the screened laboratory and field data and verification data from the PSDb-2014 database...... 19 16. Relation of scour depth to pier width for the screened laboratory and field data and verification data from the PSDb-2014 database ...... 22 17. The PSDb-2014 pier-scour envelope curve with field data grouped by recurrence interval ...... 23 18. Comparison of the South Carolina clear-water and the PSDb-2014 pier-scour envelope curves ...... 23 19. Comparison of the South Carolina live-bed and the PSDb-2014 pier-scour envelope curves ...... 24 20. Relation of the 500- and 100-year flow, theoretical pier-scour depths for selected data from level-2 bridge-scour studies in South Carolina and Missouri ...... 26 vii
21. Block diagram showing flow around a bridge abutment and embankment and resulting scour ...... 28 22. Diagram showing plan view of simplified flow field at a bridge contraction ...... 29 23. Diagram showing plan view of regions of clear-water abutment and contraction scour ...... 29 24. Diagram showing example of single scour hole at shorter bridges, as shown at structure 212030100100 on U.S. Route 301 crossing Douglas Swamp in Florence County, South Carolina, July 31, 1996 ...... 32 25. Diagram showing example of separate left and right abutment-scour holes at longer bridges in the Coastal Plain, as shown at structure 277008700100 on Road S-87 crossing the Coosawhatchie River in Jasper County, South Carolina, November 12, 1997 ...... 32 26. Graph showing the South Carolina Piedmont clear-water abutment-scour envelope curve with respect to the embankment length blocking flow ...... 33 27. Graph showing the South Carolina Coastal Plain clear-water abutment-scour envelope curve with respect to the embankment length blocking flow ...... 34 28. Diagram showing definition of the embankment length blocking flow determined by projection of the bridge cross section onto the upstream approach cross section ...... 34 29. Diagram showing definition of the geometric-contraction ratio ...... 35 30–43. Graphs showing— 30. The South Carolina Piedmont clear-water abutment-scour envelope curve with respect to the geometric-contraction ratio ...... 36 31. The South Carolina Coastal Plain clear-water abutment-scour envelope curve with respect to the geometric-contraction ratio ...... 36 32. Relation of clear-water abutment-scour depth to the geometric- contraction ratio for field data associated with selected embankment lengths used to develop the South Carolina modified abutment-scour envelope curves ...... 39 33. Relation of clear-water abutment-scour depth to the geometric- contraction ratio for selected categories of embankment lengths in the Piedmont of South Carolina ...... 40 34. Relation of clear-water abutment-scour depth to the geometric- contraction ratio for selected categories of embankment lengths in the Coastal Plain of South Carolina ...... 40 35. Relation of the relative abutment-scour depth to relative abutment length for selected laboratory data...... 44 36. Relation of the relative abutment-scour depth to relative abutment length for selected laboratory and field data ...... 44 37. Comparison of the South Carolina abutment-scour envelope curves for geometric-contraction ratio to field data from other sources ...... 45 38. Comparison of the South Carolina abutment-scour envelope curves for embankment length to field data from other sources...... 46 39. Comparison of the South Carolina abutment-scour envelope curves for embankment length to field data from other sources ...... 46 viii
40. Relation of abutment-scour depth to drainage area for selected field data ...... 47 41. Relation of the 100- and 500-year flow, theoretical abutment-scour depths for selected data from level-2 bridge-scour studies in South Carolina and Missouri...... 49 42. Relation of abutment scour-hole top width and abutment-scour depth at bridges greater than 240 feet in length and swampy and flood plain relief bridges, 240 feet or less in length, in the Piedmont and Coastal Plain of South Carolina ...... 54 43. The South Carolina clear-water contraction-scour envelope curve for the Piedmont and Coastal Plain ...... 56 44. Diagram showing example of clear-water abutment- and contraction-scour areas at structure 274000300200 on S.C. Route 3 crossing Cypress Creek in Jasper County ...... 57 45–54. Graphs showing— 45. Comparison of the South Carolina clear-water contraction-scour envelope curve with field data from other sources ...... 59 46. Relation of the 100- and 500-year flow, theoretical clear-water contraction-scour depths for selected data from level-2 bridge-scour studies in South Carolina and Missouri ...... 60 47. Example of ground-penetrating radar longitudinal profile at structure 262050110100 on U.S. Route 501 crossing the Little Pee Dee River in Horry County, South Carolina ...... 63 48. The South Carolina dimensionless live-bed contraction-scour envelope curve for live-bed and clear-water contraction-scour data, and live-bed contraction-scour data only ...... 66 49. The South Carolina live-bed contraction-scour envelope curve for live-bed contraction-scour data, and live-bed and clear-water contraction-scour data ...... 68 50. The South Carolina live-bed contraction-scour envelope curve for the “worst case scour” for the live-bed contraction-scour data ...... 69 51. Relation of contraction-scour depth to drainage area for selected field data ...... 69 52. Relation of live-bed contraction-scour depth to the geometric-contraction ratio for selected field data for drainage areas 100 square miles or less, and drainage areas 200 square miles or less...... 71 53. Relation of live-bed contraction-scour depth to the geometric-contraction ratio for selected categories of drainage area for selected data in the Piedmont and Coastal Plain of South Carolina...... 72 54. Relation of the 100- and 500-year flow, theoretical live-bed contraction- scour depths for selected data from level-2 bridge-scour studies in South Carolina and Missouri ...... 73 ix
55–66. Diagrams showing— 55. Bridge site information for example 1, Coosawhatchie River at Road S-87 in Jasper County, South Carolina...... 83 56. Approach section information for example 1, Coosawhatchie River at Road S-87 in Jasper County, South Carolina, showing bridge top width and road embankment locations projected onto the approach cross section ...... 84 57. Region of potential scour determined from South Carolina bridge-scour envelope curves for example 1, Coosawhatchie River at Road S-87 in Jasper County, South Carolina ...... 85 58. Region of potential scour determined from the South Carolina bridge-scour envelope curves for example 1, Coosawhatchie River at Road S-87 in Jasper County, South Carolina, with the 500-year flow adjustment coefficient applied ...... 85 59. Bridge site information for example 2, Big Swamp at S.C. 51 in Florence County, South Carolina ...... 86 60. Approach section information for example 2, Big Swamp at S.C. 51 in Florence County, South Carolina, showing bridge top width and road embankment locations projected onto the approach cross section ...... 87 61. Region of potential scour determined from the South Carolina bridge-scour envelope curves for example 2, Big Swamp at S.C. 51 in Florence County, South Carolina ...... 88 62. Region of potential scour determined from the South Carolina bridge-scour envelope curves for example 2, Big Swamp at S.C. 51 in Florence County, South Carolina, with the 500-year flow adjustment coefficient applied ...... 88 63. Bridge site information for example 3, Enoree River at Road S-87 in Newberry County, South Carolina ...... 89 64. Approach section information for example 3, Enoree River at Road S-87 in Newberry County, South Carolina, showing bridge top width and road embankment locations projected onto the approach cross section...... 90 65. Region of potential scour determined from South Carolina bridge-scour envelope curves for example 3, Enoree River at Road S-87 in Newberry County, South Carolina ...... 90 66. Region of potential scour determined from the South Carolina bridge-scour envelope curves for example 3, Enoree River at Road S-87 in Newberry County, South Carolina, with the 500-year flow adjustment coefficient applied...... 92 x
Tables
1. Range of selected characteristics of clear-water pier scour field data collected in the flood plains of selected sites in South Carolina ...... 7 2. Range of selected characteristics of live-bed pier scour field data collected in the main channels of selected sites in South Carolina...... 10 3. Range of selected characteristics for the laboratory measurements of pier scour compiled in the PSDb-2014...... 15 4. Range of selected characteristics for the field measurements of pier scour compiled in the PSDb-2014...... 16 5. Review of selected data points exceeding the PSDb-2014 pier-scour envelope curve...... 20 6. Descriptive statistics for the ratio of the 500- and 100-year flow, theoretical pier-scour depths used in the regression analysis for selected data from South Carolina and Missouri...... 25 . 7 Range of selected characteristics of clear-water abutment scour field data in the flood plains of selected sites in South Carolina...... 30 8. Range of selected site characteristics for field measurements of abutment scour used to develop the modified clear-water abutment-scour envelope curves...... 38 9. Equations for the modified clear-water abutment-scour envelope curves in the Piedmont and Coastal Plain of South Carolina...... 38 10. Range of characteristics for selected laboratory abutment-scour data...... 41 11. Range of characteristics for selected abutment-scour measurements from Maine, Alabama, the National Bridge Scour Database, and the Missouri River Interstate–70 Bridge...... 42 12. Descriptive statistics for the ratio of the 500- and 100-year flow, theoretical abutment-scour depths used in the regression analysis for selected theoretical data from South Carolina and Missouri...... 48 13. Range of selected characteristics of clear-water contraction scour field data collected in the flood plains of selected sites in South Carolina...... 55 14. Range of selected characteristics of clear-water contraction scour field data used for comparison with the South Carolina clear-water contraction-scour data...... 58 15. Descriptive statistics for the ratio of the 500- and 100-year flow, theoretical clear-water contraction-scour depths used in the regression analysis for selected theoretical data from South Carolina and Missouri...... 60 16. Range of selected characteristics of live-bed contraction scour field data collected in the main channels of selected sites in South Carolina...... 62 17. Range of selected characteristics for field measurements of contraction scour used to help confirm the patterns of the South Carolina live-bed contraction-scour data...... 64 18. Range of selected site characteristics for field measurements of contraction scour used to develop the modified live-bed contraction-scour envelope curves ...... 70 19. Equations used to develop the modified live-bed contraction-scour envelope curve in the Piedmont and Coastal Plain of South Carolina...... 72 xi
20. Descriptive statistics for the ratio of the 500- and 100-year flow, theoretical live-bed contraction-scour depths used in the regression analysis for selected theoretical data from South Carolina and Missouri...... 73 21. Internet addresses for the South Carolina bridge-scour databases...... 76 22. Example of comparison scour data obtained from the South Carolina bridge- scour databases for sites in Spartanburg County, South Carolina...... 76
Digital Data in Excel Format
[Available for downloading from https://doi.org/10.3133/sir20165121] Appendix 1. South Carolina bridge-scour study sites and reference numbers for figure 1. Appendix 2. Estimate of maximum historic flows at selected bridge crossings in South Carolina.
Conversion Factors
U.S. customary units to International System of Units Multiply By To obtain Length inch (in.) 25.4 millimeter (mm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) Area square foot (ft2) 0.09290 square meter (m2) square mile (mi2) 2.590 square kilometer (km2) Volume cubic foot (ft3) 0.02832 cubic meter (m3) Flow rate cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)
Datums
Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29) or North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1927 or 1983 (NAD 27 or NAD 83, respectively). Elevation, as used in this report, refers to distance above the vertical datum or to distance above an arbitrary datum. xii
Abbreviations
FHWA Federal Highway Administration GPR ground-penetrating radar HEC-18 Hydraulic Engineering Circular No. 18 NBSD National Bridge Scour Database PSDb-2014 2014 USGS Pier Scour Database SCDOT South Carolina Department of Transportation SCLBSD South Carolina Live-Bed Scour Database SCPCSD South Carolina Clear-Water Pier- and Contraction-Scour Database USGS U.S. Geological Survey WSPRO Water-Surface Profile model The South Carolina Bridge-Scour Envelope Curves
By Stephen T. Benedict, Toby D. Feaster, and Andral W. Caldwell
Abstract Highway Administration (FHWA) Hydraulic Engineering Circular No. 18 (HEC-18 [fourth edition]; Richardson and The U.S. Geological Survey, in cooperation with the Davis, 2001), and (3) develop supplementary tools derived South Carolina Department of Transportation, conducted from the field data to help assess scour potential in the a series of three field investigations to evaluate historical, Piedmont and Coastal Plain regions of South Carolina. riverine bridge scour in the Piedmont and Coastal Plain The results from these previous investigations showed regions of South Carolina. These investigations included data that the HEC-18 scour-prediction equations, in general, collected at 231 riverine bridges, which lead to the develop- overpredicted scour depths and were at times excessive. In ment of bridge-scour envelope curves for clear-water and some cases, however, substantial underprediction occurred, live-bed components of scour. The application and limitations indicating that the equations could not be relied upon to of the South Carolina bridge-scour envelope curves were consistently give conservative and reasonable estimates of documented in four reports, each report addressing selected scour. Although the HEC-18 equations provide a valuable components of bridge scour. The current investigation resource for assessing scour, the trends in the analysis (2016) synthesizes the findings of these previous reports into highlighted the need for engineering judgment to determine a guidance manual providing an integrated procedure for if predicted scour is reasonable. To assist engineers in applying the envelope curves. Additionally, the investigation developing and applying such judgment, the data collected provides limited verification for selected bridge-scour from the South Carolina field investigations were organized envelope curves by comparing them to field data collected into regional bridge-scour envelope curves that displayed outside of South Carolina from previously published sources. the range and trend for the upper bound of observed scour Although the bridge-scour envelope curves have limitations, for each scour component, including clear-water abutment, they are useful supplementary tools for assessing the potential contraction, and pier scour, as well as live-bed contraction for scour at riverine bridges in South Carolina. and pier scour. The application and limitations of the South Carolina bridge-scour envelope curves were documented in four reports (Benedict, 2003; Benedict and Caldwell, 2006, 2009, 2012), with each report addressing different components Introduction of bridge scour. A primary objective of the current investiga- tion (2016) was to synthesize the findings of these previously The U.S. Geological Survey (USGS), in cooperation with published reports into a single guidance manual providing an the South Carolina Department of Transportation (SCDOT), integrated procedure for applying the South Carolina bridge- investigated historical scour at 231 bridges in the Piedmont scour envelope curves. In addition to the guidance manual, a and Coastal Plain Physiographic Provinces of South Carolina companion spreadsheet was developed to facilitate application (Benedict, 2003; Benedict and Caldwell, 2006, 2009, 2012). of this integrated procedure. Additional objectives of the (In this report, the Piedmont and Coastal Plain Physiographic investigation were to (1) evaluate the South Carolina bridge- Provinces will be referred to simply as Piedmont and Coastal scour envelope curves by comparing them to bridge-scour data Plain.) Historical-scour measurements represent the maximum outside of South Carolina from previously published sources, scour depth, for a given scour component, that has occurred (2) develop 500-year recurrence-interval flow bridge-scour over the life of a bridge at the time of the scour measurement. envelope-curve coefficients for each scour component A series of bridge-scour studies were conducted from 2003 to (defined later in the report), and (3) merge the three previously 2012, the general objectives of which were to (1) collect field published databases (Benedict, 2003; Benedict and Caldwell, measurements of historical abutment, contraction, and pier 2006, 2009) into a format that later could be incorporated into scour at sites that could be associated with major floods and an integrated Web-based GIS application such as StreamStats (or) older bridges, (2) use the field data to assess the perfor- (Ries and others, 2008) to provide a user-friendly format for mance of the scour-prediction equations listed in the Federal accessing the data. 2 The South Carolina Bridge-Scour Envelope Curves
Purpose and Scope Description of the Study Area
The purpose of this report is to describe (1) the South Carolina encompasses an area of about previously developed bridge-scour envelope curves and their 31,100 square miles (mi2) and is divided into three physio- evaluation with data from other sources, (2) the development graphic provinces—the Blue Ridge, Piedmont, and Coastal of the 500-year flow bridge-scour envelope-curve coefficients, Plain (fig. 1). The Coastal Plain can be further divided into (3) guidance for applying the bridge-scour envelope curves, upper and lower regions (Bloxham, 1976). The study area and (4) the companion spreadsheet for applying the South for this investigation includes most of South Carolina but Carolina bridge-scour envelope curves, which is available for generally excludes the Blue Ridge and the tidally influenced download at http://dx.doi.org/10.3133/sir20165121. area of the lower Coastal Plain.
82 81
GREENVILLE 2 S 83 . 23 210 T y CHEROKEE 96 ge SPARTANBURG YORK 35 35 Enor r PICKENS e 18 98 R e 209
e 191 e 189 R 211 80 d i 20 y v 208 Greenville 188 e
North Carolina r OCONEE 1 9 18 R UNION 186 185 30 S i 36 v 9 R 25 33 1 2 a e i 198 35 161 13 r v 2 l e u r 28 1 1 d 190 19 195 LANCASTER a 1 1 CHESTER 1 0 1 0 133 200 29 3 CHESTERFIELD 95 201 L 199 32 31 26 y MARLBORO 196 131 n 9 19 c ANDERSON 136 B LAURENS 1 h 5 R r e I 135 o s P 59 V a 1 139 162 16 e E d 126 R 138 0 3 129 5 e 60
83 128 168 DILLON 9 13 166 2 KERSHAW DARLINGTON 61 163 130 W 132 58 1
13 NEWBERRY FAIRFIELD a 56 53 . t 62–63 e 6 1 3 R 16 169 r 150 159 e 92 ABBEVILLE 101 52 D e 1 9
12 e LEE 81 MARION e
e 165 R
D
i 90 e 100 v 55 160 119
2 e 8 e
1 2 89
GREENWOOD r 80 153 e FLORENCE P SALUDA 5 15 –158
R e 99 181 iv l 1 91 88 t 180 RICHLAND e t r i HORRY 3 9 113 1 8 L 3 83 183 192 r e Columbia R 8 151 115–118 iv 182 82 R R MCCORMICK 1 i 193 85 i v v e 1 5 e r 10 –108 C w N o r 38 a o LEXINGTON ng SUMTER rt ar 86–8 152 m h a S ee 109–112
c o F R 20 EDGEFIELD u o ive c t rk r h E 22 205 a d 69 is 0 203 11 W F 1 6 to 39 3 o CALHOUN 3 202 r 93 k R WILLIAMSBURG Georgia iver CLARENDON E d 21 AIKEN is Sante 5 to 1 5 e Ri R 206 9 ver iv 1 6 1 er GEORGETOWN 82 20 ORANGEBURG 1 3–1 BARNWELL 16-18
S 1 8 a lk e 66 h 6 E PLANATION S at BAMBERG 15 DORCHESTER BERKELEY a ch ie va 19 E n d Blue Ridge Physiographic Province n 6-8 i 6 ah sto R
iv 51 Piedmont Physiographic Province e R ALLENDALE r 65
C i 33 33 68 v
o 9–50 e Upper Coastal Plain Physiographic Province o 12 103 8 r s 2 a w 3– h Lower Coastal Plain Physiographic Province 10 11 a t c 105 h COLLETON 9 ie Fall Line R 10 R i 102 v 1 iv Charleston 12 Scour investigation site and bridge e e CHARLESTON r 106 r
reference number HAMPTON 125 120 122 123 121 JASPER 80 Atlantic Ocean 12 0 20 0 60 80 I ES BEAUFORT
0 20 0 60 80 KI ETERS
Base modified from U.S. Geological Survey 1:2,000,000-scale digital data, 19 2 Albers pro ection datum AD 2 central meridian 96 rotation –8.5
81
Figure 1. Location of physiographic provinces and bridge-scour study sites in South Carolina. (Refer to appendix 1, which is available for downloading from http://dx.doi.org/10.3133/sir20165121, to identify bridge with corresponding number.)
SC Bridge Scour Figure 1 Introduction 3
The Piedmont covers approximately 35 percent of South detailed analysis. In the second cooperative investigation of Carolina and lies between the Blue Ridge and Coastal Plain level-2 bridge scour (1992–95), detailed bridge-scour studies (fig. 1). Land-surface elevations range from about 400 feet (ft) of 293 bridges in South Carolina were conducted by using near the Fall Line, which is the name given to the boundary methods presented in HEC-18 (Richardson and others, 1991, between the Piedmont and upper Coastal Plain, to about 1,000 ft at 1993), and these evaluations were used to assess the vulner- the Blue Ridge boundary. The general topography includes rolling ability of a bridge to scour. The level-1 and level-2 bridge-scour hills, elongated ridges, and moderately deep to shallow valleys. studies gave a qualitative overview of scour, which helped form The drainage patterns are well developed with well-defined general concepts of the type, magnitude, and frequency of scour channels and densely vegetated flood plains. Streambed slopes throughout South Carolina. In addition, the level-2 bridge-scour in the Piedmont range from approximately 0.001 to 0.011 foot studies provided anecdotal evidence of the apparent discrepancy per foot (ft/ft) (Guimaraes and Bohman, 1992). The geology between the predicted and measured scour. These findings of the Piedmont generally consists of fractured crystalline rock indicated a need to better understand the trends of scour within overlain by moderately to poorly permeable silty-clay loams. the field, which lead to a series of three field investigations Alluvial deposits along the valley floors consist of clay, silt, and (1996–2009) of historical bridge scour in South Carolina sand, and form varying degrees of cohesive soils (Guimaraes and (Benedict, 2003; Benedict and Caldwell, 2006, 2009). Bohman, 1992). The stream channel sediments typically consist Historical bridge-scour data were collected at 231 bridges of sandy materials overlaying decomposed rock or bedrock. in South Carolina as part of the cooperative field investigations The upper Coastal Plain lies between the Piedmont and (fig. 1) and included 209 measurements (5 were excluded lower Coastal Plain, and covers approximately 20 percent of from the envelopes leaving 204 in the analysis) of clear-water the State (fig. 1). The general topography in the upper Coastal abutment scour, 179 measurements of clear-water pier scour, Plain consists of rounded hills with gradual slopes and land-surface 139 measurements of clear-water contraction scour, 151 elevations that range from less than 200 ft to more than 700 ft. measurements (10 were excluded from the envelopes leaving The geology consists primarily of sedimentary rocks composed 141 in the database) of live-bed pier scour, and 89 measure- of layers of sand, silt, clay, and gravel underlain by igneous rocks ments of live-bed contraction scour. It should be noted that (Zalants, 1990). A shallow surface layer of permeable sandy soils is figure 1 and appendix 1 show only a single location for common. Low-flow stream channels bounded by densely vegetated sites with twin bridges and do not include abandoned bridge flood plains characterize upper Coastal Plain streams, and the structures or railroad bridges near a highway bridge structure channel sediments typically consist of sandy materials overlying for which bridge-scour data may have been collected. The rock. Streambed slopes are moderate, ranging from approximately historical data represent the maximum scour depths at the 0.001 to 0.0038 ft/ft (Guimaraes and Bohman, 1992). time of the measurement that have occurred at the bridge The lower Coastal Plain covers about 43 percent of the since construction. The historical scour data were measured State (fig. 1). The topographic relief of the lower Coastal Plain is during low-flow conditions; therefore, the flood conditions less pronounced than that of the upper Coastal Plain, and land- that produced the measured scour were approximated with a surface elevations range from 0 ft at the coast to nearly 200 ft one-dimensional step-backwater model. The approximated at the boundary with the upper Coastal Plain. The geology of the hydraulic characteristics are less than ideal and may introduce lower Coastal Plain consists of loosely consolidated sedimentary uncertainty in the data analysis, but the large number of data rocks of sand, silt, clay, and gravel overlain by permeable provide a means for assessing general field trends of scour sandy soils (Zalants, 1991). As in the upper Coastal Plain, the in South Carolina. These data were used to evaluate the low-flow stream channels bounded by densely vegetated flood performance of the HEC-18 (Richardson and Davis, 2001) plains characterize the lower Coastal Plain streams, and the scour-prediction equations and to develop the South Carolina channel sediments typically consist of sandy materials overlying bridge-scour envelope curves. Additional details regarding sedimentary rock. Streambed slopes range from approximately these three field investigations can be found in the associated 0.0002 to 0.0038 ft/ft, and streamflow patterns are tidally reports (Benedict, 2003; Benedict and Caldwell, 2006, 2009). influenced near the coast (Guimaraes and Bohman, 1992). The sixth cooperative investigation of bridge scour in South Carolina involved the modification of several bridge-scour Previous Investigations envelope curves to include a family of curves that provide a refined assessment of the upper limit of observed historical scour The USGS, in cooperation with the SCDOT, has conducted depths in South Carolina, in particular for sites associated with six previous investigations of bridge scour in South Carolina. smaller drainage areas. The modified envelope curves included In the first investigation of level-1 bridge scour (1990–92), the clear-water abutment-scour and live-bed contraction-scour limited structural, hydraulic, geomorphic, and vegetative data envelope curves. Benedict and Caldwell (2012) referred to these were collected at 3,506 bridges and culverts in South Carolina, family of curves as “secondary envelope curves” to indicate their and observed- and potential-scour indexes were developed modification from the original curves. The phrase “secondary for each site (Hurley, 1996). These indexes, along with other envelope curve” also is used in this report for the same purpose. variables, were used by the SCDOT to identify bridges that A description of these envelope curves and the techniques used may be susceptible to scour and potentially require a more to develop them can be found in Benedict and Caldwell (2012). 4 The South Carolina Bridge-Scour Envelope Curves
The South Carolina Pier-Scour Numerous laboratory studies have been done to investigate the variables that influence pier scour, including Laursen and Envelope Curves Toch (1956), Neill (1964), National Cooperative Highway Research Program (1970), Breusers and others (1977), Breusers Local bridge scour is the erosion of streambed material and Raudkivi (1991), Melville and Coleman (2000), and from around flow obstructions, such as a pier. The mecha- Ettema and others (2011). These authors generally agree that nism that causes the erosion is the combined effect of flow the variables that influence pier scour include the velocity of acceleration and the resulting vortexes that are induced by the approaching flow, the depth of approaching flow, flow duration, obstructions (Arneson and others, 2012). In the case of piers, sediment characteristics, pier geometry, pier alignment with three principal flow features contribute to the development of flow, and pier width. Although the flow and sediment charac- scour: down flow at the face of the pier, the horseshoe vortex teristics at a pier have some influence on scour, the geometric at the bottom of the pier, and the wake vortexes downstream variable, pier width, has been noted by various investigators from the pier (Melville and Coleman, 2000; Arneson and to be the prominent variable that influences pier-scour depth others, 2012; fig. 2). The down flow acts like a vertical jet (Laursen and Toch, 1956; Melville and Coleman, 2000; Mueller eroding sediments at the pier face. The eroded sediments then and Wagner, 2005; Ettema and others, 2011; Arneson and are transported by the horseshoe vortex past the pier and into others, 2012). Because pier width is considered to be a strong the area of the wake vortexes. Melville and Coleman (2000) explanatory variable, many researchers have proposed simpli- describe the wake vortexes as vacuum cleaners that can erode fied pier-scour prediction equations that use pier width as the bed sediments downstream from the pier as well as continue only explanatory variable (Larras, 1963; Breusers, 1965; Neill, the downstream transportation of the sediments eroded by 1973; Norman, 1975; Breusers and Raudkivi, 1991; Ansari and the down flow. The interaction of these flow patterns creates Qadar, 1994; Melville and Coleman, 2000; Ettema and others, a scour hole that is located close to the pier base. Pier scour 2011; Arneson and others, 2012). An example of this type of typically is classified as clear-water or live-bed scour, which equation is the upper bound of pier scour recommended in indicates the sediment-transport conditions along the upstream HEC-18 (Arneson and others, 2012): bed during the scouring process, with clear-water scour conditions having no sediment transport and live-bed having y = 2.4b, for Froude number ≤0.8, and (1a) active transport. The prevailing sediment-transport conditions s will influence the rate at which pier scour develops; therefore, y = 3.0b, for Froude number >0.8, (1b) researchers typically have distinguished between live-bed s and clear-water pier scour in their investigations. Although it where is important to understand the differences between live-bed ys is the upper bound of potential pier-scour and clear-water pier scour, it should be noted that the scour depth, in feet, and processes are similar and that the maximum values of scour b is the pier width, in feet, with the pier depth associated with these types of pier scour also are similar aligned with the approaching flow. (Melville and Coleman, 2000; Arneson and others, 2012). Both types of pier scour were studied in the South Carolina Equation 1 and the equations associated with the aforemen- field investigations leading to the development of clear-water tioned investigations indicate that researchers have considered and live-bed pier-scour envelope curves (Benedict and pier-scour predictive equations that use pier width as the Caldwell, 2006, 2009, respectively). primary explanatory variable to be reasonable and useful for
Wake vortex
Horseshoe vortex
Figure 2. Illustration of scour at a cylindrical pier (from Arneson and others, 2012).
SC Bridge Scour Figure 2 The South Carolina Pier-Scour Envelope Curves 5 defining the potential maximum pier-scour depth. Following further promoted by the dense flood plain vegetation typical of this pattern, Benedict and Caldwell (2006, 2009) developed South Carolina streams that tends to limit sediment transport, field-derived envelope curves for clear-water and live-bed pier regardless of upstream flow velocities. Because sediment scour, using pier width as the primary explanatory variable. transport is limited, clear-water scour holes have minimal or An overview of the South Carolina pier-scour envelope curves no infill and, therefore, provide a good record of the maximum and a comparison with field data from other sources follow. historical scour that has occurred at a bridge since construc- tion. Additionally, the minimal infill of the scour holes allows for the direct measurement of the scour depth using standard Clear-Water Pier-Scour Envelope Curve surveying techniques. In South Carolina, clear-water scour typically occurs on the overbanks at a bridge (fig. 4) or, in Clear-water scour occurs at a bridge when upstream the case of a swamp or relief bridge, across the entire bridge approach flow velocities are smaller than the critical velocity opening (fig. 5). required to transport bed sediments into the area of scour (Arneson and others, 2012). Such conditions commonly occur on the flood plains of South Carolina streams as illustrated South Carolina Clear-Water Pier-Scour Field Data by Benedict and Caldwell (2006; fig. 3). From a sample of 116 bridges from the Coastal Plain and Piedmont regions of Benedict and Caldwell (2006) made 179 measurements South Carolina, the ratio of the 100-year recurrence interval1 of clear-water pier scour at selected sites in South Carolina, (also called the 1-percent annual exceedance probability) flood with 87 measurements in the Piedmont and 92 in the Coastal plain flow velocity to the sediment critical velocity shows that Plain (table 1). Bridge sites where clear-water pier-scour data 75 percent of the Piedmont flood plain sites are in clear-water were collected can be identified in appendix 1 and figure 1. conditions while 25 percent are in live-bed conditions. For Measurements of pier-scour depths made in both provinces the Coastal Plain sites, 96 percent of the flood plain sites are ranged from 0 to 8.0 ft and were assumed to represent the in clear-water conditions. Clear-water scour conditions are maximum historical clear-water pier-scour depth that has occurred at the bridge since construction. Nominal pier widths (the width at the frontal pier face with no adjustment for skew; 1The Federal Highway Administration’s manual on scour, HEC-18 (Arneson and others, 2012) uses the term “recurrence interval” rather than fig. 6) for these data ranged from 0.8 to 6 ft. Approximately “annual exceedance probability” to refer to flood frequency estimates.This 80 percent of the data were associated with pile bents having convention is used in this report. nominal pier widths of 1.5 ft or less. A grab sample of the
4 NOTE: Critical velocity was computed using the loose grain sediment equation in Richardson and Davis (2001). In contrast, most Piedmont soils are cohesive. Although the critical velocity equation indicates that live-bed conditions exist at y
t about 25 percent of the Piedmont sites, in reality these sites i
c are clear-water in nature because of soil cohesion. o 3 ve l
l a c i t i r c
o y t t i c o 2 Piedmont ve l Theoretical break between live-bed n i
a and clear-water scour od p l o l f Live-bed scour f o 1 o i t Clear-water scour a Coastal Plain R
0 0 20 40 60 80 100 Percentile Figure 3. Distribution of the ratio of the average flood plain velocity to the critical velocity of the median grain size for selected bridges in the Coastal Plain and Piedmont of South Carolina (from Benedict and Caldwell, 2006).
SC Bridge Scour Figure 3 6 The South Carolina Bridge-Scour Envelope Curves
Left overbank Main channel Right overbank
Bridge deck
Live-bed scour area Clear-water scour area Channel Clear-water scour area banks
Abutment toe Ground line Abutment toe
NOT TO SCALE
Figure 4. Typical bridge cross section with well-defined main channel, showing areas of clear-water and live-bed scour (from Benedict, 2003).
Swampy channel or flood plain relief bridge
Bridge deck
Clear-water scour area
Abutment toe Abutment toe Ground line
NOT TO SCALE
Figure 5. Typical bridge cross section for a swampy channel or flood plain relief bridge, showing area of clear- water scour (from Benedict, 2003).
SC Bridge Scour Figure 4
SC Bridge Scour Figure 5 The South Carolina Pier-Scour Envelope Curves 7
Table 1. Range of selected characteristics of clear-water pier scour field data collected in the flood plains of selected sites in South Carolina (Benedict and Caldwell, 2006).
[mi2, square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; mm, millimeter; <, less than]
aApproach Measured Drainage Channel Nominal aApproach Median Relative Range Pier skew flow pier-scour area slope pier width flow depth grain size scour depth value (degree) velocity depth (mi2) (ft/ft) (ft) (ft) (mm) (ft) (ft/s) (ft) South Carolina Piedmont (87 measurements) Minimum 10.7 0.00015 0.8 0.0 1.3 1.3b < 0.062 0.0 0.1 Median 81.5 0.0012 1.0 0.0 3.1 7.7b 0.105 0.85 0.7 Maximum 1,620c 0.0029 6.0 22.0 6.2 20.2b 0.99 8.0 2.0 South Carolina Coastal Plain (92 measurements) Minimum 26.3 0.00007 0.9 0.0 0.4 2.0 < 0.062 0.0 0.0 Median 586.0 0.0003 1.4 0.0 1.9 6.3 0.162 0.8 0.5 Maximum 13,000d 0.0009 5.0 20.0 5.4 17.3 0.556 1.8 1.3 aValues were estimated from a one-dimensional water-surface profile model. b This value was misreported in Benedict and Caldwell (2006), but is correctly shown here. cApproximately 94 percent of the study sites in the Piedmont have drainage areas less than 400 mi2. dApproximately 80 percent of the study sites in the Coastal Plain have drainage areas less than 1,420 mi2.
Plan view of pier
Plane normal to flow
L
Pier be Flow θ
b
E PLANATION L Is the pier length be Is the effective pier width which is the projected width normal to the approaching flow b Is the nominal pier width θ Is the pier skew angle with respect to approaching flow
Figure 6. Definition of nominal and effective pier width (modified from Sheppard and others, 2011).
SC Bridge Scour Figure 6 8 The South Carolina Bridge-Scour Envelope Curves flood plain surface sediment was obtained in the upstream models were developed for each site by using the one- flood plain at each site and was analyzed to estimate the dimensional step-backwater model, Water-Surface PROfile median grain size. Because sediment characteristics in the (WSPRO; Shearman, 1990; Arneson and Shearman, 1998). On field setting can vary substantially in the vertical and hori- the basis of the review of historic floods and the risk analysis, zontal direction, a grab sample taken at a point may not fully the 100-year flow was assumed to be an approximation of represent the sediment characteristics at a site. a common peak flow that may have occurred at all bridges, To provide some assurance that measured historical and this flow was used in the WSPRO model. The estimated scour reflected scour resulting from high flows, Benedict and hydraulic characteristics derived from the WSPRO model Caldwell (2006) collected pier-scour data at older bridges, should be viewed as approximate rather than measured data. having higher probabilities of the occurrence of large flows, For additional information on the South Carolina clear-water and bridges with known historical flows. On the basis of pier-scour field data, refer to Benedict and Caldwell (2009). risk analysis and a review of historical flood records, it was concluded that flows equal to or exceeding 70 percent of the Clear-Water Pier-Scour Envelope Curve 100-year flow likely occurred at approximately 80 percent of the bridges in the investigation, with approximately 40 percent The pier-scour data collected by Benedict and Caldwell of the bridges having known historical floods that equaled or (2006) were used to develop the South Carolina clear-water exceeded this flow value. The term “100-year flow” (or flood) pier-scour envelope curve (fig. 7) that reflects the upper is often used to describe a flood that has a 1 in 100 chance bound of clear-water pier scour with respect to the nominal of being equaled or exceeded in any given year and also is pier width (fig. 6) for the sampled bridges in South Carolina. referred to as the 1-percent annual exceedance probability Although the streams of the Coastal Plain and Piedmont flood (Holmes and Dinicola, 2010). Information on sites with have regional characteristics (table 1) that will likely influ- known historical floods can be found in appendix 2. The ence scour in some measure, Benedict and Caldwell (2006) South Carolina clear-water pier-scour depths were measured concluded that the data were insufficient to develop separate during low flows, and the flow conditions that produced the envelope curves for each region and, therefore, used a single scour are not known. To estimate the hydraulic characteristics envelope curve (fig. 7) encompassing data from both regions that may have produced the observed pier scour, numerical for assessing the upper bound of clear-water pier scour.
14 E PLANATION Piedmont data 12 Coastal Plain data Envelope for South Carolina field data b Nominal pier width, in feet ys Pier-scour depth, in feet 10
8
6 .5 0 .5b 1
Pier-scour depth, in feet y s 4
2
0 0 2 4 6 8 Pier width, in feet
Figure 7. The South Carolina clear-water pier-scour envelope curve (from Benedict and Caldwell, 2006).
SC Bridge Scour Figure 7 The South Carolina Pier-Scour Envelope Curves 9
The equation for the South Carolina clear-water pier- Live-Bed Pier-Scour Envelope Curve scour envelope curve is as follows: Live-bed scour occurs at a bridge when upstream
ys = 1.5b + 0.5, (2) approach flows transport bed sediments into the area of scour (Arneson and others, 2012). In South Carolina, live-bed where pier-scour typically occurs in the main channel of streams
ys is the upper bound of potential pier-scour (see figure 4 for the definition of main channel) as illustrated depth, in feet, and by Benedict and Caldwell (2009; fig. 8). For the 100-year flow, b is the nominal pier width (fig. 6), in feet, figure 8 shows the ratio of the approach channel flow velocity and is limited to a value of 6 or less. to the sediment critical velocity for a sample of 78 bridges from the Coastal Plain and Piedmont regions of South This equation can be applied to round- and square-shaped Carolina and demonstrates how live-bed scour processes piers with no skew to flow and to pile bents with moderate predominate the main channel. After a flood event, live-bed skews having spacings between piles of approximately 5 pile scour holes generally remain inundated and are partially widths or greater. Benedict and Caldwell (2006) originally or totally refilled with sediments. Therefore, to measure suggested that the “moderate skews” for pile bents be limited historical live-bed scour holes, the bathymetric surface of to approximately 20 degrees or less. However, to be consistent the hole, as well as the depth of sediment infill, must be with the guidance for the South Carolina live-bed pier-scour measured. One method that has been used successfully to envelope curve, as described later in the report, it is recom- concurrently measure the bathymetry and sediment infill of mended that application of equation 2 be limited to pile bents historical live-bed scour holes is ground-penetrating radar with skews of 15 degrees or less having spacings between (GPR) deployed by boat (Placzek and Haeni, 1995; Webb and piles of approximately 5 pile widths or greater. When skews others, 2000). The shallow (20 ft or less), freshwater, sandy are thought to influence scour (such as for long, solid piers, streams of South Carolina provide a favorable environment pile bents with skews exceeding 15 degrees, or pile bents with for the application of GPR, and Benedict and Caldwell (2009) relatively small spacing between piles), the skew-correction used this technique to measure live-bed pier and contraction coefficient, as defined in HEC-18 (Arneson and others, 2012), scour at selected bridges in South Carolina. An example of a can be applied to equation 2 as shown below: GPR measurement of pier scour is shown in figure 9, which displays the scour-hole bathymetric surface as well as the y = K (1.5b + 0.5), (3) s 2 sediment infill. where K2 is the HEC-18 (Arneson and others, 2012) correc- tion coefficient for skew, and other variables are as previously South Carolina Live-Bed Pier-Scour Field Data defined. The following equation can be used to defineK : 2 Benedict and Caldwell (2009) made 141 measurements K = (cosineθ + L/b sineθ)0.65, (4) of live-bed pier scour in the main channel of selected bridges 2 in South Carolina, with 42 measurements in the Piedmont and where 99 in the Coastal Plain (table 2). Bridge sites where live-bed θ is the skew angle of the major axis of pier-scour data were collected can be identified by referring to the pier with respect to approaching flow appendix 1 and figure 1. Measurements of pier-scour depths (fig. 6), in degrees; made in both provinces ranged from 1.7 to 16.9 ft and were assumed to represent the maximum historical live-bed pier- L is the pier length along the major axis scour depth that has occurred at the bridge since construction. (fig. 6), in feet; and other variables Nominal pier widths for these data ranged from 0.8 to 9 ft. are as previously defined. Approximately 50 percent of the data were associated with When the value of L/b in equation 4 exceeds 12, L/b should be pile bents having nominal pier widths of 1.6 ft or less. A grab set to a value of 12. Reference should be made to Arneson and sample of sediment from the channel-bed surface was obtained others (2012) for further guidance regarding the application in the upstream channel at each site and was analyzed to and limitations of equation 4. Additional details regarding estimate the median grain size. Because sediment character- the development of the South Carolina clear-water pier scour istics in the field setting can vary substantially in the vertical envelope curve can be found in Benedict and Caldwell (2006), and horizontal direction, a grab sample taken at a point may and reference to that report is encouraged for developing not fully represent the sediment characteristics at a site. a more thorough understanding of that envelope curve. Using an approach similar to the clear-water pier-scour Application and limitations of equation 3 are described in investigation, Benedict and Caldwell (2009) collected more detail in the section of the current report, “Application live-bed pier-scour data at older bridges, having higher and Limitations of the PSDb-2014 and South Carolina Clear- probabilities of the occurrence of large flows, and bridges Water and Live-Bed Pier-Scour Envelope Curves.” with known historical flows to provide some assurance that 10 The South Carolina Bridge-Scour Envelope Curves
4
3
Piedmont
2 Coastal Plain
Live-bed scour 1 Clear-water scour
R a t i o f appr ach chann e l flow ve ocit y crit ca vel cit Theoretical break between live-bed and clear-water scour
0 0 20 40 60 80 100 Percentile Figure 8. Distribution of the ratio of the average main-channel velocity to the critical velocity of the median grain size for selected bridges in the Coastal Plain and Piedmont of South Carolina (from Benedict and Caldwell, 2009).
Table 2. Range of selected characteristics of live-bed pier scour field data collected in the main channels of selected sites in South Carolina (Benedict and Caldwell, 2009).
[mi2, square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; mm, millimeter]
aApproach Measured Drainage Channel Nominal aApproach Median Relative Range Pier skew flow pier-scour area slope pier width flow depth grain size scour depth value (degree) velocity depth (mi2) (ft/ft) (ft) (ft) (mm) (ft) (ft/s) (ft) South Carolina Piedmont (42 measurements) Minimum 21.0 0.00015 0.8 0.0 1.7 6.8 0.5 2.1 0.6 Median 201.0 0.0007 4.0 8.0 7.1 19.3 1.0 4.6 1.6 Maximum 5,250b 0.0016 5.8 30.0 9.6 27.1 1.7 8.7 3.6 South Carolina Coastal Plain (99 measurements) Minimum 17.2 0.0001 0.9 0.0 0.8 2.8 0.24 1.7 0.5 Median 1,030.0 0.0003 1.5 0.0 4.2 15.6 0.64 3.8 1.8 Maximum 9,360c 0.002 9.0 30.0 8.8 50.8 1.7 16.9 3.7 aValues were estimated from a one-dimensional water-surface profile model. bApproximately 94 percent of the study sites in the Piedmont have drainage areas less than 760 mi2.
c 2 ApproximatelySC Bridge Scour Figure 8 80 percent of the study sites in the Coastal Plain have drainage areas less than 1,860 mi . The South Carolina Pier-Scour Envelope Curves 11
0 0 Figure 9. Example of ground- penetrating radar pier-scour 50 Flow profile at South Carolina Department of Transportation structure 364012100101 on 100 S.C. Route 121 crossing the Channel Saluda River in Newberry bed County, South Carolina. 150 Reference Centerline of 4-foot wide pier column surface Centerline of 4-foot wide pier column
200 10
250 Approximately 6 feet Depth, in feet of pier scour including
Time, nanoseconds in 2 feet of infill
300
Approximate bottom 350 of infill
400 20
450
0 12 24 36 48 60 72 Time, in seconds
measured historical scour reflected scour resulting from 100-year flow was assumed to be an approximation of the high flows. On the basis of risk analysis and a review of peak flow at sites without gage data. The estimated hydraulic historical flood records, it was concluded that flows equal to characteristics derived from the WSPRO model should or exceeding 70 percent of the 100-year flow likely occurred be viewed as approximate rather than measured data. For at approximately 96 percent of the bridges in the investigation, additional information on the South Carolina live-bed pier- with approximately 62 percent of the bridges having known scour field data, refer to Benedict and Caldwell (2009). historical floods that equaled or exceeded this flow value. Information on sites with known historical floods can be Live-Bed Pier-Scour Envelope Curve found in appendix 2. As with the clear-water pier-scour data, the South Carolina live-bed pier-scour depths were measured The pier-scour data collected by Benedict and Caldwell during low flows; therefore, the WSPRO (Shearman, 1990; (2009) were used to develop the South Carolina live-bed pier- Arneson and Shearman, 1998) model was used to estimate the scour envelope curve (fig. 10) that reflects the upper bound hydraulic characteristics that may have produced the observed of live-bed pier scour with respect to the nominal pier width pier scour. Many of the data-collection sites had gage data (fig. 6) for the sampled bridges in South Carolina. Because of at or near the bridge (appendix 2) and historical peak flows the differing regional characteristics in the Coastal Plain and based on the gage data were used in the WSPRO model. The Piedmont (table 2), the live-bed pier-scour data were initially
SC Bridge Scour Figure 9 12 The South Carolina Bridge-Scour Envelope Curves
14
E PLANATION Piedmont data 12 Coastal Plain data Envelope for South Carolina field data b Nominal pier width, in feet y Pier-scour depth, in feet 10 s
8 3.4 .1b 1 y s 6 Pier-scour depth, in feet 4
2
0 0 2 4 6 8 10 Pier width, in feet
Figure 10. The South Carolina live-bed pier-scour envelope curve (from Benedict and Caldwell, 2009).
analyzed for regional trends. However, Benedict and Caldwell flow and to pile bents with moderate skews (approximately (2009) concluded that the regional distinctions were insig- 15 degrees or less) and spacings between piles of approxi- nificant and combined the data for the analysis. The original mately 5 pile widths or greater. When skews are thought to equation for the South Carolina live-bed pier-scour envelope influence scour (such as for long, solid piers, severely skewed curve is as follows: pile bents, or pile bents with relatively small spacing between
piles), the skew-correction coefficient,K 2, as defined in
ys = 1.11b + 3.34, (5) HEC-18 (Arneson and others, 2012; eq. 4), can be applied to equation 6 as follows: where all variables are as previously defined. However, to be consistent with the significant figures of equation 2, equation 5 ys = K2 (1.1b + 3.4), (7) can be slightly modified by adjusting the equation constants to reflect one, rather than two, decimal place as represented in where all variables are as previously defined. Reference should equation 6: be made to Arneson and others (2012) for further guidance regarding the application and limitations of equation 4. Addi-
ys = 1.1b + 3.4, (6) tional details regarding the development of the South Carolina live-bed pier-scour envelope curve can be found in Benedict where all variables are as previously defined. This minor and Caldwell (2009), and reference to that report is encouraged modification produces negligible differences from the origi- for developing a more thorough understanding of that envelope nal equation over the range of applicable pier widths (6 ft curve. Application and limitations of equations 6 and 7 are or less) and is recommended for evaluating the upper bound described in more detail in the section of the current report, of live-bed pier scour in South Carolina. Equation 6 can be “Application and Limitations of the PSDb-2014 and South Car- applied to round- and square-shaped piers with no skew to olina Clear-Water and Live-Bed Pier-Scour Envelope Curves.”
SC Bridge Scour Figure 10 Comparison of the South Carolina Pier-Scour Envelope Curves With Other Data 13
Comparison of the South Carolina Pier- difficult to form strong conclusions regarding the validity of the envelope curve and its comparison with the South Carolina Scour Envelope Curves With Other Data pier-scour envelope curves. The data patterns in figure 12 indicate the need for further evaluation of the South Carolina Benedict and Caldwell (2006, 2009) made some limited live-bed pier-scour envelope curves with a larger set of data, comparisons of the South Carolina pier-scour data with which is one of the objectives of the current (2016) investi data from other sources, including selected laboratory and gation. To facilitate such an evaluation, Benedict and Caldwell field data. The comparison of the South Carolina field data (2014) conducted a literature review to identify potential with laboratory data is shown in figure 11. The laboratory sources of pier-scour data that could be compared with the measurements (no skewed piers) are those used to develop South Carolina pier-scour envelope curves. This effort led the original HEC-18 pier-scour equation (Richardson and to the development of the 2014 USGS Pier Scour Database others, 1991; laboratory data provided by J.S. Jones, Federal (PSDb-2014) that includes 2,427 field and laboratory measure- Highway Administration, written commun., October 2003). ments of both clear-water and live-bed pier scour (Benedict The dimensionless variables shown in the figure are based on and Caldwell, 2014). Selected data from this extensive a simplified form of the HEC-18 equation as represented by database were used to evaluate the upper bound of pier-scour the trend line through the laboratory data. The field measure- depth and were compared with the South Carolina clear-water ments in figure 11 represent selected data from South Carolina and live-bed pier-scour envelope curves. The following having no or minimal influence from pier skew, including report sections provide a description of (1) the PSDb-2014 clear-water (fig. 11A) and live-bed (fig. 11B) pier-scour database, (2) the development of the envelope curve defined data. The slope of the trend lines through the South Carolina with selected data from the PSDb-2014 database, and (3) the field data (fig. 11) are similar to that of the laboratory data, comparison of the PSDb-2014 envelope curve with the indicating that the trends of the field data are similar to those South Carolina pier-scour envelope curves. associated with the laboratory. In the case of the clear-water pier-scour field measurements (fig. A11 ), most of the data plot below the laboratory data thus causing the field data The PSDb-2014 Database trend line to be below the laboratory trend line. The smaller The PSDb-2014 (Benedict and Caldwell, 2014) consists relative scour depths associated with the clear-water field of 569 laboratory and 1,858 field measurements of clear-water data are likely caused by a combination of factors including and live-bed pier scour compiled from selected publications. (1) cohesive and (or) non-uniform flood plain sediments, The laboratory data (table 3) are measurements taken from (2) flood plain flow velocities that are substantially below the 17 previous investigations and originally compiled by Shep- sediment critical velocity (fig. 3), and (3) short flood-flow pard and others (2011). The field data (table 4) were compiled durations, all of which diminish the potential for scour. It from 32 previous publications and include measurements made is notable that some of the clear-water field data (fig. A11 ) in 23 states within the United States and six other countries. conform to the upper bound of the laboratory data, indicating The field data have a wide range of stream gradients, drainage that the South Carolina clear-water pier scour measurements areas, sediment sizes, flow depths, flow velocities, and pier are likely capturing the upper bound of scour. The trend line sizes, providing a large and diverse database for assessing the for the live-bed pier-scour field measurements (fig. B11 ) plots upper bound of pier scour. Several subsets of data within the close to the laboratory trend line, and the field data have an PSDb-2014 are pertinent to the evaluation of the upper bound approximate uniform scatter about the laboratory trend line. of pier scour and are described below. Additional information These patterns indicate that the live-bed field data have similar regarding the PSDb-2014 (Benedict and Caldwell, 2014) is characteristics to that of the laboratory data, confirming that available at http://pubs.usgs.gov/ds/0845/. the field data are reasonable. Some of the live-bed pier-scour measurements exceed the laboratory data, and Benedict and Caldwell (2009) note that this is likely caused by the larger South Carolina Clear-Water and Live-Bed measurement uncertainty associated with defining the scour- Pier-Scour Field Data hole infill by the interpretation of GPR data (fig. 9). Part of the analysis by Benedict and Caldwell (2009) Included in the PSDb-2014 are the clear-water and included a comparison of the South Carolina live-bed pier- live-bed pier-scour measurements made as part of the South scour envelope curve with the upper bound of pier scour on Carolina investigations (Benedict and Caldwell, 2006, 2009; the basis of 92 measurements selected from the National 335 measurements; tables 1 and 2). (Note: The PSDb-2014 Bridge Scour Database (NBSD; U.S. Geological Survey, includes 15 measurements purposefully excluded from the 2001) having similar sediment sizes to the South Carolina original analysis by Benedict and Caldwell (2006, 2009); data (fig. 12). With the exception of three data points, all of therefore, tables 1 and 2 contain only 320 measurements.) the data fall within, or near to, the South Carolina live-bed Data from the PSDb-2014 were used to evaluate the trends pier-scour envelope curve, providing some support for that of the South Carolina data; therefore, the South Carolina data curve. However, the data points that define the upper bound were excluded from the analysis of the upper bound of pier of the data are sparse (two data points; fig. 12), making it scour on the basis of the PSDb-2014 data. 14 The South Carolina Bridge-Scour Envelope Curves
10 A 3 0.215 y b s 2.0 Fr 2 y1 y1
1
0.1
0.01 0.0000001 0.000001 0.00001 0.001 0.01 0.1 1 10 3 2 (b/y1) Fr
10
B 3 0.215 y b s 2.0 Fr 2 y1 y1 Pier-scour depth divided by flow depth
1
0.1
0.01 0.000001 0.00001 0.0001 0.001 0.01 0.1 1 10 3 2 (b/y1) Fr E PLANATION Line for original HEC-18 equation b Pier width
Power trend line for South Carolina field data ys Pier-scour depth
Laboratory data for original HEC-18 equation y1 Approach flow depth (Richardson and others, 1991) Fr Approach Froude number South Carolina clear-water field data South Carolina live-bed field data
3 2 Figure 11. Relation of relative scour to the dimensionless variable, (b/y1 ) Fr , for laboratory data used to develop the original HEC-18 pier-scour equation (Richardson and others, 1991) and (A) clear-water and (B) live-bed pier-scour data from selected sites in South Carolina (from Benedict and Caldwell, 2006 and 2009, respectively)
SC Bridge Scour Figure 11 Comparison of the South Carolina Pier-Scour Envelope Curves With Other Data 15
30
25
20 (National Bridge Scour Database) (South Carolina) 1.5b 4.1 y S 1.1b 3.34 y 15 S
Pier-scour depth, in feet 10
5
0 0 5 10 15 20 Pier width, in feet E PLANATION Projected envelope curve of National South Carolina live-bed field data Bridge Scour Database data National Bridge Scour Database data Envelope curve of South Carolina field data y S Scour depth, in feet Projected envelope curve of South Carolina b Pier width, in feet (b≤6 feet for the field data South Carolina envelope curve)
Figure 12. Comparison of the South Carolina live-bed pier-scour envelope curve with the envelope curve developed from selected field data from the National Bridge Scour Database (from Benedict and Caldwell, 2009).
Table 3. Range of selected characteristics for the laboratory measurements of pier scour compiled in the PSDb-2014 (Benedict and Caldwell, 2014).
[ft, foot; ft/s, foot per second; mm, millimeter]
Measured Nominal Approach Approach Median Relative Range pier-scour pier width flow velocity flow depth grain size scour depth value depth (ft) (ft/s) (ft) (mm) (ft) (ft) All laboratory data in the PSDb-2014 (569 measurements) Minimum 0.1 0.5 0.1 0.2 0.0 0.1 Median 0.2 1.4 0.6 0.8 0.3 1.5 Maximum 3.0 7.1 6.2 7.8 4.6 3.1 Screened laboratory data in the PSDb-2014 (441 measurements) Minimum 0.1 0.5 0.1 0.2 0.0 0.1 Median 0.2 1.6 0.6 0.8 0.3 1.5 Maximum 3.0 7.1 6.2 7.8 4.6 3.1
SC Bridge Scour Figure 12 16 The South Carolina Bridge-Scour Envelope Curves
Table 4. Range of selected characteristics for the field measurements of pier scour compiled in the PSDb-2014 (Benedict and Caldwell, 2014).
[mi2, square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; mm, millimeter]
Pier width Approach Approach Measured Relative Drainage Channel Nominal Median normal Pier skew flow flow pier-scour scour Range value area slope pier width grain size to flow (degree) velocity depth depth depth (mi2) (ft/ft) (ft) (mm) (ft) (ft/s) (ft) (ft) (ft) All field data in the PSDb-2014 (1,858 measurements) Minimum 3.1 0.00007 0.8 0.7 0 0 0 0.001 0.0 0.0 Median 1,081 0.0005 3 5 0 4.2 9.8 0.94 2.3 0.5 Maximum 708,600 0.02 64 94 85 18 73.9 228.6 34.1 5.4 Missing data 556 775 336 0 341 34 28 72 0 0 Screened field data in the PSDb-2014 (727 measurements)a Minimum 64 0.00008 0.95 0.7 0 0.3 0.4 0.1 0.0 0.0 Median 2,400 0.00060 3.6 7.7 0 4.1 7.7 1.3 2.8 0.5 Maximum 708,600 0.00700 28 55.3 85 15.4 73.9 108 25.6 1.8 Missing data 381 343 317 0 279 0 0 0 0 0 Selected unscreened verification field data from the PSDb-2014 (569 measurements) Minimum 3 0.0001 1 1 0 0.0 0.0 0.01 0.0 0 Median 1,350 0.00019 3 4 0 4.1 11.8 0.74 2.0 0.4 Maximum 697,000 0.005 64 94 66 12.8 64.0 108 34.1 4.2 Missing data 86 260 0 0 0 6 0 13 0.0 0 aSheppard and others (2011) published 791 measurements of screened field data; however, selected data were removed, reducing the number of measure- ments of screened field data to 727.
Screened Laboratory and Field Data Therefore, the 569 unscreened field measurements in the PSDb-2014 made during flood conditions (table 4) were used Sheppard and others (2011) screened selected labora- as the dataset to verify the upper bound of pier scour defined tory and field data to identify pier-scour measurements with the screened data. considered to represent equilibrium scour depths, and these data, consisting of 441 laboratory and 727 field measure- ments (tables 3 and 4, respectively), were included in the Skewed Piers PSDb-2014. The screened laboratory and field data provide The PSDb-2014 includes piers aligned with the a large and high-quality set of equilibrium pier-scour depths approaching flow (569 laboratory measurements; 1,033 field that can be used to investigate the upper bound of pier measurements) as well as skewed piers (0 laboratory scour to assist in evaluating the South Carolina pier-scour measurements; 536 field measurements; unknown at 289 field envelope curves. measurements), and when evaluating the upper bound of pier scour, special consideration must be given to the skewed piers. Verification Data Skewed piers tend to produce larger scour depths; therefore, in an upper-bound analysis, the scour depths should be adjusted The remaining unscreened field measurements in the to reflect scour depths comparable to piers aligned with the PSDb-2014, excluding the South Carolina data, provide flow. This adjustment can be accomplished in several ways, a good source of data (796 measurements) to verify the including (1) normalizing the scour measurement by dividing upper bound of pier scour defined by the screened data. it by the skew correction coefficient,K 2, as determined The unscreened field data represent various types of scour from equation 4, or (2) using the effective pier width (fig. 6) measurements, including post-flood and during-flood rather than the nominal pier width to represent a comparable measurements. Pier-scour measurements obtained during a unskewed pier that would likely have produced the scour of flood are considered to have less measurement uncertainty the skewed pier. The effective pier width can be determined than post-flood measurements (Benedict and Caldwell, 2014). from the following equation: Comparison of the South Carolina Pier-Scour Envelope Curves With Other Data 17
be = b cosineθ + Lsineθ, (8) Envelope Curve for the PSDb-2014 Screened Laboratory Pier-Scour Data where be is the effective pier width perpendicular to flow, in feet, and other variables are as previously defined in figure 6. The relation of pier-scour depth to nominal pier width, When a pier is aligned with the flow (no skew), the using the 441 screened laboratory measurements, is shown in effective pier width is considered identical to the nominal pier figure 13. A log-log scale is used to better display the upper width. Although there are differences in the various methods for bound of the data. With only two exceptions, the upper bound adjusting for skew, Sheppard and others (2011) noted that when of the data displays a well-defined relation. The two data incorporated into the scour-prediction equations, the methods points that slightly exceed the line are from the investigation yielded similar results. Benedict and Caldwell (2015) evaluated by Jain and Fischer (1979) and have high Froude numbers the upper bound of pier scour using the nominal pier width (1.2 and 1.5). Jain and Fischer (1979) note that the flow with selected data from the PSDb-2014 database. (Note: Some conditions associated with these two measurements were of the measurements in the PSDb-2014 database provide only very turbulent and unsteady, making the measurements the effective pier width, requiring some of the field data to be questionable and justifying their exclusion in defining the excluded when limiting the analysis to nominal pier widths.) upper bound of the data. The envelope curve in figure 13 Similarly, an evaluation of the upper bound of pier scour using can be represented by the following equation: the effective pier width for all of the PSDb-2014 data was made by Benedict and Caldwell (2016). Although the plotting positions 0.9 ys = 2.1b , (9) for skewed piers differ with these methods, the analysis yielded identical upper-bound envelope curves, confirming in some where all variables are as previously defined. If the labora- measure the observation of Sheppard and others (2011) that tory data plotted in figure 13 are capturing the upper bound the two methods for adjusting for pier skew are similar. For of pier-scour depth that is anticipated in the field, it would additional details regarding these differing approaches for evalu- be reasonable to expect that the upper bound of the field data ating the upper bound of pier scour with the PSDb-2014 data, would conform to the extension of the envelope curve shown reference can be made to Benedict and Caldwell (2015, 2016). in figure 13.
The PSDb-2014 Pier-Scour Envelope Curve Extending the Envelope Curve With the PSDb-2014 Screened Field Data The South Carolina clear-water and live-bed pier-scour envelope curves (figs. 7, 10) were developed using the nominal Figure 14 shows the relation of scour depth to pier width; therefore, the analysis of the upper bound of pier pier width for the 727 screened field measurements, as scour for the PSDb-2014 presented in this report used that previously described, using the same format as figure 13, approach. In the case of skewed piers, the measured scour with an extension of the envelope curve derived from depth was adjusted by dividing by K2, as described in the the laboratory data. The nominal pier width was used to preceding report section “Skewed Piers.” The exception to this plot the screened field data, with the exception of the 317 approach was for 317 measurements of the screened field data measurements where the effective pier width was used where the effective pier width was used in the analysis because because of data limitations as previously described. The data the nominal pier width was not published in the original source. associated with the nominal and effective piers widths for As noted previously, the analysis by Benedict and Caldwell the screened field data are identified in figure 14. Although (2015, 2016) indicates that using the nominal or effective pier there is more scatter in the upper bound of the screened width yields the same envelope curve for the PSDb-2014 data, field data than in the upper bound of the laboratory data, the which demonstrates that using the effective pier width for these screened field data conform well to the envelope curve of 317 measurements rather than the nominal pier width, will not the laboratory data, indicating that the laboratory envelope bias the analysis of the upper bound of pier scour. The upper curve provides a reasonable definition of the upper bound bound analysis of the PSDb-2014 presented in this report uses of pier scour in the field as well as in the laboratory. There the same general approach presented in Benedict and Caldwell are four field measurements that exceed the envelope curve (2015, 2016) and yields the same envelope curve. The primary with exceedance values ranging from 0.3 and 2.5 ft and difference in these analyses is the differing datasets selected an average value of 1.1 ft. The complexities of the field, from the PSDb-2014. The evaluation of the upper bound combined with the harsh measuring environment, make it of pier scour using the PSDb-2014 data was conducted in a difficult to measure pier scour to the same degree of accuracy stepwise process using the high-quality, screened laboratory as in the laboratory; therefore, it is expected that some field data (table 3) to initially define the envelope curve, extending measurements may exceed the upper bound of the laboratory that curve with the screened field data (table 4), and then data. Because the exceedance values are small, no adjustment verifying the curve with the unscreened verification field data was made to the envelope curve to account for these (table 4). The results of these steps are presented below. four data points. 18 The South Carolina Bridge-Scour Envelope Curves
10
E PLANATION Screened laboratory data Envelope for laboratory data 0.9 2.1b b Pier width, in feet y s ys Pier-scour depth, in feet
1 Froude number 1.2
e t Froude number 1.5 f i n
h , d e p t o u r S c 0.1
0.01 0.01 0.1 1 10 Pier width, in feet
Figure 13. Relation of scour depth to pier width for the screened laboratory data from the PSDb-2014 database.
100 E PLANATION Screened field data (nominal pier width) Screened field data (effective pier width) Screened laboratory data Envelope for laboratory data Projection of laboratory envelope 10 b Pier width, in feet ys Pier-scour depth, in feet 0.9 2.1b y s e t f i n
h , 1 d e p t o u r S c
0.1
0.01 0.01 0.1 1 10 100 Pier width, in feet Figure 14. Relation of scour depth to pier width for the screened laboratory and field data from the PSDb-2014 database.
SC Bridge Scour Figure 13
SC Bridge Scour Figure 14 Comparison of the South Carolina Pier-Scour Envelope Curves With Other Data 19
Verifying the Envelope Curve With the PSDb-2014 have insufficient information to evaluate the possible cause Unscreened Field Data of the exceedance, three of those measurements have small exceedance values less than 1.5 ft. Seven measurements have The 569 unscreened verification field measurements exceedances that fall within the published accuracy of the made during high flows, as previously described, are plotted measurement, and the remaining five measurements have in figure 15 along with the screened laboratory and field data. some plausible explanation for their exceedance. The large Twelve of the verification measurements exceeded the enve- number of measurements (441 laboratory and 1,296 field lope curve, with exceedance values ranging from 0.2 to 3.3 ft measurements) used to develop the PSDb-2014 pier-scour and a median exceedance of 1.1 ft. As noted above, measure- envelope curve, in conjunction with the information in table 5, ment error associated with field data possibly accounts for this indicates that it is a reasonable approximation of the upper exceedance, and because the exceedance is small, no adjust- bound of pier scour for both the laboratory and field. The ment was made to the envelope curve. The verification data PSDb-2014 pier-scour envelope curve is defined most clearly provide further confirmation that the envelope curve shown in for nominal pier widths of approximately 15 ft or less. figures 14 and 15 is a reasonable upper bound for pier scour. The equation for the PSDb-2014 pier-scour envelope curve is expressed in equation 9 where nominal pier widths Review of Field Data That Exceed the are limited to 15 ft or less. The data used to develop the PSDb-2014 envelope curve include variously shaped piers and Envelope Curve pile bents having live-bed and clear-water scour conditions; Figure 16 shows the same data that are in figure 15 but therefore, it is assumed that the PSDb-2014 envelope curve is using an arithmetic rather than logarithmic scale to provide applicable to piers with those conditions. When the skew of a perspective on the data relations and envelope curve on the pier to flow is thought to influence scour, the skew-correction differing scales. The scale of the horizontal axis in figure 16 coefficient, as defined in HEC-18 (Arneson and others, 2012; has been truncated to a value of 30 ft for clarity. Additionally, eq. 4), can be applied to equation 9 as follows: a 2.5-ft offset from the envelope curve defined by equation 9 0.9 is shown in figure 16 to provide perspective on the data that ys = K2(2.1b ), (10) exceed the curve. The data exceeding the envelope curve were reviewed to evaluate if there were site conditions that where all variables are as previously defined. Reference may have contributed to the exceedance, and table 5 lists the should be made to Arneson and others (2012) for further guid- findings of that review. Although four of the measurements ance regarding the application and limitations of equation 4.
100 E PLANATION erification field data Screened laboratory and field data Envelope for laboratory data Projection of laboratory envelope 10 b Pier width, in feet ys Pier-scour depth, in feet
0.9
e t 1b f 2.
i n y s
h , 1 d e p t o u r S c
0.1
0.01 0.01 0.1 1 10 100 Pier width, in feet
Figure 15. Relation of scour depth to pier width for the screened laboratory and field data and verification data from the PSDb-2014 database.
SC Bridge Scour Figure 15 20 The South Carolina Bridge-Scour Envelope Curves
Table 5. Review of selected data points exceeding the PSDb-2014 pier-scour envelope curve.— Continued
[ID, identification; ft, foot; ft/s, foot per second; —, missing data]
Measured ID number Exceedance Published Pier Pier Skew Flow Flow scour depth for data of envelope measurement width length angle velocity depth adjusted Comment points in curve accuracy (ft) (ft) (degree) (ft/s) (ft) for skew figure 16 (ft) (ft) (ft) 1 2.5 14 54 0 9.5 32.1 25.1 1 A small error in the measurement of the skew angle at this long, solid pier could account for the exceedance. For example, a skew angle of 2 degrees brings the exceedance of the scour depth adjusted for skew within the pub- lished accuracy of the measure- ment. A skew angle of 3 degrees brings the measurement adjusted for skew below the PSDb-2014 envelope curve. 2 0.3 14 54 0 10.4 39.2 22.9 2 The exceedance is within the published accuracy of the measurement. 3a 1.3 10 — 6 7.2 13.8 18 — There is insufficient information to evaluate; however, the exceedance is small. 4a 0.4 10 — 6 8.9 13.1 17.1 — There is insufficient information to evaluate; however, the exceedance is small. 5 0.3 7.23 24.75 0 6.84 18.2 12.8 0.5 The exceedance is within the published accuracy of the measurement. Additionally, this site is noted to have substantial debris which tends to create larger than normal scour depths. 6 2.5 7 -- 0 9.5 35.3 14.6 0.5 Southard (1992) noted that there was substantial contraction scour at this site, and this could possibly contribute to the exceedance. 7 1.5 6 31 0 7.28 30.8 12 — There is an exposed footing at this pier that could possibly contribute to the exceedance. 8 1.6 4.2 23.7 26 9.6 18.6 9.3 2 The exceedance is within the published accuracy of the measurement. 9 0.9 4 24 0 4.9 29.3 8.2 1 The exceedance is within the published accuracy of the measurement. 10 0.2 4 24 0 5.6 32.4 7.5 0.5 The exceedance is within the published accuracy of the measurement. Comparison of the South Carolina Pier-Scour Envelope Curves With Other Data 21
Table 5. Review of selected data points exceeding the PSDb-2014 pier-scour envelope curve.— Continued
[ID, identification; ft, foot; ft/s, foot per second; —, missing data]
Measured ID number Exceedance Published Pier Pier Skew Flow Flow scour depth for data of envelope measurement width length angle velocity depth adjusted Comment points in curve accuracy (ft) (ft) (degree) (ft/s) (ft) for skew figure 16 (ft) (ft) (ft) 11 0.2 4 26 0 5.3 27.2 7.5 0.5 The exceedance is within the published accuracy of the measurement. 12 1.2 3.3 26.9 0 7.6 27.6 7.4 1 A small error in the measurement of the skew angle at this long, solid pier could account for the exceedance. For example, a skew angle of 1 degree brings the exceedance of the scour depth adjusted for skew within the published accuracy of the measurement. A skew angle of 3 degrees brings the measure- ment adjusted for skew below the PSDb-2014 envelope curve. 13 0.4 2.6 -- 0 11 21.2 5.4 — There is insufficient information to evaluate; however, the exceed- ance is small. 14 0.4 2.5 43 0 1.13 26.2 5.2 1 The exceedance is within the pub- lished accuracy of the measurement. 15 3.3 1.5 -- 0 3.3 12.4 6.3 — Insufficient information to evaluate.
16 2.1 1.4 1.4 23 3.2 8.4 4.9 — Southard (1992) indicated that this pier is in the abutment region, and therefore, abutment scour could possibly contribute to the exceedance. a This measurement only published the effective pier width, and it is assumed that the measured scour depth reflects the field measurement unadjusted for skew. 22 The South Carolina Bridge-Scour Envelope Curves
50
E PLANATION erification field data Screened laboratory and field data 2.5-foot offset 40 Envelope for laboratory data b Pier width, in feet ys Pier-scour depth, in feet 1 Site number in table 5 0.9 2.1b y s e t 30 f i n 1 h , 2
d e p t 3 4 o u r 20 S c 6 5 10 and 11 7 9 12 8 13 10 14 15 16
0 0 10 20 30 Pier width, in feet Figure 16. Relation of scour depth to pier width for the screened laboratory and field data and verification data from the PSDb-2014 database (arithmetic scale).
Flow Recurrence Intervals for Data Used in the (1) unknown recurrence interval, (2) recurrence interval less PSDb-2014 Pier-Scour Envelope Curve than the 25-year flow magnitude, and (3) recurrence interval equal to or greater than the 25-year flow magnitude. The data Through risk analysis and review of historic flow from category 3 show scatter patterns similar to the analyses records, Benedict and Caldwell (2006, 2009) concluded of the South Carolina data presented in Benedict and Caldwell that flows equal to or exceeding 70 percent of the 100-year (2006, 2009), having a large scatter within the data and the flow (approximately the 25-year flow) likely occurred at a upper bound of the data falling in close proximity to the large percentage of the bridges used to develop the South envelope cure. Although these data are limited, the indication Carolina clear-water and live-bed pier-scour envelope curves is that the PSDb-2014 pier-scour envelope curve, similar to the (80 and 96 percent, respectively). Additionally, scour measure- South Carolina envelope curves, likely represents pier scour ments in these studies (81 and 73 data points, respectively) for flows near the 100-year flow magnitude. The South Caro- associated with known historical floods, equal to or exceeding lina and PSDb-2014 pier-scour envelope curves provide useful 70 percent of the 100-year flow, plotted within the envelope tools for understanding scour associated with flows near curves with some of the data defining the envelope curve. On the 100-year flow magnitude; however, the envelope curves the basis of these observations, Benedict and Caldwell (2006, should not be considered as a definitive estimate of the pier 2009) concluded that the South Carolina pier-scour envelope scour associated with the 100-year flow. curves represented pier scour for flows near the 100-year flow magnitude. With respect to the selected field data used to develop Comparison of the South Carolina and PSDb-2014 the PSDb-2014 envelope curve (screened and verification Pier-Scour Envelope Curves field data; fig. 15), about 40 percent of the data have estimates of the flow recurrence interval ranging from a 1-year to The South Carolina clear-water and live-bed pier-scour 500-year flow magnitude. Sixty-one of those measurements envelope curves and the PSDb-2014 pier-scour envelope have flow magnitudes equaling or exceeding the 25-year flow, curve are shown in figures 18 and 19, respectively. Arithmetic comparable to the range associated with much of the data used scales were used in these figures to better display the differ- to develop the South Carolina envelope curves. Figure 17 ences between the South Carolina and PSDb-2014 envelope shows the PSDb-2014 pier-scour envelope curve with the field curves. The South Carolina clear-water envelope curve data (screened and verification) grouped by the categories of (fig. 18) plots slightly below the PSDb-2014 envelope curve,
SC Bridge Scour Figure 16 Comparison of the South Carolina Pier-Scour Envelope Curves With Other Data 23
100 E PLANATION Flow > 25-year flow magnitude Flow 25-year flow magnitude Screened laboratory data Unknown flow recurrence interval Envelope for laboratory data 10 Projection of laboratory envelope b Pier width, in feet ys Pier-scour depth, in feet 0.9 1b e t 2. f y s i n
h , 1 d e p t o u r S c
0.1
0.01 0.01 0.1 1 10 100 Pier width, in feet
Figure 17. The PSDb-2014 pier-scour envelope curve with field data grouped by recurrence interval.
30 E PLANATION Screened field and laboratory data South Carolina clear-water data (Benedict and Caldwell, 2006) South Carolina clear-water envelope (Benedict and Caldwell, 2006) PSDb-2014 envelope b Pier width, in feet ys Pier-scour depth, in feet
20 e t f i n 0.9
h , 1b 2. y s d e p t o u r y 1.5b 0.5
S c s 10
0 0 10 20 Pier width, in feet
Figure 18. Comparison of the South Carolina clear-water and the PSDb-2014 pier-scour envelope curves.
SC Bridge Scour Figure 17
SC Bridge Scour Figure 18 24 The South Carolina Bridge-Scour Envelope Curves
30 E PLANATION Screened field and laboratory data South Carolina live-bed data (Benedict and Caldwell, 2009) South Carolina live-bed envelope (Benedict and Caldwell, 2009) PSDb-2014 envelope b Pier width, in feet ys Pier-scour depth, in feet
20 e t f i n
0.9 h , 1b 2. y s d e p t o u r y 1.1b 3.4
S c s 10
0 0 10 20 Pier width, in feet Figure 19. Comparison of the South Carolina live-bed and the PSDb-2014 pier-scour envelope curves. having an identical scour depth for a nominal pier width be used to assess scour potential for such flow conditions. of 0.8 ft and being 1 ft smaller for a nominal pier width of (Note: Although the South Carolina and PSDb-2014 pier-scour 6 ft. The South Carolina live-bed envelope curve (fig. 19) envelope curves include a number of measurements likely primarily plots above the PSDb-2014 envelope curve, having associated with flows near the 100-year flow magnitude, an identical scour depth for a nominal pier width of 5 ft, and the envelope curves should not be considered as a definitive being 2.6 ft larger for a nominal pier width of 0.8 ft. The larger estimate of the pier scour associated with the 100-year values associated with the South Carolina live-bed envelope flow.) The South Carolina pier-scour envelope curves were curve (fig. 19) can be attributed to the use of GPR that tends not recommended for assessing scour potential for extreme to have a larger measurement uncertainty, potentially leading floods such as the 500-year flow (Benedict and Caldwell, to overestimates of measured scour (Benedict and Caldwell, 2006, 2009), and this guidance also is appropriate for the 2009). Although there are some differences, the South PSDb-2014 envelope curve. In order to gain insights on Carolina pier-scour envelope curves fall in close proximity the relative increase in pier-scour depth associated with the to the PSDb-2014 envelope curve, indicating that the South 100- to 500-year flows, it is possible to develop theoretical Carolina pier-scour envelope curves are reasonable. adjustment coefficients that approximate that relative change. Such coefficients can be applied to the pier-scour envelope curves to approximate the relative increase in scour associated with the 100- to 500-year flows. Although such adjustments Relative Increase in Theoretical Pier do not provide a definitive estimate of the 500-year pier-scour Scour Associated With the 100- to depth, they do provide perspective on the relative increase. To accomplish this task, theoretical pier-scour computations from 500-Year Flows previous level-2 scour evaluations, conducted by the USGS, were compiled into a database and used to evaluate the relative As described previously, the South Carolina and increase in scour depth associated with the 100-year flow to PSDb-2014 pier-scour envelope curves included scour the 500-year flow for theoretical pier-scour depths. A summary measurements likely associated with historic flows near the of the compiled theoretical scour data and the analysis of the 100-year flow magnitude; therefore, the envelope curves can 500-year flow adjustment coefficient is presented below.
SC Bridge Scour Figure 19 Relative Increase in Theoretical Pier Scour Associated With the 100- to 500-Year Flows 25
Theoretical Pier-Scour Data coefficient that is a constant, which simplifies the application of the coefficient. The resulting curves as shown in figure 20 The USGS, in cooperation with the SCDOT, conducted and similar curves from analyses for the other scour compo- level-2 bridge-scour studies at 293 bridges in South Carolina nents presented later in the report indicate this is a reasonable (1992–95), using methods presented in HEC-18 (Richardson approach. Summary statistics for the data are provided in and others, 1991, 1993). These studies included the develop- table 6. The scatter about the trend line for the South Carolina ment of one-dimensional step-backwater flow models for the data is small, and the coefficient of determination is high 100- and 500-year flows (Guimaraes and Bohman, 1992) and (0.99) indicating that there is a strong correlation in the theoretical bridge-scour computations for both flow conditions data. The trend line through the Missouri data falls in close using the HEC-18 scour equations (Richardson and others, proximity to the South Carolina trend line indicating that the 1991, 1993). Selected theoretical bridge-scour data for the South Carolina trend line is reasonable. The equation for the 100- and 500-year flows, as determined in the level-2 scour South Carolina trend line is as follows: investigations, were compiled into a digital spreadsheet and called the South Carolina Theoretical Bridge Scour Database. Scour500 =1.09 Scour100 (11) This database is included as a companion spreadsheet with this report and is available at https://doi.org/10.3133/ where sir20165121. The South Carolina Theoretical Bridge Scour Scour500 is the 500-year flow theoretical scour Database includes theoretical scour computations for both the depth, in feet; and 100- and 500-year flows at 1,645 piers. In addition to the South Scour100 is the 100-year flow theoretical scour Carolina data, theoretical pier-scour data from level-2 studies depth, in feet. conducted in Missouri (Huizinga and Rydlund, 2004) also were Equation 11 can be divided by Scour and simplified to the used in the analysis. The Missouri data included theoretical 100 scour computations for 187 piers and were used as a means to following form: confirm the trends of the South Carolina data. The Missouri K = 1.09 (12) data are available at http://pubs.usgs.gov/sir/2004/5213/. 500
where K500 is the theoretical 500-year flow adjustment coeffi- The 500-Year Flow Adjustment Coefficient cient that represents the ratio of the 500-year flow to 100-year flow theoretical pier-scour depths. The K500 coefficient can The relation of the theoretical pier scour for the 100- and be applied to the South Carolina and PSDb-2014 pier-scour 500-year flows for the South Carolina and Missouri data envelope curves to increase the envelope pier-scour depth by is shown in figure 20 including trend lines through each the relative increase in theoretical pier scour associated with dataset. As previously noted, the purpose of this analysis is the 100- to 500-year flow condition. The K500 is a helpful tool to provide perspective on the relative increase in scour depth for gaining perspective on the relative increase of theoretical for the 100- and 500-year flows and not to provide a defini- scour associated with the 100- to 500-year pier-scour depth. tive estimate of the 500-year scour depth. Consequently, the However, the adjusted envelope curve values, should not be analysis was done such that the intercept in the linear equation considered a definitive estimate of the pier scour associated would be zero, resulting in a 500-year flow adjustment with the 500-year flow.
Table 6. Descriptive statistics for the ratio of the 500- and 100-year flow, theoretical pier-scour depths used in the regression analysis for selected data from South Carolina and Missouri.
[N, number of measurements; Q500, 500-year flow; Q100, 100-year flow; 2mi , square mile]
South Carolina (N = 1,645) Missouri (N = 187) Descriptive statistic Ratio of Q500 and Q100 Drainage area Ratio of Q500 and Q100 Drainage area theoretical pier-scour depths (mi2) theoretical pier-scour depths (mi2) Minimum 1.00 0.91 1.00 3.70 25th percentile 1.06 46.6 1.08 15.1 Mean 1.10 760 1.19 244 Median 1.08 144 1.13 44.4 75th percentile 1.12 553 1.20 170 Maximum 1.81 8,312 2.40 14,000 26 The South Carolina Bridge-Scour Envelope Curves
35 E PLANATION South Carolina data Missouri data Scour500 1.19Scour100 30 Trend line for South Carolina data R2 0.93 Trend line for Missouri data
Scour500 Scour depth for the 500-year flow
Scour100 Scour depth for the 100-year flow 25 R2 Coefficient of determination
20
r-scour depth, in feet Scour 1.09Scour e 500 100 i
p 2 R 0.99 a l c
i 15 t e r o e h t
a r 10 e 500- y 5
0 0 5 10 15 20 25 30 100-year theoretical pier-scour depth, in feet
Figure 20. Relation of the 500- and 100-year flow, theoretical pier-scour depths for selected data from level-2 bridge-scour studies in South Carolina and Missouri.
Application and Limitations of the tidally influenced sites. The envelope curves were developed from a limited sample of bridges, and scour depths possibly PSDb-2014 and South Carolina could exceed the envelope curves; therefore, applying a safety factor to the envelope curves may be prudent. Application of Clear-Water and Live-Bed Pier-Scour the South Carolina clear-water and live-bed envelope curves Envelope Curves is limited to nominal pier widths of approximately 6 ft or less. The PSDb-2014 envelope curve can be applied to nominal pier Estimating pier-scour depth with current scour-prediction widths approximately 15 ft or less. The following guidance methods from HEC-18 (Arneson and others, 2012) is an impre- for applying the pier-scour envelope curves is separated into cise science, and the practitioner must use judgment to evaluate categories for nominal pier widths less than or equal to 6 ft the reasonableness of the results. The PSDb-2014 and the and for nominal pier widths greater than 6 ft. South Carolina clear-water and live-bed pier-scour envelope curves are useful supplementary tools that can be used to assist Evaluating Scour Depth at Pier Widths Less in such evaluations. However, the limitations associated with the envelope curves, as noted below, should be kept in mind Than or Equal to 6 Feet when using them to assess scour potential in South Carolina. The pier-scour envelope curves are empirical formulations, and The South Carolina clear-water and live-bed pier-scour therefore, their application should be limited to sites having envelope curves (figs. 7 and 10, respectively) can be used as similar characteristics to those used to develop the envelope supplementary tools to evaluate pier-scour potential for flood curves. The envelope curves do not account for adverse field plain or main-channel piers, respectively, having nominal pier conditions, such as debris, severe channel bends, and unusual widths less than or equal to 6 ft. The PSDb-2014 envelope flow distributions, and judgment must be used to assess the curve (fig. 14) also is applicable for this range of nominal increased potential for pier scour when such conditions exist pier widths. Following is guidance for application of these or are anticipated. The envelope curves are not intended for envelope curves.
SC Bridge Scour Figure 20 Application and Limitations of the PSDb-2014 and South Carolina Clear-Water and Live-Bed Pier-Scour Envelope Curves 27
Clear-Water Pier Scour on the Flood Plain as recommended in HEC-18 (Arneson and others, 2012) may be appropriate. 1. Determine if the site of interest has characteristics within ◦◦ For pile bents with skews exceeding approximately the range of the data used to develop the envelope curve 15 degrees or with spacings between piles smaller by reviewing the corresponding regional data in table 1. than 5 pile widths, evaluate K2 with equation 4 following the guidance as recommended in HEC-18 2. Evaluate the correction coefficient for skew K( 2). (Arneson and others, 2012). • Pile bents: • Piers: ◦◦ For pile bents with skews approximately 15 degrees or less and spacings between piles of approximately ◦◦ For skewed piers, evaluate K2 with equation 4 following the guidance as recommended in HEC-18 5 pile widths or greater, K2 has a value of 1.0. If adverse field conditions exist or are anticipated at (Arneson and others, 2012). the site, use of equation 4 following the guidance 3. Evaluate the upper bound of live-bed pier scour in the as recommended in HEC-18 (Arneson and others, main channel with equation 7. 2012) may be appropriate. 4. Select comparison sites from the South Carolina ◦◦ For pile bents with skews exceeding approximately Live-Bed Scour Database (SCLBSD; Benedict and 15 degrees or with spacings between piles smaller Caldwell, 2009) having characteristics similar to than 5 pile widths, evaluate K with equation 4 2 the site of interest, and evaluate the reasonable- following the guidance as recommended in HEC-18 ness of the assessment of potential pier scour. The (Arneson and others, 2012). PSDb-2014 database (Benedict and Caldwell, 2014) • Piers: may be consulted for additional comparisons if deemed appropriate. ◦◦ For skewed piers, evaluate K2 with equation 4 following the guidance as recommended in HEC-18 5. Evaluate the relative increase in theoretical pier scour (Arneson and others, 2012). associated with the 100- to 500-year flow by multiplying the results from equation 7 by the K coefficient as 3. Evaluate the upper bound of clear-water pier scour on 500 the flood plain with equation 3. determined from equation 12. 4. Select comparison sites from the South Carolina Clear-Water Pier- and Contraction-Scour Database The PSDb-2014 Pier-Scour Envelope Curve (SCPCSD; Benedict and Caldwell, 2006) having It is possible to use the PSDb-2014 pier-scour envelope characteristics similar to the site of interest, and evaluate curve to assess clear-water and live-bed pier-scour potential the reasonableness of the assessment of potential pier for pier widths less than or equal to 6 ft. The PSDb-2014 scour. The PSDb-2014 database (Benedict and Caldwell, envelope curve will tend to provide slightly larger estimates of 2014) may be consulted for additional comparisons if pier-scour depth than the South Carolina clear-water pier-scour deemed appropriate. envelope curve (fig. 18) and slightly lower values than the 5. Evaluate the relative increase in theoretical pier scour South Carolina live-bed pier-scour envelope curve (fig. 19). associated with the 100- to 500-year flow by multiplying 1. Determine if the site of interest has characteristics within the results from equation 3 by the K500 coefficient from the range of the data used to develop the envelope curve equation 12. by reviewing table 4.
2. Evaluate the correction coefficient for skew, K2, Live-Bed Pier Scour in the Main Channel following the guidance as recommended in HEC-18 (Arneson and others, 2012). 1. Determine if the site of interest has characteristics within 3. Evaluate the upper bound of pier scour by applying the the range of the data used to develop the envelope curve nominal pier width and K to equation 10. by reviewing the corresponding regional data in table 2. 2 4. Select comparison sites from the PSDb-2014 database 2. Evaluate the correction coefficient for skew K( ). 2 having characteristics similar to the site of interest, • Pile bents: and evaluate the reasonableness of the assessment of potential pier scour. ◦◦ For pile bents with skews approximately 15 degrees or less and spacings between piles of approximately 5. Evaluate the relative increase in theoretical pier scour
5 pile widths or greater, K2 has a value of 1.0. If associated with the 100- to 500-year flow by multiplying
adverse field conditions exist or are anticipated at the results from equation 10 by the K500 coefficient as the site, use of equation 4 following the guidance determined from equation 12. 28 The South Carolina Bridge-Scour Envelope Curves
Evaluating Scour Depth at Pier Widths Greater regions of clear-water abutment and contraction scour on a Than 6 Feet bridge overbank that correspond to the flow patterns depicted in figure 22. The flow patterns in the abutment-scour region The PSDb-2014 envelope curve (fig. 13) can be used as will tend to produce larger scour depths than those in the a supplementary tool to evaluate pier-scour potential for flood clear-water contraction-scour region, and the more severe the plain and main-channel piers having nominal pier widths of contraction of flow, the larger the potential for scour depth in 15 ft or less. The guidance presented in the previous report both regions. Although abutment scour can be classified as section can be followed for applying the PSDb-2014 envelope clear-water or live-bed scour, clear-water abutment scour was curve. (Note: Benedict and Caldwell [2009] recommended the focus of the South Carolina investigation (Benedict, 2003). using the pier-scour envelop curve shown in figure 12 on As previously described in the report section “Clear-Water the basis of a limited set of pier-scour data from the NBSD Pier-Scour Envelope Curve,” clear-water scour conditions for nominal pier widths greater than 6 ft. However, the prevail on the flood plains of South Carolina streams, and PSDb-2014 envelope curve is based on a much larger dataset clear-water abutment scour typically occurs in close proximity and should supersede the limited NBSD envelope curve.) to the abutment toe (fig. 23) in the clear-water scour areas as defined in figures 4 and 5. The complexity of the abutment scour processes (fig. 21) and the difficulty of scaling long road embankments to the The South Carolina Clear-Water laboratory flume present challenges for investigating abut- Abutment-Scour Envelope Curves ment scour in the laboratory setting. As such, the state-of-the knowledge for abutment scour processes is not as advanced as Local scour at an abutment is caused by the obstruction that for pier scour (Ettema and others, 2010; Sturm and others, of flow created by a road embankment. The flow contraction 2011). A number of laboratory studies have been conducted to forced by the road embankment creates a complex flow investigate the variables that influence abutment scour. Some field causing accelerated flows and vortexes near the toe of the more notable publications on this topic include Laursen of the abutment (fig. 21); these are considered the primary and Toch (1956), Das (1973), Melville (1992), Dongol (1993), mechanisms that produce abutment scour (Ettema and others, Palaviccini (1993), Melville and Coleman (2000), Sturm 2010; Sturm and others, 2011). Figure 22 shows a simplified (2004), Briaud and others (2009), Ettema and others (2010), flow field at a bridge contraction, identifying the zone of flow and Sturm and others (2011). Although some have differing acceleration and severe flow curvature near the abutment toe, views regarding the most prominent variables that influence which produces the abutment scour. With lateral distance from abutment scour, researchers generally agree that influencing the abutment, the flow curvature and acceleration diminishes variables include the velocity of approaching flow, the depth decreasing the potential for scour. This decreased potential of approaching flow, flow duration, sediment characteristics, of scour is typically associated with clear-water contraction abutment geometry, abutment alignment with flow, embank- scour on bridge overbanks. Figure 23 shows the general ment length blocking flow, and the degree of flow contraction.
Scour area High turbulence Wake region Abutment Rollers Water level Embankment
Eddy
Shear layer NOT TO SCALE
Flood plain soil Contracted flow region Bed sediment
Figure 21. Flow around a bridge abutment and embankment and resulting scour (from Ettema and others, 2010).
SC Bridge Scour Figure 21 The South Carolina Clear-Water Abutment-Scour Envelope Curves 29
Plan view one of flow acceleration and severe curvature
Streamlines
Bridge deck Road embankment Axis of symmetry
Flow
Recirculation area NOT TO SCALE
Figure 22. Plan view of simplified flow field at a bridge contraction (from Benedict, 2003).
Plan view Contraction-scour region
Bridge deck
Abutment-scour Flow Channel region banks
NOT TO SCALE Stream
Figure 23. Plan view of regions of clear-water abutment and contraction scour (from Benedict, 2003).
SC Bridge Scour Figure 22
SC Bridge Scour Figure 23 30 The South Carolina Bridge-Scour Envelope Curves
Some researchers have considered the embankment length from laboratory investigations, Benedict (2003) developed blocking flow, a geometric variable, as a primary explanatory field-derived envelope curves for clear-water abutment scour, variable for abutment scour (Melville, 1992; Dongol, 1993; using the embankment length blocking flow and the degree of Melville and Coleman, 2000). (Note: Some researchers use contraction (expressed by the geometric-contraction ratio) as the term “abutment length” rather than “embankment length.” the primary explanatory variables. An overview of the South These terms should be considered synonymous in this report.) Carolina abutment-scour envelope curves and a comparison Others have considered the degree of flow contraction created with laboratory and field data from other sources follows. by the road embankment as a primary explanatory variable (Laursen and Toch, 1956; Das, 1973; Chang and Davis, 1999; Sturm, 2004; Ettema and others, 2010), with scour depth South Carolina Clear-Water Abutment-Scour generally increasing as the severity of contraction increases. Field Data Although the embankment length blocking flow and the degree of contraction are different variables, they are strongly Benedict (2003) made 209 measurements of clear-water correlated such that as one increases the other generally abutment scour at selected sites in South Carolina, with increases. A notable example of an abutment-scour prediction 100 measurements in the Piedmont (table 7) and 109 in equation that utilizes the embankment length blocking flow the Coastal Plain (table 7). Bridge sites where clear-water as a primary explanatory variable is the Froehlich (1989) abutment-scour data were collected can be identified in equation, which is the predictive equation recommended appendix 1 and figure 1. Measurements of abutment-scour in HEC-18 (Arneson and others, 2012). An example of an depths made in both provinces ranged from 0 to 23.6 ft abutment-scour prediction equation that utilizes the degree and were assumed to represent the maximum clear-water of flow contraction as a primary explanatory variable is the abutment-scour depth that has occurred at the bridge since NCHRP 24-20 equation (Ettema and others, 2010), which construction. All measurements were associated with abutment is an alternate predictive equation recommended in HEC-18 scour in the flood plain in the clear-water scour area located (Arneson and others, 2012). On the basis of the findings at the bridge (figs. 4, 5), and the reference surface used to
Table 7. Range of selected characteristics of clear-water abutment scour field data in the flood plains of selected sites in South Carolina (Benedict, 2003).
2 [mi , square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; mm, millimeter; Dsadj , abutment-scour depth adjusted for abutment shape and skew; <, less than]
a,bAverage aEmbankment aAverage Measured cRelative cRelative a,cGeometric Drainage Channel approach length Median Range approach scour abutment scour contraction area slope flow blocking grain size value velocity depth (D ) length depth ratio (mi2) (ft/ft) depth (y ) flow (L) (mm) s (ft/s) (ft) (L /y ) (D /y ) (m ) (ft) (ft) sadj South Carolina Piedmont (100 measurements) Minimum 11 0.0002 0.1 1.0 18 < 0.062 0 3.0 0.0 0.0 Median 74.9 0.0012 0.85 5.4 276 0.073 1.3 37 0.2 0.7 Maximum 1,620d 0.0029 3.2 14.6 953e 0.99 18 230 3.0 0.9 South Carolina Coastal Plain (109 measurements) Minimum 6.1 0.00007 0.1 1.5 87 < 0.062 0 20.5 0 0.51 Median 120 0.00054 0.5 4.7 557 0.18 7 125.6 1.51 0.86 Maximum 8,830f 0.0024 1.6 17.4 7,440g 0.78 23.6 760.5 4.21 0.98 a Values were estimated from a one-dimensional water-surface profile model. b For setback abutments on the flood plain, flow depth is the average approach flood plain flow depth. For abutments set at the channel bank or protruding into the channel, flow depth is the average approach channel flow depth. cValues reported in Benedict (2016). dApproximately 97 percent of the study sites in the Piedmont have drainage areas less than 400 mi2 (fig. 40). eThree observations had embankment lengths exceeding 950 ft and were significantly outside the range for the majority of the Piedmont data.These sites were excluded from development of the embankment-length envelope curve; therefore, the range of Piedmont embankment lengths used in this study should be limited to 950 ft. fApproximately 80 percent of the study sites in the Coastal Plain have drainage areas less than 426 mi2 (fig. 40). g Only seven measurements have embankment lengths that exceed 2,000 ft. The South Carolina Clear-Water Abutment-Scour Envelope Curves 31 determine the scour depth was the average undisturbed flood Field Conditions That Influence Clear-Water plain elevation in the vicinity of the observed scour. Because Abutment-Scour Potential of clear-water scour conditions, infill sediments within the scour holes were, in general, negligible. The dominant Benedict (2003) identified selected field conditions in abutment geometry was the spill-through abutment, which was the South Carolina abutment-scour data that at times were observed at all but three bridges. The remaining three bridges associated with large abutment-scour depths thus indicating had vertical wingwall abutments. A grab sample of the flood an increased potential for abutment scour. These conditions plain surface sediment was obtained in the upstream flood included (1) wide, flat flood plains where bridges often plain at each site and was analyzed to estimate the median create more severe contractions of flow; (2) flood plain grain size. Because sediment characteristics in the field setting land cover (clear cut and pastures) that reduced flood plain can vary substantially in the vertical and horizontal direction, flow resistance and thereby increased the flow velocities a grab sample taken at a point may not fully represent the approaching the abutment; and (3) severe upstream channel sediment characteristics at a site. bends that redirected concentrated channel flows during a The South Carolina clear-water abutment-scour depths flood toward an abutment that typically would not experi- were measured during low flows, and the flow conditions that ence this magnitude of flow if the channel were straight. produced the scour are not known. To estimate the hydraulic A review of aerial photography and topographic maps can characteristics that may have produced the observed abutment help identify these field conditions, providing insights into scour, WSPRO (Shearman, 1990; Arneson and Shearman, the potential for abutment scour at a given site. In addition 1998) models were developed for each site. A review of to these field characteristics, large abutment scour depths historic floods in South Carolina and a risk analysis associated were often associated with bridges having long embankment with the bridge age indicated that about 90 percent of the lengths blocking flow and large geometric-contraction ratios, bridges in the study likely had experienced flows equaling which are highlighted in the abutment-scour envelope curves or exceeding approximately 70 percent of the 100-year flow, described later in the report. with approximately 25 percent of the bridges having known Benedict (2003) also noted that bridges over swampy historical floods that equaled or exceeded 70 percent of channels or flood plain relief bridges (fig. 5) approximately the 100-year flow value. Information on sites with known 240 ft or less in length tended to form a large, single scour historical floods can be found in appendix 2. On the basis of hole that encompassed the entire bridge opening from the review of historic floods and the risk analysis, the 100-year abutment toe to abutment toe (fig. 24). In contrast, bridges flow was assumed to be representative of a common flow that greater than 240 ft in length generally formed separate may have occurred at all bridges, and this flow was used in abutment scour holes at the left and right abutments (fig. 25). the WSPRO model to estimate the flow characteristics that This phenomenon at shorter bridges appeared to be caused may have produced the measured scour. These estimated by the overlap of the accelerated, turbulent, and curvilinear flow characteristics, which include the embankment length flow (figs. 21, 22) coming from around the left and right blocking flow and the geometric-contraction ratio, should be abutments. At longer bridges, the overlap of the left and right viewed as approximate rather than measured data. abutment flow patterns does not occur, and separate left and The clear-water abutment-scour data collected in South right abutment-scour holes are formed. Although the large, Carolina were grouped into two datasets based on regional single scour holes associated with the shorter bridges could location within the State (fig. 1). One dataset contained be classified as contraction scour, the accelerated, turbulent, data collected in the Piedmont and the other contained data and curvilinear flow patterns that create the scour at these collected in the Coastal Plain. (Note: The Coastal Plain dataset shorter bridges is similar to the flow patterns associated with did not include bridges that were tidally influenced during abutment scour (figs. 21, 22) at the longer bridges; therefore, high flows.) This division of the data was justified because of it was assumed that these scour holes should be classified as the distinct regional characteristics associated with the streams abutment scour. To associate the single scour holes at these of the Piedmont and Coastal Plain Provinces. The Piedmont shorter bridges with abutment variables, it was assumed that generally has cohesive flood plain soils, moderate stream the longer of the left or right embankment length blocking gradients, relatively narrow flood plains, and relatively short flow provided the stronger influence in developing the scour flood-flow durations. In contrast, the Coastal Plain generally hole and was associated with the observed scour. Benedict has sandy flood plain soils, low gradient streams, relatively (2003) also noted that the upper bound of scour depth for the wide flood plains, and relatively long flood-flow durations. To shorter bridges (fig. 24) generally was about 3 to 4 ft larger provide some understanding of the differences between these than for the longer bridges, indicating that shorter bridges, regions, the median and range of selected site characteristics which are often associated with large flow contractions, for the clear-water abutment-scour field data collected in the tend to have a larger potential for abutment-scour depth. Piedmont and Coastal Plain are listed in table 7. For additional Additional details on the above noted field conditions details regarding the South Carolina clear-water abutment- and their influence on abutment scour can be found in scour data, reference can be made to Benedict (2003). Benedict (2003). 32 The South Carolina Bridge-Scour Envelope Curves
Figure 24. Example of single scour hole at shorter bridges, as shown at structure 212030100100 on U.S. Route 301 crossing Douglas Swamp in Florence County, South Carolina,
July 31, 1996 (from Benedict, 2003).
Elevation
at bridge deck corner is 99.9 feet
E PLANATION Bridge deck
Topographic contour–Interval 0.5 foot. Arbitrary datum Bent location Surveyed data point 91.9 Average unscoured reference-
Flow EET surface elevation, in feet ETE S
Flow
Elevation at bridge
1
deck corner is 30.6 feet
EET
ETE S
E PLANATION Bridge deck Topographic contour–Interval 1.0 foot. Arbitrary datum Bent location Surveyed data point 19.8 Average unscoured reference-surface elevation, in feet
Figure 25. Example of separate left and right abutment-scour holes at longer bridges in the Coastal Plain, as shown at structure 277008700100 on Road S-87 crossing the Coosawhatchie River in Jasper County, South Carolina, November 12, 1997 (from Benedict, 2003). SC Bridge Scour Figure 24
SC Bridge Scour Figure 25 The South Carolina Clear-Water Abutment-Scour Envelope Curves 33
Clear-Water Abutment-Scour Envelope Curves that the estimates are comparable to those used in Benedict With Respect to Embankment Length (2003), which were based on the WSPRO model. Guidance regarding this matter is presented in the report section “Guid- As noted previously, various researchers concur with ance for Applying the South Carolina Bridge-Scour Envelope Dongol’s (1993) conclusion, that “abutment length is one of Curves.”) Because of the distinct regional characteristics of the most important parameters influencing the process of local the data collected in the Piedmont and Coastal Plain (table 7), abutment scour.” On the basis of this finding, Benedict (2003) separate envelope curves were developed for each region. used the South Carolina abutment-scour data (table 7) to develop several envelope curves that reflect the upper bound Piedmont Clear-Water Abutment-Scour of clear-water abutment scour with respect to the embank- ment length blocking flow for the sampled bridges in South Envelope Curve Carolina (figs. 26, 27). The embankment length blocking flow The equation for the Piedmont abutment-scour envelope was evaluated with the WSPRO flow model (Shearman, 1990; curve with respect to embankment length (fig. 26) is as follows: Arneson and Shearman, 1998) for the 100-year flow condition by projecting the bridge cross section onto the upstream y = – 0.000009L2 + 0.0276L, (13) approach cross section and using the distance from the edge s of the water to the projected abutment toe to represent the where embankment length (fig. 28). (See report section “Guidance y is the upper bound for potential abutment- for Applying the South Carolina Bridge-Scour Envelope s scour depth, in feet; and Curves” for additional information on evaluating the location of the approach cross section and the embankment length.) L is the embankment length blocking flow, The WSPRO model generally uses the full flood plain width in feet. (left edge of water to right edge of water) for the natural, Because the largest observation of abutment scour in the unconstricted approach cross section with no adjustment for Piedmont (18.0 ft) had an embankment length of 950 ft, Bene- ineffective flow areas, thus providing the largest justifiable dict (2003) recommended that the Piedmont envelope curve estimate for embankment lengths. (Note: The bridge hydrau- (fig. 26), as expressed in equation 13, be limited to sites with lics algorithm and associated cross sections in the HEC-RAS embankment lengths less than or equal to 950 ft. Additionally, model (Brunner, 2016) differ from that of the WSPRO model, the Piedmont embankment-length envelope curve (fig. 26) and when using the HEC-RAS model to estimate the embank- should not be used for evaluating abutment-scour potential ment length blocking flow, judgment must be used to assure at a multiple-bridge opening because of the uncertainty in
20 E PLANATION Equation for envelope curve Envelope curve based on a trend line y –0.000009L2 0.0276L through the points identified by circles s for L 950 Observed abutment scour from field ≤ investigation eet f 15 Data points arbitrarily chosen to develop n i
, envelope curve and equation h ys Estimate of abutment-scour depth ep t along envelope curve, in feet r d
u L 100-year flow embankment length, in feet o (not to exceed 950 feet)
t-s c 10 me n t ab u
ve d 5 bse r O
0 0 200 400 600 800 1,000 Embankment length, in feet
Figure 26. The South Carolina Piedmont clear-water abutment-scour envelope curve with respect to the embankment length blocking flow (from Benedict, 2003).
SC Bridge Scour Figure 26 34 The South Carolina Bridge-Scour Envelope Curves
25 Equation for upper segment of envelope curve ys 14.4 0.00131(L–426), where 426 L≤7,440
20 eet f
E PLANATION n i
, Envelope curve based on a trend line h through the points identified by circles
ep t Observed abutment scour from field 15 r d investigation u
o Data points arbitrarily chosen to develop envelope curve and equation t-s c ys Estimate of abutment-scour depth me n t 10 along envelope curve, in feet L 100-year flow embankment length, in feet ab u
ve d Equation for lower segment of envelope curve
bse r 5 ys 0.0338L, where L 426 O
0 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 Embankment length, in feet Figure 27. The South Carolina Coastal Plain clear-water abutment-scour envelope curve with respect to the embankment length blocking flow (from Benedict, 2003).
Embankment length Embankment length
Projected bridge deck
Water surface
Obstructed area Obstructed area Approach cross section ground line
NOT TO SCALE Projected abutment toe
Figure 28. Definition of the embankment length blocking flow determined by projection of the bridge cross section onto the upstream approach cross section.
SC Bridge Scour Figure 27
SC Bridge Scour Figure 28 The South Carolina Clear-Water Abutment-Scour Envelope Curves 35 determining the embankment length at such sites. Additional respect to the embankment length can be found in Benedict details regarding the development of the Piedmont abutment- (2003). Application and limitations of equations 14a and 14b scour envelope curve with respect to the embankment length are described in more detail in the report section “Application can be found in Benedict (2003). Application and limitations and Limitations of the South Carolina Clear-Water Abutment- of equation 13 are described in more detail in the report sec- Scour Envelope Curves.” tion “Application and Limitations of the South Carolina Clear- Water Abutment-Scour Envelope Curves.” Clear-Water Abutment-Scour Envelope Curves With Respect to the Geometric-Contraction Ratio Coastal Plain Clear-Water Abutment-Scour Envelope Curve As noted previously, various researchers have concluded that the degree of flow contraction created by the embankment The Coastal Plain clear-water abutment-scour envelope length blocking flow is an important variable influencing curve with respect to embankment length (fig. 27) is expressed abutment scour. Although the degree of flow contraction can in the following series of equations: be expressed as a ratio of the constricted to unconstricted flow, it also can be approximated by a geometric variable called the y = 0.0338L, where L ≤ 426, and (14a) s geometric-contraction ratio (fig. 29), expressed as
ys =14.4+ 0.00131(L– 426), where 426 < L ≤ 7,440, (14b) m = 1 – b2 /B1, (15) where all variables are as previously defined. Although the where envelope curve as expressed in equation 14b is limited to m is the geometric-contraction ratio; embankment lengths of 7,440 ft, the field data for embank- b is the flow top width in the bridge ment lengths exceeding 2,000 ft are sparse. Because of the 2 opening, in feet; and sparse data, Benedict (2003) noted that the Coastal Plain B is the flow top width at the upstream abutment-scour envelope curve (fig. 27) may not fully 1 represent the upper bound of abutment scour for embankment unconstricted approach cross section, lengths greater than about 2,000 ft. Therefore, caution must be in feet. used when the embankment length exceeds this value. Addi- This approach was used by Das (1973) in his investigation of tionally, the Coastal Plain embankment-length envelope curve abutment scour. On the basis of the findings from other investi (fig. 27) should not be used for evaluating abutment-scour gations, Benedict (2003) used the South Carolina abutment-scour potential at a multiple-bridge opening unless the embankment data (table 7) to develop several envelope curves that reflect the length for the abutment of interest exceeds 426 ft, which is upper bound of clear-water abutment scour with respect to the the embankment length for which all of the Coastal Plain field geometric-contraction ratio for the sampled bridges in South Car- data associated with multiple-bridge openings plotted within olina (figs. 30, 31). Because of the distinct regional characteristics the envelope curve. Additional details regarding the develop- of the data collected in the Piedmont and Coastal Plain (table 7), ment of the Coastal Plain abutment-scour envelope curve with separate envelope curves were developed for each region.
Flow top width at approach cross section ( )
Projected bridge deck Flow top width Approach Water surface cross at bridge ( ) section ground line
Projected abutment toe m is the geometric-contraction ratio m 1 – b /B NOT TO SCALE
Figure 29. Definition of the geometric-contraction ratio.
SC Bridge Scour Figure 30 36 The South Carolina Bridge-Scour Envelope Curves
20 E PLANATION Envelope curve based on a trend line through the points identified by circles Observed abutment scour from field
eet 15 f
investigation n i
, Data points arbitrarily chosen to develop h envelope curve and equation
ep t ys Estimate of abutment-scour depth
r d along envelope curve, in feet u o m 100-year flow geometric-contraction ratio (not to exceed 0.82) t-s c 10 me n t Equation for envelope curve ab u
2 ys 19.96m 6.163m
ve d for m 0.82 5 bse r O
0 0 0.2 0.4 0.6 0.8 1.0 Geometric-contraction ratio
Figure 30. The South Carolina Piedmont clear-water abutment-scour envelope curve with respect to the geometric-contraction ratio (from Benedict, 2003).
25 E PLANATION Envelope curve based on a trend line through the points identified by circles Observed abutment scour from field 20 investigation eet f
Data points arbitrarily chosen to develop n i
envelope curve and equation , h ys Estimate of abutment-scour depth
ep t along envelope curve, in feet 15 r d m 100-year flow geometric-contraction ratio u
o (not to exceed 0.98) t-s c me n t 10 Equation for envelope curve
ab u 3 2 ys 29.62m –10.182m 5.538m where m ≤ 0.98 ve d bse r
O 5
0 0 0.2 0.4 0.6 0.8 1.0 Geometric-contraction ratio
Figure 31. The South Carolina Coastal Plain clear-water abutment-scour envelope curve with respect to the geometric-contraction ratio (from Benedict, 2003).
SC Bridge Scour Figure 30
SC Bridge Scour Figure 31 The South Carolina Clear-Water Abutment-Scour Envelope Curves 37
The geometric-contraction ratio was evaluated with the Coastal Plain (23.4 ft) had the largest geometric-contraction WSPRO flow model (Shearman, 1990; Arneson and Shearman, ratio of 0.98, as well as the largest embankment length of 1998) for the 100-year flow condition. To estimate the 7,440 ft. Because these characteristics are at the upper bound geometric-contraction ratio, the WSPRO model uses a standard for the Coastal Plain data, caution must be used when applying location of the approach cross section at one bridge length equation 17 to such severe contractions. Additional details upstream from the bridge. In WSPRO, the approach cross regarding the development of the Coastal Plain abutment- section represents the full, natural flood plain width with no scour envelope curve with respect to the geometric-contrac- adjustment for ineffective flow areas. (Note: The bridge hydrau- tion ratio can be found in Benedict (2003). Application and lics algorithm and associated cross sections in the HEC-RAS limitations of equation 17 are described in more detail in the model (Brunner, 2016) differ from that of the WSPRO model, report section “Application and Limitations of the South Caro- and when using the HEC-RAS model to estimate the geometric- lina Clear-Water Abutment-Scour Envelope Curves.” contraction ratio, judgment must be used to assure that the estimates are comparable to those used in Benedict (2003), which were based on the WSPRO model. Guidance regarding The Modified Clear-Water Abutment-Scour this matter is presented in the report section “Guidance for Envelope Curves Applying the South Carolina Bridge-Scour Envelope Curves.”) Ballio and others (2009) conducted a limited investigation Piedmont Clear-Water Abutment-Scour on the combined effect of embankment length and the geometric- contraction ratio on abutment-scour depth by conducting a series Envelope Curve of experiments that held the embankment length constant while The equation for the Piedmont clear-water abutment- varying the geometric-contraction ratio. Findings from Ballio scour envelope curve (fig. 30) is as follows: and others (2009) indicated that for geometric-contraction ratios of approximately 0.33 or less, the effect on local abutment-scour y = 19.96m2 + 6.163m, (16) depth was negligible. For geometric-contraction ratios greater s than 0.33, however, local abutment-scour depth increased where m is the geometric-contraction ratio, and other variables substantially. It is noteworthy that the upper bound of the are as previously defined. Because the largest observation of South Carolina abutment-scour data (Benedict, 2003; figs. 30, abutment scour in the Piedmont (18.0 ft) had a geometric- 31) has similar patterns to the laboratory data, with relatively contraction ratio of 0.82, Benedict (2003) recommended that small abutment-scour depths for geometric-contraction ratios the Piedmont envelope curve (fig. 30), as expressed in equa- of approximately 0.4 or less and the upper bound of scour tion 16, be limited to sites with geometric-contraction ratios substantially increasing for larger values. This provides further less than or equal to this value. Additional details regarding confirmation that the patterns displayed in the field data are the development of the Piedmont abutment-scour envelope reasonable. On the basis of the work of Ballio and others (2009), curve with respect to the geometric-contraction ratio can Benedict and Caldwell (2012) modified the South Carolina be found in Benedict (2003). Application and limitations of abutment-scour envelope curves (Benedict, 2003) to include equation 16 are described in more detail in the report section a family of envelope curves (secondary or modified envelope “Application and Limitations of the South Carolina Clear- curves) that displays the combined effect of embankment length Water Abutment-Scour Envelope Curves.” and the geometric-contraction ratio on abutment-scour depth. This was accomplished by grouping selected field data from South Carolina and the NBSD (table 8) into categories based on Coastal Plain Clear-Water Abutment-Scour embankment-length and developing modified envelope curves Envelope Curve for each embankment-length category. Examples of the modified envelope curves for selected embankment lengths and the associ- The equation for the Coastal Plain clear-water abutment- ated field data for the Piedmont and Coastal Plain are shown in scour envelope curve (fig. 31) is as follows: figure 32. The full family of curves for the Piedmont and Coastal Plain are shown in figures 33 and 34, respectively, and the equa- 3 2 ys = 29.62m – 10.182m + 5.538m, (17) tions for these curves are listed in table 9. The application of the modified abutment-scour envelope curves are limited to embank- where all variables are as previously defined, andm is limited ment lengths of 500 ft or less and to geometric-contraction ratios to values less than 0.98. As previously noted, there are sparse less than or equal to 0.85 and 0.9 for the Piedmont and Coastal data for embankment lengths greater than 2,000 ft. Although Plain, respectively. Additional details regarding the development the Coastal Plain abutment-scour envelope curve with respect of the South Carolina modified abutment-scour envelope curves to the geometric-contraction ratio does not utilize embank- can be found in Benedict and Caldwell (2012). Application and ment length, caution still should be used when evaluating limitations of equations in table 9 are described in more detail scour potential at sites with embankment lengths exceeding in the report section “Application and Limitations of the South that value. The largest observation of abutment scour in the Carolina Clear-Water Abutment-Scour Envelope Curves.” 38 The South Carolina Bridge-Scour Envelope Curves
Table 8. Range of selected site characteristics for field measurements of abutment scour used to develop the modified clear-water abutment-scour envelope curves (from Benedict and Caldwell, 2012).
[mi2, square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; mm, millimeter; <, less than; —, not available]
Average aAverage Embankment Measured Drainage Channel Geometric Median Range approach approach length block- scour area slope contraction grain size value velocity flow depth ing flow depth (mi2) (ft/ft) ratio (mm) (ft/s) (ft) (ft) (ft) South Carolina Piedmont (Benedict, 2003) (74 measurements) Minimum 11 0.0004 0.14 1.0 18 0.02 < 0.062 0 Median 76 0.0012 0.92 5.1 208 0.59 0.095 0.7 Maximum 395 0.0029 3.16 13.8 497 0.96 0.99 9.7 South Carolina Coastal Plain (Benedict, 2003) (39 measurements) Minimum 6 0.0002 0.2 1.5 127 0.56 < 0.062 1.4 Median 43 0.00076 0.5 4.5 374 0.86 0.21 8.6 Maximum 426 0.0024 1.57 7.1 489 1 0.78 14.4 National Bridge Scour Database (U.S. Geological Survey, 2001) (16 measurements) Minimum 836 0.00015 — — 8 0.41 0.001a 0 Median 845 0.0006 — — 396 0.87 0.15a 3 Maximum 1,963 0.0046 — — 546 0.93 35.0a 10 aData missing for seven measurements.
Table 9. Equations for the modified clear-water abutment-scour envelope curves in the Piedmont and Coastal Plain of South Carolina (from Benedict and Caldwell, 2012).
[Note: If the geometric-contraction ratio for a given bridge is less than the minimum value, the minimum value should
be used in the equation. ft, foot; ≤, less than or equal to; L, embankment length; ys, scour depth, in feet; m , geometric- contraction ratio; <, less than]
Embankment-length Limits of the geometric- Equation category contraction ratio Piedmont
2 0 ft ≤ L ≤ 100 ft ys = 3.27m – 1.12m + 1.29 0.136 ≤ m ≤ 0.85 2 100 ft < L ≤ 200 ft ys = 6.27m – 2.83m + 3.16 0.255 ≤ m ≤ 0.85 2 200 ft < L ≤ 300 ft ys = 8.33m – 4.06m + 4.84 0.343 ≤ m ≤ 0.85 2 300 ft < L ≤ 400 ft ys = 11.54m – 6.78m + 6.93 0.423 ≤ m ≤ 0.85 2 400 ft < L ≤ 500 ft ys = 15.38m – 10.83m + 9.61 0.503 ≤ m ≤ 0.85 Coastal Plain
2 0 ft ≤ L ≤ 100 ft ys = 4.64m – 1.99m + 1.43 0.252 ≤ m ≤ 0.9 2 100 ft < L ≤ 200 ft ys = 9.12m – 5.55m + 4.37 0.496 ≤ m ≤ 0.9 2 200 ft < L ≤ 300 ft ys = 13.14m – 9.57m + 8.07 0.649 ≤ m ≤ 0.9 2 300 ft < L ≤ 400 ft ys = 21.30m – 19.22m + 13.54 0.757 ≤ m ≤ 0.9 2 400 ft < L ≤ 500 ft ys = 57.60m – 77.53m + 39.42 0.837 ≤ m ≤ 0.9 The South Carolina Clear-Water Abutment-Scour Envelope Curves 39 1 8 . 0 envelope curve envelope curve (Benedict, 2003) (Benedict, 2003) Original Coastal Plain Original Coastal Plain o i t a r
6 . io n 0 t c th th a r t 300 feet) 500 feet) en g en g
n ≤ ≤ - l - l o L L ( (
en t en t ≤ 300 feet) ≤ 500 feet) e e c- c v v L L i r r k m k m ( ( r
n n t e e c u c u a a
v v e r r b b 4 ) ) . w w m m o o 0 c u c u l l op e op e l l f f 00 3 00 3
2 2 e e g g , , v v n n Geo m ope ope TION TION n n l l c t c t i i e e e e Embankment length ≤ 500 feet d d v v ck i ck i y y Embankment length ≤ 300 feet r r e e n n o o original e original e l l a a n n e e
b b d d e e m m y y n n B B o o envelope curve envelope curve th th (Benedict, 2003) (Benedict, 2003) ( ( r r Original Piedmont Original Piedmont f f c o c o
da r da r r r E PLAN A E PLAN A n n v e v e u u en g en g r r s e s e l l
2 c o c o t t n n . c u c u i i n n
sc o sc o 0 a a s e s e
e e e e l l p p m m P P
o o u u l l k m k m l l ont ont a a n n m m m m a a s t s t d d x i x i b b a a e e a a ≤ 300 feet (Piedmont Benedict, 2003) ≤ 300 feet (National Bridge Scour Database ≤ 300 feet (Coastal Plain Benedict, 2003) ≤ 500 feet (Piedmont Benedict, 2003) ≤ 500 feet (National Bridge Scour Database ≤ 500 feet (Coastal Plain Benedict, 2003) i i o o P P E m E m M M en v e en v e L L U.S. Geological Survey, 2001) L L L U.S. Geological Survey, 2001) L C C L L 0 1 8 . 0 envelope curve envelope curve (Benedict, 2003) (Benedict, 2003) Original Coastal Plain Original Coastal Plain o i t a r
6 . 0 io n t th th c a 200 feet) 400 feet) r en g en g
t ≤ ≤ - l - l n L L o ( (
en t en t ≤ 200 feet) ≤ 400 feet) e e v v L L r r k m k m ( ( c- c
i n n e e r c u c u a a t
v v r r b b ) ) 4 e w w . m m o o c u c u 0 l l op e op e l l f f 00 3 00 3
2 2 e e g g , , v v n n ope ope TION TION n n l l c t c t Geo m i i e e e e d d v v ck i ck i Embankment length ≤ 400 feet Embankment length ≤ 200 feet y y r r e e n n o o original e original e l l a a n n e e
b b d d e e m m y y n n envelope curve envelope curve B B (Benedict, 2003) (Benedict, 2003) o o th th ( ( Original Piedmont Original Piedmont r r f f c o c o
da r da r r r E PLAN A E PLAN A n n v e v e u u en g en g r r s e s e l l
c o c o t t n n 2 . c u c u i i n n
sc o sc o 0 a a s e s e
e e e e l l p p m m P P
o o Relation of clear-water abutment-scour depth to the geometric-contraction ratio for field data associated with selected embankment lengths used Relation of clear-water u u l l k m k m l l ont ont a a n n m m m m a a
s t s t d d x i x i b b 200 feet (Piedmont Benedict, 2003) 200 feet (National Bridge Scour Database 200 feet (Coastal Plain Benedict, 2003) 400 feet (Piedmont Benedict, 2003) 400 feet (National Bridge Scour Database 400 feet (Coastal Plain Benedict, 2003) a a e e a a ≤ ≤ ≤ ≤ ≤ ≤ i i o o P P E m E m M M L L en v e en v e U.S. Geological Survey, 2001) U.S. Geological Survey, 2001) L L L L C C L L Figure 32. develop the South Carolina modified abutment-scour envelope curves (from Benedict and Caldwell, 2012). 0
5 0 5 0 5 0 5 0 5 0 5 0
2 2 1 1 2 2 1 1
t e e f n i , h t ep d ur o c s t- n e m t u b A t e e f n i , h t ep d ur o c s t- n e m t u b A SC Bridge Scour Figure 32 40 The South Carolina Bridge-Scour Envelope Curves
20 E PLANATION L Embankment-length category for secondary envelope curves
Original Piedmont envelope curve 15 (Benedict, 2003) t e f e
n i
, h t p
e Secondary envelope curves d
r
u 10 o L 500 feet c t- s
n L 400 feet e m t L 300 feet Ab u 5 L 200 feet
L 100 feet
0 0 0.2 0.4 0.6 0.8 1 Geometric-contraction ratio Figure 33. Relation of clear-water abutment-scour depth to the geometric-contraction ratio for selected categories of embankment lengths in the Piedmont of South Carolina (from Benedict and Caldwell, 2012).
25 E PLANATION L Embankment-length category for secondary envelope curves Original Coastal Plain envelope curve 20 (Benedict, 2003) t e f e L 500 feet n i
, Secondary envelope curves h
t 15 p e d
r
u L 400 feet o c t- s
n 10 e m
t L 300 feet Ab u
5 L 200 feet
L 100 feet
0 0 0.2 0.4 0.6 0.8 1 Geometric-contraction ratio
Figure 34. Relation of clear-water abutment-scour depth to the geometric-contraction ratio for selected categories of embankment lengths in the Coastal Plain of South Carolina (from Benedict and Caldwell, 2012).
SC Bridge Scour Figure 33
SC Bridge Scour Figure 34 Comparison of the South Carolina Clear-Water Abutment-Scour Envelope Curves With Other Data 41
Comparison of the South Carolina laboratory data have a wide range of hydraulic, sediment and abutment geometry characteristics (table 10), providing a Clear-Water Abutment-Scour Envelope good dataset for assessing the upper bound of abutment scour in the laboratory setting. Additional details about the labora- Curves With Other Data tory abutment-scour data can be found in Benedict (2016) and the cited sources. Benedict (2003) and Benedict and Caldwell (2012) made The compiled field data are listed in table 11 and some limited comparisons of the South Carolina abutment- include 133 measurements: 93 measurements from the small, scour data with selected laboratory and field data from other steep-gradient, coarse sediment streams of Maine (Lombard sources. To expand upon this previous work, Benedict (2016) and Hodgkins, 2008), 23 from the low-gradient, cohesive compiled additional laboratory and field data from other sediment streams of the Alabama Black Prairie Belt (Lee existing sources and compared the upper-bound patterns of and Hedgecock, 2008), 15 from the NBSD (U.S. Geological those data to the South Carolina data. The compiled labora- Survey, 2001), and 2 from the Missouri River at Interstate 70 tory data consisted of 446 selected measurements as given (I–70; Parola and others, 1998). (Note: Seven of the NBSD in table 10. These data primarily represent clear-water scour scour measurements are associated with meandering streams conditions with unskewed abutments and a variety of abut- that are challenging to simulate with a one-dimensional ment shapes. The laboratory flumes included simple rectan- flow model. These models were reviewed and modified, gular channels for the data published in or prior to 1993 and where appropriate, to better represent the unconstricted primarily compound channels for the other data. The compiled approach section. This adjustment caused some of the
Table 10. Range of characteristics for selected laboratory abutment-scour data (from Benedict, 2016).
[ft/s, foot per second; ft, foot; mm, millimeter; Dsadj , abutment-scour depth adjusted for abutment shape and skew; —, not available] Embankment Average aAverage Measured Relative Geometric length Median Relative Range approach approach scour abutment contraction blocking grain size scour depth value velocity flow depth (y ) depth (D ) length ratio flow (L) (mm) s (D /y ) (ft/s) (ft) (ft) (L /y ) sadj (m ) (ft) Melville (1992) (96 measurements) Minimum — 0.07 0.3 — 0.3 0.25 0.5 — Median — 0.33 1.5 — 0.7 3.95 3.21 — Maximum — 1.97 4.6 — 2.4 69 9.7 — Palaviccini (1993) (191 measurements) Minimum 0.31 0.09 0.2 0.29 0.0 0.41 0.01 0.1 Median 1.02 0.4 1 0.9 0.4 3.43 2.07 0.27 Maximum 2.03 1.64 3.7 3.3 1.4 13 6.3 0.47 Sturm (2004) (80 measurements) Minimum 0.73 0.06 2.6 1.1 0.0 13.7 0.36 0.19 Median 1.23 0.11 6.9 3.3 0.6 54.1 5.04 0.5 Maximum 2.04 0.19 12 3.3 1 162 11 0.87 bBriaud and others (2009) (17 measurements) Minimum 0.68 0.6 3.3 0.004 0.2 2.77 0.24 0.28 Median 1.34 0.96 6 0.004 1.4 6.22 3.02 0.5 Maximum 1.9 1.31 9 0.004 4.7 10 9.09 0.75 Ettema and others (2010) (62 measurements) Minimum 1.08 0.49 1.1 0.45 0.2 2.15 0.81 0.08 Median 1.08 0.49 4.3 0.45 0.8 6.65 2 0.33 Maximum 1.38 0.98 9.9 0.45 1.3 18.4 5.48 0.76 aFor setback abutments on the flood plain, flow depth is the average approach flood plain flow depth. For abutments set at the channel bank or protruding into the channel, flow depth is the average approach channel flow depth. bSediments for this investigation were cohesive clays. 42 The South Carolina Bridge-Scour Envelope Curves
Table 11. Range of characteristics for selected abutment-scour measurements from Maine, Alabama, the National Bridge Scour Database, and the Missouri River Interstate–70 Bridge (from Benedict, 2016).
2 [mi , square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; mm, millimeter; Dsadj , abutment-scour depth adjusted for abutment shape and skew; —, not available]
aEmbankment aAverage a,bAverage Measured Relative Relative Geometric Drainage Channel length Median Range approach approach scour abutment scour contraction area slope blocking grain size value velocity flow depth (y ) depth (D ) length depth ratio (mi2) (ft/ft) flow (L) (mm) s (ft/s) (ft) (ft) (L/y) (D / y) (m ) (ft) sadj Maine (Lombard and Hodgkins, 2008) (93 measurements) Minimum 4.1 0.00041 0.18 1.11 0 0.25 0 0 0 0.08 Median 20.1 0.00275 1.04 7.06 40.1 45 0 5.4 0 0.79 Maximum 95.1 0.04446 5.62 15.25 808 109 6.8 96.5 3.36 0.98 Alabama ( Lee and Hedgecock, 2008) (23 measurements) Minimum 10 0.0004 0.18 3.21 43 0.001 1.4 9.8 0.15 0.75 Median 80.2 0.0008 0.62 5 400 0.009 4.7 74.9 1.01 0.83 Maximum 607 0.0016 1.31 9.34 1,141 0.17 10.4 204.3 2.11 0.94 National Bridge Scour Database (U.S. Geological Survey, 2001) (15 measurements) Minimum 836 0.0001 0.49 3.95 15 0.001 0 2 0 0.49 Median 970 0.0006 0.71 8.81 560 0.15 4.5 56.3 0.59 0.86 Maximum 16,010 0.0046 3.37 36.56 3,522 35 18 143.7 1.25 0.94 Missouri River at Interstate 70 (Parola and others, 1998) (2 measurements) — 500,000 — — 14.1 6,890 — c56/30 488 c3.95/2.09 0.72 aValues were generally estimated from a one-dimensional water-surface profile model. bFor setback abutments on the flood plain, flow depth is the average approach flood plain flow depth. For abutments set at the channel bank or protruding into the channel, flow depth is the average approach channel flow depth. cTwo scour measurements were made at this site: the first value represents the measurement at the levee breach approximately 350 feet upstream from the bridge and the second value is at the bridge. hydraulic properties to vary from those originally published makes these data less than ideal. These limitations should be in the NBSD.) The NBSD data primarily have spill-through kept in mind when using these field data in any analysis. abutments, and the Maine and Alabama data primarily have The two scour measurements at the I–70 crossing the wing-wall abutments. The NBSD and Maine data are primarily Missouri River, are associated with the 1993 flood and are bankline abutments or abutments that protrude into the main perhaps the largest measured riverine abutment-scour depths in channel, while the Alabama data are primarily flood plain the United States (30 ft at the bridge and 56 ft upstream from the relief bridges. The Alabama data were originally classified bridge) and were influenced by a levee breach located approxi- as contraction scour (Lee and Hedgecock, 2008); however, mately 350 ft upstream from the abutment. Additionally, the they are similar to the relief bridges in the South Carolina site has a large drainage area of 500,000 mi2 that produces long abutment-scour investigation (Benedict, 2003), making it flood-flow durations and, therefore, a larger potential for scour. appropriate to classify the data as abutment scour. All of the In contrast, the maximum drainage area for the South Carolina Alabama and many of the Maine data represent clear-water data is 8,830 mi2 with a median value less than 100 mi2. The scour conditions, while most of the NBSD data represent adverse flow conditions and substantially larger drainage area of live-bed scour conditions. Most of these data represent the Missouri River site contribute to the larger scour depths than historical scour measurements similar to the South Carolina those of the South Carolina data. Although the Missouri River data, and one-dimensional flow models were used to estimate data do not represent typical abutment scour, they were included the hydraulic properties. The post-flood nature of the scour in the analysis for perspective. Additional details about the field measurements, in conjunction with the estimated hydraulics, data can be found in Benedict (2016) and the cited sources. Comparison of the South Carolina Clear-Water Abutment-Scour Envelope Curves With Other Data 43
Comparison of the Clear-Water Abutment-Scour providing a measure of confidence that the South Carolina Field Data to Laboratory Data data are reasonable. The field envelope curve for cohesive sediments encompasses all of the cohesive data from the Benedict (2016) compared the laboratory and field data South Carolina Piedmont and Alabama. These data had using dimensionless relations, and a summary of the analysis varying degrees of cohesion, with the less cohesive sediments and findings follows. Melville (1992) used 96 laboratory tending to have larger scour depths that fall near the envelope measurements (table 10), collected in rectangular channels at curve while the more scour-resistant cohesive sediments can threshold clear-water scour conditions, to develop an envelope be substantially below the envelope curve. The upper bound curve of abutment scour (fig. 35) based on the relation of of the non-cohesive field data is 4.25 times the approach flow depth with the cohesive field data having a value of three relative scour depth (ysadj /y) to relative abutment length (L/y), times the approach flow depth. where ysadj is the measured abutment-scour depth adjusted for the effect of abutment shape, y is the approach flow depth, and L is the embankment length blocking flow. The adjusted scour depth represents the equivalent scour depth associated Discrepancies in the Upper Bound of the Clear- with the abutment shape of a vertical wall and is obtained Water Abutment-Scour Laboratory and Field Data by dividing the measured scour depth by the appropriate abutment shape correction factors as described in Melville Figure 36 highlights the distinct difference in the upper bounds of relative scour for the laboratory and field data, (1992). Melville (1992) noted that the upper bound of ysadj /y increased with increasing L/y and identified three abutment- with the maximum relative abutment-scour depth for the length categories where the rate at which scour increased laboratory data being 11 in contrast to 4.25 for the field data. varied. These categories, as identified in figure 35, included The maximum relative abutment-scour depth is the measured short abutments (L/y ≤ 1) with the smallest scour potential, abutment-scour depth adjusted for the effect of abutment long abutments (L/y ≥ 25) with the largest scour potential, shape (ysadj ) divided by the approach flow depth y( ). Benedict and intermediate abutments between these values. The other (2016) noted that this large discrepancy is likely caused by laboratory data listed in table 10 were adjusted in a manner the oversimplification of the laboratory models in contrast similar to the Melville (1992) data and also are shown in to the complexity of the field. In particular, the small to figure 35. All of the laboratory data fall within or close to the moderate drainage areas associated with most of the field data Melville (1992) envelope curve, indicating that the envelope (tables 7, 11) will tend to have peak-flow durations lasting curve is a reasonable representation of the approximate upper only hours (Benedict, 2003), thus preventing the attainment of bound of abutment scour for laboratory data. Additionally, the equilibrium abutment-scour depths similar to those associated data shown in figure 35 confirm the general pattern that the with laboratory investigations that often run for days (Sturm, potential for abutment-scour depth increases at a decreasing 2004). Additionally, the sediments in the field are non-uniform rate as the embankment length increases. It also is noteworthy in grain size and often have some measure of cohesion. This that the maximum scour depth based on the envelope curve in is particularly the case for flood plain sediments that consist figure 35 is about 10 times the approach flow depth. of varying mixtures of clays, silts, and sands, along with The South Carolina field data (table 7) and the field data organic matter, and this is undoubtedly why the upper bound from table 11 were adjusted for abutment shape and skew of the South Carolina Piedmont and Alabama data, generally similar to the manner of the Melville (1992) laboratory data having cohesive sediments, are smaller than the non-cohesive and plotted with the previously presented laboratory envelope data. Furthermore, sediment characteristics typically change curve (fig. 36). The Melville (1992) laboratory data were with depth, often becoming more resistant to scour and thus included in the figure to provide perspective between the limiting scour depth. It is common to see abutment-scour holes laboratory and field data patterns. Benedict (2016) developed cut through the more erodible alluvium sediments on a flood approximate field envelope curves for both non-cohesive plain and then encounter a subsurface, scour-resistant sedi- and cohesive sediments as shown in figure 36. The field data ment that impedes or limits scour. This was the pattern seen in encompass the range of the three abutment-length categories; the deeper scour holes in the South Carolina data (Benedict, however, the data are heavily weighted toward the long- 2003). Similarly, this was the pattern for the scour hole at abutment category, where scour potential is greatest, with I–70 crossing the Missouri River (Parola and others, 1998). few short abutments. The Missouri River measurements are These field patterns indicate that scour-resistant subsurface the largest scour depths for the field data and are associated sediments will frequently impede or limit scour and highlights with prolonged flood flows and adverse field conditions (levee the importance of understanding the sediment characteristics breach) that tend to create severe scour. These data fall in and subsurface strata at a site to gain insights regarding close proximity to the upper bound of the South Carolina data, scour potential. 44 The South Carolina Bridge-Scour Envelope Curves
100 Short Intermediate Long E PLANATION abutment abutment abutment Envelope (Melville, 1992) Palaviccini (1993) Sturm (2004) Briaud and others (2009) 10 Ettema and others (2010) Melville (1992) ysadj Adjusted scour depth L Embankment length y Flow depth
1
0.1 Adjusted scour depth divided by flow
0.01 0.001 0.01 0.1 1 10 100 1,000 Embankment length divided by flow depth
Figure 35. Relation of the relative abutment-scour depth (ysadj /y ) to relative abutment length (L /y ) for selected laboratory data (from Benedict, 2016).
100 Short Intermediate Long E PLANATION abutment abutment abutment Envelope (Melville, 1992) Field envelope (non-cohesive) Field envelope (cohesive) Melville (1992) 10 Missouri River at levee breach (1993 flood) Missouri River at bridge (1993 flood) South Carolina Piedmont South Carolina Coastal Plain Maine National Bridge Scour Database Alabama y Adjusted scour depth 1 sadj L Abutment length y Flow depth
0.1 Adjusted scour depth divided by flow
0.01 0.001 0.01 0.1 1 10 100 1,000 Abutment length divided by flow depth
Figure 36. Relation of the relative abutment-scour depth (ysadj /y ) to relative abutment length (L /y ) for selected laboratory and field data (from Benedict, 2016).
SC Bridge Scour Figure 35
SC Bridge Scour Figure 36 Comparison of the South Carolina Clear-Water Abutment-Scour Envelope Curves With Other Data 45
Comparison of the South Carolina Abutment- explanation for the exceedance. With respect to the Missouri Scour Envelope Curves to Other Abutment-Scour River data, Benedict (2016) noted that the levee breach associated with those measurements provide some explanation Field Data for their exceedance of the South Carolina envelope curves. Moreover, the large drainage area (500,000 mi2), which greatly To provide some support of the South Carolina abutment- exceeds those of the South Carolina data, having a median scour envelope curves, Benedict (2015) plotted the field data value less than 100 mi2, is indicative of longer flow durations from table 11 and the South Carolina data from table 7 against and deeper deposits of alluvium flood plain sediments, both of the envelope curves shown in figures 37 through 39. (Note: which will promote larger scour depths than those of the South The larger of the two Missouri River data points [56 ft of Carolina data. Figure 40 shows the relation of abutment-scour scour] was excluded from these figures to provide a better depth with respect to drainage area, using the same field plotting scale.) The Maine data (table 11) have characteristics data as figure 37 and including both measurements from the that are distinctly different from those of the South Carolina Missouri River. The upper bound of the abutment-scour data data (table 7) and, therefore, do not provide an ideal compar- clearly increases with increasing drainage area and demon- ison. However, the Maine data do provide some perspective on strates how the drainage area for the Missouri River data is the range of abutment-scour depths for small, steep gradient substantially beyond the range of the South Carolina data. streams. The Alabama data are associated with cohesive sedi- The upper-bound pattern of the abutment-scour data shown in ments, providing a good comparison for the Piedmont data. figure 40 highlights the importance of limiting application of The NBSD data have site characteristics similar to the Coastal the South Carolina abutment-scour envelope curves to sites Plain data, providing a good comparison for those data. having characteristics well within the range of the South Caro- The Piedmont and Coastal Plain geometric-contraction lina data. It is notable that the Coastal Plain envelope curve ratio abutment-scour envelope curves and the field data are in figure 37 encompasses all of the NBSD field data, which shown in figure 37. Regarding the data patterns in figure 37, have site characteristics similar to the Coastal Plain data, Benedict (2016) made the following observations. Three data providing support for the Coastal Plain geometric-contraction points substantially exceed the envelope curves, including ratio envelope curve. Excluding the previously noted outliers, the two Missouri River data points (the largest point is not all of the field data fall within or very near the Piedmont displayed) and one data point from Maine. The data point from envelope curve (fig. 37), providing support for that curve. If Maine was collected with GPR, which tends to have a larger the Piedmont envelope curve is extended to the limits of the measurement uncertainty (Benedict, 2003), providing some Coastal Plain envelope curve (m=0.98), the extended curve
30 E PLANATION Piedmont envelope curve Piedmont envelope curve extension Coastal Plain envelope curve Missouri River at bridge (1993 flood) South Carolina Piedmont
t South Carolina Coastal Plain e
f Maine 20 n
i National Bridge Scour Database
,
h Alabama t d e p
r o u
10 Abutment-s c
0 0 0.2 0.4 0.6 0.8 1 Geometric-contraction ratio
Figure 37. Comparison of the South Carolina abutment-scour envelope curves for geometric-contraction ratio to field data from other sources (from Benedict, 2016).
SC Bridge Scour Figure 37 46 The South Carolina Bridge-Scour Envelope Curves
30 E PLANATION Piedmont envelope curve Coastal Plain envelope curve Missouri River at bridge (1993 flood) South Carolina Piedmont South Carolina Coastal Plain t Maine e e f 20 National Bridge Scour Database n i Alabama
, h t p d e
r u o
10 Abutment-s c
0 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 Embankment length blocking flow, in feet
Figure 38. Comparison of the South Carolina abutment-scour envelope curves for embankment length to field data from other sources (from Benedict, 2016).
20 E PLANATION Piedmont envelope curve Coastal Plain envelope curve South Carolina Piedmont South Carolina Coastal Plain Maine t
e National Bridge Scour e f Database n i
, Alabama h t p e d
r
u 10 o c Abutment- s
0 0 200 400 600 800 1,000 1,200 Embankment length blocking flow, in feet
Figure 39. Comparison of the South Carolina abutment-scour envelope curves for embankment length to field data from other sources (truncated scale; from Benedict, 2016).
SC Bridge Scour Figure 38
SC Bridge Scour Figure 39 Comparison of the South Carolina Clear-Water Abutment-Scour Envelope Curves With Other Data 47
60 E PLANATION Excessive scour Missouri River at levee breach (1993 flood) caused by levee Missouri River at bridge (1993 flood) breach 50 South Carolina Piedmont South Carolina Coastal Plain Maine National Bridge Scour Database t Alabama e f 40 n i
, h t d e p
r 30 o u
20 Abutment-s c
10
0 1 10 100 1,000 10,000 100,000 1,000,000 Drainage area, in square miles
Figure 40. Relation of abutment-scour depth to drainage area for selected field data.
provides a conservative envelope curve for the field data, with openings did not always conform to the abutment-length the exception of the previously noted outliers. Although the envelope curves, in part, because of the difficulty in deter- South Carolina geometric-contraction ratio envelope curves mining the embankment length blocking flow at such sites, can provide perspective on scour potential for the small, providing some explanation for their exceedance. There coarse-sediment streams of Maine, the curves generally will are 18 Maine data points that exceed the envelope curves. be conservative for such sites, especially at larger values of the Thirteen data points are small scour depths (3 ft or less), geometric-contraction ratio. falling in close proximity to the curves and are not of major The Piedmont and Coastal Plain embankment-length concern. The remaining five data points are associated with abutment-scour envelope curves and the field data are shown GPR measurements and fall approximately 5 to 6 ft away from in figures 38 and 39, with figure 39 having a truncated scale the curves. As noted previously, GPR tends to have a larger to highlight the data patterns for smaller abutment lengths. measurement uncertainty, providing some explanation for the Regarding the data patterns in figures 38 and 39, Benedict exceedance of these data. (2016) makes the following observations. Most of the data fall Although the field data (table 11) used to compare with within the envelope curves, providing support for the curves. the South Carolina abutment-scour envelope curves are The exceedance of the Missouri River data is to be expected, limited, the data indicate that the envelope curves are reason- and reasons for the exceedance were noted previously. The able for sites having similar characteristics to those of the NBSD field data have similar site characteristics to the South South Carolina data. The exceedance of data for the Missouri Carolina data and fall close to or within the Piedmont and River at I–70 indicates that the South Carolina abutment- Coastal Plain envelope curves, providing support for the scour envelope curves are not appropriate for sites with large curves. The Alabama data that exceed the South Carolina drainage areas and highlights the importance of limiting their envelope curves are associated with multiple-bridge openings. application to sites having similar characteristics to the data Benedict (2003) noted that field data from multiple-bridge listed in table 7.
SC Bridge Scour Figure 40 48 The South Carolina Bridge-Scour Envelope Curves
Relative Increase in Theoretical the 100- and 500-year flows that were used to evaluate the 500-year flow adjustment coefficient. In addition to the Abutment Scour Associated With the South Carolina data, the Missouri level-2 data (Huizinga and Rydlund, 2004) included theoretical scour computations at 223 100- to 500-Year Flows abutment-scour depths that were used as a means to confirm the trends of the South Carolina data. The Missouri data are As previously described in the report section “South available at http://pubs.usgs.gov/sir/2004/5213/. Carolina Clear-Water Abutment-Scour Field Data,” a substan- tial portion of the field data used to develop the South Carolina abutment-scour envelope curves included scour measurements The 500-Year Flow Adjustment Coefficient likely associated with historic flows near the 100-year flow magnitude. Therefore, the envelope curves can be used to The relation of the theoretical abutment scour for the assess scour potential for such flow conditions (Benedict, 100- and 500-year flows for the South Carolina and Missouri 2003). (Note: The abutment-scour envelope curves should data is shown in figure 41, including trend lines through not be considered a definitive estimate of the abutment-scour each dataset. Summary statistics for the data are provided depth associated with the 100-year flow.) However, the South in table 12. The scatter about the trend line for the South Carolina abutment-scour envelope curves were not recom- Carolina data is small, and the coefficient of determination is mended for assessing scour potential for extreme floods such high (0.96) indicating that there is a strong correlation in the as the 500-year flow (Benedict, 2003). In order to gain insights data. The trend line through the Missouri data falls in close on the relative increase in abutment-scour depth associated proximity to the South Carolina trend line indicating that the with the 100- to 500-year flows, a theoretical adjustment South Carolina trend line is reasonable. The equation for the coefficient similar to those developed for pier scour (see the South Carolina trend line is as follows: report section “Relative Increase in Theoretical Pier Scour Scour = 1.21Scour (18) Associated With the 100- to 500-Year Flows”) can be devel- 500 100 oped for abutment scour. Although such adjustments do not provide a definitive estimate of the 500-year abutment-scour where depth, they do provide perspective on the relative increase. Scour500 is the 500-year flow theoretical scour A summary of the compiled theoretical abutment-scour data depth, in feet; and and the analysis of the 500-year flow adjustment coefficient is Scour100 is the 100-year flow theoretical scour presented below. depth, in feet.
Equation 18 can be divided by Scour100 and simplified to the Theoretical Abutment-Scour Data following form:
The South Carolina Theoretical Bridge Scour Database Scour500= 1.21 (19) (http://dx.doi.org/10.3133/sir20165121), as previously described in the report section “Theoretical Pier-Scour Data,” where K500 is the theoretical 500-year flow adjustment includes 134 theoretical abutment-scour depths for both coefficient that represents the ratio of the 500-year flow
Table 12. Descriptive statistics for the ratio of the 500- and 100-year flow, theoretical abutment-scour depths used in the regression analysis for selected theoretical data from South Carolina and Missouri.
[N, number of measurements; Q500, 500-year flow; Q100, 100-year flow; 2mi , square mile]
South Carolina (N = 134) Missouri (N = 223) Descriptive Ratio of Q500 and Q100 Drainage Ratio of Q500 and Q100 Drainage statistic theoretical abutment- area theoretical abutment- area scour depths (mi2) scour depths (mi2) Minimum 0.83 5.12 1.01 1.30 25th percentile 1.16 15.3 1.22 12.6 Mean 1.24 643 1.65 269 Median 1.21 46.5 1.32 28.1 75th percentile 1.29 108.0 1.70 141 Maximum 1.93 12,990 9.60 14,000 Application and Limitations ofthe South Carolina Clear-Water Abutment-Scour Envelope Curves 49
90 E PLANATION Trend line for South Carolina data 80 Trend line for Missouri data South Carolina data Missouri data 70 500 Scour depth for the 500-year flow 100 Scour depth for the 100-year flow R2 Coefficient of determination 60 500 = 1.26 100 R2 = 0.91 50
abutment-scour depth, in feet 40
a l c i t e r
o 30 e h t
a r
e 20 = 1.21
500- y 500 100 2 10 R = 0.96
0 0 10 20 30 40 50 60 70 80 100-year theoretical abutment-scour depth, in feet
Figure 41. Relation of the 100- and 500-year flow, theoretical abutment-scour depths for selected data from level-2 bridge-scour studies in South Carolina and Missouri. to 100-year flow theoretical abutment-scour depths. Other geometric-contraction ratio, select the appropriate abutment- variables are as previously defined. TheK 500 coefficient can scour envelope curve, and evaluate other scour components be applied to the South Carolina abutment-scour envelope in the abutment region. It also is important that the user be curves to increase the abutment-scour depth by the relative mindful of the limitations and potential uncertainty of these increase in theoretical abutment scour associated with the curves. The clear-water abutment scour envelope curves were
100- to 500-year flow condition. TheK 500 is a helpful tool developed using modeled hydraulic data to provide an esti- for gaining perspective on the relative increase of theoretical mate of the “true” hydraulic conditions that may have created scour associated with the 100- to 500-year abutment-scour the observed scour. It is important that site characteristics at depth. However, the adjusted envelope curve values should the bridge site to which the envelop curves are being applied not be considered a definitive estimate of the abutment scour have similar characteristics to the sites used to develop the associated with the 500-year flow. envelope curves. The envelop curves were developed using field data from sites with flows approaching the 100-year flow but should not be interpreted as representing the “100-year” scour estimate and also should not be used to evaluate clear- Application and Limitations of water abutment scour depths for extreme conditions, such as the South Carolina Clear-Water those approaching the 500-year flow. To gain insights on the relative change in the theoretical clear-water abutment scour Abutment-Scour Envelope Curves depth associated with the 100- to 500-year flow, the 500-year scour coefficient (eq. 19) may be applied to the envelope curve The South Carolina clear-water abutment-scour estimate. Data used to develop the clear-water abutment scour envelope curves (figs. 26, 27, 30, 31, 33, 34) can be useful envelope curves did not include failure from embankment supplementary tools for assessing clear-water abutment-scour washout and, therefore, the envelope curves should not be depth at bridges in South Carolina. When using these envelope used to assess this type of abutment scour. The envelope curves to assess abutment-scour potential, one must select a curves are not intended for tidally influenced sites that may reference surface, estimate the embankment-length and the be encountered in the Coastal Plain.
SC Bridge Scour Figure 41 50 The South Carolina Bridge-Scour Envelope Curves
Selecting a Reference Surface Benedict [2003], which were based on the WSPRO model. Guidance regarding this matter is presented in the report Benedict (2003) used the average, undisturbed flood plain section “Guidance for Applying the South Carolina Bridge- elevation in the abutment-scour region as the reference surface Scour Envelope Curves.”) to determine the abutment-scour depth for the South Carolina data, and this reference surface should be used when assessing abutment scour with the South Carolina abutment-scour Selecting the Appropriate Clear-Water envelope curves. The reference surface can be determined by Abutment-Scour Envelope Curve reviewing flood plain elevations from SCDOT road and bridge plans, surveyed cross sections, and (or) site visit observations. Clear-water abutment-scour depth can be assessed In many cases, the flood plain in the region of the abutment using the original clear-water abutment-scour envelope is relatively flat, so estimating an average flood plain eleva- curves (Benedict 2003; figs. 26, 27, 30, 31; table 7) or tion in the region of anticipated scour is not a difficult task. the modified clear-water abutment-scour envelope curves However, there can be cases where the flood plain slopes (Benedict and Caldwell, 2012; figs. 33, 34; tables 8, 9). substantially in the lateral and (or) longitudinal direction The original envelope curves have broader application than making the determination of a reference surface more difficult. the modified envelope curves, with the modified envelope In such cases, judgment must be applied, bearing in mind curves providing some refinement for assessing the potential that lower reference-surface elevations will produce lower for abutment scour at smaller embankment lengths. Criteria scour-hole elevations. for selecting the appropriate envelope curve will be based on the regional location and site characteristics for the Estimating the Embankment Length and the bridge of interest. Selection guidance with respect to the Geometric-Contraction Ratio modified and original envelope curves follows.
The explanatory variables used in the original and Original Clear-Water Abutment-Scour modified abutment-scour envelope curves are the embankment Envelope Curves length and the geometric-contraction ratio. The envelope curves can be sensitive to the selection of these variables; In general, the original clear-water abutment-scour therefore, it is important that accurate estimates of these envelope curves (Benedict, 2003) have broader application variables be obtained. When developing the envelope curves, than the modified clear-water envelope curves (Benedict and the embankment length and the geometric-contraction ratio Caldwell, 2012). When assessing abutment-scour depths were determined by using the WSPRO flow model (Shearman, using the original envelope curves, one must select envelope 1990; Arneson and Shearman, 1998) for the 100-year flow curves for the appropriate physiographic region (Piedmont condition (figs. 28, 29). When applying the envelope curves or Coastal Plain) and use both the embankment-length at specific sites, these variables also should be estimated (figs. 26, 27) and the geometric-contraction ratio (figs. 30, for the 100-year flow at the site of interest. The WSPRO 31) envelope curves for the selected region to assess model generally uses the full flood plain width (left edge of potential abutment-scour depth. These two envelope curves water to right edge of water) for the natural, unconstricted often provide different estimates for the upper bound of approach cross section, with no adjustment for ineffective abutment-scour depth, so one must use judgment to select flow areas, thus providing the largest justifiable estimate for the most appropriate depth. Application of these envelope embankment lengths and geometric-contraction ratio. It is curves to selected sites in South Carolina has shown that the important that a similar approach be used when determining geometric-contraction-ratio envelope for both physiographic these variables to assure that the largest justifiable estimate regions will often provide higher estimates of the upper is obtained on the basis of the full flood plain width for bound of abutment-scour depth than the embankment-length the natural, unconstricted approach cross section. It also is envelope curves. The larger estimates can be attributed, recommended that topographic maps and road plans, when in part, to the fact that the geometric-contraction-ratio available, be used to verify the embankment lengths and envelope curves cannot account for variation in embank- the geometric-contraction ratio. When discrepancies exist ment length at a given abutment. For example, a 100-ft-long between these sources, judgment should be used to determine bridge crossing a 500-ft-wide Piedmont flood plain has a a reasonable, yet conservative, estimate of the explanatory geometric-contraction ratio of 0.8. Similarly, a 200-ft-long variables. (Note: The bridge hydraulics algorithm and associ- bridge crossing a 1,000-ft-wide flood plain will have the ated cross sections in the HEC-RAS model [Brunner, 2016] same geometric-contraction ratio. If these bridges are differ from that of the WSPRO model, and when using the symmetrically located on the flood plain, the embankment HEC-RAS model to estimate the embankment lengths and lengths will be 200 and 400 ft, respectively. Using the the geometric-contraction ratio, judgment must be used to geometric-contraction-ratio envelope curve (fig. 30) to assure that the estimates are comparable to those used in estimate abutment-scour depth, the scour depth will be Application and Limitations ofthe South Carolina Clear-Water Abutment-Scour Envelope Curves 51 identical for both bridges. It is reasonable, however, to assume Single-Bridge Openings (with all other variables the same) that the bridge with the shorter embankments will have smaller scour depths than the For the bridge sites associated with the Piedmont and bridge with longer embankments. Coastal Plain data used to develop the original abutment-scour Another example of where excessive estimates of envelope curves (Benedict, 2003), 94 and 62 percent of the abutment-scour depth may occur is at sites having eccentric bridges, respectively, were associated with single-bridge open- bridge crossings where one embankment is substantially ings. All of the single-bridge opening data for the Piedmont shorter in length. In such cases, the geometric-contraction- and Coastal Plain regions plotted within their respective ratio envelope will give identical scour depths at both embankment length and geometric-contraction ratio envelope abutments; however, with other variables remaining constant, curves. Therefore, if a single-bridge opening in the Piedmont it is reasonable to assume that scour depths will be less at or Coastal Plain has site conditions similar to bridges included the shorter embankment of an eccentric bridge crossing. To in this study (table 7), it is appropriate to use the embankment avoid overestimates of the upper bound of abutment-scour length or the geometric-contraction ratio envelope curves depth, it may be reasonable to use the embankment-length to estimate the upper bound of abutment-scour potential. envelope curve as the primary tool for estimating abutment- As previously discussed, it may be reasonable to use the scour potential in the Piedmont and Coastal Plain of South embankment-length envelope curve as the primary tool for Carolina. If a conservative estimate of the upper bound of estimating abutment-scour depth at single-bridge openings in abutment-scour potential is needed, the largest estimate of the the Piedmont and Coastal Plain of South Carolina. If, however, upper bound of abutment scour obtained from the geometric- a conservative estimate of the upper bound of abutment-scour contraction-ratio and embankment-length envelope curves potential is needed, the larger scour depth obtained from the may be more appropriate. geometric-contraction ratio or embankment-length envelope curves may be more appropriate. Benedict (2003) noted that bridges crossing swamps Modified Clear-Water Abutment-Scour with poorly defined low-flow channels, with bridge lengths Envelope Curves approximately 240 ft or less, tend to develop a single large scour hole that encompasses the entire bridge opening (fig. 24; The modified abutment-scour envelope curves (Benedict see the report section “Field Conditions That Influence and Caldwell, 2012) can be used to provide refined estimates Clear-Water Abutment-Scour Potential”). To associate these of the upper bound of abutment scour potential for smaller single scour holes with abutment variables, Benedict (2003) embankment lengths. The modified envelope curves are recommended using the longer of the left or right embank- limited to embankment lengths less than or equal to 500 ft, and ment lengths in the assessment. Therefore, when assessing the geometric-contraction ratios should not exceed 0.85 or 0.9 abutment-scour potential at single-bridge openings crossing for the Piedmont or Coastal Plain, respectively. The limits of swamps, with bridge lengths approximately 240 ft or less, the explanatory variables for the modified abutment-scour the longest of the left or right embankment lengths should be envelope curves are presented in table 9. If the geometric- used in conjunction with the appropriate embankment-length contraction ratio for the bridge of interest is less than the envelope curves that utilize this variable (figs. 26, 27, 33, 34). minimum value specified in table 9, the minimum value The geometric-contraction-ratio envelope curves (figs. 30, 31) should be used in the equation. Additionally, the site charac- also may be used to assess abutment-scour potential at such teristics of the bridge of interest should fall within the range bridges, using judgment to select the final value. The estimated of the South Carolina regional data (Piedmont or Coastal upper bound of the scour depth will be assumed to represent Plain) used to develop the modified envelope curves as listed the depth of the large, single scour hole that will likely extend in table 8. If the above criteria are not met, then the original from abutment toe to abutment toe. clear-water abutment-scour envelope curves (Benedict, 2003) must be used. Multiple-Bridge Crossings For embankment lengths that fall between the embank- ment-length categories of the modified envelope curves Special consideration must be given to the selection of (figs. 33, 34; table 9), it is possible to interpolate between the appropriate envelope curve when dealing with multiple- the modified envelope curves in order to refine the estimate bridge openings, and a discussion of assessing abutment-scour of the upper bound of abutment-scour potential; however, it depth at such bridges follows. Flood plain widths in the may be prudent to apply the modified envelope curves only Piedmont are relatively narrow, and therefore, multiple-bridge by embankment-length category. Using this category applica- openings are not common. In contrast, flood plains in the tion, there may be cases when the modified envelope curves Coastal Plain are typically flat and wide and often have provide larger values of abutment-scour potential compared multiple-bridge openings. The South Carolina abutment-scour to the original abutment-scour envelope curves. In such cases, data included 2 bridges associated with a multiple-bridge it is recommended that the value from the original abutment- crossing in the Piedmont in contrast to 31 in the Coastal scour envelope curve be used in the assessment. Plain. Five abutment-scour measurements associated with 52 The South Carolina Bridge-Scour Envelope Curves multiple-bridge crossings plotted above the original embank- judgment must be used to assure that conservative yet reason- ment-length envelope curves (figs. 26, 27). This anomaly able estimates of L and m are obtained. Reference should be is possibly caused by the irregular distribution of flow that made to Arneson and Shearman (1998) and Brunner (2016) for commonly occurs at a multiple-bridge site. In addition, stagna- additional details regarding the hydraulics of multiple-bridge tion points for flow between multiple bridges, as estimated openings for the WSPRO and HEC-RAS models, respectively. by the simplified routine in the WSPRO (Shearman, 1990; Arneson and Shearman, 1998) model, may be in error. Any Contraction and Pier Scour Within the Clear-Water error in this estimate will provide improper estimates of the embankment lengths. In contrast, all data (single and multiple Abutment-Scour Region bridges) plotted near or within the geometric-contraction-ratio Benedict (2003) concluded that contraction scour should envelope curves. Therefore, to assure that abutment-scour not be considered a contributing component to total scour in potential is not underestimated at a multiple-bridge crossing, the abutment-scour region. It is notable that Sturm (2004) Benedict and Caldwell (2012) recommended that the modified and Ettema and others (2010) made similar conclusions on abutment-scour envelope curves (figs. 33, 34) not be used to the basis of laboratory investigations, giving support for assess multiple-bridge openings. Additionally, Benedict (2003) this recommendation. Therefore, when using the original recommended that only the original geometric-contraction- (Benedict, 2003) or modified (Benedict and Caldwell, 2012) ratio envelope curves (figs. 30, 31) be used to assess abutment- clear-water abutment-scour envelope curves to assess total scour potential at multiple-bridge crossings rather than the scour depth at abutments, no adjustment for contraction scour original embankment-length envelope curves (figs. 26, 27). in the abutment region is required. The exception to this recommendation is for multiple-bridge With regard to pier scour, Benedict (2003) concluded that openings in the Coastal Plain where the embankment length is the South Carolina abutment-scour data included any influ- greater than or equal to 426 ft. This value represents the break- ence of pier scour for the piers located in the abutment-scour point for the Coastal Plain embankment-length envelope curve region at the time of the measurement. A review of the field (fig. 27) where the upper bound of abutment-scour potential data indicated that piers located in the abutment-scour region becomes less sensitive to the selection of the embankment were limited to multiple-column bents, having pier widths length. Additionally, the multiple-bridge opening data that 2.3 ft or less with minimal skew. Therefore, when using the exceeded the Coastal Plain embankment-length envelope original (Benedict, 2003) or modified (Benedict and Caldwell, curve were all associated with embankment lengths less than 2012) clear-water abutment-scour envelope curves to assess 426 ft. Therefore, when a multiple-bridge site in the Coastal total scour depth at abutments, no adjustment is required for Plain has an embankment length that equals or exceeds pier scour associated with the pier characteristics of the South 426 ft, Benedict (2003) noted that the original embankment Carolina data as previously described. The South Carolina length and geometric-contraction ratio envelope curves for abutment-scour data did not include sufficient information to the Coastal Plain could be used to assess the upper bound of draw conclusions regarding the influence of piers having pier abutment-scour potential following the guidance for a single- widths greater than 2.3 ft. Therefore, when pier widths exceed bridge opening. For a multiple-bridge site in the Coastal Plain 2.3 ft, judgment should be used to determine if the results of with an embankment length less than 426 ft, assessment of the the clear-water abutment-scour envelope curves should be upper bound of abutment-scour potential should be limited to adjusted to account for the effects of the wider piers. Judgment the original geometric-contraction ratio envelope curve. also should be used to account for the effects of debris and (or) The determination of the embankment length and pier skew if they are thought to substantially influence pier geometric-contraction ratio for a multiple-bridge opening scour, regardless of the pier width. differs from that of a single-bridge opening. In the case of The above guidance for piers is slightly modified for a single-bridge opening, the unconstricted approach cross the Piedmont. When the upper bound of abutment-scour section is used to evaluate both variables. In the case of a potential in the Piedmont is estimated to be 5 ft or less with multiple-bridge opening, the WSPRO model estimates the the original or modified envelope curves, judgment should be portion of the unconstricted approach cross section that used to account for the effect of pier scour within the abutment contributes flow to each bridge and subdivides the approach region regardless of the pier width. Ettema and others (2010) cross section accordingly into slices. The geometric- conducted a limited laboratory investigation regarding the contraction ratio is determined for each bridge by using influence of a pier on abutment-scour depth and concluded that the associated slice section rather than the entire approach pier presence in the abutment-scour region did not substantially cross section. Each bridge opening can be projected onto the increase abutment-scour depth. The limited nature of the corresponding slice section to determine the left and right investigation makes it difficult to draw broad applications from embankment lengths. The multiple-bridge opening hydraulics the investigation, but reference can be made to that report to algorithm in the HEC-RAS model (Brunner, 2016) is similar gain insights on the influence of a pier in the abutment-scour but not identical to the algorithm in the WSPRO model. region. For additional information on the recommendations Therefore, when using the HEC-RAS model to evaluate L for contraction and pier scour in the abutment-scour regions, and m at a multiple-bridge opening, caution and engineering reference can be made to Benedict (2003). Application and Limitations ofthe South Carolina Clear-Water Abutment-Scour Envelope Curves 53
Top Width of Abutment-Scour Hole • The abutment-scour envelope curves were developed from a limited sample of bridges in the Piedmont Benedict (2003) observed that the toe of the abutment and Coastal Plain, and it is possible that scour depths frequently coincided with the edge of the abutment-scour hole. could exceed the envelope curves. Additionally, Therefore, using the abutment toe as a reference location for the the explanatory variables, embankment length and bank of the abutment-scour hole, the lateral extent of the area geometric-contraction ratio, used to develop the affected by scour can be assessed by estimating the scour-hole abutment-scour envelope curves, were based on top width. Figure 42 shows the relation of observed scour-hole modeled hydraulic data that will have some degree top widths and observed scour depths for two abutment-scour of error. On the basis of these limitations, it may be hole patterns: (1) individual abutment-scour holes at the left prudent to apply a safety factor to the abutment-scour and (or) right abutments that do not overlap (figs. 21 and 25) envelope curves. and, (2) single abutment-scour holes that encompassed the entire bridge opening, abutment toe to abutment toe (fig. 24). • Application of the abutment-scour envelope curves Figure 42A represents the envelope for pattern 1 and includes should be limited to bridges having site characteristics bridges in the Piedmont and Coastal Plain. As noted previously similar to those used to develop the envelope curves (see report section, “Field Conditions That Influence Clear- (tables 7, 8). Also, the abutment-scour database Water Abutment-Scour Potential”) bridges over poorly defined (Benedict, 2003) can be used to review studied sites to swampy channels or flood plain relief bridges (fig. 5) approxi- gain insights about potential scour at unstudied sites. mately 240 ft or less in length tended to form a large, single Limits for embankment length and the geometric- scour hole that encompassed the entire bridge opening from contraction ratio have been defined for each envelope, abutment toe to abutment toe (fig. 24). These types of bridges and care should be given to remain within those limits. with scour-hole pattern 2 are represented in figure 42B. (Note: When site characteristics are substantially different Bridges approximately 240 ft or less and having well defined from the site characteristics of the sites used in the channels will tend to have abutment-scour holes patterns like current study, other methods for assessing abutment- pattern 1 as represented in figure 42A.) scour potential should be considered. The scatter in the depth-to-width ratio of the observed • The abutment-scour envelope curves were developed scour is large, making estimates of scour-hole top width by using field data that represent scour resulting difficult. Using the envelope curves in figure 42, a conservative from flows near the 100-year flow but should not estimate of top width can be obtained. These estimates could be interpreted as representing the “100-year” scour be unreasonable under some site conditions. At Piedmont sites estimate and should not be used to evaluate clear-water where the overbank width (at setback abutments) is less than the abutment-scour depths for larger flows, such as the estimated abutment scour-hole top width, it may be reasonable 500-year flow. To gain insights on the relative change to limit the estimated scour-hole top width to the width of the in the theoretical abutment-scour depth associated overbank. For bridges having site characteristics comparable to with the 100-year to 500-year flow, the 500-year scour abutment-scour hole pattern 2 (poorly defined swampy channels coefficient (eq. 19) may be applied to the envelope or flood plain relief bridges (fig. 5) and approximately 240 ft curves but should not be interpreted as representing or less in length), it may be reasonable to assume that the the “500-year” scour estimate. scour-hole top width equals the toe-to-toe width. • An additional scour problem that frequently occurs in South Carolina is the washout of road embankments. Limitations of the South Carolina Clear-Water Typically, washout occurs at smaller bridges that Abutment-Scour Envelope Curves create a large contraction of flow. The magnitude of this problem is seen in bridge failure statistics The South Carolina clear-water abutment-scour envelope from the October 1990 flood. This event caused curves (figs. 26, 27, 30, 31, 33, 34) can be useful supplemen- 80 bridges to fail in South Carolina with 79 of these tary tools for assessing clear-water abutment-scour potential at failures attributed to embankment washout (Hurley, bridges in South Carolina. However, the following limitations 1996). Data collected in the current study did not of these empirical envelope curves can restrict their use. include failure from embankment washout; therefore, Therefore, the envelope curves should not be relied upon as envelope curves in this report should not be used to the only tool for assessing abutment scour but rather should be assess this type of abutment scour. used in conjunction with the prevailing scour-prediction tech- nology (currently HEC-18). To best assess potential scour, one • The abutment-scour envelope curves do not account should compile and study the available information for a given for adverse field conditions that may affect abutment site, evaluate scour with the South Carolina envelope curves scour at bridges, such as channel bends, debris, and the prevailing scour technology (currently HEC-18), and or field conditions that may create unusual flow then bring sound engineering principles to bear on the final distributions that direct flow into the abutment-scour estimate of potential abutment-scour depth. region during high flows. 54 The South Carolina Bridge-Scour Envelope Curves
A 0
n elope cur e 0
0
0
40
30
20 E T 0 Envelope curve ased on points identi ied circles served a ut ent scour ro ield investigation oints ar itraril chosen to develop envelope curve 0 0 0 20 bser ed abutment-scour depth, in feet B 40 E T Envelope curve ased on points identi ied circles n elope cur e 20 served a ut ent scour ro ield investigation oints ar itraril chosen to develop envelope curve
bser ed abutment-scour-hole top width, in feet top abutment-scour-hole bser ed 00
0
0
40
20
0 0 0 20 2 bser ed abutment-scour depth, in feet
Figure 42. Relation of abutment scour-hole top width and abutment-scour depth at bridges (A) greater than 240 feet in length and (B) swampy and flood plain relief bridges, 240 feet or less in length, in the Piedmont and Coastal Plain of South Carolina (from Benedict, 2003). The South Carolina Clear-Water Contraction-Scour Envelope Curve 55
The South Carolina Clear-Water from 0.0 to 4.5 ft. The flood plain soils of the Piedmont are generally clayey with varying degrees of cohesion, and the Contraction-Scour Envelope Curve median grain size ranged from less than 0.062 to 0.99 milli- meter (mm). The bridge sites where data were collected also Bridge contraction scour occurs when a bridge substan- were part of the abutment-scour investigation by Benedict tially constricts the natural flow of a stream and causes the (2003), and reference can be made to the report section streambed to erode in the general region of the bridge. The “South Carolina Clear-Water Abutment Scour Field Data” mechanisms that create the contraction scour are associated for additional information about these sites. Expanding with increased flow velocities and vortexes generated by the on the work by Benedict (2003), Benedict and Caldwell contraction of flow. Contraction scour is generally classified as (2006) made 64 measurements of clear-water contraction clear-water or live-bed, which refers to the sediment-transport scour at selected sites in the Coastal Plain of South Carolina conditions at the time of scour (Arneson and others, 2012). (table 13), with scour depths ranging from 0.0 to 3.9 ft. The Clear-water contraction scour occurs when approach-flow soils of the Coastal Plain are generally sandy (with some velocities are insufficient to transport sediments along the clayey soils), and the median grain size ranged from less bed and into the scoured region. Equilibrium conditions for than 0.062 to 0.56 mm. These bridges also were part of the clear-water contraction scour are attained when flow velocities clear-water pier-scour investigation by Benedict and Caldwell in the contraction have been reduced by bed degradation to (2006), and reference can be made to the report section the critical velocity of the bed sediments. In South Carolina, “Clear-Water Pier-Scour Envelope Curve” for additional clear-water contraction scour typically occurs on the flood information about these sites. Bridge sites where the South plain (also called overbank) of a bridge (figs. 4, 5). Live-bed Carolina clear-water contraction-scour data were collected contraction scour is discussed later in the report. can be identified in appendix 1 and figure 1. All measurements were associated with contraction scour occurring in the clear-water scour regions on the bridge South Carolina Clear-Water Contraction-Scour overbanks (fig. 4) or at flood plain relief bridges and bridges Field Data over swampy channels (fig. 5). The scour measurements were made in the contraction-scour region, as defined in figure 23, Benedict (2003) made 75 measurements of clear- beyond the deeper scour depths typically associated with the water contraction scour at selected sites in the Piedmont abutment-scour region. The reference surface used to deter- of South Carolina (table 13), with scour depths ranging mine the contraction-scour depth was the average undisturbed
Table 13. Range of selected characteristics of clear-water contraction scour field data collected in the flood plains of selected sites in South Carolina (Benedict and Caldwell, 2006).
[mi2, square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; mm, millimeter; <, less than]
aAverage aAverage Measured Drainage Channel overbank flow overbank flow Median clear-water Geometric Range area slope velocity at depth at grain size contraction- contraction value (mi2) (ft/ft) the bridge the bridge (mm) scour depth ratio (ft/s) (ft) (ft) South Carolina Piedmont (75 measurements) Minimum 11.0 0.00015 1.7 1.3 < 0.062 0.0 0.02 Median 81.0 0.0012 3.1 7.9 0.09 0.8 0.63 Maximum 1,620b 0.0029 5.8 20.5 0.99 4.5 0.96 South Carolina Coastal Plain (64 measurements) Minimum 26.3 0.00007 0.5 4.2 < 0.062 0.0 0.50 Median 586.0 0.0004 2.0 6.6 0.17 1.8 0.84 Maximum 13,000c 0.0009 6.7 17.9 0.56 3.9 0.96 aValues were estimated from a one-dimensional water-surface profile model. bApproximately 97 percent of the study sites in the Piedmont have drainage areas less than 400 mi2. cApproximately 80 percent of the study sites in the Coastal Plain have drainage areas less than 1,420 mi2. 56 The South Carolina Bridge-Scour Envelope Curves flood plain elevation in the vicinity of the observed scour. have occurred at all bridges. The estimated hydraulic charac- The measurements were assumed to represent the maximum teristics derived from the WSPRO model should be viewed clear-water contraction-scour depth that has occurred at the as approximate rather than measured data. bridge since construction. Because of clear-water scour condi- tions, infill sediments within the scour holes were, in general, Clear-Water Contraction-Scour Envelope Curve negligible. A grab sample of the flood plain surface sediment was obtained in the upstream flood plain at each site and was As described in the report section “Clear-Water analyzed to estimate the median grain size. Because sediment Abutment-Scour Envelope Curves With Respect to the characteristics in the field setting can vary substantially in Geometric-Contraction Ratio,” the geometric-contraction ratio the vertical and horizontal direction, a grab sample taken at is an indicator of the severity of flow contraction created by a a point may not fully represent the sediment characteristics bridge. In general, as the geometric-contraction ratio increases, at a site. The overview of the historical flows as described in the flow velocity through a bridge opening increases, thereby the report sections “Clear-Water Pier-Scour Envelope Curve” increasing the potential for scour. Therefore, it is reasonable and “South Carolina Clear-Water Abutment Scour Field Data” to expect an increase in clear-water contraction-scour depth provide information regarding the historic flows associated with increasing geometric-contraction ratios. Using this with the clear-water contraction-scour sites in the Coastal concept, Benedict (2003) developed an envelope curve for the Plain and Piedmont, respectively, and reference can be made clear-water contraction-scour data collected in the Piedmont to those sections and appendix 2 to gain insight regarding flow (table 13) by using the 100-year flow geometric-contraction conditions that likely created the measured scour. The South ratio as the primary explanatory variable. Benedict and Carolina clear-water contraction-scour depths were measured Caldwell (2006) expanded on the work by Benedict (2003) during low flows, and the flow conditions that produced the by combining the Piedmont and Coastal Plain data (table 13) scour are not known. As with the previously described pier- and evaluating the upper bound of clear-water contraction and abutment-scour data, hydraulic characteristics that may scour for the larger dataset. The analysis yielded the envelope have produced the observed contraction scour were estimated curve shown in figure 43, which varied only slightly from the with the WSPRO (Shearman, 1990) model, assuming the envelope curve developed by Benedict (2003). The equation 100-year flow to be representative of a common flow that may associated with the envelope curve is as follows:
6 E PLANATION 2 Measured clear-water contraction scour for selected ys –6m 10m 0.6 sites in the Coastal Plain of South Carolina where m 0.95 Measured clear-water contraction scour for selected sites in the Piedmont of South Carolina (from Benedict, 2003) ys Estimate of contraction-scour depth along t
e envelope curve, in feet e f m n 100-year flow geometric-contraction ratio
i 4
,
r (not to exceed 0.95) u o c
2 Measured contraction s
0 0.0 0.2 0.4 0.6 0.8 1.0 Geometric-contraction ratio
Figure 43. The South Carolina clear-water contraction-scour envelope curve for the Piedmont and Coastal Plain (from Benedict and Caldwell, 2006).
SC Bridge Scour Figure 43 The South Carolina Clear-Water Contraction-Scour Envelope Curve 57
2 ys = –6m + 10m + 0.6, (20) figure 22, provides some explanation for this pattern, with the largest velocities occurring in the abutment-scour region and where diminishing with lateral distance from the abutment toe in the y is the upper bound for potential clear-water s contraction scour region. A field example of the scour regions contraction-scour depth, in feet; and shown in figure 23 can be seen at structure 274000300200 m is the geometric-contraction ratio based on S.C. Route 3 crossing Cypress Creek in Jasper County on the 100-year-flow (eq. 15). (fig. 44). The bridge is 210 ft long over a swampy channel On the basis of the limits of the Piedmont data, Benedict similar to the conditions shown in figure 5. The left and right (2003) recommended limiting the application of the envelope abutment-scour holes are 10.8 and 14.4 ft deep, respectively, curve equation to geometric-contraction ratios less than or and are located in close proximity to the abutment toes. equal to 0.85. Because of the additional data from the Coastal Beyond the abutment-scour holes, the clear-water contraction- Plain that exceed this value, Benedict and Caldwell (2006) scour depth is substantially less, with a maximum depth of noted that it may be reasonable to increase this limit to 0.95. approximately 2 ft. The clear-water contraction-scour data However, they noted that because data are sparse for geo- collected in the Piedmont (Benedict, 2003) and the Coastal metric-contraction ratios between 0.90 and 0.95, the equation Plain (Benedict and Caldwell, 2006) of South Carolina were should be used with caution within this range. generally collected in the clear-water contraction-scour region, Clear-water abutment and contraction scour both can as defined in figures 23 and 44, outside the region of abutment occur on the bridge overbank (fig. 23), with abutment scour scour. Because clear-water abutment-scour depths can be generally occurring in close proximity to the abutment toe substantially larger than clear-water contraction-scour, it is and the contraction scour occurring beyond the region of important to differentiate these types of scour and the general abutment scour. The abutment scour typically will be deeper regions of the overbank where they are most likely to occur than the contraction scour and at times substantially so. The (figs. 23 and 44) when applying the South Carolina clear-water velocity flow field at a bridge contraction, as illustrated in contraction- and abutment-scour envelope curves.
Figure 44. Example of Abutment scour clear-water abutment- and contraction-scour areas at
Flow structure 274000300200 on S.C. Route 3 crossing Cypress Creek in Jasper County (December 9, 1996; from Benedict and Caldwell, 2006).
Contraction scour
E PLANATION Bridge deck Topographic contour–Interval 1.0 foot. Arbitrary datum Bent location Surveyed data point 87.7 feet Average unscoured reference-surface elevation
EET
ETE S
SC Bridge Scour Figure 44 58 The South Carolina Bridge-Scour Envelope Curves
Comparison of the South Carolina rather than clear-water contraction scour, as was done for such bridges in the South Carolina abutment-scour investigation Clear-Water Contraction-Scour (Benedict, 2003). However, seven measurements (table 14) in the Alabama data have bridge lengths greater than 240 ft Envelope Curve With Other Data with lengths ranging from 246 ft to 894 ft. Scour at such sites should represent clear-water contraction scour similar to the A literature review for field measurements of clear-water South Carolina data, making them appropriate for comparison. contraction scour identified several sources of data including Lee and Hedgecock (2008) concluded that the Alabama bridge the NBSD (U.S. Geological Survey, 2001) and the scour sites had historical peak flows that likely equaled or exceeded investigation in Alabama (Lee and Hedgecock, 2008). The the 50-year flow magnitude. The NBSD and Alabama data NBSD included nine measurements of clear-water contraction represent historical scour measurements similar to the South scour on bridge overbanks. A review of the data indicated Carolina data, and one-dimensional flow models were used that the two measurements with the largest scour depths to estimate the hydraulic properties. The post-flood nature of (14 ft) were associated with scour in the abutment-scour the scour measurements, in conjunction with the estimated region (fig. 23) and, therefore, would be better classified as hydraulics, makes these data less than ideal. These limitations abutment scour rather than contraction scour. The remaining should be kept in mind when using these field data in any seven measurements had contraction-scour depths ranging analysis. Table 14 lists the median and range of selected from 0.5 to 4.5 ft; however, only two of these measurements site characteristics for the NBSD and Alabama field data. had supporting information for estimating the geometric- For additional details regarding these data, reference can be contraction ratio, and these data were selected (table 14) to made to NBSD (U.S. Geological Survey, 2001) and Lee and compare with the South Carolina data. These two NBSD Hedgecock (2008). measurements were associated with relief bridges at a The NBSD and Alabama clear-water contraction-scour multiple-bridge crossing, cohesive sediments, and historical data (table 14) and the South Carolina field data (table 13) are peak flows near the 100-year flow magnitude. The Alabama plotted with the South Carolina clear-water contraction-scour data included 37 measurements of clear-water contraction envelope curve in figure 45. Most of the NBSD and Alabama scour collected in the cohesive, flood plain sediments of the data fall within the envelope curve, with the exception of one Black Prairie Belt of the Coastal Plain of Alabama (Lee and point that slightly exceeds the curve. Although the number of Hedgecock, 2008). As noted in the report section “Comparison comparison data are small (table 14), the data provide some of the South Carolina Clear-Water Abutment-Scour Envelope support that the South Carolina clear-water contraction-scour Curves With Other Data,” most of the Alabama data were envelope curve is reasonable. The NBSD and Alabama data associated with short flood plain relief bridges or bridges are associated with cohesive sediments, similar to the Pied- crossing swampy channels (240 ft or less) making it appro- mont data (table 13); therefore, the comparison in figure 45 is priate to classify these data as clear-water abutment scour most applicable to the Piedmont data.
Table 14. Range of selected characteristics of clear-water contraction scour field data used for comparison with the South Carolina clear-water contraction-scour data.
[mi2, square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; mm, millimeter; NBSD, National Bridge Scour Database; —, not available]
aAverage aAverage Measured Drainage Channel overbank flow overbank flow bMedian clear-water Geometric Range area slope velocity at depth at grain size contraction- contraction value (mi2) (ft/ft) the bridge the bridge (mm) scour depth ratio (ft/s) (ft) (ft) NBSD (U.S. Geological Survey, 2001) (2 measurements) Minimum 1,327 0.000145 — — 0.001 0.0 0.92 Median — — — — — — — Maximum 1,327 0.000145 — — 0.001 4.5 0.92 Alabama (Lee and Hedgecock, 2008) (7 measurements) Minimum 9.55 0.0004 2.10 2.33 0.001 2.13 0.42 Median 21.3 0.001 2.86 3.94 0.0041 2.71 0.76 Maximum 114 0.0035 4.45 7.06 0.13 4.93 0.88 aValues were estimated from a one-dimensional water-surface profile model. bAll sediments are cohesive. Relative Increase in Theoretical Clear-Water Contraction Scour Associated With the 100- to 500-Year Flows 59
6 E PLANATION 2 South Carolina Piedmont ys –6m 10m 0.6 South Carolina Coastal Plain where m 0.95 National Bridge Scour Database Alabama ys Contraction-scour depth, in feet m Geometric contraction (not to exceed 0.95)
4
2 Measured contraction scour, in feet
0 0 0.2 0.4 0.6 0.8 1.0 Geometric-contraction ratio Figure 45. Comparison of the South Carolina clear-water contraction-scour envelope curve with field data from other sources. Relative Increase in Theoretical Clear- contraction-scour data and the analysis of the 500-year flow adjustment coefficient is presented below. Water Contraction Scour Associated With the 100- to 500-Year Flows Theoretical Clear-Water Contraction-Scour Data
As previously described in the report sections “Clear- The South Carolina Theoretical Bridge Scour Database Water Pier-Scour Envelope Curve” and “South Carolina (http://dx.doi.org/10.3133/sir20165121), as previously Clear-Water Abutment-Scour Field Data,” a substantial described in the report section “Theoretical Pier-Scour portion of the field data used to develop the South Carolina Data,” includes 309 theoretical clear-water contraction-scour clear-water contraction-scour envelope curve included scour depths for both the 100- and 500-year flows that were used measurements likely associated with historic flows near the to evaluate the 500-year flow adjustment coefficient. In 100-year flow magnitude. Therefore, the envelope curves addition to the South Carolina data, the Missouri level-2 data can be used to assess scour potential for such flow conditions (Huizinga and Rydlund, 2004) included theoretical computa- (Benedict and Caldwell, 2006). (Note: The contraction-scour tions for 137 clear-water contraction-scour depths that were envelope curves should not be considered a definitive estimate used as a means to confirm the trends of the South Carolina of the contraction-scour depth associated with the 100-year data. The Missouri data are available at http://pubs.usgs.gov/ flow.) The South Carolina clear-water contraction-scour sir/2004/5213/. envelope curve, however, was not recommended for assessing scour potential for extreme floods such as the 500-year flow The 500-Year Flow Adjustment Coefficient (Benedict and Caldwell, 2006). In order to gain insights on the relative increase in contraction-scour depth associated with The relation of the theoretical clear-water contraction the 100- to 500-year flows, a theoretical adjustment coefficient scour for the 100- and 500-year flows for the South Carolina similar to those developed for pier scour (see report section and Missouri data is shown in figure 46, including trend lines “Relative Increase in Theoretical Pier Scour Associated With through each dataset. Summary statistics for the data are the 100- to 500-Year Flows”) can be developed for clear- provided in table 15. The scatter about the trend line for the water contraction scour. Although such adjustments do not South Carolina data is small, and the coefficient of determina- provide a definitive estimate of the 500-year contraction-scour tion is high (0.94) indicating that there is a strong correlation depth, the estimates do provide perspective on the relative in the data. The trend line through the Missouri data falls in increase. A summary of the compiled theoretical clear-water close proximity to the South Carolina trend line indicating that SC Bridge Scour Figure 45 60 The South Carolina Bridge-Scour Envelope Curves
60
Figure 46. Relation of the 100- t
e E PLANATION e
and 500-year flow, theoretical f Trend line for South Carolina data n i
clear-water contraction-scour , Trend line for Missouri data h t South Carolina data depths for selected data from p 50 e Missouri data d level-2 bridge-scour studies in r 500 Scour depth for the 500-year flow u 500 = 1.46 100 South Carolina and Missouri. o 100 Scour depth for the 100-year flow 2 c 2 R = 0.96
- s R Coefficient of determination n 40 o i t a c r t n o c 30 500 = 1.46 100 R2 = 0.94
20
10 500-year theoretical clear-water 0 0 10 20 30 40 50 100-year theoretical clear-water contraction-scour depth, in feet
Table 15. Descriptive statistics for the ratio of the 500- and 100-year flow, theoretical clear-water contraction-scour depths used in the regression analysis for selected theoretical data from South Carolina and Missouri.
[N, number of measurements; Q500, 500-year flow; Q100, 100-year flow; 2mi , square mile]
South Carolina (N = 309) Missouri (N = 137) Descriptive statistic Ratio of Q500 and Q100 theoretical Drainage area Ratio of Q500 and Q100 theoretical Drainage area contraction-scour depths (mi2) contraction-scour depths (mi2) Minimum 1.03 4.12 1.00 3.70 25th percentile 1.42 21.4 1.40 16.9 Mean 1.96 286 1.63 158 Median 1.60 52.4 1.50 34.3 75th percentile 1.87 174.0 1.65 159 Maximum 20.0 8,312 4.33 1,790
the South Carolina trend line is reasonable. The equation for where K500 is the theoretical 500-year flow adjustment coeffi- the South Carolina trend line is cient that represents the ratio of the 500-year flow to 100-year flow theoretical clear-water contraction-scour depths, and Scour500 = 1.46 Scour100 (21) other variables are as previously defined. TheK 500 coefficient can be applied to the South Carolina clear-water contraction- where scour envelope curve (fig. 46) to increase the contraction- Scour is the 500-year flow theoretical scour 500 scour depth by the relative increase in theoretical clear-water depth, in feet; and contraction scour associated with the 100- to 500-year flow
Scour100 is the 100-year flow theoretical scour condition. The K500 is a helpful tool for gaining perspective depth, in feet. on the relative increase of theoretical scour associated with the 100- to 500-year pier-scour depth. However, the adjusted Equation 21 can be divided by Scour100 and simplified to the following form: envelope curve values should not be considered a defini- tive estimate of the contraction scour associated with the
K500 = 1.46, (22) 500-year flow. SC Bridge Scour Figure 46 Application and Limitations of the South Carolina Clear-Water Contraction-Scour Envelope Curve 61
Application and Limitations of Selecting a Reference Surface for Clear- the South Carolina Clear-Water Water Contraction Scour Contraction-Scour Envelope Curve Benedict (2003) used the average, undisturbed flood plain elevation in the contraction-scour region (fig. 23) The evaluation of clear-water contraction scour using as the reference surface to determine the clear-water the South Carolina contraction-scour envelope curves should contraction scour depth for the South Carolina data, and this be limited to sites having similar characteristics to those reference surface should be used when assessing clear-water listed in table 13. The limitations of the envelope curves that contraction scour with the South Carolina clear-water were described in the previous sections should be followed contraction-scour envelope curves. The reference surface carefully, and caution should be used when characteristics can be determined by reviewing flood plain elevations from at a bridge approach the limits of the site characteristics SCDOT road and bridge plans, surveyed cross sections, and used to develop the envelope curves. Because the envelope (or) site visit observations. In many cases, the flood plain curves were developed from a limited sample of bridges in the region of the contraction scour is relatively flat so in the Coastal Plain and Piedmont, scour depths could estimating an average flood plain elevation in the region exceed the envelope curves; therefore, it may be prudent to of anticipated scour is not a difficult task. However, there apply a safety factor to the envelope curves. When using can be cases where the flood plain slopes substantially the envelope curves, it is critical to properly estimate the in the lateral and (or) longitudinal direction making the geometric-contraction ratio for the 100-year flow. To ensure determination of a reference surface more difficult. In that the geometric-contraction ratio is properly evaluated, such cases, judgment must be applied, bearing in mind various sources of data should be reviewed, including but not that lower reference-surface elevations will produce limited to topographic maps, hydraulic models, road plans, lower scour-hole elevations. and field measurements. When discrepancies exist between these sources, judgment should be used to determine the most reasonable estimate of the geometric-contraction ratio. The Pier Scour Within Clear-Water Contraction- geometric-contraction ratio was evaluated with the WSPRO Scour Areas flow model (Shearman, 1990; Arneson and Shearman, 1998) for the 100-year flow condition. To estimate the geometric- Because of the shallow nature of clear-water contrac- contraction ratio, the WSPRO model uses a standard location tion-scour holes, it generally was possible to distinguish the of the approach cross section at one bridge length upstream area of pier scour from the area of clear-water contraction from the bridge, which represents the full, natural flood plain scour. When collecting data at these sites, scour around width with no adjustment for ineffective flow areas. (Note: piers generally was not included in the measurement of The bridge hydraulics algorithm and associated cross sections clear-water contraction scour. Therefore, the envelope curve in the HEC-RAS model [Brunner, 2016] differ from that of shown in figure 43 represents contraction scour only and not the WSPRO model, and when using the HEC-RAS model to total scour. These envelope curves can be used to evaluate estimate the geometric-contraction ratio, judgment must be anticipated ranges of clear-water contraction scour in used to assure that the estimates are comparable to those used Coastal Plain and Piedmont overbanks, but judgment must in Benedict and Caldwell [2006], which were based on the be used to account for any additional scour created by piers WSPRO model. Guidance regarding this matter is presented in and pile bents. Guidance previously given in this report the report section “Guidance for Applying the South Carolina can be used to evaluate potential clear-water pier scour on Bridge-Scour Envelope Curves.”) The South Carolina bridge overbanks in the Coastal Plain and Piedmont. clear-water contraction-scour envelope curve (fig. 43) was A potential threat to overbank piers is channel developed using field data from sites with flows approaching widening (see Benedict [2003] for more details) or migra- the 100-year flow but should not be interpreted as representing tion. Channel widening and migration can undermine the “100-year” scour estimate and also should not be used overbank piers that are located near a channel bank. to evaluate clear-water contraction-scour depths for extreme Therefore, it is important to evaluate overbank piers for the conditions, such as the 500-year flow. To gain insights on possibility of channel widening or migration. One should the relative change in the theoretical clear-water contraction- be aware of this potential problem and use judgment when scour depth associated with the 100-year to 500-year flow, evaluating scour at overbank piers or bents. Judgment also the 500-year scour coefficient (eq. 22) may be applied to the should be used if the effects of debris and (or) severe skews envelope curve estimate. must be considered. 62 The South Carolina Bridge-Scour Envelope Curves
Estimate of Clear-Water Contraction-Scour time of scour (Arneson and others, 2012). Live-bed contraction Hole Location scour occurs when approach-flow velocities are sufficient to transport bed sediments into the scoured region. Equilibrium Benedict and Caldwell (2006) noted that the general conditions for live-bed contraction scour are attained when shape of clear-water contraction-scour holes in the overbank bed-sediment transport into and out of the scoured region are region consists of shallow parabolic depressions running at equilibrium. Live-bed contraction scour in South Carolina perpendicular to flow (fig. 23) and covering most of the streams typically occurs in the main channel of a stream (fig. 4). overbank region unaffected by abutment scour. The low point of the scour hole typically is in close proximity to the roadway South Carolina Live-Bed Contraction-Scour centerline beneath the bridge deck. The left and right lateral Field Data extent of the clear-water contraction scour typically begins at the edge of the abutment-scour hole and extends toward the Benedict and Caldwell (2009) made 89 measurements bank. Scour depths over the lateral extent of the scour hole of live-bed contraction scour in the main channel of selected vary. It is reasonable to assume, however, that scour potential bridges in South Carolina, with 35 measurements in the determined from the clear-water contraction-scour envelope Piedmont and 54 in the Coastal Plain (table 16). Bridge sites curve (fig. 43) will occur under the bridge and will extend where live-bed contraction-scour data were collected can laterally from the edge of the abutment-scour hole to the be identified by referring to appendix 1 and figure 1. (Note: channel bank. Because the edge of the abutment-scour hole Twelve of the measurements were taken at secondary sites is a limiting boundary for the clear-water contraction scour, that were located near the primary bridge of interest. These abutment scour at the bridge should be evaluated first. secondary sites included old abandoned highway bridges, other highway bridges, or railroad bridges and are not included in the list of sites in appendix 1.) A grab sample of sediment The South Carolina Live-Bed from the channel bed surface was obtained in the upstream channel at each site and was analyzed to estimate the median Contraction-Scour Envelope Curves grain size. Because sediment characteristics in the field setting can vary substantially in the vertical and horizontal direction, As noted previously, contraction scour occurs when a grab sample taken at a point may not fully represent the increased flow velocities and vortexes, generated by a bridge sediment characteristics at a site. The data-collection sites contraction, cause the streambed to erode in the general region for live-bed contraction scour also were part of the live-bed of the bridge. Contraction scour is classified as clear-water or pier-scour investigation by Benedict and Caldwell (2009), and live-bed, which refers to the sediment-transport conditions at the as with the live-bed pier-scour data, GPR was used to make
Table 16. Range of selected characteristics of live-bed contraction scour field data collected in the main channels of selected sites in South Carolina (Benedict and Caldwell, 2009).
[mi2, square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; (ft3/s)/ft, cubic foot per second per foot; mm, millimeter]
aAverage aUnit width Measured contraction- aAverage Approach Median Drainage Channel approach flow in scour depth based on Geometric Range approach channel grain area slope flow approach estimate of scour contraction value velocity width size (mi2) (ft/ft) depth channel (ft) ratio (ft/s) (ft) (mm) (ft) ([ft3/s]/ft) Most likely Worst case South Carolina Piedmont (35 measurements) Minimum 21 0.00015 2.4 7.7 41.0 19.5 0.51 0 2.8 0.14 Median 148 0.001 5.6 15.7 87.0 88.6 0.78 3.4 7.7 0.61 Maximum 5,250b 0.0021 11.6 28.3 788.0 291.2 1.7 16.7 16.7 0.92 South Carolina Coastal Plain (54 measurements) Minimum 17.2 0.00007 1.1 4.7 21.0 6.7 0.18 0 2.7 0.29 Median 521 0.00031 2.7 12.5 92.5 34.6 0.59 4.6 5.4 0.82 Maximum 9,360c 0.002 7.1 39.0 785.0 267.5 1.7 17.1 17.1 0.95 aValues were estimated from a one-dimensional water-surface profile model. bApproximately 94 percent of the study sites in the Piedmont have drainage areas less than 760 mi2. cApproximately 80 percent of the study sites in the Coastal Plain have drainage areas less than 1,860 mi2. The South Carolina Live-Bed Contraction-Scour Envelope Curves 63
these measurements during low-flow conditions. An overview the historic contraction scour, if any, had previously occurred. of the use of GPR can be found in the report section “Live- The small contraction ratios, in conjunction with the limited Bed Pier-Scour Envelope Curve,” and an example of a GPR evidence of remnant scour holes, indicated that substantial measurement of contraction scour that displays the remnant contraction scour likely had not occurred at such sites; scour-hole bathymetry, as well as the sediment infill, is shown however, the GPR data at these sites was at times ambiguous. in figure 47. The overview of the historical flows and the esti- At such sites, (8 measurements in the Coastal Plain and 19 in mate of hydraulic characteristics with the WSPRO (Shearman, the Piedmont), Benedict and Caldwell (2009) made two 1990; Arneson and Shearman, 1998) model, as described in estimates of live-bed contraction-scour depth, including (1) the the report section “Live-Bed Pier-Scour Envelope Curve,” also “most likely estimate of measured scour” (referred to as the is applicable to the live-bed contraction-scour sites. “most likely scour” for the remainder of the report), repre- Benedict and Caldwell (2009) noted that the complex senting the interpreter’s judgment of the maximum historic bed bathymetry often associated with the main channel of a live-bed contraction scour that likely had occurred at that site, stream, in conjunction with bed mobility, can make it difficult and (2) a more conservative interpretation of live-bed contrac- to define the live-bed contraction-scour depth. In particular, tion scour called the “worst case estimate of measured scour” these characteristics can make it difficult to determine the (referred to as the “worst case scour” for the remainder of the most appropriate reference surface for estimating the depth report) that assumed scour depth extended to the subsurface of scour. (Benedict and Caldwell [2009] used the average scour-resistant layer as identified in the GPR data and bridge- thalweg elevation in the region of scour to estimate a reference plan borings. Twenty-five of these sites had little evidence of surface. The thalweg is the lowest channel elevation at a given contraction scour, and the “most likely scour” ranged from cross section.) Additionally, the complex bathymetry of the 0 to 3 ft. In contrast, the “worst case scour” at these 25 sites channel bed can make it difficult to isolate contraction scour ranged from 2.8 to 11.6 ft. Benedict and Caldwell (2009) used caused by the bridge from scour caused by natural field condi- the “most likely scour” in their development of the South tions such as confluences, bends, natural channel constrictions, Carolina live-bed contraction-scour envelope curves. Benedict channel migration, debris, dune bedforms, and the natural and Caldwell (2009) noted that the challenges associated with thalweg meander. The difficulty of estimating live-bed assessing historical live-bed contraction scour, as previously contraction scour is further increased by the interpretive nature described, introduce uncertainty and error into the South of GPR data. Although interpretation of GPR data often was Carolina live-bed contraction-scour data. These limitations conclusive, the determination of the sediment infill in some should be kept in mind when assessing the trends associated cases was unclear. In particular, for sites with relatively small with these data. Additional information regarding the data geometric-contraction ratios (0.5 or less), there often was limitations and the estimate of scour can be found in Benedict little evidence of remnant scour holes that identified where and Caldwell (2009).
0 0 Bridge Bridge 50 feet 100 feet 150 feet 50 feet 50 150 feet
100 5 Flow
150 Approximate reference surface for channel bottom
200 10
250 Depth, in feet Time, in nanoseconds 300 SCDOT borings 15 indicate clay overlain by thin 350 Approximately 12 feet of contraction scour layer of sand with 2 feet of infill 400 20
12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204 216 Time, in seconds Figure 47. Example of ground-penetrating radar longitudinal profile at structure 262050110100 on U.S. Route 501 crossing the Little Pee Dee River in Horry County, South Carolina (from Benedict and Caldwell, 2009). Depth scale through sediments is unadjusted and approximate only.
SC Bridge Scour Figure 47 64 The South Carolina Bridge-Scour Envelope Curves
Other Field Data plain relief bridges or bridges over swampy channels with large contractions of flow, bridge lengths of approximately 240 ft or Benedict and Caldwell (2009) used selected data from less, and substantial scour that often created a single scour hole other sources to help confirm the trends observed in the South that encompassed the entire bridge opening. Although Benedict Carolina live-bed contraction-scour field data. These sources (2003) classified these data as clear-water abutment scour, included the NBSD (U.S. Geological Survey, 2001), Hayes technically the data are a form of clear-water contraction scour. (1996), and Benedict (2003); table 17 lists the median and Even though these are not live-bed scour data, they include range of selected site characteristics for these data. Not all of some of the largest scour depths from the abutment-scour the data from these sources could be used in the analysis, and investigation, and Benedict and Caldwell (2009) used the data Benedict and Caldwell (2009) provide details on the selected for perspective and limited confirmation. The data are referred data. The selected NBSD measurements were associated with to as clear-water contraction scour in this section of the report floods that were near to or exceeded the 100-year flow magni- but should not be confused with the data used to develop the tude. (Note: One of the NBSD measurements was excluded in overbank clear-water contraction-scour envelope curve (fig. 43; the dimensionless analysis by Benedict and Caldwell [2009] table 13). A literature review was conducted for the current because of missing flow-depth data. However, the review of (2016) investigation to identify additional live-bed contraction- supporting files for that site during the current [2016] investi scour data for comparison with the South Carolina live-bed gation identified the missing data, allowing it to be used in the contraction-scour envelope curves. However, no additional current analysis.) The selected field data from Hayes (1996) data that could be used in the comparison were identified. included three measurements of live-bed contraction scour collected at the same bridge site, and the data were associated Live-Bed Contraction-Scour Envelope Curves with flows ranging from approximately 50 to 60 percent of the 100-year flow magnitude. The geometric-contraction ratio Using a similar approach for the development of the for the Hayes (1996) data was modified from the published South Carolina clear-water abutment- and contraction-scour value of 0.29 to 0.6 based on topographic features at the site as envelope curves, Benedict and Caldwell (2009) used the described in Benedict and Caldwell (2009). In addition to the geometric-contraction ratio as the primary explanatory NBSD and Hayes (1996) data, 42 measurements of clear-water variable to develop several envelope curves for the live-bed abutment scour selected from Benedict (2003) were used contraction-scour data. The envelope curves include dimen- to help confirm the trends in the live-bed contraction-scour sionless, or relative, scour depths, and the dimensional, or field measurements. The selected data were associated with flood scour depths, and are described below.
Table 17. Range of selected characteristics for field measurements of contraction scour used to help confirm the patterns of the South Carolina live-bed contraction-scour data (Benedict and Caldwell, 2009).
[mi2, square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; (ft3/s)/ft, cubic foot per second per foot; mm, millimeter; NBSD, National Bridge Scour Database; —, not available]
aUnit width Average Average Approach Measured Drainage Channel flow in Median Geometric Range approach approach channel contraction- area slope approach grain size contraction value velocity flow depth width scour depth (mi2) (ft/ft) channel (mm) ratio (ft/s) (ft) (ft) (ft) ([ft3/s]/ft) Hayes (1996) and NBSD (U.S. Geological Survey, 2001) (12 measurements) Minimum 10.3 — 0.7 7.9 42 25.7 0.1 0 0.60 Median 1,078 — 3.4 20.2 130 120 0.3 7 0.91 Maximum 16,010 — 5.2 43 300 243 1.6 15 0.95 bBenedict (2003) (42 measurements) Minimum 6.1 0.00015 0.05 2 — 0.4 0.06 0.9 0.77 Median 32.2 0.001 0.5 4.3 — 2.1 0.2 9.6 0.91 Maximum 8,230c 0.0021 0.94 11.7 — 6.3 0.78 23.6 0.98 aThis is the unit width flow on the approach flood plain for the Benedict (2003) data. bThe Benedict (2003) data were originally classified as abutment scour; however, the data are technically a form of clear-water contraction scour. These data represent some of the largest scour depths in Benedict (2003) and were used for perspective and limited confirmation of the patterns in the live-bed contraction-scour data. cApproximately 95 percent of the clear-water sites have drainage areas less than 265 mi2 (Benedict, 2003). The South Carolina Live-Bed Contraction-Scour Envelope Curves 65
Dimensionless Live-Bed Contraction-Scour flow depth for application to the equation. Although the clear- Envelope Curves water contraction-scour data (table 17) provides some support for the dimensionless envelope curve for live-bed contraction- Assuming a simple rectangular channel, comparable to a scour (fig. 48A), it is difficult to make strong conclusions laboratory flume, Benedict and Caldwell (2009) simplified and because of the differing nature of the clear-water contraction- rearranged the Laursen (1960) theoretical live-bed contraction- and live-bed contraction-scour data. scour equation to the following form: Figure 48B only shows the live-bed contraction-scour data along with the dimensionless envelope curves for the 0.64 Coastal Plain and Piedmont as presented in Benedict and y 1 s –1, (23) Caldwell (2009). The equations for these dimensionless y1 1 m envelope curves are presented below. Coastal Plain live-bed contraction-scour equation: where y is the theoretical live-bed contraction- s ys 2 scour depth, in feet; 1.27mm0.43 , (24) y1 y1 is the approach flow depth, in feet; and m is the geometric-contraction ratio as Piedmont live-bed contraction-scour equation: defined in equation 15. The quotient of the variables on the left side of the equals y 2 symbol in equation 23 (y /y ) is the dimensionless, or relative, s 1.21mm0.19 (25) s 1 y live-bed contraction-scour depth, and the degree of contrac- 1 tion is expressed in terms of the geometric-contraction ratio
(m). The theoretical live-bed contraction-scour, ys , can be where ys is the upper bound of potential live-bed contraction- obtained by multiplying the relative scour depth, ys /y1, by the scour data, in feet, and the other variables are as defined appropriate approach flow depth,y 1. Figure 48A shows the in equation 23. The upper bound for potential live-bed theoretical curve for equation 23, as well as the relative scour contraction-scour, ys, for equations 24 and 25 can be obtained depths for the South Carolina, NBSD, and Hayes (1996) live- by multiplying the relative scour depth, ys /y1, by the appro- bed contraction-scour data (tables 16, 17). Additionally, the priate approach flow depth,y 1. In general, the upper bound selected clear-water contraction-scour data for the flood plain of the South Carolina data fall near the simplified Laursen relief bridges or bridges over swampy channels (table 17), (1960) equation (eq. 23) for geometric-contraction ratios of as described previously, are included. (Note: The additional about 0.5 or less but begin to fall below equation 23 as values NBSD data point not in the original analysis by Benedict and increase beyond 0.5. Perhaps one reason for this pattern is Caldwell [2009], as described previously, is shown in figure 48 that the subsurface, scour-resistant layers can impede or limit with an x-y plotting position of 0.93 and 1.42, respectively.) scour, which is described in more detail later in the report. The vertical axis represents scour depth normalized by the As noted previously, the one Coastal Plain data point that approach channel flow depth, and the horizontal axis repre- exceeds the dimensionless envelope curve is associated with sents the geometric-contraction ratio. The theoretical curve a channel bend that will tend to increase scour depths. The for equation 23 is essentially an envelope curve for the upper dimensionless envelope curves indicate that the upper bound bound of the field data with only one data point exceeding the of relative scour for the Coastal Plain data exceeds that of the curve. The point that exceeds the curve is not excessive and is Piedmont. Benedict and Caldwell (2009) note that the reason associated with a channel bend that will tend to increase scour for this pattern is uncertain but suggest that the differing field depths, providing some explanation for its exceedance. The characteristics between these regions may contribute in some South Carolina clear-water contraction-scour data (table 17) measure. In particular, the Coastal Plain tends to have longer are encompassed well by the dimensionless envelope curve flow durations, a higher frequency of bridges with large flow defined by equation 23 (fig. A48 ), providing limited support contractions, and subsurface sediment layers that are more for the dimensionless envelope curve. The relative scour susceptible to scour than the subsurface bedrock commonly depths for the clear-water contraction-scour data tend to associated with Piedmont streams. These characteristics will plot above the live-bed contraction-scour data. Benedict and tend to produce larger scour depths in the Coastal Plain. Caldwell (2009) noted that the smaller flood plain flow depths Although the amount of data from Hayes (1996) and the used to normalize the clear-water contraction-scour data, in NBSD (table 17) is small (only 9 of the 12 measurements contrast to the channel flow depths used to normalize the live- could be used), all the data fall within the envelope curves, bed contraction-scour data, strongly contributes to this pattern. thus providing some support for the curves. This pattern highlights how the relative-scour depth in equa- The dimensionless envelope curves shown in figure 48 tion 23 can be sensitive to the approach-flow depth, indicating and their associated equations (eqs. 23–25) can be used the need to be cautious in selecting an appropriate estimate of to help assess live-bed contraction-scour potential in the 66 The South Carolina Bridge-Scour Envelope Curves
4 EXPLANATION Laursen (1960) equation (Flow ratio = 1) Coastal Plain Piedmont National Bridge Scour Database Hayes (1996) 3 South Carolina clear-water field data y Contraction-scour depth 0.64 s ys /y1 (1/(1–m)) –1 y Approach flow depth m Geometric-contraction ratio
2
Little Pee Dee River U.S. Route 501 h t 1 p e d
w o l f
y b
d e d i v i
d 0
h t
p e
d 4
r
u EXPLANATION o c
s Laursen (1960) equation (Flow ratio = 1) -
n Envelope curve of Piedmont o i
t Envelope curve of Coastal Plain c
a Coastal Plain r t
n Piedmont
o 0.64
C National Bridge Scour Database 3 ys /y (1/(1–m)) –1 Hayes (1996) ys Contraction-scour depth y1 Approach flow depth m Geometric-contraction ratio
y /y 1.27m 2 0.43m 2 s
2 ys /y 1.21m 0.19m
1
0 0 0.2 0.4 0.6 0.8 1.0 Geometric-contraction ratio
Figure 48. The South Carolina dimensionless live-bed contraction-scour envelope curve for (A) live- bed and clear-water contraction-scour data, and (B) live-bed contraction-scour data only (modified from Benedict and Caldwell, 2009).
SC Bridge Scour Figure 48 The South Carolina Live-Bed Contraction-Scour Envelope Curves 67
Coastal Plain and Piedmont of South Carolina. Equation 23 within the envelope curve, thus providing some support for reflects the Laursen (1960) theoretical equation for live-bed the curve. Figure 49B shows the same data in figure 49A, but contraction scour by using the simplifying assumption of includes the clear-water contraction-scour data (table 17). a rectangular channel and is a general equation that can be Even though the clear-water contraction-scour data differ in applied to assessing live-bed contraction-scour potential. nature from the live-bed contraction-scour data, they con- Equations 24 and 25 reflect the trends for the upper bound of form well to the live-bed contraction-scour envelope curve, the field data for the Coastal Plain and Piedmont, respectively, suggesting that the curve is reasonable. and can be used to refine the assessment of scour potential for Figure 50 shows the South Carolina data and envelope sites within those regions. The upper bound of the field data curve for the “worst case scour.” These data include the same for the Coastal Plain and Piedmont (fig. 48B) have geometric- data as figure 49A, but substitute the “worst case scour” at the contraction ratios limited to approximately 0.87 and 0.85, 27 measurements previously described. For comparison, the respectively; therefore, the application of equations 24 and 25 envelope curve for the “most likely scour” from figure 49A beyond these values is questionable. In general, the application also is included in figure 50. Benedict and Caldwell (2009) of these equations should be used with caution and should be developed an envelope curve for the “worst case scour” by limited to sites that have characteristics well within the range adding a value of 5.5 ft to equation 26. The two “worst case of the South Carolina live-bed contraction-scour data used to scour” data that substantially exceed the “worst case scour” develop the dimensionless envelope curves (table 16). Because envelope curve (fig. 50) were purposely excluded because of the limited nature of the NBSD and Hayes (1996) data, the they are associated with small geometric-contraction ratios characteristics associated with those data (table 17) should not less than 0.3 where substantial contraction scour would not be used to extend the application limits of the equations. As be anticipated. Benedict and Caldwell (2009) also noted noted previously, relative scour depth as defined in equa- that the “worst case scour” data that exceed the “most likely tions 23–26 is sensitive to the selection of the approach-flow scour” envelope curve are associated with smaller geometric- depth (y1 ), indicating the need to be cautious in selecting this contraction ratios (approximately 0.5 or less) where substantial variable for application to the envelope curves. contraction scour would not be anticipated, suggesting that the “worst case scour” estimates are likely over estimating the actual scour depths. Although Benedict and Caldwell (2009) Field Envelope Curve for Live-Bed acknowledge the uncertainty of the live-bed contraction-scour Contraction Scour data on the basis of the observation above and the fact that the verification data from table 17 all fall within the “most likely Similar to the dimensionless envelope curves (fig. 48), scour” envelope curve (fig. 49), they conclude that the “most Benedict and Caldwell (2009) developed a field envelope likely scour” data and the associated envelope curve shown curve for the dimensional form of the live-bed contraction- in figure 49A appear to be the more reasonable representation scour data, using the geometric-contraction ratio as the of live-bed contraction scour in South Carolina. The Coastal explanatory variable (fig. 49). Figure 49A includes the live-bed Plain and Piedmont data for the envelope curve in figure 49A contraction-scour data for South Carolina, NBSD, and Hayes have geometric-contraction ratios limited to approximately (1996). In the development of the dimensionless live-bed 0.82, indicating that the application of equation 26 beyond this contraction-scour envelope curve (fig. 48), differences in the value is questionable. Although the clear-water contraction- upper bound of the Coastal Plain and Piedmont data were scour data (fig. 49B) suggest that it may be appropriate to sufficient to justify separate envelope curves. However, Bene- extend the envelope curve beyond a geometric-contraction dict and Caldwell (2009) concluded that regional envelope ratio of 0.82, the differing nature of the clear-water contrac- curves could not be justified for the data patterns shown in tion- and live-bed contraction-scour data makes it difficult to figure 49A and used a single envelope curve to encompass make a strong conclusion, and any extension of equation 26 all of the data. The equation for the field envelope curve of beyond a geometric-contraction ratio of 0.82 should be used live-bed contraction scour is as follows: with caution. In general, the application of equation 26 should be used with caution and limited to sites well within the range y = 24.7m 2 + 1.3m, (26) s of the data used to develop the envelope curves. where ys is the upper bound of potential live-bed contraction- scour data, in feet, and m is as previously defined. The two The Modified Live-Bed Contraction-Scour Coastal Plain measurements that exceed the envelope curve Envelope Curve are associated with channel bends that will increase the potential for scour, providing some explanation for their Using selected field data from tables 16 and 17 exceedance. Additionally, the one Piedmont measurement (excluding the Hayes [1996] data), Benedict and Caldwell that exceeds the curve is associated with substantial debris, (2012) noted that the upper bound of clear-water and live-bed which likely causes the exceedance. Although the amount contraction scour increases with increasing drainage area of data from Hayes (1996) and the NBSD (table 17) is small (fig. 51). This upper-bound pattern is distinct in the clear-water (11 of the 12 measurements could be used), all the data fall and NBSD data. Two outliers in the South Carolina live-bed 68 The South Carolina Bridge-Scour Envelope Curves
25 E PLANATION Envelope of South Carolina data Coastal Plain Piedmont Enoree River 20 National Bridge Scour Database Road S-263 Black Mingo Creek Hayes (1996) S.C. Route 41 y s Contraction-scour depth Congaree River m Geometric-contraction ratio U.S. Route 601
15
Little Pee Dee River U.S. Route 501
10
2 ys 24.7m 1.3m 5 in feet , h t p e d 0 r u o
c s
- 25 n o i t E PLANATION c a
r Envelope curve t
n Envelope curve extension o
C Coastal Plain 20 Piedmont National Bridge Scour Database Hayes (1996) South Carolina clear-water data (Benedict, 2003) ys Contraction-scour depth m Geometric-contraction ratio 15
10
2 ys 24.7m 1.3m 5
0 0 0.2 0.4 0.6 0.8 1.0 Geometric-contraction ratio
Figure 49. The South Carolina live-bed contraction-scour envelope curve for (A) live-bed contraction- scour data, and (B) live-bed and clear-water contraction-scour data (from Benedict and Caldwell, 2009).
SC Bridge Scour Figure 49 The South Carolina Live-Bed Contraction-Scour Envelope Curves 69
25 E PLANATION Envelope curve of most likely scour Envelope curve of worst-case scour Coastal Plain y 24.7m2 1.3m 5.5 Piedmont s 20 National Bridge Scour Database Hayes (1996)
t ys Contraction-scour depth e
e m Geometric-contraction ratio f South Tyger River n i
, Road S-242 h t 15 p e d
r Wateree River u
o U.S. Route 1 c y 24.7m 2 1.3m s s - n o i t 10 Rocky Creek c a
r U.S. Route 21 t n o C
5
0 0 0.2 0.4 0.6 0.8 1.0 Geometric-contraction ratio Figure 50. The South Carolina live-bed contraction-scour envelope curve for the “worst case scour” for the live-bed contraction-scour data (from Benedict and Caldwell, 2009).
30 E PLANATION Coastal Plain (Benedict and Caldwell, 2009) Piedmont (Benedict and Caldwell, 2009) 25 National Bridge Scour Database (U.S. Geological Survey, 2001) Clear-water data (Benedict, 2003)
Significant debris accumulation 20 Swampy site similar to conditions associated Envelope curve for clear-water data with the clear-water data 15 Envelope curve for live-bed data
10 Contraction-scour depth, in feet
5
0 1 10 100 1,000 10,000 100,000 Drainage area, in square miles Figure 51. Relation of contraction-scour depth to drainage area for selected field data (modified from Benedict and Caldwell, 2012).
SC Bridge Scour Figure 50
SC Bridge Scour Figure 51 70 The South Carolina Bridge-Scour Envelope Curves contraction-scour data exceed the envelope curve for the envelope curves) with field data grouped by categories of live-bed data, but fall near the clear-water contraction-scour drainage area. A review of the field data identified two catego- envelope curve (fig. 51). These two data points are associated ries including sites with drainage areas less than or equal to with substantial debris accumulation or swampy conditions 100 mi2 and sites with drainage areas greater than 100 mi2 similar to the clear-water scour data, providing some explana- but less than or equal to 200 mi2. Plots of these grouped data tion of their exceedance of the other live-bed contraction-scour and their associated envelope curves are shown in figure 52. data. Excluding these two data points, the upper bound of the Figure 52B includes the data from both drainage area catego- South Carolina live-bed contraction-scour data show a similar ries. The data used to develop the modified envelope curves pattern of increasing scour depth with increasing drainage are shown in table 18. The full family of curves is shown in area. It is notable that a similar pattern is observed in the upper figure 53, and the equations for these curves are shown in bound of the abutment-scour data (fig. 40), providing support table 19. that the upper bound patterns in figure 51 are reasonable. The The application of the modified live-bed contraction- reasons for this pattern are not fully understood; however, scour envelope curves is limited to geometric-contraction Benedict and Caldwell (2012) suggested that smaller drainage ratios less than or equal to 0.90 for the modified envelope basins tend to have shorter flow durations and thinner layers curves and 0.82 for the original curve. Additional details of channel alluvium sediments deposited over subsurface regarding the development of the South Carolina modified scour-resistant materials, both of which may contribute to live-bed contraction-scour envelope curves can be found in smaller contraction-scour depths. On the basis of the above- Benedict and Caldwell (2012). Application and limitations of noted patterns of the field data, it was concluded that the South equations in table 19 are described in more detail in the report Carolina live-bed contraction-scour envelope curve could be section “Application and Limitations of the South Carolina modified to form a family of curves (secondary or modified Live-Bed Contraction-Scour Envelope Curves.”
Table 18. Range of selected site characteristics for field measurements of contraction scour used to develop the modified live-bed contraction-scour envelope curves (from Benedict and Caldwell, 2012).
[mi2, square mile; ft/ft, foot per foot; ft/s, foot per second; ft, foot; mm, millimeter; NBSD, National Bridge Scour Database]
Measured Average Average Approach contraction-scour Drainage Channel Geometric Median Range approach approach channel depth based area slope contraction grain size value velocity flow depth width on most likely (mi2) (ft/ft) ratio (mm) (ft/s) (ft) (ft) estimate of scour (ft) South Carolina Piedmont (24 measurements) Minimum 21 0.0005 2.4 7.7 41 0.14 0.51 0 Median 105 0.0012 5.5 13.7 72.5 0.64 0.77 3.7 Maximum 199 0.0021 8.8 18.1 210 0.92 1.7 9.4 South Carolina Coastal Plain (15 measurements) Minimum 17.2 0.00013 1.3 5.2 21 0.44 0.23 3.1 Median 124 0.00074 2.5 9.5 53 0.84 0.59 5.2 Maximum 198 0.002 7.1 15.8 140 0.93 1.7 17.1 Benedict (2003) (38 measurements) Minimum 6.1 0.0003 0.2a 3.1b Flood plain 0.77 0.06 3.2 Median 30.9 0.00076 0.5a 5.4b Flood plain 0.91 0.22 6.8 Maximum 120 0.0024 0.9a 8.0b Flood plain 0.97 0.78 12.8 NBSD (2 measurements) Minimum 10.3 0.001 0.7 13.8 42 0.95 0.1 4 Maximum 10.3 0.001 1 14.2 42 0.95 0.1 8 aThese sites are associated with swampy flood plains with shallow, poorly defined channels; the average approach flow velocity was determined on the basis of the average approach flood plain velocity. bThese sites are associated with swampy flood plains with shallow, poorly defined channels; the average approach flow depth was determined on the basis of the average approach flood plain depth. The South Carolina Live-Bed Contraction-Scour Envelope Curves 71
25 E PLANATION Drainage area ≤ 100 square miles (Piedmont Benedict and Caldwell, 2009) Drainage area ≤ 100 square miles (Coastal Plain Benedict and Caldwell, 2009) Drainage area ≤ 100 square miles (National Bridge Scour Database U.S. Geological Survey, 2001) 20 Drainage area ≤ 100 square miles (clear-water data Benedict, 2003) ys Scour depth, in feet m Geometric-contraction ratio
2 15 ys 16m
Original envelope curve (Benedict and Caldwell, 2009) 10
2 ys 24.7m 1.3m
t 5 e f e
Secondary envelope curve (drainage area is ≤ 100 square miles)
0 ur depth , i n
o c
- s 25 E PLANATION io n t
c Drainage area ≤ 200 square miles (Piedmont Benedict and Caldwell, 2009) a
r Drainage area 200 square miles (Coastal Plain Benedict and Caldwell, 2009) t ≤ n Drainage area 200 square miles (National Bridge Scour Database o ≤
C U.S. Geological Survey, 2001) 20 Drainage area ≤ 200 square miles (clear-water data Benedict, 2003) Black Mingo Creek ys Scour depth, in feet m Geometric-contraction ratio
15 2 ys 20m m
Original envelope curve (Benedict and Caldwell, 2009) 10
2 ys 24.7m 1.3m 5
Secondary envelope curve (drainage area is ≤ 200 square miles)
0 0 0.2 0.4 0.6 0.8 1.0 Geometric-contraction ratio Figure 52. Relation of live-bed contraction-scour depth to the geometric-contraction ratio for selected field data for (A) drainage areas 100 square miles or less, and (B) drainage areas 200 square miles or less (modified from Benedict and Caldwell, 2012).
SC Bridge Scour Figure 52 72 The South Carolina Bridge-Scour Envelope Curves
Figure 53. Relation of live-bed 25 E PLANATION contraction-scour depth to the ys Scour depth, in feet geometric-contraction ratio for m Geometric-contraction ratio selected categories of drainage Secondary envelope curve (drainage area is ≤ 200 square miles) area for selected data in the 20 Piedmont and Coastal Plain of t
South Carolina (from Benedict e f e and Caldwell, 2012). 15 m 2
depth , i n Original envelope curve r
u (Benedict and Caldwell, 2009)
o 20m y s c 2 - s
io n 10 16m
t y s c a r t n
o 2 1.3m C
24.7m 5 y s Secondary envelope curve (drainage area≤ 100is square miles)
0 0 0.2 0.4 0.6 0.8 1.0 Geometric-contraction ratio
Table 19. Equations used to develop the modified live-bed flow.) However, the South Carolina live-bed contraction- contraction-scour envelope curve in the Piedmont and Coastal scour envelope curve was not recommended for assessing Plain of South Carolina. scour potential for extreme floods such as the 500-year flow (Benedict and Caldwell, 2009). In order to gain insights on 2 [mi , square mile; ≤, less than or equal to; DA, drainage area; ys, scour depth, the relative increase in contraction-scour depth associated in feet; m, geometric-contraction ratio; <, less than; >, greater than] with the 100- to 500-year flows, a theoretical adjustment coefficient similar to that developed for pier scour (see report Limits of the Drainage-area section “Relative Increase in Theoretical Pier Scour Associ- Equation geometric- category contraction ratio ated With the 100- to 500-Year Flows”) can be developed for live-bed contraction scour. Although such adjustments do not 0 mi2 ≤ DA ≤ 100 mi2 y = 16m2 0 ≤ m ≤ 0.9 s provide a definitive estimate of the 500-year contraction-scour 2 2 2 100 mi < DA ≤ 200 mi ys = 20m + m 0 ≤ m ≤ 0.9 depth, they do provide perspective on the relative increase. DA > 200 mi2 y = 24.7m2 + 1.3m 0 ≤ m ≤ 0.82 A summary of the compiled theoretical live-bed contraction- s scour data and the analysis of the 500-year flow adjustment coefficient are presented below. Relative Increase in Theoretical Live-Bed Contraction Scour Associated With the Theoretical Live-Bed Contraction-Scour Data 100- to 500-Year Flows The South Carolina Theoretical Bridge Scour Database (http://dx.doi.org/10.3133/sir20165121), as previously As previously described in the report section “Live- described in the report section “Theoretical Pier-Scour Data,” Bed Pier-Scour Envelope Curve,” a substantial portion of includes 99 theoretical live-bed contraction-scour depths for the field data used to develop the South Carolina live-bed both the 100- and 500-year flows that were used to evaluate contraction-scour envelope curve included scour measure- the 500-year flow adjustment coefficient. In addition to the ments likely associated with historic flows near the 100-year South Carolina data, the Missouri level-2 data (Huizinga flow magnitude; therefore, the envelope curves can be used and Rydlund, 2004) included theoretical computations for to assess scour potential for such flow conditions (Benedict 103 live-bed contraction-scour estimates and were used as and Caldwell, 2009). (Note: The live-bed contraction-scour a means to confirm the trends of the South Carolina data. envelope curves should not be considered a definitive estimate The Missouri data are available at http://pubs.usgs.gov/ of the contraction-scour depth associated with the 100-year sir/2004/5213/.
SC Bridge Scour Figure 53 The South Carolina Live-Bed Contraction-Scour Envelope Curves 73
The 500-Year Flow Adjustment Coefficient be applied to the South Carolina live-bed contraction-scour envelope curves (figs. 48, 49, 53) to increase the contraction- The relation of the theoretical live-bed contraction scour scour depth by the relative increase in theoretical live-bed for the 100- and 500-year flows for the South Carolina and contraction scour associated with the 100- to 500-year flow Missouri data is shown in figure 54, including trend lines condition. The K500 is a helpful tool for gaining perspec- through each dataset. Summary statistics for the data are tive on the relative increase of theoretical scour associated provided in table 20. The scatter about the trend line for the with the 100- to 500-year pier-scour depth. However, the South Carolina data is small, and the coefficient of determi adjusted envelope curve values should not be considered a nation is high (0.97) indicating that there is a strong correla- definitive estimate of the contraction scour associated with tion in the data. The trend line through the Missouri data falls the 500-year flow. in close proximity to the South Carolina trend line indicating that South Carolina trend line is reasonable. The equation for Table 20. Descriptive statistics for the ratio of the 500- and 100- the South Carolina trend line is as follows: year flow, theoretical live-bed contraction-scour depths used in the regression analysis for selected theoretical data from South
Scour500= 1.32 Scour100 (27) Carolina and Missouri. [N, number of measurements; Q500, 500-year flow; Q100, 100-year flow; where mi2, square mile]
Scour500 is the 500-year flow theoretical scour depth, in feet; and South Carolina (N = 99) Missouri (N = 103) Ratio of Q500 Ratio of Q500 Scour is the 100-year flow theoretical scour 100 Descriptive and Q100 Drainage and Q100 Drainage depth, in feet. statistic theoretical area theoretical area contraction- (mi2) contraction- (mi2) Equation 27 can be divided by Scour100 and simplified to the following form: scour depths scour depths Minimum 1.00 0.91 1.00 3.64
K500= 1.32, (28) 25th percentile 1.28 10.6 1.46 12 Mean 1.94 230 2.62 121 where K is the theoretical 500-year flow adjustment coeffi- 500 Median 1.43 26.3 1.75 25.7 cient that represents the ratio of the 500-year flow to 100-year flow theoretical live-bed contraction-scour depths, and other 75th percentile 1.68 60.8 2.30 101 Maximum 12.5 8,312 19.50 1,790 variables are as previously defined. TheK 500 coefficient can
80 Figure 54. Relation of the 100- E PLANATION
t and 500-year flow, theoretical
e 500 = 1.59 100
e Trend line for South Carolina data
f 2 70 R = 0.83 live-bed contraction-scour depths
n Trend line for Missouri data i
, for selected data from level-2
h South Carolina data t
p Missouri data bridge-scour studies in South e d 60 Scour depth for the 500-year flow Carolina and Missouri. r 500 u
o 100 Scour depth for the 100-year flow c R2 Coefficient of determination - s n
o 50 i t a c r t
n 500 = 1.32 100 o 40 c R2 = 0.97
30
20
10 500-year theoretical live-bed
0 0 10 20 30 40 50 60 100-year theoretical live-bed contraction-scour depth, in feet
SC Bridge Scour Figure 54 74 The South Carolina Bridge-Scour Envelope Curves
Application and Limitations of the be a difficult task; however, the channel-bed topography at selected sites can be complex, making the determination of South Carolina Live-Bed Contraction- a reference surface more difficult. In such cases, judgment should be applied, bearing in mind that lower reference- Scour Envelope Curves surface elevations will produce lower scour-hole elevations and more conservative scour assessments. For additional Benedict and Caldwell (2009, 2012) noted that substan- details, refer to Benedict and Caldwell (2009). tial uncertainty can be associated with assessing the potential for live-bed contraction scour in South Carolina, making 2. Estimate the drainage area and geometric- judgment a critical component in evaluating this component of contraction ratio: scour. Although the South Carolina live-bed contraction-scour envelope curves (figs. 48B, 49A, 53) can be useful supplemen- The original and modified envelope curves can be sensi- tary tools for assessing live-bed contraction-scour at bridges tive to the selection of drainage area and the geometric- in South Carolina, the difficulty in evaluating this scour contraction ratio; therefore, it is important that accurate component should be considered carefully. Application of the estimates of these variables be obtained. The drainage area live-bed contraction-scour envelope curves should be limited for a given site can be determined by using standard com- to sites having similar characteristics to those of sites used to puter software for determining topographic features. The develop the curves. Because the envelope curves were devel- geometric-contraction ratio can be determined by using flow oped from a limited sample of bridges in the Coastal Plain models, topographic maps, and road plans, and when pos- and Piedmont of South Carolina, scour depths could exceed sible, all three sources should be used for verification. When those determined from the envelope curves. The envelope discrepancies exist between these sources, judgment should curves are not intended for tidally influenced sites that may be be used to determine the most reasonable estimate of the encountered in the Coastal Plain. Additionally, the potential geometric-contraction ratio. error and uncertainty associated with the live-bed contraction- The geometric-contraction ratio was evaluated with scour measurements also is a source of uncertainty in the the WSPRO flow model (Shearman, 1990; Arneson and estimates from the envelope curves. Applying a safety factor Shearman, 1998) for the 100-year flow condition. To estimate to the envelope curve estimates would be considered prudent. the geometric-contraction ratio, the WSPRO model uses a Because the envelope curves were derived from measured standard location of the approach cross section at one bridge data, with many of the sites having historic flows close to length upstream from the bridge, which represents the full, the 100-year flow magnitude, the determination of hydraulic natural flood plain width with no adjustment for ineffective variables should be based on flows near this magnitude. The flow areas. (Note: The bridge hydraulics algorithm and envelope curves should not be used to evaluate live-bed associated cross sections in the HEC-RAS model [Brunner, contraction-scour depths for extreme conditions, such as the 2016] differ from that of the WSPRO model, and when using 500-year flow. To gain insights on the relative change in the the HEC-RAS model to estimate the geometric-contraction theoretical live-bed contraction-scour depth associated with ratio, judgment must be used to assure that the estimates are the 100- to 500-year flow, the 500-year coefficient (eq. 28) comparable to those used in Benedict [2003], which were may be applied to the envelope curve estimates. based on the WSPRO model. Guidance regarding this matter When applying the live-bed contraction-scour envelope is presented in the report section “Guidance for Applying the curves, one must select a reference surface, estimate the South Carolina Bridge-Scour Envelope Curves.”) drainage area and geometric-contraction ratio, select the appropriate contraction-scour envelope curve, evaluate 3. Select the appropriate live-bed contraction-scour other scour components in the contraction-scour region, and envelope curve: consider the limitations associated with the envelope curves. Live-bed contraction-scour depth can be assessed by using The following guidance may provide some assistance in the original envelope curves (figs. 48B, 49A) or the modi- applying the modified envelope curves. fied envelope curve (fig. 53). To utilize the modified enve- lope curves, the drainage area must be 200 mi2 or less, and 1. Select a reference surface: the geometric-contraction ratio should not exceed 0.90. The reference surface should be the average thalweg elevation Additionally, the site characteristics of the bridge of interest along the profile of the channel in the live-bed contraction- should fall within the range of the appropriate South Carolina scour region at the bridge. The thalweg is defined as the low regional data used to develop the modified envelop curves point of the channel bed and should represent the natural (table 18). If these criteria are not met, then the original live- conditions unaffected by scour. This reference surface can be bed contraction-scour envelope curves must be used follow- determined by plotting the thalweg elevation at selected cross ing the application guidance from Benedict and Caldwell sections along the channel profile and then placing a best-fit (2009). Because of the uncertainty of assessing live-bed con- line through those data to determine a reference surface. In traction scour, interpolation between the modified envelope many cases, defining the average thalweg elevation should not curves for live-bed contraction scour is not recommended. Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases 75
4. Evaluate the pier scour within the contraction- Guidance for Using the South Carolina scour region: Because of the complexity associated with pier and contrac- Bridge-Scour Envelope Curves and tion scour in a river channel, Benedict and Caldwell (2009) Databases concluded that it was uncertain if the original live-bed contraction-scour envelope curves (figs. 48B, 49A) would The South Carolina bridge-scour envelope curves and account for pier scour. Therefore, judgment must be used the large databases used in their development can serve as to account for potential pier scour in addition to the compo- valuable supplementary tools for assessing bridge scour in nent of live-bed contraction scour when using the original South Carolina. The three databases compiled in the previous or modified live-bed contraction-scour envelope curves. For investigations (Benedict, 2003; Benedict and Caldwell, 2006, additional guidance, refer to Benedict and Caldwell (2009). 2009) can be used for site comparisons to gain understanding of historical scour depths at particular sites having similar char- 5. Consider the limitations of the live-bed contraction- acteristics to a site of interest. In contrast to site comparisons, scour envelope curves: the South Carolina bridge-scour envelope curves provide a The modified South Carolina live-bed contraction-scour broader illustration of the upper-bound patterns of scour based envelope curves can be useful supplementary tools for assess- on a large sample of bridges in South Carolina, with a limited ing live-bed contraction-scour potential at bridges in South measure of verification based on field data from sources Carolina; however, the following limitations of these empirical outside of South Carolina. The South Carolina envelope envelope curves should be considered. curves can be used in several ways, including (1) evaluation of • The live-bed contraction-scour envelope curves were predicted scour from laboratory equations by comparing them developed from a limited sample of bridges in the with the envelope curves, and (2) evaluation of the upper- Piedmont and Coastal Plain of South Carolina, and it bound of scour potential at a given site as represented in the is possible that scour depths could exceed the envelope envelope curves. This section of the report provides guidance curves. Therefore, applying a safety factor to the for using the South Carolina bridge-scour envelope curves and modified envelope curves may be prudent. databases to assess scour potential in South Carolina. • Application of the live-bed contraction-scour envelope curves should be limited to bridges having site Using the Bridge-Scour Databases for characteristics similar to those used to develop the Site Comparisons envelope curves. • The South Carolina live-bed contraction-scour The South Carolina bridge-scour databases published envelope curves were developed using field data from with their associated reports (Benedict, 2003; Benedict sites with flows approaching the 100-year flow and, and Caldwell, 2006, 2009) provide a valuable resource for therefore, should not be used to evaluate live-bed understanding historical scour depths in South Carolina. contraction-scour depths for extreme conditions, such Links for downloading these three databases are provided in as the 500-year flow. To gain insights on the relative table 21. (Note: The Internet address for the PSDb-2014 pier change in the theoretical live-bed contraction-scour scour database [Benedict and Caldwell, 2014] also is provided depth associated with the 100-year to 500-year flow, in table 21.) These three databases include photographs, some the 500-year scour coefficient (eq. 28) may be applied figures, measured scour depths, predicted scour depths, limited to the envelope curve. basin characteristics, limited soil data, and selected hydraulic data determined from one-dimensional flow models and can • The live-bed contraction-scour envelope curves do not be viewed using Microsoft® Access®. The databases provide account for adverse field conditions, such as channel automated report formats that can be used to view data for a bends, natural channel constrictions, channel migra- given site. The raw data also can be viewed in tabular format. tion, dune bedforms, and debris, which may increase In addition to the bridge-scour databases, appendix 2 identifies scour depths. historical peak flows that have occurred at selected bridge sites Although the South Carolina live-bed contraction-scour in the South Carolina studies, providing insights into the flow envelope curves presented in this report can serve as a conditions that likely created the measured historical scour. valuable supplementary tool in assessing live-bed contraction- Arneson and others (2012) recommend that predicted scour scour depths in South Carolina, the potential errors and limita- obtained from laboratory-prediction equations be evaluated for tions restrict their use. Therefore, the envelope curves should reasonableness by comparing it with available historical data not be relied upon as the only tool for assessing live-bed at or near the site of interest, and the South Carolina databases contraction scour. To best assess anticipated scour, one should provide a useful tool to assist with such comparisons. compile and study the available information for a given site Furthermore, the review of historical scour at sites near a site and then bring sound engineering principles and judgment to of interest help the practitioner develop judgment with regard bear on the final estimate of live-bed contraction-scour depth. to ranges for anticipated scour under differing field conditions. 76 The South Carolina Bridge-Scour Envelope Curves
Various approaches can be used to compare sites, including data, and photographs, can be retrieved from the databases. the use of figure 1 and appendix 1 to identify and select sites Comparing such data with the characteristics for a site of interest near a site of interest, or querying the databases to identify and can assist in further promoting understanding of scour potential. select sites having similar characteristics to the site of interest. Additionally, review of topographic maps, aerial photographs, Or perhaps it may be appropriate to retrieve selected field data road and bridge plans, bridge inspection reports, and soil from a given county, or neighboring counties, where a site of borings can yield insights regarding the potential for scour at interest is located. Table 22 provides an example of selected the selected comparison sites and the site of interest. (Note: scour data retrieved from the South Carolina databases for The South Carolina databases include latitude and longitude for bridges located in Spartanburg County, South Carolina. The each site that can be used to locate the site on topographic maps data example in table 22 indicates relatively moderate values and aerial photography.) Compiling and reviewing the above of the geometric-contraction ratio (all but one are 0.6 or less), noted information for a group of comparison sites helps identify relatively small embankment lengths (most are less than 200 ft), scour patterns and field characteristics associated with such and all scour depths, except one, are less than 5 ft, with the patterns. Assuming that a site of interest in or near Spartanburg largest scour depth of 7.8 ft being associated with the largest County has characteristics similar to the bridges listed in table geometric-contraction ratio of 0.7. It also is noteworthy that 22, it would be reasonable to expect similar scour patterns. most of these sites have historical flow data from the 1995 Although site comparisons, as described above, is not the only flood when flows exceeded the 100-year flow magnitude, tool for assessing scour potential at sites in South Carolina, it is a indicating that the measured scour reflects scour associated valuable tool when used in conjunction with the South Carolina with a large flow event. Other information for the sites listed bridge-scour envelope curves and the prevailing scour-prediction in table 22, such as hydraulic model data, predicted scour, soil technology (currently HEC-18; Arneson and others, 2012).
Table 21. Internet addresses for the South Carolina bridge-scour databases.
Database Scour components Source Internet address The South Carolina Bridge Clear-water abutment Benedict (2003) http://pubs.usgs.gov/wri/wri034064/ Scour Database (SCBSD) scour The South Carolina Clear-Water Clear-water overbank Benedict and Caldwell (2006) https://doi.org/10.3133/sir20055289 Pier- and Contraction-Scour contraction and Database (SCPCSD) pier scour The South Carolina Live-Bed Live-bed channel Benedict and Caldwell (2009) http://pubs.usgs.gov/sir/2009/5099/ Pier- and Contraction-Scour contraction and Database (SCLBSD) pier scour 2014 USGS Pier-Scour Clear-water and live- Benedict and Caldwell (2014) http://pubs.usgs.gov/ds/0845/ Database (PSDb-2014) bed pier scour
Table 22. Example of comparison scour data obtained from the South Carolina bridge-scour databases for sites in Spartanburg Table 22. Example of comparison scour data obtained from the South Carolina bridge-scour databases for sites in Spartanburg County, South Carolina. County, South Carolina.—Continued
2 [SCDOT, South Carolina Department of Transportation; mi , square mile; ft, foot; S.C., South Carolina Route; S, secondary road; U.S., United States Route; [SCDOT, South Carolina Department of Transportation; mi2, square mile; ft, foot; S.C., South Carolina Route; S, secondary road; U.S., United States Route; —, missing data] —, missing data]
Ratio of Calendar Geometric Left Left overbank Right overbank Geometric Drainage maximum Left Right Right Clear-water Live-bed Live-bed SCDOT Bridge year for contraction abutment clear-water clear-water Clear-water Live-bed contraction area at historic flow embankment embankment abutment pier scour pier scour contraction Road Stream structure length maximum ratio for scour contraction contraction pier width pier width ratio for live- bridge to 100-year length length scour depth depth depth scour number (ft) historic flow abutment depth scour depth scour depth (ft) (ft) bed contrac- (mi2) flow at (ft) (ft) (ft) (ft) (ft) (ft) at bridge scour (ft) (ft) (ft) tion scour bridge U.S. 29 South Tyger River 422002900100 76 270 1995 1.1 0.5 63 0 216 0.4 0 0 0.9 0.8 1.6 4.6 0.5 0 U.S. 221 South Tyger River 422022110300 137 250 1995 1.35 — — — — — — — — — 3 4.5 0.3 0 S.C. 146 Enoree River 424014600100 127 292 — — 0.5 187 0 119 3.1 0.5 1.2 0.8 0.4 — — 0.5 1.5 S.C. 296 Enoree River 424029600100 119 250 — — 0.5 63 0.7 121 1.1 — — — — — — — — S.C. 417 South Tyger River 424041700200 113 240 1995 1.18 — — — — — — — — — — — 0.7 7.8 S-62 South Tyger River 427006200500 91.8 270 1995 1.82 0.4 114 0 37 0 0.9 — 1.5 0.7 1.5 3.5 0.5 3 S-118 Enoree River 427011800001 186 370 1995 4.65 0.6 — — 234 3.1 — 1.3 0.8 1.6 — — — — S-242 South Tyger River 427024200200 94.4 200 1995 1.09 0.02 43 0.6 29 — 0.6 — 0.8 0.5 0.8 2.5 0.3 0 Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases 77
Guidance for Applying the South Carolina contraction-scour region (fig. 23). As noted in the report Bridge-Scour Envelope Curves section “The South Carolina Clear-Water Contraction-Scour Envelope Curve,” clear-water abutment-scour depths can The South Carolina bridge-scour envelope curves repre- be substantially larger than clear-water contraction-scour sent the upper-bound patterns of scour based on a large sample depths. Therefore, it is important to differentiate these types of field data collected in South Carolina. The comparison of of scour and the general regions of the overbank where they the South Carolina envelope curves with field data from other are most likely to occur. When assessing scour on the bridge sources, as described previously in this report, provides some overbank, clear-water abutment scour should be evaluated first measure of verification that the envelope curves are reason- followed by an evaluation of clear-water contraction scour. able. As such, the envelope curves offer a valuable resource The evaluation of abutment scour generally includes (1) the for understanding the upper-bound trends of scour and can be assessment of the upper bound of abutment scour using the used as supplementary tools for assessing scour potential at appropriate abutment-scour envelope curve (figs. 26, 27, 30, bridges in South Carolina. This report section provides guid- 31, 33, or 34), (2) the assessment of clear-water pier scour ance for applying the South Carolina bridge-scour envelope in the abutment-scour region, if required (fig. 7 or 18), and curves to assess scour potential at sites in South Carolina. The (3) an assessment of the extent, or top width, of the abutment- guidance is presented in an outline form that gives application scour hole on the overbank. The remaining overbank area is steps. Additionally, examples for applying the envelope curves assumed to be the region of contraction scour (fig. 23). The are given at the end of this report section. A bridge opening evaluation of clear-water contraction-scour includes (1) the generally has two regions of scour (figs. 4, 5): (1) clear-water assessment of the upper bound of clear-water contraction scour on the bridge overbank, and (2) live-bed scour in the scour using the clear-water contraction-scour envelope curve main channel. The application of the South Carolina bridge- (fig. 43), and (2) the assessment of clear-water pier scour in scour envelope curves has unique considerations for these the contraction-scour region, if required (fig. 7 or 18). If the regions and, therefore, application guidance is described assessed scour depth in the clear-water contraction-scour separately for these regions. In all cases and all steps when region (fig. 23) exceeds that of the assessed scour depth in evaluating scour using the envelope curves, engineering the abutment-scour region (fig. 23), then the scour depth in judgment should be used to evaluate the reasonableness and the clear-water contraction-scour region should be assumed application of the results. to represent the best estimate of scour for the abutment and applied to the abutment-scour region as well. The general Assessing Clear-Water Scour in the application steps are outlined below. For additional details, reference can be made to the report sections, “Application and Bridge Overbank Limitations of the South Carolina Abutment-Scour Envelope There are two primary regions of scour on bridge Curves” and “Application and Limitations of the Clear-Water overbanks including the abutment-scour region and Contraction-Scour Envelope Curve.”
Table 22. Example of comparison scour data obtained from the South Carolina bridge-scour databases for sites in Spartanburg Table 22. Example of comparison scour data obtained from the South Carolina bridge-scour databases for sites in Spartanburg County, South Carolina. County, South Carolina.—Continued
2 [SCDOT, South Carolina Department of Transportation; mi , square mile; ft, foot; S.C., South Carolina Route; S, secondary road; U.S., United States Route; [SCDOT, South Carolina Department of Transportation; mi2, square mile; ft, foot; S.C., South Carolina Route; S, secondary road; U.S., United States Route; —, missing data] —, missing data]
Ratio of Calendar Geometric Left Left overbank Right overbank Geometric Drainage maximum Left Right Right Clear-water Live-bed Live-bed SCDOT Bridge year for contraction abutment clear-water clear-water Clear-water Live-bed contraction area at historic flow embankment embankment abutment pier scour pier scour contraction Road Stream structure length maximum ratio for scour contraction contraction pier width pier width ratio for live- bridge to 100-year length length scour depth depth depth scour number (ft) historic flow abutment depth scour depth scour depth (ft) (ft) bed contrac- (mi2) flow at (ft) (ft) (ft) (ft) (ft) (ft) at bridge scour (ft) (ft) (ft) tion scour bridge U.S. 29 South Tyger River 422002900100 76 270 1995 1.1 0.5 63 0 216 0.4 0 0 0.9 0.8 1.6 4.6 0.5 0 U.S. 221 South Tyger River 422022110300 137 250 1995 1.35 — — — — — — — — — 3 4.5 0.3 0 S.C. 146 Enoree River 424014600100 127 292 — — 0.5 187 0 119 3.1 0.5 1.2 0.8 0.4 — — 0.5 1.5 S.C. 296 Enoree River 424029600100 119 250 — — 0.5 63 0.7 121 1.1 — — — — — — — — S.C. 417 South Tyger River 424041700200 113 240 1995 1.18 — — — — — — — — — — — 0.7 7.8 S-62 South Tyger River 427006200500 91.8 270 1995 1.82 0.4 114 0 37 0 0.9 — 1.5 0.7 1.5 3.5 0.5 3 S-118 Enoree River 427011800001 186 370 1995 4.65 0.6 — — 234 3.1 — 1.3 0.8 1.6 — — — — S-242 South Tyger River 427024200200 94.4 200 1995 1.09 0.02 43 0.6 29 — 0.6 — 0.8 0.5 0.8 2.5 0.3 0 78 The South Carolina Bridge-Scour Envelope Curves
Assessing Clear-Water Abutment-Scour Potential approximately one bridge length upstream from the bridge should be used. If substantial variation 1. Select the reference surface. of the flood plain width in the river reach extends approximately one bridge length upstream from the • The reference surface should be selected on the basis bridge, judgment must be used to select a natural, of the natural flood plain elevation and not an existing unconstricted approach cross section that will scour hole. Use field surveys and bridge/road plans provide a conservative but reasonable estimate of L to determine the average flood plain elevation in the and m. In some cases, it may be prudent to go further abutment-scour region (fig. 23). upstream beyond the one bridge width to assure 2. Estimate abutment geometric variables. conservative estimates of these variables. In many cases, the flood plain and channel in the approaching • The envelope curves can be sensitive to the selection reach upstream from the bridge is relatively uniform, of the embankment length (L) and the geometric- and in such cases, it may be appropriate to use the contraction ratio (m). Therefore, it is important natural, unconstricted cross section near the bridge to obtain accurate estimates of these variables. In to evaluate L and m. Because field conditions can general, L and m should be determined conservatively present unique site characteristics that deviate and should be based on (1) flow conditions near the from the ideal estimate, application of engineering 100-year flow, and (2) the full flood plain width (left judgment is critical to assure that conservative yet edge of water to right edge of water) for the natural, reasonable estimates of L and m are obtained. unconstricted approach cross section with no adjust- ment for ineffective flow areas associated with the ◦◦ In the case of a multiple-bridge opening, the bridge contraction. Additionally, unusual approach WSPRO model estimates the portion of the uncon- flow conditions, such as levees or skewed abutments, stricted approach cross section that contributes flow that may tend to narrow the width of the approaching to each bridge and subdivides the approach cross flow should generally be neglected to assure conser section accordingly into slices. The geometric- vative estimates of L and m. Use hydraulic models to contraction ratio is determined for each bridge by determine L for left and right abutments (edge of water using the associated slice section rather than the to abutment toe; fig. 28) andm (fig. 29) and check entire approach cross section. (In figure 29, the slice topographic maps and road plans. section would be substituted for the full approach cross section.) Each bridge opening can be projected ◦◦ In the original investigation (Benedict, 2003) onto the corresponding slice section to determine the WSPRO model (Shearman, 1990; Arneson the left and right embankment lengths. (In figure 28, and Shearman, 1998) was used to estimate these the slice section would be substituted for the full variables. The bridge hydraulics algorithm in the approach cross section.) The multiple-bridge WSPRO model assumes that the contraction of flow opening hydraulics algorithm in the HEC-RAS into a bridge begins at a cross section located one model (Brunner, 2016) is similar, but not identical, bridge-length upstream from the bridge, called the to the algorithm in the WSPRO model. Therefore, approach cross section. The WSPRO approach cross when using the HEC-RAS model to evaluate L section represents the natural upstream cross section and m at a multiple-bridge opening, caution and with no effects from the bridge contraction or adjust- engineering judgment must be used to assure that ments for ineffective flow areas. The WSPRO model conservative yet reasonable estimates of L and m are calculates the value of m for the bridge using the obtained. Reference should be made to Arneson and general approach shown in figure 29. The values of Shearman (1998) and Brunner (2016) for additional L were determined by projecting the bridge section details regarding the hydraulics of multiple-bridge onto the approach section as shown in figure 28. In openings for the WSPRO and HEC-RAS models, determining L and m, it is important to understand respectively. the cross-section configuration used in the WSPRO model and reference should be made to Arneson ◦◦ Old bridge and road plans often provide a survey of and Shearman (1998) for additional details. The the original ground line at the centerline of the road bridge hydraulics algorithm in the HEC-RAS model that gives a reasonable representation of the natural (Brunner, 2016) differs from that of the WSPRO unconstricted cross section at a bridge. This cross model in that the cross sections upstream from the section, in conjunction with an approximation of the bridge are adjusted to account for ineffective flow water-surface elevation, can be used to estimate L areas associated with the bridge contraction. The and m. Similarly, the edge of the unconstricted flood adjusted approach cross sections in the HEC-RAS plain is often a distinct feature on a topographic map. model should not be used to determine L and This feature, in conjunction with the bridge location m. Instead, a natural, unadjusted cross section on the map, can be used to estimate L and m. Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases 79
◦◦ When discrepancies exist between L and m, as figure 42, extends to the channel, use the largest determined by hydraulic models, topographic maps, scour depth from the clear-water abutment-scour and and road plans, evaluate why these discrepancies contraction-scour estimates. exist and use judgment in the final selection of • Special Case 1: Single-bridge opening crossing swamp these variables. with a poorly defined channel having a bridge length • The clear-water abutment-scour potential should of approximately 240 ft or less (see report section be estimated for both the left and right abutments “Single-Bridge Openings) separately unless the site meets the criteria for ◦◦ Assume a large, single scour hole develops across Special Cases 1 and 2 described later in this the entire bridge opening. report section. ◦◦ Use the longest L from the left and right abutments 3. Select the appropriate envelope curves based on the along with geometric-contraction ratio and apply regional location of the site (Piedmont or Coastal Plain). to the selected envelope curves to assess abutment- • Embankment-length envelope curves. scour potential. ◦◦ Piedmont (fig. 26; table 7; eq. 13):L ≤ 950 ft. ◦◦ Use judgment in selecting the final estimate of abutment scour based on the specific characteristics ◦◦ Coastal Plain (fig. 27; table 7; eq. 14):L ≤ 7,440 ft; of the site being assessed. because there are few data with L > 2,000 ft, caution should be used for L > 2,000 ft. ◦◦ If a conservative estimate of the range of abutment- scour depth is needed, then the larger scour depth • Geometric-contraction ratio envelope curve obtained from the geometric-contraction ratio or (figs. 30 or 31; table 7; eqs. 16 and 17). embankment-length envelope curves may be more ◦◦ Piedmont (fig. 30; table 7; eq. 16):m ≤ 0.82. appropriate (Benedict, 2003). ◦◦ Coastal Plain (fig. 31; table 7; eq. 17):m ≤ 0.98; ◦◦ If values between the various envelope curves differ because there are few data with L > 2,000 ft, substantially, assess why and consider use of the caution should be used for L > 2,000 ft. larger value if deemed appropriate. • Modified envelope curve. • Special Case 2: Multiple-bridge opening (see report ◦◦ Piedmont (fig. 33; table 8; table 9 for equations): section “Multiple-Bridge Openings) L ≤ 500 ft; see table 9 for limits of m. ◦◦ Piedmont sites: ◦◦ Coastal Plain (fig. 34; table 8; table 9 for equations): -- Use only the m envelope curve. L ≤ 500 ft; see table 9 for limits of m. ◦◦ Coastal Plain sites: • Evaluate if site characteristics are within the range of the data for the envelope curve by comparing with -- For L < 426 ft, use only the m envelope curve. noted tables above. -- For L ≥ 426 ft, evaluate with m and L envelope ◦◦ If yes, proceed; if no, use of the specific envelope curves; generally use the smallest value unless curve becomes questionable. a more conservative estimate is considered more reasonable; if values differ substantially, ◦◦ Uncertainty of the envelope curve increases near assess why and consider use of the larger value the limits of the data range. if deemed appropriate. If the bridge of interest is 4. Apply the selected curves and evaluate the results. a flood plain relief bridge with a poorly defined channel having a bridge length of approximately • To avoid overestimates of the range for abutment-scour 240 ft or less (similar to Special Case 1), use the depth, it may be reasonable to use the embankment- longest L from the left and right abutments along length envelope curve as the primary tool for with the geometric-contraction ratio and apply estimating abutment-scour depth in the Piedmont and to the selected Coastal Plain envelope curves to Coastal Plain of South Carolina (Benedict, 2003). assess abutment-scour potential. However, judgment should be used in selecting the final estimate of abutment scour based on the specific -- Use judgment in selecting the final estimate of characteristics for the site of interest. abutment scour based on the specific characteris- tics of the site being assessed. • If a conservative estimate of the range of abutment- scour depth is needed, then the larger scour depth -- If a conservative estimate of the range of abut- obtained from the geometric-contraction ratio or ment-scour depth is needed, then the larger scour embankment-length envelope curve may be more depth obtained from the geometric-contraction appropriate (Benedict, 2003). If the top width of the ratio or embankment-length envelope curves potential abutment-scour hole, as determined from may be more appropriate (Benedict, 2003). 80 The South Carolina Bridge-Scour Envelope Curves
-- If values between the various envelope curves to determine the average flood plain elevation in the differ substantially, assess why and consider use contraction-scour region (fig. 23). of the larger value if deemed appropriate. 2. Estimate the geometric-contraction ratio. 5. No adjustment is required for contraction scour in the • Follow the guidance in the report section “Assessing abutment-scour region. Clear-Water Abutment-Scour Potential” for evaluating 6. Adjust for pier scour in the abutment region. this variable. • If pier width ≤ 2.3 ft and skew is minimal, do not add 3. Apply the clear-water contraction-scour envelope curve pier-scour depth. (fig. 43; table 13; eq. 20):m ≤ 0.95. • For larger piers or piers with large skews, add pier- • The envelope curve is applicable to both the Piedmont scour depth. and Coastal Plain. ◦◦ Use guidance for assessing pier scour as described • Evaluate if site characteristics are within the range later in this report section. of the data for the envelope curve by comparing the • Special Case in Piedmont: If abutment-scour depth characteristics with noted tables above. ≤ 5 ft, add pier scour regardless of pier width; for ◦◦ If yes, proceed; if no, use of the envelope curve abutment-scour depth > 5 ft, follow the above-noted becomes questionable. guidance for adjusting for pier scour. ◦◦ Uncertainty of the envelope curve increases near the 7. Evaluate site characteristics that may adversely affect limits of the data range. In particular, use caution abutment scour and factor into final assessment of scour when applying the curve between m values of potential, including but not limited to the following: 0.90 and 0.95. • Adverse effects from debris. 4. Adjust for pier scour in the contraction-scour region. • Field conditions that may create unusual flow distribu- • Pier-scour depth as determined from the South Carolina tions that direct flow into the abutment-scour region clear-water or PSDb-2014 pier-scour envelope curves during high flows and thus may increase scour depths. should be added to the assessment of clear-water Such conditions may include natural channel constric- contraction-scour potential. tions, channel migration, dune bedforms, and debris. • Use guidance for assessing pier scour as described 8. Apply the 500-year flow adjustment coefficients to the later in this report section. selected values of abutment and pier scour. 5. Evaluate site characteristics that may adversely affect • Pier-scour adjustment (eq. 12). contraction scour and factor into the final assessment of scour potential, including but not limited to the • Abutment-scour adjustment (eq. 19). following: • This adjustment provides perspective on the relative • Adverse effects from debris. increase in theoretical abutment and pier scour associ- ated with the 100- to 500-year flow condition. • Field conditions that may create unusual flow distribu- tions, such as channel bends, which may direct flow 9. Use abutment-scour-hole top-width envelope curves to into the contraction-scour region during high flows, evaluate the extent of abutment scour on the overbank thus increasing the potential for scour. (fig. 42). 6. Apply the 500-year flow adjustment coefficients to the • The lateral reference point for applying the estimated selected values of clear-water contraction and pier scour. top width of the abutment-scour hole should be the abutment toe; the remaining overbank area beyond the • Pier-scour adjustment (eq. 12). abutment-scour-hole top width is considered the region • Clear-water contraction-scour adjustment (eq. 22). of clear-water contraction scour (fig. 23). • This adjustment provides perspective on the relative increase in theoretical clear-water contraction and Assessing Clear-Water Contraction-Scour Potential pier scour associated with the 100- to 500-year flow condition. 1. Select the reference surface. 7. Determine the extent of clear-water contraction-scour • The reference surface should be selected on the basis on the overbank. If abutment scour encompasses the of the natural flood plain elevation and not an existing entire overbank, then no contraction scour is applied to scour hole. Use field surveys and bridge/road plans the overbank. Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases 81
Assessing Live-Bed Contraction-Scour Potential ◦◦ Uncertainty of the envelope curve increases near the limits of the data range. Benedict and Caldwell (2009) did not identify a definitive 4. Adjust for pier scour in contraction-scour region. method for qualitatively assessing the potential for live-bed contraction scour and noted that there is a substantial measure • Pier-scour depth as determined from the South of uncertainty associated with assessing the potential for Carolina live-bed or PSDb-2014 pier-scour envelope live-bed contraction scour in South Carolina. Therefore, using curves should be added to the assessment of live-bed judgment and caution is critical when making such assess- contraction-scour potential. ments. This should be kept in mind when assessing live-bed • Use guidance for assessing pier scour as described scour potential with the South Carolina live-bed contraction- later in this report section. scour envelope curves. 1. Select the reference surface. 5. Evaluate site characteristics that may adversely affect live-bed contraction and pier scour and factor into the • The reference surface should be selected on the basis final assessment of scour potential, including but not of the natural average thalweg elevation and not an limited to the following: existing scour hole. Use field surveys and bridge/ road plans to determine the average thalweg elevation • Adverse effects from debris. (channel low point) in the main channel at the bridge. • Field conditions that may create unusual flow distribu- 2. Estimate the geometric-contraction ratio. tions that direct flow into the contraction-scour region during high flows and thus may increase scour depths. • Follow the guidance in the report section “Assessing Such conditions may include natural channel constric- Clear-Water Abutment-Scour Potential” for evaluating tions, channel migration, dune bedforms, and debris. this variable. 6. Apply the 500-year flow adjustment coefficients to the 3. Apply the live-bed contraction-scour envelope curves. selected values of live-bed contraction and pier scour. • Dimensionless envelope curve (for comparison • Pier-scour adjustment (eq. 12). purposes). • Live-bed contraction-scour adjustment (eq. 28). ◦◦ Piedmont curve (fig.48B ; table 16; eq. 25): m ≤ 0.85. • This adjustment provides perspective on the relative increase in theoretical live-bed contraction and pier ◦◦ Coastal Plain curve (fig. 48B; table 16; eq. 24): scour associated with the 100- to 500-year flow m ≤ 0.87. condition. ◦◦ Theoretical curve for conservative assessment (fig. 48B; eq. 23): limits of m are undefined. 7. Because the channel bed and thalweg can shift during high-flow events, it is assumed that the live-bed • Field envelope curve. contraction scour will extend across the entire main ◦◦ Piedmont and Coastal Plain (fig. 49A; table 16; channel. In addition, for piers in the overbank area, eq. 26): m ≤ 0.82. the possibility of channel widening and (or) shifting occurring over time might also need to be considered. ◦◦ The clear-water contraction-scour data used for comparison with the live-bed contraction-scour envelope curve (fig. 49B) suggest it may be Assessing Pier-Scour Potential appropriate to extend the envelope curve beyond a geometric-contraction ratio of 0.82; however, the 1. Estimate the nominal pier width (b). differing nature of the clear-water contraction- and live-bed contraction-scour data makes it difficult to • The pier-scour envelope curves do not account for make a strong conclusion. unusual or complex pier geometries or for exposed footings, and so judgment must be used to assess the • Modified envelope curve (for drainage areas ≤ 200 mi2). applicability of the pier-scour envelope curves for ◦◦ Piedmont and Coastal Plain (fig. 53; table 18; these types of piers. table 19 for equations): m ≤ 0.9. 2. Select appropriate pier-scour envelope curve. • Evaluate if site characteristics are within the range of the data for the envelope curve by comparing the • Clear-water pier scour on overbank (fig. 7; table 1; characteristics with noted tables above. eq. 2): b ≤ 6 ft. ◦◦ If yes, proceed; if no, use of the envelope curve • Live-bed pier scour in main channel (fig. 10; table 2; becomes questionable. eq. 6): b ≤ 6 ft. 82 The South Carolina Bridge-Scour Envelope Curves
• PSDb-2014 pier scour (fig. 14; table 4; eq. 9): Limitations of the South Carolina Bridge-Scour 6 ft < b ≤ 15 ft. Envelope Curves • Evaluate if site characteristics are within the range of 1. The envelope curves are empirical. the data for the envelope curve by comparing with noted tables above. • The envelope curves are empirical methods that are based on limited field samples; it is possible that ◦◦ If yes, proceed; if no, use of the envelope curve scour could exceed the envelope curves; therefore, becomes questionable. application of a safety factor may be prudent. ◦◦ Uncertainty of the envelope curve increases near • Application of the envelope curves is constrained the limits of the data range. to the range of data and site characteristics used to 3. Because the channel bed and thalweg can shift during develop the envelope curves. high-flow events, it is assumed that the live-bed • The uncertainty associated with the envelope curves contraction scour will extend across the entire main increases near the limits of the data range. channel. In addition for piers in the overbank area, the possibility of channel widening and (or) shifting occur- • The live-bed scour data have the largest uncertainty. ring over time might also need to be considered. 2. The envelope curves do not account for unusual site conditions such as: 4. Account for pier skew. • Debris • Pile bents • Channel bank failures ◦◦ For pile bents with skews approximately 15 degrees or less and spacings between piles of approximately • Pressure flow 5 pile widths, no adjustment for skew is required; • Unusual flow patterns created by unique site conditions if adverse field conditions exist or are anticipated at the site, use of equation 4 following the guidance • Unusual pier geometries as recommended in HEC-18 (Arneson and others, • Washout of approach road embankments 2012) may be appropriate. 3. The field data generally represent scour associated with ◦◦ For pile bents with skews exceeding approximately flows approaching the 100-year flow magnitude but 15 degrees or with spacings between piles smaller should not be interpreted as representing the “100-year” than 5 pile widths, adjust for skew using equation 4 scour estimate. Additionally, the envelope curves should following the guidance as recommended in HEC-18 not be used to evaluate scour potential for larger flows, (Arneson and others, 2012). such as the 500-year flow. The 500-year flow adjustment • Piers coefficients can be used to provide perspective on the relative increase in theoretical scour associated with ◦◦ Adjust for skew using equation 4 following the the 100- to 500-year flow condition but should not be guidance as recommended in HEC-18 (Arneson interpreted as providing a “500-year” scour estimate. and others, 2012). 4. Subsurface soils can impede or promote scour. There- 5. Evaluate site characteristics that may adversely impact fore, assessing the potential effect of subsurface soils pier scour and factor into the final assessment of scour on scour is important. potential, including, but not limited to the following: • Adverse effects from debris. Example Problems • Channel bends that may create a larger potential The following example problems are provided as a general for scour. overview of a few of the types of assessments that could be • Channel widening and shifting (for piers near the made using the South Carolina bridge-scour envelope curves. channel banks). The examples should not be viewed as being all inclusive with respect to the steps that might need to be taken for an actual 6. Apply the 500-year flow adjustment coefficients to the bridge-scour assessment. For such assessments, the reader selected values of pier scour. should closely follow the steps listed in the “Guidance for • Pier-scour adjustment (eq. 12). Applying the South Carolina Bridge-Scour Envelope Curves” section and other sections in this document providing greater • This adjustment provides perspective on the relative details for such analyses. The information needed to apply increase in theoretical pier scour associated with the the South Carolina bridge-scour envelope curves should be 100- to 500-year flow condition. obtained using all available sources and based on engineering Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases 83 judgment. The reader is once again reminded that the curves Clear-Water Abutment-Scour Potential represent empirical methods that have a number of limitations Drainage area (DA) = 383 mi2 as previously discussed. The South Carolina bridge-scour envelope curves are useful tools for assessing potential for scour L = 1,720 ft (for the left embankment) at riverine bridges in South Carolina; however, the envelope L = 1,181 ft (for the right embankment) curves should not be relied upon as the only tool for assessing b2 = 559 ft (flow top width in the bridge opening) bridge scour but rather should be used in conjunction with the B1 = 3,434 ft (flow top width at the upstream prevailing scour-prediction technology (currently HEC-18). To unconstricted approach cross section) best assess potential scour, one should compile and study the m = 1 – (559/3434) = 0.84 (geometric-contraction available information for a given site, evaluate scour with the ratio, eq. 15) South Carolina bridge-scour envelope curves and the prevailing scour-prediction technology (currently HEC-18), and then The above characteristics were verified to be within the bring sound engineering principles to bear on the final estimate range of data used to develop the envelope curves (table 7). of potential abutment-scour depth. Each bridge site will likely Because the bridge is longer than 240 ft, both the left and right have unique characteristics requiring that sound engineering embankment lengths will be used to evaluate each respective judgment be applied for a thorough assessment of the various abutment-scour potential. Because L for both the left and right potential scour components. (Note: Potential scour estimates for embankments is greater than 500 ft, the modified envelope these examples have been rounded to two significant digits.) curves will not be used. Using left embankment length and equation 14
(fig. 27), ys = 16 ft Example 1: Coosawhatchie River at Road S-87 in Using right embankment length and equation 14
Jasper County (fig. 27), ys = 15 ft The site for example 1 is located in the Coastal Plain of Using geometric-contraction ratio and equation 17 (fig. 31), y = 15 ft South Carolina. Figure 55 shows the bridge-site data for this s example, and figure 56 shows the approach section informa- The pier widths are 1.4 ft. Because they are less than tion. The flood plain reference surface was determined to be 2.3 ft and skew is assumed minimal, the pier-scour depth will 19.5 ft (fig. 55). not be added to the piers in the abutment region.
35
High steel
30 t
e f Low steel i n um, t
a 25 d Top width at bridge for the 100-year water surface ary r i t ar b e v
o 20 a b
io n t a
v Abutment scour at
El e time of survey 15 Abutment scour at time of survey E PLANATION Ground line Water-surface elevation NOT TO SCALE Reference surface 10 0 100 200 300 400 500 Station from left end of bridge, in fe et Figure 55. Bridge site information for example 1, Coosawhatchie River at Road S-87 in Jasper County, South Carolina.
SC Bridge Scour Figure 55 84 The South Carolina Bridge-Scour Envelope Curves
35
Left embankment Bridge top width Right embankment
30 Left edge of water to abutment toe Abutment toe to right edge of water t e
f
i n um, t
a 25 100-year water surface d
ary Approach top width r i t ar b e v
o 20 a b
Ground line io n t a v El e 15
NOT TO SCALE 10 0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 Station from left end of bridge, in fe et Figure 56. Approach section information for example 1, Coosawhatchie River at Road S-87 in Jasper County, South Carolina, showing bridge top width and road embankment locations projected onto the approach cross section.
Abutment Scour-Hole Top Width abutment-scour region, is 4.8 (previously computed clear- water contraction scour) + 2.6 = 7.4 ft. To estimate the abutment scour-hole top width for the The region of scour determined from these computations left abutment, it is assumed that a conservative estimate is is shown in figure 57. It is important to remember that the desired and thus, the larger y of 16 ft from the embankment- s scour estimates shown in figure 57 are based on hydraulic length envelope curve will be used instead of the estimate of data determined from the 100-year recurrence interval flow 15 ft from the geometric-contraction ratio envelope curve. estimated for this site in conjunction with the appropriate Therefore, using figure 42A and the y of 16 ft for the left s South Carolina bridge-scour envelope curve; and although embankment length, the left abutment scour-hole top width the estimated scour depths from the envelope curves reflect is estimated to be about 70 ft. scour depths associated with flows near the 100-year flow, the For the right abutment, the embankment-length envelope estimates should not be interpreted as the “100-year” scour curve and the geometric-contraction ratio envelope curve estimates. In an actual analysis, the user also should utilize the provided the same estimate for y of 15 ft. Using figure 42A, s available South Carolina bridge-scour databases to compare the right abutment scour-hole top width also is estimated to the estimated scour results with observed results from sites be about 70 ft. with similar characteristics and in similar areas of the State. Clear-Water Contraction-Scour Potential Applying the appropriate 500-year flow adjustment coefficient to the previously computed scour components From the geometric-contraction ratio computed results in the following (fig. 58): previously along with equation 20 (fig. 43), the clear-water contraction-scour depth is estimated: y = 4.8 ft. s Clear-Water Abutment Region Pier-Scour Potential Left abutment: Using equation 18 (fig. 41), potential clear-water Nominal pier width = 1.4 ft with minimal skew (verified abutment scour = 1.21 × 16 = 19 ft. that the characteristics are within the range of the data shown in table 1). Using equation 2 (fig. 7), the clear-water pier scour Right abutment: depth is 2.6 ft. The total scour depth for the piers located in Using equation 18 (fig. 41), potential clear-water pier the clear-water overbank region, outside of the clear-water scour = 1.21 × 15 = 18 ft.
SC Bridge Scour Figure 56 Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases 85
35
High steel 30
t Low steel E PLANATION e f
Ground line i n 25 Top width at bridge for the 100-year water surface Water-surface
um, elevation t a
d Reference surface 20
ary Scour template based r i t on the 100-year flow ar b e
v 15 o a b
io n t 10 a
v Region of clear-water contraction scour El e Region of and pier scour Region of 5 abutment scour abutment scour
NOT TO SCALE 0 0 100 200 300 400 500 Station from left end of bridge, in feet Figure 57. Region of potential scour determined from South Carolina bridge-scour envelope curves for example 1, Coosawhatchie River at Road S-87 in Jasper County, South Carolina.
45 EXPLANATION 40 Ground line Scour template with the 500-year flow Water-surface elevation adjustment coefficient applied Scour template based on the 35 Reference surface 100-year flow High steel t e f
30
i n Low steel um, t Top width at bridge for the 100-year water surface a 25 d ary r i t 20 ar b e v o 15 a b
io n t a v 10 El e
5 Region of clear-water Region of contraction scour Region of abutment scour and pier scour abutment scour 0
NOT TO SCALE –5 0 100 200 300 400 500 Station from left end of bridge, in feet Figure 58. Region of potential scour determined from the South Carolina bridge-scour envelope curves for example 1, Coosawhatchie River at Road S-87 in Jasper County, South Carolina, with the 500-year flow adjustment coefficient applied.
SC Bridge Scour Figure 57
SC Bridge Scour Figure 58 86 The South Carolina Bridge-Scour Envelope Curves
Clear-Water Overbank Region Clear-Water Abutment-Scour Potential Pier Scour: DA = 16.6 mi2 Using equation 11 (fig. 20), potential clear-water pier L = 552 ft (for the right embankment) scour = 1.09 × 2.6 = 2.8 ft. L = 332 ft (for the left embankment) Contraction Scour: b = 74 ft (flow top width in the bridge opening) Using equation 21 (fig. 46), potential clear-water 2 B1 = 944 ft (flow top width at the upstream unconstricted contraction scour = 1.46 × 4.8 = 7.0 ft. approach cross section) Total potential scour at piers outside of the m = 1 – (74/944) = 0.92 (geometric-contraction ratio, abutment-scour region = 2.8 + 7.0 = 9.8 ft. eq. 15) Applying the 500-year flow adjustment coefficients The above characteristics were verified to be within provides perspective on the relative increase in theoretical the range of the data used to develop the envelope curves clear-water abutment, contraction, and pier scour associated (table 7). This site is a single-bridge opening that crosses a with the 100- to 500-year flow conditions but should not be swamp with a poorly defined channel, having a bridge length interpreted as the “500-year” scour estimates. less than 240 ft. Therefore, abutment scour at this site will be assessed as Special Case 1, where a large single scour hole Example 2: Big Swamp at S.C. 51 in is assumed to form across the entire bridge opening. For this case, the longest L from the left and right abutments, along Florence County with the geometric-contraction ratio, will be applied to the Example 2 is a swampy site located in the Coastal Plain selected envelope curves to assess abutment-scour potential. of South Carolina. Figure 59 shows the bridge-site data Using the longest embankment length (right embank- for this example, and figure 60 shows the approach section ment) and equation 14 (fig. 27),y s = 15 ft. information. The flood plain reference surface was determined Using the geometric-contraction ratio and equation 17 to be 85.6 ft (fig. 59). (fig. 31), ys = 20 ft.
110
105
High steel t 100 e f
i n Low steel um, t 95 a d ary
r Top width at bridge for the 100-year water surface i t 90 ar b e v o a b
85 io n t
a Swampy channel v
El e 80 E PLANATION Ground line Water-surface elevation 75 Reference surface
NOT TO SCALE 70 0 10 20 30 40 50 60 70 80 90 100 Station from left end of bridge, in feet
Figure 59. Bridge site information for example 2, Big Swamp at S.C. 51 in Florence County, South Carolina.
SC Bridge Scour Figure 59 Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases 87
105
Bridge top Left embankment width Right embankment
100 Left edge of water to abutment toe Abutment toe to right edge of water
t
e f 95 i n
um, 100-year water surface t
a Approach top width d 90 ary r i t ar b e
v 85
o Ground line a b
io n t a
v 80 El e
75
70 0 200 400 600 800 1,000 1,200 Station from left end of bridge, in fe et Figure 60. Approach section information for example 2, Big Swamp at S.C. 51 in Florence County, South Carolina, showing bridge top width and road embankment locations projected onto the approach cross section.
Because the longest embankment (used for Special The region of scour determined from these computa- Case 1) is greater than 500 ft, the modified envelope curves tions is shown in figure 61. It is important to remember that will not be used. the scour estimates shown in figure 61 are based on hydraulic The pier widths are 1.0 ft. Because they are less than data determined from the 100-year recurrence interval flow 2.3 ft and skew is assumed minimal, the pier-scour depth will estimated for this site in conjunction with the appropriate not be added to the piers in the abutment region. South Carolina bridge-scour envelope curve; and although Benedict (2003) noted that it may be reasonable to give the estimated scour depths from the envelope curves reflect preference to the embankment-length envelope curve rather scour depths associated with flows near the 100-year flow, than the geometric-contraction ratio envelope curve in order the estimates should not be interpreted as the “100-year” to minimize overestimates of abutment-scour depth. Addition- scour estimates. In an actual analysis, the user also should ally, this site has a relatively small drainage area (16.6 mi2), utilize the available South Carolina bridge-scour databases which will tend to produce smaller abutment-scour depths to compare the estimated scour results with observed results (fig. 40). On the basis of these observations, the smaller scour from sites with similar characteristics and in similar areas of estimate of 15 ft was assumed to be the more reasonable value the State. and was selected for this site. Applying the appropriate 500-year flow adjustment
Using figure 42B and the ys of 15 ft computed using the coefficient to the previously computed scour components right embankment length, the abutment scour-hole top width is results in the following (fig. 62): estimated to be about 100 ft, which exceeds the bottom width Using equation 19, potential clear-water abutment of the channel at the bridge. This pattern gives some support scour = 1.21 × 15 = 18 ft. that the assumption of Special Case 1 is reasonable and that Applying the 500-year flow adjustment coefficients the abutment-scour can be assumed to encompass the bridge provides perspective on the relative increase in theoretical opening from the left abutment toe to the right abutment toe as clear-water abutment scour associated with the 100- to shown in figure 61. Because the abutment scour extends from 500-year flow conditions but should not be interpreted as the left abutment toe to the right abutment toe, no evaluation the “500-year” scour estimate. of clear-water contraction scour was required.
SC Bridge Scour Figure 60 88 The South Carolina Bridge-Scour Envelope Curves
110
105
High steel 100 Low steel t e f
95 i n Top width at bridge for the 100-year water surface um, t 90 a d ary r 85 i t ar b e
v 80 o a b
Region of io n
t 75
a abutment scour
v E PLANATION
El e Ground line 70 Water-surface elevation Reference surface Scour template based on 65 the 100-year flow NOT TO SCALE 60 0 10 20 30 40 50 60 70 80 90 100 Station from left end of bridge, in feet Figure 61. Region of potential scour determined from the South Carolina bridge-scour envelope curves for example 2, Big Swamp at S.C. 51 in Florence County, South Carolina.
110
105
High steel 100
t Low steel e f 95 i n um,
t Top width at bridge for the 100-year water surface
a 90 d ary r
i t 85 ar b e v
o 80 a b
EXPLANATION io n t 75 a Ground line v Region of Water-surface elevation El e Reference surface 70 abutment scour Scour template based on the 100-year flow 65 Scour template with the 500-year flow adjustment coefficient applied NOT TO SCALE 60 0 10 20 30 40 50 60 70 80 90 100 Station from left end of bridge, in feet Figure 62. Region of potential scour determined from the South Carolina bridge-scour envelope curves for example 2, Big Swamp at S.C. 51 in Florence County, South Carolina, with the 500-year flow adjustment coefficient applied.
SC Bridge Scour Figure 61
SC Bridge Scour Figure 62 Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases 89
Example 3: Enoree River at Road S-87 in Because there is a substantial variation in the length of the Newberry County left and right abutments, estimating the potential abutment- scour depth using the embankment length curves instead The site for example 3 is located in the Piedmont of of the geometric-contraction ratio curves is likely more South Carolina. Figure 63 shows the bridge-site data for this reasonable for this site (Benedict, 2003). For the right example, and figure 64 shows the approach section informa- abutment, the potential scour depth was the same for both tion. The flood plain reference surface was determined to be curves (13 ft), indicating that this estimate is reasonable for 86.5 ft (fig. 63). the right abutment. For the left embankment, however, the left abutment scour-depth estimate is 1.7 ft, much less than Clear-Water Abutment-Scour Potential the 13 ft estimated by the geometric-contraction ratio curve. The left embankment length is relatively small (61 ft) having DA = 677 mi2 a scour potential much less than the right embankment with L = 61 ft (for the left embankment) a length of 600 ft. Thus the smaller estimate of abutment- scour potential (1.7 ft) for the left abutment appears reason- L = 600 ft (for the right embankment) able and is selected to be the estimate of abutment scour
b2 = 309 ft (flow top width in the bridge opening) depth (fig. 65). The flood plain pier widths are 1.4 ft. Because they are B1 = 934 ft (flow top width at the upstream unconstricted approach cross section) less than 2.3 ft and skew is assumed minimal, the potential pier-scour depth for the piers in the abutment region will not m = 1 – (309/934) = 0.67 be included in the total potential scour depth.
The above characteristics were verified to be within Using figure 42A and the ys of 13 ft, the abutment scour- the range of the data used to develop the envelope curves hole top width for the right abutment is estimated to be about
(table 7). 70 ft. For the left abutment using the ys of 1.7 ft, the left Using the left embankment length and equation 13 abutment scour-hole top width is estimated to be about 45 ft.
(fig. 26), ys = 1.7 ft. Because the left abutment scour-hole width encompasses the Using the right embankment length and equation 13 entire overbank width, the estimated abutment-scour depth
(fig. 26), ys = 13 ft. should be compared to the left clear-water overbank scour Using the geometric-contraction ratio and equation 16 depth and the largest value used (see item 3 in report section
(fig. 30), ys = 13 ft. “Assessing Clear-Water Abutment-Scour Potential”).
120
110 High steel Low steel t e f
i n 100 Top width at bridge for the 100-year water surface um, t a d 90 ary r i t ar b e v
o 80 a b
io n t a v 70 Abutment scour at El e time of survey E PLANATION Channel Main Ground line 60 thalweg channel Water-surface elevation Reference surface NOT TO SCALE Channel reference surface (thalweg elevation) 50 0 50 100 150 200 250 300 Station from left end of bridge, in feet Figure 63. Bridge site information for example 3, Enoree River at Road S-87 in Newberry County, South Carolina.
SC Bridge Scour Figure 63 90 The South Carolina Bridge-Scour Envelope Curves
130
125
120 Left embankment 115 t e
f 110 Bridge top width Right embankment i n Abutment toe to right edge of water
um, 105 t a d 100 100-year water surface ary r i t 95 Approach top width ar b e
v 90 o a b 85 io n t a
v 80 El e 75
70 Ground line 65
60 0 200 400 600 800 1,000 1,200 1,400 Station from left end of bridge, in feet Figure 64. Approach section information for example 3, Enoree River at Road S-87 in Newberry County, South Carolina, showing bridge top width and road embankment locations projected onto the approach cross section.
140 EXPLANATION Ground line Water-surface elevation 120 Flood plain reference surface (average elevation) Channel reference surface (thalweg elevation) Scour template based on the 100-year flow t
e Low steel f High steel
i n 100 Top width at bridge for the 100-year water surface um, t a d ary r
i t 80 ar b
e Region of v
o abutment scour a b Region of 60 clear-water Region of io n
t abutment scour
a contraction scour v and pier scour El e
40 Region of live-bed contraction scour and pier scour NOT TO SCALE 20 0 50 100 150 200 250 300 Station from left end of bridge, in feet Figure 65. Region of potential scour determined from South Carolina bridge-scour envelope curves for example 3, Enoree River at Road S-87 in Newberry County, South Carolina.
SC Bridge Scour Figure 64
SC Bridge Scour Figure 65 Guidance for Using the South Carolina Bridge-Scour Envelope Curves and Databases 91
Clear-Water Contraction-Scour Potential The region of potential scour determined from these computations is shown in figure 65. It is important to From the geometric-contraction ratio computed remember that the scour estimates shown in figure 65 are previously along with equation 20 (fig. 43), the clear-water based on hydraulic data determined from the 100-year contraction-scour depth for the right overbank (outside the recurrence interval flow estimated for this site in conjunction region of the abutment scour) is estimated: y = 4.6 ft. Because s with the appropriate South Carolina bridge-scour envelope the estimate of scour depth in the clear-water contraction scour curve; and although the estimated scour depths from the region exceeds that of the left clear-water abutment-scour envelope curves reflect scour depths associated with flows region, and because the top width of the abutment-scour hole near the 100-year flow, the estimates should not be interpreted encompasses the entire overbank, the estimated clear-water as the “100-year” scour estimates. In an actual analysis, the contraction scour for the overbank will be used for the esti- user also should utilize the available South Carolina bridge- mated scour at the left abutment (see item 3 in report section, scour databases to compare the estimated scour results with “Assessing Clear-Water Abutment-Scour Potential”). observed results from sites with similar characteristics and in similar areas of the State. Clear-Water Pier Scour for the Right Overbank Applying the appropriate 500-year flow adjustment Using equation 2 (fig. 7), the potential pier-scour depth coefficient to the previously computed scour components for the right overbank piers, which have a width of 1.4 ft, results in the following (fig. 66): is 2.6 ft. The total scour depth for the piers located in the Clear-Water Abutment Region clear-water overbank region, outside of the clear-water Left abutment: abutment-scour region, is 4.6 (previously computed clear- Using equation 18 (fig. 41), potential clear-water water contraction scour) + 2.6 = 7.2 ft. abutment scour for the left abutment =1.21 × 4.56 = 5.6 ft. Live-Bed Contraction-Scour Potential Right abutment: Benedict and Caldwell (2009) indicated that their inves- Using equation 18 (fig. 41), potential clear-water tigation did not identify a definitive method for qualitatively abutment scour for the right abutment assessing the potential live-bed contraction scour. They noted =1.2 × 13 = 16 ft. that “…the practitioner must use caution and judgment when Clear-Water Overbank Region qualitatively assessing this type of scour” and recommended a Left overbank: number of steps for making the assessment. The more detailed The left abutment top width encompasses the assessment that should be done is beyond the scope of this entire overbank, and therefore, the estimated example problem. For this example, a comparison of the clear-water abutment scour depth is used in live-bed contraction-scour potential using the dimensionless this region. equation for the Piedmont (eq. 25; fig. 48B) will be made with the estimate from the field envelope curve (eq. 26; fig. A49 ). Right overbank: Using the dimensionless equation for the Piedmont Using equation 21 (fig. 46), potential clear-water (eq. 25; fig. 48B) with the approach depth of 23.2 ft, the contraction scour = 1.46 × 4.6 = 6.7 ft. potential live-bed contraction-scour depth is about 16 ft. In Using equation 11 (fig. 20), potential clear-water contrast, using the field envelope curve (eq. 26; fig. A48 ), pier scour for the right overbank = 1.09 × 2.6 = 2.8 ft. the potential live-bed contraction-scour depth is about 12 ft. Total potential scour at piers in the clear-water (Note: The drainage area for this site is greater than 200 mi2, overbank region = 6.7 + 2.8 = 9.5 ft. which precludes the use of the modified live-bed contraction- Live-Bed Channel Region scour envelope curve as shown in figure 53.) Because of the Using equation 27 (fig. 54), potential live-bed higher degree of uncertainty in the live-bed contraction-scour contraction scour = 1.32 × 16 = 21 ft. envelope curves (Benedict and Caldwell, 2009), the more Using equation 11 (fig. 20), potential live-bed pier conservative estimate of 16 ft was selected as the estimate scour = 1.09 × 6.7 = 7.3 ft. of live-bed contraction scour for this site (fig. 65). Total potential scour at piers in the live-bed channel region = 21 + 7.3 = 28.3 ft. Live-Bed Pier-Scour Potential Applying the 500-year flow adjustment coefficients Using equation 6 (fig. 10) and the channel pier width of provides perspective on the relative increase in theoretical 3 ft, the live-bed pier-scour potential is 6.7 ft. There is no skew clear-water abutment, contraction, and pier scour associated associated with this pier and, therefore, no adjustment for pier with the 100- to 500-year flow conditions but should not be skew is required. interpreted as the “500-year” scour estimates. 92 The South Carolina Bridge-Scour Envelope Curves
140
120 EXPLANATION Ground line Low steel Water-surface elevation High steel
t Flood plain reference surface
e 100 (average elevation) f Top width at bridge for the 100-year water surface
i n Channel reference surface (thalweg elevation) um, t
a Scour template based on d 80 the 100-year flow ary
r Scour template with the i t 500-year flow adjustment ar b Region of coefficient applied e
v abutment scour Region of clear-water o 60 contraction scour
a b Region of and pier scour abutment scour io n t
a v
El e 40 Region of live-bed contraction scour and pier scour NOT TO SCALE 20 0 50 100 150 200 250 300 Station from left end of bridge, in feet
Figure 66. Region of potential scour determined from the South Carolina bridge-scour envelope curves for example 3, Enoree River at Road S-87 in Newberry County, South Carolina, with the 500-year flow adjustment coefficient applied.
Summary the envelope curves for the various scour components to help assess scour potential at riverine bridges in South Carolina. Current practices for predicting scour at bridges are based Limited verification of selected envelope curves was made by on laboratory-derived equations and provide an important comparing the data with previously published field data from resource for assessing scour potential; however, there is a sources outside of South Carolina, including the U.S. Geological measure of uncertainty when applying such procedures to Survey National Bridge Scour Database, the U.S. Geological field conditions. One way to evaluate the reasonableness of Survey 2014 Pier Scour Database, and selected field measure- predicted scour from the laboratory-derived equations is to ments of scour from other States. In general, the comparisons compare the results with field measurements of historic scour. showed similar trends in the South Carolina data and indicated The U.S. Geological Survey has been partnering with the that the South Carolina envelope curves were reasonable. South Carolina Department of Transportation for more than The South Carolina bridge-scour envelope curves were two decades to improve the understanding of bridge scour and developed from field data having historic flows approaching enhance methods for assessing potential bridge scour in South the 100-year flow magnitude and, therefore, are considered Carolina. As a part of this effort, the U.S. Geological Survey appropriate for assessing scour potential for flows near the previously conducted a series of three field investigations to 100-year flow but are not recommended for assessing scour collect and analyze historic scour measurements in order to potential for extreme flows such as the 500-year flow. In order better understand regional trends of scour in South Carolina. to extend the application of the envelope curves to flow condi- As part of those studies, historic scour data were collected at tions near the 500-year flow, a correlation analysis was done 231 riverine bridges and included measurements of clear-water using theoretical scour computations from previous level-2 abutment-scour, contraction-scour, and pier-scour depths, scour computations that were based on hydraulic information as well as live-bed contraction- and pier-scour depths. From for the 100- and 500-year flows. From the analyses of the those investigations, regional bridge-scour envelope curves various scour components, theoretical 500-year flow adjust- were developed for the various scour components, providing ment coefficients were developed. The adjustment coefficients supplementary tools for assessing those components of scour are a useful tool for gaining perspective on the relative increase at riverine bridges in South Carolina. of theoretical scour associated with the 100- to 500-year flows On the basis of the previous scour investigations, this report but should not be considered as a definitive estimate of the provides a guidance manual for an integrated approach for using scour associated with the 500-year flow. SC Bridge Scour Figure 66 Selected References 93
The bridge-scour envelope curve guidance manual Basak, V., Baslamisli, Y., and Ergun, O., 1977, Local scour provides information on the application and limitations of the depths around circular pier groups aligned with the flow: various bridge-scour envelope curves developed using field Turkey General Directorate of State Hydraulic Works data from South Carolina. In addition, guidance for applying Report No. 641. [In Turkish.] the South Carolina bridge-scour envelope curves also is Bedient, P.B., and Huber, W.C., 1988, Hydrology and flood provided. The South Carolina bridge-scour envelope curves plain analysis: Reading, Mass., Addison-Wesley Publishing are useful tools for assessing potential for scour at riverine Co., 650 p. bridges in South Carolina; however, the envelope curves are empirical methods with limitations and, therefore, should Benedict, S.T., 2003, Clear-water abutment and contraction not be relied upon as the only tool for assessing bridge scour scour in the Coastal Plain and Piedmont Provinces of but should be used in conjunction with the prevailing scour- South Carolina, 1996–99: U.S. Geological Survey Water- prediction technology (currently HEC-18). To best assess Resources Investigations Report 03–4064, 137 p. potential scour, one should compile and study the available Benedict, S.T., 2016, The upper bound of abutment scour in information for a given site, evaluate scour with the South laboratory and field data, Transportation Research Record: Carolina bridge-scour envelope curves and the prevailing Journal of the Transportation Research Board, No. 2588, scour-prediction technology (currently HEC-18), and then Transportation Research Board, Washington, D.C., 2016, bring sound engineering principles to bear on the final p. 154–162. DOI: 10.3141/2588-17 estimate of potential scour at a given site. Benedict, S.T., Abrahamsen, T.A., and Caldwell, A.W., 2007, Collection of historical live-bed scour data at selected bridges in South Carolina using ground-penetrating radar, in Acknowledgments The World Environmental and Water Resources Congress, Tampa, Fla., 2007, Proceedings: Reston, Va., American The authors acknowledge and appreciate the long- Society of Civil Engineers, p. 1–11. standing partnership between the USGS South Atlantic Water Science Center (formerly the South Carolina Water Science Benedict, S.T., and Caldwell, A.W., 2006, Development and Center), and the South Carolina Department of Transportation. evaluation of clear-water pier and contraction scour enve- The two agencies have cooperated on numerous bridge-scour lope curves in the Coastal Plain and Piedmont Provinces of investigations for more than two decades. These investigations South Carolina: U.S. Geological Survey Scientific Investi- have resulted in improved understanding of bridge-scour gations Report 2005–5289, 98 p. processes and development of supplementary tools for the Benedict, S.T., and Caldwell, A.W., 2009, Development and assessment of bridge-scour potential based on measured data evaluation of live-bed pier and contraction scour envelope at bridge crossings of streams throughout South Carolina. curves in the Coastal Plain and Piedmont Provinces of South Carolina: U.S. Geological Survey Scientific Investi- gations Report 2009–5099, 108 p. Selected References Benedict, S.T., and Caldwell, A.W., 2012, Modification of selected South Carolina bridge-scour envelope curves: U.S. Geological Survey Scientific Investigations Ansari, S.A., and Qadar, A., 1994, Ultimate depth of scour Report 2012–5029, 37 p. around bridge piers, in The 1994 National Conference on Hydraulic Engineering, Buffalo, N.Y., 1994, Proceedings: Benedict, S.T., and Caldwell, A.W., 2014, A pier-scour Reston, Va., American Society of Civil Engineers, p. 51–55. database—2,427 field and laboratory measurements of pier scour: U.S. Geological Survey Data Series 845, 22 p., Arneson, L.A., and Shearman, J.O., 1998, User’s manual accessed July 29, 2016, at http://dx.doi.org/10.3133/ds845. for WSPRO—A computer model for water surface profile computations: Federal Highway Administration, Publication Benedict, S.T., and Caldwell, A.W., 2015, The upper bound No. FHWA–SA–98–080, 323 p. of pier scour defined by selected laboratory and field data, in SEDHYD 2015, Reno, Nev., 2015, Proceedings, accessed Arneson, L.A., Zevenbergen, L.W., Lagasse, P.F., and July 29, 2016, at http://www.sedhyd.org/2015/openconf/ Clopper, P.E., 2012, Evaluating scour at bridges (5th ed.): modules/request.php?module=oc_program&action= Federal Highway Administration Hydraulic Engineering summary.php&id=158. 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