Clear-Water Abutment and Contraction Scour in the Coastal Plain and Piedmont Provinces of South Carolina, 1996-99 By Stephen T. Benedict U.S. Geological Survey Water-Resources Investigations Report 03-4064 Prepared in cooperation with the SOUTH CAROLINA DEPARTMENT OF TRANSPORTATION Columbia, South Carolina 2003 U.S. DEPARTMENT OF THE INTERIOR GALE A. NORTON, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, Director Use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey For additional information Copies of this report can be write to: purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Branch of Information Services Suite 129 Box 25286 720 Gracern Road Denver, CO 80225-0286 Columbia, SC 29210-7651 888-ASK-USGS Additional information about water resources in South Carolina is available on the internet at http://sc.water.usgs.gov CONTENTS Abstract...................................................................................................................................................................................... 1 Introduction................................................................................................................................................................................ 1 Purpose and Scope ........................................................................................................................................................... 2 Acknowledgments............................................................................................................................................................ 2 Previous Investigations..................................................................................................................................................... 2 Description of Study Area................................................................................................................................................ 4 Data Assumptions ...................................................................................................................................................................... 4 Justification for the Assumption of Large Flood Flows................................................................................................... 7 Justification for the Assumption of Equilibrium-Scour Conditions .............................................................................. 11 Basin Equilibrium Scour................................................................................................................................................ 11 Site selection ............................................................................................................................................................................ 12 SCDOT List of Bridges with Observed Scour............................................................................................................... 12 Selection of Previously Studied Level 2 Bridge-Scour Sites......................................................................................... 13 Selection of Piedmont Sites Influenced by the August 1995 Flood............................................................................... 14 Selection of Piedmont Bridges with Wide, Flat Floodplains......................................................................................... 14 Selection of Coastal Plain Sites Influenced by the September 1999 Flood ................................................................... 14 Techniques for the Collection and Interpretation of Field Data .............................................................................................. 15 Data Collection in the Coastal Plain .............................................................................................................................. 15 Data Collection in the Piedmont .................................................................................................................................... 18 Flow Model Data............................................................................................................................................................ 19 Sediment Samples.......................................................................................................................................................... 19 Contraction Scour in the Abutment Area....................................................................................................................... 20 Pier Scour in the Abutment Area ................................................................................................................................... 21 Estimating Hydraulic Data....................................................................................................................................................... 22 Development of WSPRO Models .................................................................................................................................. 24 Estimates of Hypothetical Flows ................................................................................................................................... 25 Historical Flows ............................................................................................................................................................. 27 Flood of October 1992 ......................................................................................................................................... 27 Flood of August 1995........................................................................................................................................... 27 Flood of September 1999..................................................................................................................................... 29 Development of Theoretical Bridge-Scour Database .............................................................................................................. 31 Theoretical Pier Scour.................................................................................................................................................... 31 Theoretical Live-Bed Contraction Scour ....................................................................................................................... 36 Theoretical Clear-Water Contraction Scour................................................................................................................... 38 Theoretical Abutment Scour .......................................................................................................................................... 40 Variables Influencing Abutment Scour.................................................................................................................................... 43 Time and Flow Duration ................................................................................................................................................ 46 Flow Velocity ................................................................................................................................................................. 47 Flow Depth..................................................................................................................................................................... 52 Sediment Size................................................................................................................................................................. 54 Sediment Gradation........................................................................................................................................................ 57 Embankment Length...................................................................................................................................................... 57 Abutment Shape............................................................................................................................................................. 61 Embankment Skew......................................................................................................................................................... 62 Channel Geometry ......................................................................................................................................................... 65 Geometric-Contraction Ratio......................................................................................................................................... 65 Conclusions.................................................................................................................................................................... 66 Contents III Abutment Scour in the Piedmont .............................................................................................................................................69 Comparison of Observed and Theoretical Abutment-Scour Depths ..............................................................................70 Piedmont Floodplain Geometry as an Indicator of Potential Scour ...............................................................................71 Embankment Length as an Indicator of Potential Scour ................................................................................................71 Geometric-Contraction Ratio as an Indicator of Potential Scour...................................................................................75