SCOUR AT SELECTED BRIDGE SITES IN MISSISSIPPI by K. Van Wilson, Jr. Prepared in cooperation with the MISSISSIPPI DEPARTMENT OF TRANSPORTATION Jackson, Mississippi 1995 Water-Resources Investigations Report 94-4241 TABLE OF CONTENTS Abstract Introduction     Purpose and scope     Method of study     Description of bridge-scour sites     Acknowledgments Pier-scour data Pier geometry Pier-scour data analysis     Effect of debris piles     Effect of heterogeneous bed material     Determination of live-bed or clear-water scour Comparison of computed and measured pier-scour depths Measured total-scour depths Summary References ILLUSTRATIONS 1. Map showing location of bridge-scour sites in Mississippi 2. Graph showing relation between measured pier-scour depth and drainage area for selected bridge sites in Mississippi 3. Sketch showing four typical locations of pier footing in relation to approach flow 4. Relation between measured pier-scour depth divided by normal pier (Ys/a') and measured approach-flow depth divided by normal pier width (Yl/a') for selected bridge sites in Mississippi 5. Relation between measured pier-scour depth (Ys) and normal pier width (a') for selected bridge sites in Mississippi 6. Relation between measured pier-scour hole top width and measured pier-scour depth for selected bridge sites in Mississippi 7. Relation between net pier-scour depth through clay divided by normal pier width (Yscl/a') and approximate shear strength of clay for selected bridge sites in Mississippi 8. Relation between measured pier-scour depth divided by normal pier width (Ys/a') and approach velocity divided by critical velocity (V1/Vc) for selected bridge sites in Mississippi 9. Relation between pier-scour depth predicted by the HEC-18 equation and measured pier-scour depth for selected bridge sites in Mississippi 10. Relation between pier-scour depth predicted by the Mississippi envelope-curve equation and measured pier-scour depth for selected bridge sites in Mississippi 11. Sketch showing scour depth at minimum-bed elevation (a) with no lateral movement and (b) with significant lateral movement of the channel 12. Relation of measured stage and minimum-bed elevation to time for Chunky River at U.S. Highway 80 near Chunky (site 5), Misssissippi 13. Relation of measured stage and minimum-bed elevation to time for Leaf River at U.S. Highway 11 at Hattiesburg (site 1), Mississippi 14. Relation of measured stage and minimum-bed elevation to time for Homochitto River at State Highway 33 at Rosetta (site 21), Mississippi 15. Relation of measured stage and minimum-bed elevation to time for Buffalo River at U.S. Highway 61 near Woodville (site 22), Mississippi TABLES 1. Selected bridge sites in Mississippi where scour data were collected 2. Summary of stage and discharge data at selected bridge sites in Mississippi 3. Pier-scour data collected at selected bridge sites in Mississippi 4. Selected pier-scour measurements possibly affected by consolidated cohesive material in Mississippi 5. Pier-shape correction factor (K1) for the HEC-18 equation 6. Approach flow-angle correction factor (K2) for the HEC-18 equation 7. Bed-condition correction factor (K3) for the HEC-18 equation 8. Summary of total-scour data collected at selected bridge sites in Mississippi ABSTRACT Scour data were collected during 1938-94 at 22 selected bridge sites in Mississippi. The drainage area of the bridge-scour sites ranged from 60.8 to 5,720 square miles, and the slope in the vicinity of each site ranged from 0.00011 to 0.00163 foot per foot. Measured pier-scour depths ranged from 0.6 to 20.4 feet. Measured total-scour depths at minimum-bed elevation ranged from 5.2 to 29.8 feet. Recurrence internals of measured discharges ranged from less than 2 to about 500 years. At several sites, measured scour depths were possibly affected by heterogeneous bed material, primarily where a clay stratum was overlain by sand or gravel. Limited data indicated the pier-scour depths decreased as shear strength of the clay increased. Debris piles significantly obstructed more of the approach flow than the pier for some measurements. The normal width of the largest debis pile was as much as 1.5 times the actual pier width. All of the Mississippi pier-scour depths were within 2.3 time the normal pier width, which agreed with previous research. Only 12 (6 percent) of the 190 measured pier-scour depths were greater than 1.1 times the normal pier width. Measured pier-scour depths were as much as 2.24 times a normal pier width of 3.3 feet. However, for pier widths greather than about 4 feet, measured pier-scour depths were significantly less than 2.3 times the normal pier width. An envelope-curve equation for the Mississippi pier-scour data was developed by relating pier-scour depth divided by normal pier width to approach-flow depth divided by normal pier width. Measured pier-scour depths were compared to computed pier-scour depths using this envelope-curve equation and using the scour-prediction equation currently (1994) recommended in the Federal Highway Administration Hydraulic Engineering Circular No. 18 (HEC-18). The HEC-18 equation predicted pier-scour depths ranging from 3.9 to 25.7 feet with residuals (measured pier scour minus computed pier scour) ranging from -21.7 to 0.2 feet. The envelope-curve equation developed during this study, excluding one distorted measurement, predicted pier-scour depths ranging from 2.2 to 19.7 feet with residuals ranging from -16.8 to 0.5 feet. The envelope-curve equation predictions could be used for reasonable verifications of the HEC-18 pier scour predictions, which currently are required for the design and maintenance of bridges in Mississippi. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.