Evaluation of the Performance of Scour Prediction Equations with Regards to Flow and Sediment Characteristics

Scour around bridges and other structures is a major problem in the United States. The available scour prediction equations common in practice have the tendency to over predict the ultimate scour hole depth for design river flows by up to 6 times observed depths. Although these estimates are safe for design purposes, they result in expensive foundation designs if scour hole depth is a limiting design criteria for pile embedment length. In this study four specific scour prediction equations were examined: the HEC-18, the equation suggested by the Florida Department of Transportation (FDOT), the equation developed by Texas A&M University (TAMU), and the Froehlich prediction equations. The performance with regard to flow and sediment conditions that yield the most accurate scour depth predictions was tested for each of these equations. In each case, the prediction accuracy increased significantly for specific site conditions. For example, the HEC-18 equation yielded overall coefficients of determination of R²= 0.61, but improved to R²=0.81 when 0.5 mm < D₅₀ < 5 mm. The presented results may provide future design guidelines by providing suitable ranges of applicability for different scour models under different environments.