Scour Depth at Bridges: Method Including Soil Properties. II: Time Rate of Scour Prediction

Scour of the soil by flowing water around bridge supports is the number one reason for bridge collapse. Predicting the depth of the scour hole is an integral part of bridge foundation design, as it impacts the depth of the piles. Indeed, the scour depth must be ignored in the vertical and horizontal resistance of the piles. This paper presents a method to predict the progression of the scour depth around bridge supports as a function of time when subjected to a velocity hydrograph. It also addresses the issue of layered soils. The equations make use of the commonly used water velocity and dimensions of the obstacle, but add a new and important component: the soil erosion function to characterize the degree of soil resistance to erosion. The equations apply to pier scour, contraction scour, and abutment scour. The predicted output is the scour depth as a function of time for a given velocity hydrograph and soil layer stratigraphy. A comparison between measured and predicted scour depth is presented, but more comparisons are necessary, particularly at full scale. (C) 2014 American Society of Civil Engineers.