Method for Estimating Clear-Water Local Scour Rate at Complex Piers

When estimating design local scour depths at structures where the design flows have short durations, equilibrium scour depths can be too conservative. Attempts to develop methods to accurately estimate scour rates and times to reach near equilibrium scour depths have only been moderately successful. The objective of the work reported here was to develop an improved, but still conservative, mathematical model for estimating local scour depth as a function of time for a known flow event. This model also estimates scour rates and the time required for the scour to reach near equilibrium (80%-90%) depths. When used in conjunction with the recently published equilibrium local scour depth model, Sheppard et al., it can be applied to both simple and complex bridge pier structures. When using the scour rate equations presented here, changes in sediment properties with sediment depth can be included in the analysis. Due to the lack of local scour rate data at large structures subjected to live-bed flow conditions, the proposed methodology is currently limited to clear-water scour conditions.