Local Scour at Complex Piers

A methodology to predict local scour depth at a complex pier is presented that recognises the relative scouring potentials of the components of complex piers, and the transition of scouring processes occurring for varying pile-cap elevation. Scour depths are predicted over the entire range of possible pile-cap elevations using a combination of existing expressions for scouring respectively at uniform piers, caisson-founded piers, pile groups with debris rafts, and pile groups alone. The validity of the method is confirmed using measurements of local scour at complex piers, and a case study is used to highlight application of the methodology. For design purposes, the method highlights respective pile-cap elevations that maximise (i.e. to be avoided over the pier life) and minimise local scour at complex piers. The method reinforces that where the pile-cap elevation relative to the bed can vary with time at a bridge site, potential local-scour depths need to be assessed over the range of possible pile-cap elevations for the pier.