Clearwater Local Scour at Complex Piers

Abstract

A new methodology to predict local scour depth at a complex pier is presented herein that combines existing expressions for scouring respectively at uniform piers, caisson-founded piers, pile groups with debris rafts, and pile groups alone. The method recognises the relative scouring potentials of the components of complex piers and the transition of scouring processes occurring for varying pile-cap elevation. The validity of the method is confirmed herein using the present and also historical measurements of local scour at complex piers. The proposed methodology has the advantages of being conceptually consistent with observed scour behaviours, relatively simple to apply, applicable to wide ranges of flow and sediment conditions (through incorporation into a more general analysis framework), and applicable over the entire range of possible pile-cap elevations. For design purposes, the present method highlights respective pile-cap elevations that maximize (i.e., to be avoided over the pier life) and minimize local scour at complex piers. The present 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.