Clearwater Local Scour at Complex Piers

TLDR

The study introduces a new method that predicts local scour depth at complex piers by integrating existing scour expressions for uniform piers, caisson‑found piers, pile groups with debris rafts, and pile groups alone. The method accounts for the relative scouring potentials of pier components and the transition of scouring processes as pile‑cap elevation varies. Validation against current and historical measurements shows the approach is conceptually sound, simple to use, broadly applicable across flow and sediment conditions, and identifies pile‑cap elevations that either maximize or minimize scour, highlighting the need to evaluate scour over the full range of possible elevations.

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.