Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction

Abstract

This report reviews the present state of knowledge regarding bridge-abutment scour and the veracity of the leading methods currently used for estimating design scour depth. It focuses on research information obtained since 1990, which is to be considered in updating the scour estimation methods that are recommended by AASHTO, and used generally by engineering practitioners. Though considerable further progress has been made since 1990, the findings indicate that several important aspects of abutment scour processes remain inadequately understood and therefore, are not included in current methods for scour depth estimation. The state-of-the-art for abutment scour estimation is considerably less advanced than for pier scour. Moreover, there is a need for design practice to consider how abutment design should best take scour into account, as scour typically results in the geotechnical failure of an abutment’s earthfill embankment, possibly before a maximum potential scour depth is attained hydraulically. Abutment scour herein is taken to be scour at the bridge-opening end of an abutment, and directly attributable to the flow field developed by flow passing around an abutment. This definition excludes other flow and channel-erosion processes such as lateral geomorphic shifting of the bridge approach channel but includes contraction and abutment scour as part of the same physical processes that should be treated together rather than separately in their estimation. The review shows that, since 1990, advances have been made in understanding abutment-scour processes, and in (1) estimating scour depth at abutments with erodible compacted earthfill embankments, and at those with solid-body (caisson-like) foundations; (2) identifying the occurrence of at least three distinct abutment scour conditions depending on abutment location and construction; and (3) utilizing the capacity of numerical modeling to reveal the flow field at abutments in ways that laboratory work heretofore has been unable to provide. The review identifies and evaluates leading scour formulas and suggests a framework for developing a unified abutment scour formula that depends on satisfying several targeted future research needs.