Clear-water scour development at bridge abutments

Abstract

Development of local scour depths at vertical-wall bridge abutments of varying lengths was investigated in several series of experiments of ranges of uniform sediments and clear-water flow intensities. For each flow-abutment-sediment combination tested, the change with time in maximum local scour depth from plane-bed to equilibrium conditions was recorded and analysed. The results of similar earlier abutment-scour experimental studies were also incorporated in analyses of scour development. Variations in scour rates and depths with flow and sediment parameters are found to be different for short (flow depth/abutment length = y/L > 1) and long (y/L ≤ 1) abutments. In order to provide a consistent framework for assessment of the development of scour depth with time, a revised definition of the time to achieve equilibrium conditions is developed. Based on this definition, the dimensionless time to equilibrium for scour development from plane-bed conditions can be expressed as a function of relative flow intensity and relative abutment length. This expression can be adopted into existing methodologies to predict scour magnitudes occurring at bridge abutments. Utilising extensive data sets, expressions enabling the determination of scour depths at abutments, and also piers, are reassessed for the present definition of time to achieve equilibrium conditions. Application of the developed expressions is highlighted in an example.