Effects of Streamwise Abutment Length on Scour at Riprap Apron-Protected Setback Abutments in Compound Channels

Abstract

Scour at bridge abutments usually consists of local and contraction scour that correlate because of the flow field that develops both scours. Flume experiments were conducted using simulated compound channels to investigate the effect of a setback vertical-wall abutment’s streamwise contraction length on scour depth and pattern. The adequacy of the current design method for riprap aprons was also examined. The prescour flow measurements showed that an abutment with small streamwise length may divert the highly turbulent zone toward the main channel and downstream that beyond the protection of the apron. By contrast, the turbulent zone of a long abutment tends to occur near the abutment face and within the apron area. The erodible bed experiments showed that the abutment length may significantly affect scour depth, morphology, and temporal development for apron-protected abutments but only has a minor influence on unprotected abutments. For apron-protected abutments, an increased abutment length reduces the magnitude of the deepest scour and moves the scour hole closer to the contracted section and the abutment face, which is consistent with the prescour flow field. Depending on the extent of setback, scour may also extend into the main channel. It was found that the existing abutment scour predictors should consider streamwise abutment length in order to reduce underestimation for short contractions and overestimation for long contractions. The design of riprap aprons should also be improved accordingly and integrated with the predictors.