Wake Vortex Scour at Bridge Piers

Abstract

At most bridges, wake vortex scour is insignificant and confluence scour does not exist. In clear‐water rivers flowing on fine sand, these two forms of local scour can be very large. The deep scour holes downstream from the large circular piers (10 and 15 m in diameter) at the Tahrir and Imbaba Bridges over the Nile River in Cairo, Egypt were produced by the conflicting velocity fields at the intersection of the wake vortex streams from adjacent piers. The depths of scour due to these wake vortices are now 8 to 11 m where the normal depth of flow is approximately 8 m. Confluence scour where the main and side channel join upstream from the Imbaba Bridge was 9 m in 1981 and 1987 and was aligned with the bisector of the intersection angle of the two channels. These depths of scour can be considered nearly clear‐water scour as the bridges are in the backwater of the Delta Barrages, the velocity is less than 1 m/s, and sediment transport is very low.