Scour and three-dimensional flow field measurement around short vertical-wall abutment protected by collar

Abstract

Installing collar on abutment is an applicable method to protect the abutment against scouring. In this study, effect of using collar on reduction of scour depth at short vertical-wall abutment (ratio of abutment length to flow depth of less than 1, La/y < 1) under clear-water conditions and uniform bed materials was investigated experimentally. Different sizes and various elevations of installing collar were studied. Results showed that using collar with 2.25La in width, situated under the bed elevation, led to 88.9% scour depth reduction for threshold flow condition. Three-dimensional velocity components around the abutment, for both with/without collar conditions, were measured by Acoustic Doppler Velocimeter (ADV). Turbulent intensity components, Reynolds stresses and bed shear stress on a rough rigid bed were studied at different sections. A primary vortex, associated with the downflow, existed at upstream face of the abutment. In the downstream, the flow field was turbulent due to the vortex shedding, and wake vortices occurred near the bed level next to the abutment. Installing collar led to diminish the vortex power below the collar. Also, turbulent intensity at upstream face of the abutment and dimensionless shear stress was reduced by using collar. The data presented in this paper could be useful for development and validation of a flow field model.