Abstract:
The local clear water scour at a wing-wall abutment in a uniform cohesionless bed was investigated experimentally. The main objectives of the study were to measure the complex three-dimensional flow field at the abutment and to determine the effect of flow depth on the velocity field and the choice of an appropriate normalising parameter for abutment scour depth. The experimental programme was confined to steady approach flows at near threshold conditions. An abutment length of 475 mm and a mean sediment size of 0.85 mm in a 1.52 m wide rectangular channel were used. The temporal scour development was monitored for flow depths of 50, 75, 100, 150 and 200 mm. Fixed-bed equilibrium scour models were constructed for flow depths of 75 and 150 mm and the hydrogen bubble technique was used for detailed qualitative and quantitative measurements of the three-dimensional flow at equilibrium conditions for these models. The experimental results have shown that the primary vortex and associated downflow are the dominant flow structures at the abutment. Near the abutment edge the strong downflow, which has a peak value at equilibrium of 75% of the free stream velocity, impinges on the bed of the scour hole and removes bed material. This flow which is deflected away from the abutment carries the entrained sediment further downstream where it is deposited. During the early stages of scour the bed material deposited downstream is continually washed away. Later, as scour progresses a ridge is formed. The ridge reduces the flow velocities ahead of it and leads to the formation of a secondary counter vortex adjacent to the primary vortex. The flow splits at the ridge and the primary and secondary vortices occur on either side of it. The secondary vortex is small and relatively weak. The study has also shown that the flow patterns are relatively unaffected by changes in approach flow depth and are similar to those at piers. In particular the primary vortex and downflow are confined predominantly within the scour hole beneath the original bed level and the maximum downflow is of the same order of magnitude. It is concluded, as for piers, that the abutment length is the appropriate normalising parameter for abutment scour depths. This assumes that the abutment length to flow depth ratio is less than about 30. It is proposed that the maximum scour depth dsm for a given abutment length L and uniform flow depth yQ can be represented by the following relationship: for L/y0 >2, where the coefficient k is 1.1 on average and is normally less than 1.36.