Abutment scour

Abstract

The aim of the study was to determine the effect of important parameters on the local scour depth at abutments. The principal parameters were identified from a review of data from several previous studies and from the similarities that exist between the local scour at abutments and piers. The principal parameters are the flow depth, abutment length, shear velocity, sediment size and gradation. The local scour depth varies with the flow depth and the abutment length in the same manner. Both the flow depth and the abutment length Influence the approach flow to the abutment and therefore the scour depth. The extent of the dead water region, which forms ahead of the abutment and against the channel wall, is important. The two independent parameters, flow depth and abutment length and the dependent parameter, scour depth, form a three dimensional relationship. The relationship has two limits, one at high flow depth and small abutment length and the other at large abutment length and small flow depth. In addition pier data show a similar trend. The ratio yo/L and its Inverse, L/yo, are adequate in representing the effects of flow depth and abutment length. The experimental data can be plotted in term of Yo/L versus ds/L or L/yo versus ds/Yo which can be used for design. In live bed conditions, u*/u*c > 1.0, experiments were conducted using the 'sediment feed' method i.e. dry sediment were fed into the upstream flow at a constant rate. In the range of shear velocity ratio tested the equilibrium scour depth, dsavg, shows an approximately similar variation with shear velocity ratio as in piers. In addition the ratio of the scour depth at threshold conditions to that at higher shear velocity ratios is larger at smaller yo/L ratios. This is due to the manner in which the dead water region contracts with shear velocity, flow depth and abutment length.