Scour Countermeasures for Wing-Wall Abutments

Abstract

An experimental study of scour countermeasures for wing-wall abutments under mobile-bed conditions is reported. The purpose of the study was to determine the required apron size to protect a wing-wall abutment from scour under mobile-bed conditions, for different flow depths, flow velocities, and apron placement level. Riprap and cable-tied block countermeasures were investigated. It was observed that scour at the abutment undermines the apron, causing the outer edge of the apron to settle. Scour at the abutment can be a combination of localized scour (i.e., contraction scour and local scour) and scour due to the troughs of the bed forms that migrate past the abutment, the latter being the focus of the study presented. As the flow depth and flow velocity are increased, the outer edge of the apron settles into the sand, to a depth proportional to the height of the bed forms developed in the flume. Localized scour may additionally influence the depth of settlement. When the apron width is increased, the depth to which the outer edge settles remains reasonably constant. When the apron is preburied below the average bed level, the settlement of the outer edge is less, but the depth of the outer edge of the apron below the average bed level is the same as for the corresponding apron placed on top of the bed. Riprap and cable-tied block aprons respond in different ways. Stones at the outer edge of riprap aprons tend to roll away from the abutment as they settle, pushing the erosion zone further away from the abutment. Conversely, because cable-tied block mats remain attached to the abutment, apron settlement allows the scour to occur closer to the abutment face. It is concluded that the scour at abutments under mobile-bed conditions is often governed by the size of the bed forms that pass the abutment. Therefore, the design of armored scour countermeasures for abutments under mobile-bed conditions should be based upon the largest expected bed forms, which can be predicted based on existing expressions, together with any effects of localized scour. This will enable the settlement of the outer edge of the apron to be determined, and the required apron width to be chosen, depending on the tolerable distance between the deepest scour and the abutment. An equation is given for prediction of the minimum width of apron remaining horizontal after erosion.