Toe protection for spill-through and vertical-wall abutments

This study addresses the design of riprap aprons as a scour countermeasure near abutments under clear-water conditions. It deals with the lateral extent of riprap aprons and the geometry of the scour hole prevailing at the apron edge. The study applies to riprap aprons acting as granular filters. The scour depth appears to be independent for a sufficiently long relative abutment length. Scour holes develop farther away from spill-through abutments than from vertical-wall abutments; the distance between the point of maximum scour depth and the abutment increases with the relative abutment length. The effect of contraction on this distance was not identified. The angle defining the position of the deepest scour point is close to 308. Neither the abutment shape nor the flow contraction seems to influence the minimum stable apron width.