A multi-parameter design formula for riprap size selection at wing-wall abutments

Abstract

A numerically-based approach is used to obtain a size selection formula for the riprap stone part of the apron used to protect wing-wall abutments against erosion. Several series of simulations with a given mean riprap diameter are conducted to estimate the maximum bed shear stress over the riprap apron and then the maximum (critical) Froude number at which riprap stones will resist shear failure by the flow for cases where the abutment is placed in straight and curved channels with or without a floodplain. Simulation results show that some of the existing design formulas for wing-wall abutments placed in straight river channels are not conservative enough for relatively large floodplain widths. Using data from the numerical simulations, a modified, two parameter design formula for riprap size selection is proposed that explicitly incorporates the effects of bank curvature and floodplain width.