Sizing Dumped Rock Riprap

An easy-to-use, uncomplicated, and rapid statistical procedure using the Weibull distribution as a probabilistic model of rock particle resistance is developed for sizing loosely dumped rock riprap. Parameters of the Weibull distribution describing critical particle shear stress were found for individual particle size classes for various beds composed of loosely dumped rounded and subrounded gravel and coarse sand from the results of an experimental study. Fitted parameters were related to the relative size of an individual particle and to a measure of the gradation of the bed mixture. The fraction of particles on the surface of a revetment displaced by a specified hydrodynamic shear stress can then be calculated based on the model of critical shear stress and a relation for the particle angle of initial yield. This calculation procedure is suggested for sizing dumped rock riprap and for evaluating the stability of existing dumped rock riprap facings. Borrowing from existing riprap design methods, we extend the calculation procedure to include angular particles. Qualitative damage assessments are formed based on 36 observations of stable and unstable riprap-lined stream banks at 24 different sites and the corresponding predicted fraction of surface particles displaced. The calculations are condensed in a single graph for a standard riprap gradation that gives a quick estimate of the reliability of a protected slope.