The authors of this technical paper developed a quick and easy statistical procedure using the Weibull distribution as a probabilistic model of rock particle resistance for sizing loosely dumped rock riprap. Parameters of the Weibull distribution that describe critical particle shear stress were determined for individual particle size classes for various beds consisting 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 percentage of particles on the surface of a revetment displaced by a specified hydrodynamic shear stress was then computed based on the model of critical shear stress and a relation for the particle angle of initial yield. This computation is recommended for sizing dumped rock riprap and for assessing the stability of existing dumped rock riprap facings. Borrowing from existing riprap design methods, the calculation can be extended to include angular particles. Qualitative damage assessments are established based on 36 observations of stable and unstable riprap-lined stream banks at 24 sites and the corresponding predicted percentage of surface particles displaced. The computations are reduced to a single graph for a standard riprap gradation that provides a quick estimate of the reliability of a protected slope.


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  • Accession Number: 00728762
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Nov 28 1996 12:00AM