Enhanced Stone Sizing for Overtopping Flow

Rock riprap is one of the most widely used erosion control techniques for protecting embankments, levees, spillways, and in-stream structures subjected to overtopping flow conditions. The literature indicates that over 21 riprap design expressions exist to appropriately size the stone. However, each of these relationships was developed from limited databases with narrow bands of boundary conditions. A power regression analysis was performed using 102 overtopping observations from 10 separate investigations; the analysis related median stone size to slope, unit discharge, coefficient of uniformity, rock layer thickness, and stone specific gravity. A unique, riprap-sizing expression was developed with a coefficient of determination (R2) of 0.97 and a variance of 11%. The resulting design relationship expands the boundary conditions of the independent variables beyond earlier efforts, thereby reducing the need for extrapolating previous relationships.