Roughness of Loose Rock Riprap on Steep Slopes

A study was conducted to determine the roughness of rock riprap laid on steep slopes. Tests were conducted in a 1.07 m wide flume and two 2.74 m wide field-scale structures using angular riprap with median diameters D50 ranging from 52 to 278 mm and bed slopes S0 ranging from 0.028 to 0.333 m/m. Empirical relationships were developed to predict the Manning roughness coefficient as a function of D50 and S0. The Darcy-Weisbach friction factor was predicted as a function of d/D84, the relative submergence. Newly developed equations are applicable for riprap placed on slopes from 2.8%–33%. Roughness coefficients calculated using the developed equations were compared with roughness coefficients calculated using equations developed from observations on natural mountain streams.