ROUGHNESS OF LOOSE ROCK RIPRAP ON STEEP SLOPES

A study was conducted to determine the roughness of rock riprap laid o n 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 diam eters D-50 ranging from 52 to 278 mm and bed slopes S-o ranging from 0 .028 to 0.333 m/m. Empirical relationships were developed to predict t he Manning roughness coefficient as a function of D-50 and S-o. The Da rcy-Weisbach friction factor was predicted as a function of d/D-84, th e relative submergence. Newly developed equations are applicable for r iprap placed on slopes from 2.8%-33%. Roughness coefficients calculate d using the developed equations were compared with roughness coefficie nts calculated using equations developed from observations on natural mountain streams.