EFFECTS OF SURFACE-ROUGHNESS AND RAINFALL IMPACT ON OVERLAND-FLOW

Bed roughness and rainfall impact affect hydraulic friction in overlan d flow. The relationship between friction, velocity, discharge, and ch annel slope was examined for the case of shallow, spatially varied flo w on rough surfaces. Rainfall was simulated on artificially roughened surfaces in a laboratory. Measured flow velocities were compared to th eoretical velocities computed from a dynamic equation for spatially va ried flow and from the Darcy-Weisbach uniform flow equation. The measu rements showed that the constant that relates the Darcy-Weisbach frict ion coefficient to Reynolds number can become larger than the theoreti cal value for smooth laminar flow. An analysis of the dynamic equation showed that frictional forces were much larger than forces resulting from changes in hydrostatic pressure and momentum. Water-surface profi les based on the Darcy-Weisbach friction equation nearly matched those from the dynamic equation for spatially varied flow. A regression was developed from the Darcy-Weisbach equation. The regression accurately predicted flow velocity when absolute roughness, rainfall rate, chann el discharge, slope, slope length, and viscosity were known.