EFFECTS OF ROCK FRAGMENT COVERS ON EROSION AND TRANSPORT OF NONCOHESIVE SEDIMENT BY SHALLOW OVERLAND-FLOW

Rock fragments on soil surfaces can have a variety of contrasting effects on the hydraulics of overland flow and soil erosion. This makes modeling soil erosion difficult. Systematic studies are needed to assess the effects of rock fragments on the hydraulics of overland flow and to specifically relate these hydraulics to sediment yield. Therefore flume experiments were conducted in which overland flow was applied to a highly erodible sediment surface covered by various percentages of rock fragments. High local turbulence was caused by horseshoe vortex erosion around individual rock fragments. For a given discharge and slope, the dimension of the scour was affected by the space between the rock fragments. For low cover percentages the scour can develop undisturbed, and sediment yield increases with percent of rock fragment cover up to about 20%. Sediment yield then declines as percent cover increases above 20%. Beyond a cover threshold value of 20%, the larger opportunity for scour hole development is counteracted by limited inter-rock fragment space that does not allow scour holes to develop to their full extent. As sediment yield varies nonmonotonically with rock fragment cover percent, sediment yield cannot be predicted by spatially averaged flow hydraulics.