HYDRAULICS AND EROSION IN ERODING RILLS

Rills often act as sediment sources and the dominant sediment and wate r transport mechanism for hillslopes. Six experiments were conducted o n two soils and a uniform sand using three experimental methodologies, The results of this study challenge the assumption often used in hydr ologic and erosion models that relationships derived for sheet flow or larger channel flow are applicable to actively eroding rills. Velocit y did not vary with slope, and Reynolds number was not a consistent pr edictor of hydraulic friction. This result was due to interactions of slope gradient, flow rate, erosion, and the formation of rill roughnes s, bed structures, and head cuts. A relationship for rill flow velocit ies was proposed, Stream power was found to be a consistent and approp riate predictor for unit sediment load for the entire data set, while other hydraulic variables were not. The data for stream power and sedi ment load fit the form of a logistic curve (r(2)=0.93), which is promi sing relative to recently proposed erosion models which are based on p robabilistic particle threshold theory.