Estimation of Washload Produced on Certain Small Watersheds

Present knowledge on the prediction of suspended load production on small watersheds reveals that with the exception of the universal soil-loss equation, and sediment-rating techniques, a rational method does not exist. A method is presented that is analogous to Sherman's unit-hydrograph method. The ordinates of a sediment discharge graph are divided by the excess runoff that mobilized it, producing a unit sediment discharge graph. For many storm events, unit sediment discharge graphs are generated that vary considerably in peak value and shape. The ordinates of the latter graphs are then plotted logarithmically against their respective excess runoff, yielding data points that can be fitted by straight lines. Estimation of sediment discharge or of a sediment discharge graph for a given excess runoff can therefore be accomplished. Bixler Run Watershed, Pa., 15 sq miles (39 sq km) in area, was selected as a data source. Granulometric tests disclosed that the suspended sediment in Bixler Run is predominantly washload.