Sediment supply and local scouring at bed sills in high gradient streams

This paper presents the results of an experimental study about the pattern of local scouring generated at the toe of bed sills, which are commonly built in steep, gravel bed mountain streams. The aim of this study is to evaluate the effect of upstream sediment supply on the scour depth and shape. The experiments simulated conditions of a steady upstream sediment supply which had the same grain size composition as the sediment deposit placed on the bed of the flume. The geometric characteristics of three scour holes developed under conditions of steady flow discharge and steady upstream sediment supply were measured during 48 different tests. This provided a data set containing measurements of 144 scour holes. Three different sediment feed rates were used, and tests were also performed with clear water conditions. Geometric interference between sills was not observed. A natural river gravel with a limited grain size was used. The collected data were analyzed using nondimensional groups developed in previous clear water studies. It is shown that the imposed sediment transport does not require the introduction of new dimensionless parameters into existing scour depth and length prediction formula. The effects of sediment feed are shown to be incorporated in the existing dimensionless parameters. The new data set was used to recalibrate existing scour depth formula. The influence of jet erosion on scour geometry is also discussed. Inspection of the data indicates a possible two-stage behavior, with the threshold between behaviors related to the ratio of the value of the morphological jump created at a scour hole and the flow specific energy at the bed sill.