Abstract
This paper presents the results of an experimental study about the pattern of local scouring generated downstream bed sills commonly used in mountain rivers to limit bed degradation. The effect of sediment supply on the scour depth at the toe of bed sills displaced along a staircase-like sequence is evaluated for the first time. The experiments simulated conditions of steady upstream sediment supply which had the same grain size composition as the deposit placed on the bed of the flume. Results of forty-eight new tests are presented. The geometry of three scour holes developed under conditions of steady flow discharge and steady upstream sediment supply are examined for each test, giving a total of 144 scour holes. Three sediment feed rates were examined. Twelve tests were performed with clear water conditions. The collected data were examined using nondimensional groups developed by the authors in previous no-feed studies. It is shown that the imposed sediment transport, appearing as a new independent variable, does not require the introduction of new non-dimensional parameters in existing prediction formula calibrated with clear water conditions data. The effects of sediment feed are shown to be incorporated in the existing non-dimensional parameters. The new data set was used to re-calibrate existing scour depth formula. Closer inspection of the data indicates a possible two-stage behaviour, dependent on the range of the ratio between the morphological jump created at a scour hole and the flow specific energy.