Time evolution of scouring downstream of bed sills

Results of laboratory experiments conducted on time evolution of the scouring phenomenon at bed sills are presented. Starting from previous findings on long-term depth and length of the local scour hole downstream of bed sills, 12 long-duration tests were designed and performed in the Sloping Sediment Duct at HR Wallingford Ltd, using uniform sand. No sediment feeding system was adopted. Video-recordings of the scour holes through the glass-sided duct allowed successive image analysis to obtain water surface and bed profiles over time. Results confirmed the validity of the assessment method of the maximum scour depth, ys, at equilibrium; uncertainties arose about the length of the scour hole, ls. A cyclic phenomenon perturbing the scour hole was also observed and described. The investigation of the time evolution of ys showed that the scour hole develops rapidly and conditions close to equilibrium are achieved in a short time. The introduction of a morphological time, ts, permitted the non-dimensional description of the increasing of ys over time through a unique curve. The short- and the long-term local scour regions are clearly distinguished. An application of the results is shown in a numerical example.