Morphological Effects of Bed Sills in Degrading Rivers

Experimental results are presented about long term local scouring downstream of bed sills in monogranular gravel beds. Two sets of tests were performed in a tilting flume at HR Wallingford, UK, in 1996 and 1997, the main difference between the sets of tests being the size of sediments used. A dimensional analysis of the problem of local scour at bed sills was carried out and used to analyse the experimental results. The introduction of a new variable, the ‘morphological jump’, permits the derivation of two empirical formulas to predict the maximum depth and length of the scour hole at equilibrium. In addition, the Froude number seems to have no influence on the dimensions of the scour hole, but does have an effect on the development of a ‘sill step’ upstream of each sill. Non-dimensional scour hole profiles were found to be affine. As a consequence, it is possible to predict the location of the deepest scour relative to the bed sill.