Laboratory study of maximum scour depth downstream of sills

Bed sills have been widely used as a sequence of grade-control structures in mountain streams where incision has to be prevented. Major local scouring downstream of these structures is, however, often observed. In order to investigate the characteristics of local scouring, a number of experiments have been carried out with 105 cases in different situations. The initial bed slopes were 0·176 and 0·268; sediments were coarse sand, fine gravel and medium gravel. The sill spacings were 0·4, 0·6, 0·8 and 0·9 m, and the sill heights above initial stream bed level were 0, 0·02, 0·04 and 0·06 m, respectively. Three stages of scour evolution could be classified. The size of scour hole depends on the bed slope, sediment diameter, sill height and sill spacing. The slope of the stream bed was found to be the most important parameter affecting the maximum scour depth. Using dimensional analysis, the relative maximum scour depth at the equilibrium stage was formulated as functions of the relative morphological jump to ...