Sill-controlled hydraulic jump in a gradually expanding channel

The gradually diverging stilling basin is attractive to the designer due to its compatibility with a large range of upstream and downstream sections, and to having a small sequent depth ratio and jump length. However, the hydraulic jump in such stilling basins often encounters instability and asymmetrical flow problems. In the present study, the effect of broad and sharp-crested sills in improving hydraulic jump characteristics, including sequent depth ratio, jump length and the free surface profile of a jump in a gradually diverging stilling basin was investigated. For this purpose, sills with various heights and different locations relative to the jump toe were installed in stilling basins with diverging angles of 3, 5 and 9°. In total, 284 tests were carried out in the range of initial Froude numbers of 3·1 to 10·4. The results indicate that the end sill has an insignificant effect on the sequent depth ratio, but it may considerably decrease the length of the hydraulic jump. It was also found that the sill could considerably improve the general conditions and features of an expanding hydraulic jump and could stabilise the position of this type of jump.