Effects of Bed Roughness Shape on the Length of Hydraulic Jump

In this study, effects of shape of roughness elements on the length of hydraulic jump have
been investigated experimentally. For the purpose of this study, prismatic roughness elements
with different shapes (rectangular, triangular, circular, lozenge and hexangular) have been
tested in a rectangular flume 7.5m long and 30cm wide. Experiments were performed in the
hydraulic laboratory of Shahid Chamran University, Ahwaz, Iran. The roughed elements are
glued on the bed of flume downstream of ogee spillways in such a way that the incoming
water jet is just above the element surface. It should be noted that in previous studies the
rough elements have been tested such that the incoming jet was at the same level of the
element bottom so the elements act as baffle blocks. Each rough element was tested under
different flow conditions. The incoming Froude number was in the range of 4.5 to 12. During
each test the water surface profile, the roller length and the jump length were measured. In
few tests the longitudes and vertical flow velocity were measured. The results indicate that the
presence of rough element can reduce the hydraulic jump length and the amount of the
reduction depends on the Froude number and the roughness shape. The lozenge element shape
was found to produce lesser jump length (up to 40% of the smooth bed jump). Relations have
been presented in this study to predict jump length, roller length and the amount of energy
which is dissipated. Comparison of this study with previous results reveals that using the
rough element can reduce the hydraulic jump as much as the block elements do, and the rough
element can not cause separation of flow which might be subject to cavitations.