Local Scour at Downstream Sloped Submerged Weirs

Abstract

This paper presents an experimental study of local scour at submerged weirs with different downstream slopes. Thirty-three tests of coarse sand (d5=.85 mm) and 24 tests of fine sand (d5=.26 mm) were carried out in a sand-recirculating flume. The tests for each sand type used four weirs with different downstream weir slopes (π/6, π/4, π/3, and π/2 to horizontal for the fine sand; π/12, π/6, π/4, and π/2 to horizontal for the coarse sand). The results show that the upstream scour depth is independent of the downstream weir slope β. For both sand types, the downstream weir slope has no effect on the downstream scour depth when it is greater than the upstream slope of the downstream scour hole θ. For β<θ, the downstream weir surface is partly exposed to flow, resulting in a downstream scour depth at the weir surface dw, which can be increased by the increase in either downstream scour depth or θ. In this situation, the upstream water level rises because of the reduced flow cross-section area, and the overflow jet is deflected to be approximately aligned with the downstream weir surface. For the coarse sand tests, when β=π/6, the effect of the rising upstream water level is predominant because β is close to θa (time-averaged value of θ). This results in larger downstream scour depths. When β is considerably smaller than θa, the effect of jet deflection starts to be predominant in the coarse sand tests, generating lower downstream scour depths when U/Uc<2.6. Those two effects balance each other when 2.6<U/Uc<3.1 for β=π/12, resulting in the same downstream scour depths as those for a rectangular weir. For the fine sand tests, the jet deflection effect is predominant because the downstream weir slope has limited effect on the upstream water level. A conservative design method accounting for both effects is proposed for the local scour at downstream sloped submerged weirs.