Local Scour at Downstream Sloped Submerged Weirs: Fid-Bau Portal

Local Scour at Downstream Sloped Submerged Weirs

Wang, Lu / Melville, Bruce W. / Guan, Dawei / Whittaker, Colin N.

This paper presents an experimental study of local scour at submerged weirs with different downstream slopes. Thirty-three tests of coarse sand ( $d_{50} = 0.85 mm$ ) and 24 tests of fine sand ( $d_{50} = 0.26 mm$ ) 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 $d_{w}$ , 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 $θ_{a}$ (time-averaged value of $θ$ ). This results in larger downstream scour depths. When $β$ is considerably smaller than $θ_{a}$ , the effect of jet deflection starts to be predominant in the coarse sand tests, generating lower downstream scour depths when $U_{0} / U_{c} < 2.6$ . Those two effects balance each other when $2.6 < U_{0} / U_{c} < 3.1$ 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.