Local scour at submerged weirs

Abstract

Weirs are river training structures conventionally used for upstream water level and grade control. The flow over weirs can be fully submerged during high flow rate, inducing local scour both upstream and downstream of the submerged weir. The scour downstream of unsubmerged weirs has been broadly studied. The research on local scour at submerged weirs is limited to a single rectangular shaped weir. The thesis experimentally studied the local scour at sloped submerged weirs and the local scour interaction between a submerged weir and an upstream bridge pier. A scalable design method for predicting the equilibrium scour depth at submerged weirs based on the new presented data and the existing data in literature is proposed. Also, the clear-water scour development downstream of a submerged weir is analysed, proposing predictors for the temporal scour depth at submerged weirs. For clear-water scour, no upstream scour depth was observed at the upstream sloped weirs, and downstream scour depth is independent of the upstream weir slope. For live-bed scour, a flatter upstream weir slope reduces upstream scour depth for both sands. The existence of upstream weir slope slightly reduces the downstream scour depth when the sediment is transported as bedload. When the sediment transport mode is suspension load, the downstream scour depth is independent of the upstream weir slope. The upstream scour depth is independent of the downstream weir slope β. The downstream weir slope has no effect on the downstream scour depth for β > θ, where θ is the upstream slope of the downstream scour hole. In this case, the downstream weir surface is fully covered by sand. For β < θ, the downstream weir surface is partly exposed to flow, resulting in a downstream scour depth at the weir surface dw. 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. The water level rising and the jet deflection can increase and reduce the downstream scour depth, respectively. Design suggestions are given for sloped submerged weirs. II Through the dimensional analysis taking all variables into account, the dimensionless scour depth at a submerged weir in a uniform sediment bed is expressed as a function of dimensionless weir height, flow intensity, sediment coarseness parameter and weir slope. The existing data in literature and the new presented data are synthesized to quantitatively evaluate the dependence of scour depth on each dimensionless parameter, for clear-water and live-bed scour conditions. Preliminary design methods for local scour estimation at submerged weirs are proposed based on the evaluated relationships. The impacts of flow intensity, weir height, sand coarseness parameter and weir slope on the development of downstream scour depth are analysed. An equilibrium time scale ∗ is defined, ∗ increases linearly with flow intensity, it also increases with increasing weir height and decreasing sediment coarseness parameter. Based on the evaluated relationships, predictors for the temporal downstream scour depth at submerged weirs are proposed. The interference of local scour between a circular pier and a downstream submerged weir with different separation distances L is experimentally studied. For clear-water scour, the scour depth at the pier can be reduced significantly by the downstream submerged weir when L = 0. This reduction decreases with increasing L. For live-bed scour, besides the upstream bed aggradation, the downstream submerged weir can affect the local scour at a pier through two mechanisms: (i) superposition of local scour areas; and (ii) altering flow patterns. The two mechanisms were analysed based on experimental data and design suggestions for building a submerged weir downstream of bridge piers are proposed.