River training structures are conventionally used to improve river stability and ecological conditions and to protect the upstream instream infrastructures from scour and erosion. A submerged weir is a typical river training structure usually built downstream of bridges for grade control. Flow over the weir can cause local scour, destabilizing both the structure and the trained river reach. It is therefore important for safe design to estimate the scour accurately and understand the effects of an upstream bridge pier on local scour at submerged weirs. This study describes experiments on the scour process at a submerged weir with an upstream bridge pier, varying the distance L between the two. In this study, 56 experiments were conducted using uniform coarse sand with a median grain size d₅₀ = 0.85 mm in a sediment‐recirculating flume. For clear‐water scour, the experiments showed that the bedform induced by the pier scour process creates a scour‐and‐fill process upstream of the submerged weir; and the upstream pier can also affect the scour downstream of the submerged weir, through two mechanisms: (i) upstream sediment replenishment and (ii) flow section constriction. The mechanisms (i) and (ii) can reduce and increase the downstream clear‐water scour depth at the submerged weir, respectively. For live‐bed scour, a pier upstream can reduce and increase the scour depth upstream and downstream of the submerged weir, respectively. Based on the experimental data, the dependence of the scour depth at the submerged weir on L is evaluated.

This paper presents an experimental study of local scour at submerged weirs with different downstream slopes. Thirty-three tests of coarse sand ($d_{50}=0.85\mathrm{mm}$ ) and 24 tests of fine sand ($d_{50}=0.26\mathrm{mm}$ ) were carried out in a sand-recirculating flume. The tests for each sand type used four weirs with different downstream weir slopes ($\pi /6$ , $\pi /4$ , $\pi /3$ , and $\pi /2$ to horizontal for the fine sand; $\pi /12$ , $\pi /6$ , $\pi /4$ , and $\pi /2$ to horizontal for the coarse sand). The results show that the upstream scour depth is independent of the downstream weir slope $\beta$ . 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 $\theta$ . For $\beta <\theta$ , 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 $\theta$ . 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 $\beta =\pi /6$ , the effect of the rising upstream water level is predominant because $\beta$ is close to ${\theta }_a$ (time-averaged value of $\theta$ ). This results in larger downstream scour depths. When $\beta$ is considerably smaller than ${\theta }_a$ , 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 $\beta =\pi /12$ , 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.

The variation of upstream slope of a submerged weir could significantly alter the local flow pattern and sediment transport. This paper presents an experimental investigation into local scour at submerged weirs with different upstream weir slope angles. In this study, 62 tests are carried out in a tilting recirculating flume, including 38 tests using coarse sand and 24 tests using fine sand, to investigate the effects of upstream weir slope on the local scour at a submerged weir. The tests of each sand use four weirs with different upstream weir slopes. For clear-water scour, no upstream scour depth is observed at the 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 results for fine sand show that when the slope is gentler than a critical angle ${\alpha }_0$ , the effects of upstream weir slope on upstream scour depth are minimized, such that the upstream scour depth no longer decreases with decreasing weir slope angle. 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 weir slope. New predictors for the average scour depth including the effects of upstream weir slope are developed. A relationship between the average and maximum scour depth is also proposed.

Equilibrium clear-water scour depths predicted by existing equations based on the peak flow rate of a flood hydrograph can be overly conservative for submerged weirs in mild-slope sandy rivers, because the equilibrium timescale normally is much larger than the timescale of a flood event. Therefore it is important to understand the clear-water scour development at submerged weirs in order to improve the accuracy of scour estimation. This paper presents new data on the temporal evolution of the downstream clear-water scour depth at a submerged weir under steady flow conditions and analyzed the impacts of flow intensity, weir height, sand coarseness parameter, and weir slope on the development of the downstream clear-water scour depth. This study defined a dimensionless equilibrium time scale $t_e^{*}$ and evaluated its dependencies on the aforementioned parameters. Predictors for $t_e^{*}$ and the temporal downstream clear-water scour depth at submerged weirs are proposed.

New laboratory data of local scour at submerged weirs are presented. The dimensionless scour depth downstream of a submerged weir in a uniform sediment bed is expressed as a function of dimensionless weir height, flow intensity, sediment coarseness, and weir slope. The existing and newly presented data are combined to evaluate the dependencies of downstream clear-water scour depth on each of the relevant dimensionless parameters. For live-bed scour, the limitations of the existing predictors are summarized.