Submerged weirs are river training structures that are used for raising upstream water level, bed stabilization and reducing flow velocity. This paper presents an experimental study of local scour at submerged weirs in sand-bed channels. Two types of tests (coarse sand tests and fine sand tests) were conducted to investigate the effects of sediment size and tailwater depth on scouring at submerged weirs. The flow regimes over the weir are found to be independent of the sediment size, and the transition flow regime boundary can be expressed as a function of upstream Froude number and the ratio of weir height to tailwater depth. New equations, including the effects of sediment size, tailwater depth, flow intensity, and weir height, are proposed for prediction of equilibrium scour depths both upstream and downstream of the submerged weir. A new design method is given for estimating the maximum scour depths at the weir.

The erosion threshold of sand-mud mixtures is investigated by analyzing the momentum balance of a sand particle or a mud parcel in the mixture bed surface, and a formula for the critical shear stress of sand-mud mixtures is developed, which also applies for pure sand and mud. The developed formula suggests that the variation of the critical shear stress of sand-mud mixtures over mud content is mainly caused by the varying dry bulk density of the mud component in the mixture. The developed formula reproduces well the variation of the critical shear stress of sand-mud mixtures over mud content and can predict the critical shear stress of both sand-mud mixtures and pure mud in the process of consolidation. The developed formula promises to be convenient for application by relating the critical shear stress to mud content and the dry bulk density of sediment.