The contextualization of macroplastics in a budget framework can help to improve our understanding of the physical processes determining macroplastic behaviour and impacts in rivers. Hence, better management practices can be adopted.

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.