Backwater rise upstream of a stream-crossing culvert due to large wood accumulation and sediment deposition

•The large wood (LW) accumulation process at the stream-crossing culvert (SCC) has three stages: culvert blockage, triangle formation, and LW carpet.•The backwater height upstream of the SCC increases nearly linearly with the increasing sediment supply rate.•A design equation is proposed for computing the backwater height and length upstream of the SCC under the coupled effects of LW accumulation and sediment supply. Stream-crossing culverts (SCCs) are widely used transportation facilities in mountain streams. The sediment or large wood (LW) transported by the flow may block the SCC, resulting in a water level rise to increase the flood risk. Until now, the coupled impacts of sediment supply and LW accumulation on the backwater rise upstream of SCCs have remained unclear, which greatly restricts the flood control in streams with SCCs. In this study, a series of flume experiments were conducted to investigate the coupled impacts of sediment supply and LW accumulation on the backwater rise upstream of an SCC. The experimental results showed that: 1) the process of LW accumulation at the SCC has three stages: culvert blockage, triangle formation, and LW carpet; 2) the water level upstream of the SCC increases with increasing LW volume added to the flume in the culvert blockage and triangle formation stages, whereas it is almost unaffected by LW volume in the LW carpet stage; 3) the backwater height and length upstream of the SCC increases with the increase in the sediment supply rate under the combined effects of LW accumulation and sediment deposition, and the LW volumes corresponding to the LW carpet stage at the SCC are greater than those without sediment supply. Finally, predictors are proposed for the backwater height and length upstream of the SCC owing to the sediment supply and LW accumulation.