Bedload continuity and wood retention at inclined bar screens

In forested mountain catchment areas, both bedload and large wood (LW) can be transported during ordinary and high flows. Given settlements or important infrastructures are located within the catchment area, retention structures such as sediment traps or rack structures are built to mitigate potential hazards downstream. Currently, the design of these structures focuses on either the management of bedload or LW. LW retention racks are often designed using vertical poles positioned perpendicular to the flow direction and spanning the entire river cross-section. Due to this setup, they tend to retain both LW and bedload, thereby significantly affecting bedload transport continuity during ordinary flows. Hence, the natural regimes downstream of such structures are altered, with important consequences in terms of changes in biodiversity. Therefore, the design of LW retention structures needs to be adapted to enable LW retention and bedload continuity during ordinary flows. Flume experiments were conducted by modeling an inclined bar screen that consists of poles spanning the river cross-section but are inclined with a specific angle to the downstream and have a bottom clearance height to enable bedload transport. The experiments were performed to study the effect of LW accumulations at an inclined bar screen by adding a specific LW volume to the flow and measuring the resulting changes in upstream water depth (backwater rise) and bedload transport continuity. The focus was put on testing different LW characteristics, including different LW sizes, LW shapes, LW densities, and adding model fine material. The results demonstrated that a few logs are sufficient to reduce the bedload transport capacity to below 75% compared to the condition without LW. Leaves and smaller wood sizes further reduced bedload transport and increased backwater rise. In contrast, LW density and LW shape had a negligible effect. Our results indicate that the design of such structures may need to be adapted to enable both LW retention and bedload continuity during ordinary flows.