Numerical modeling of sediment transport in sediment bypass tunnels: Influence of transverse slope in tunnel bend

Sediment Bypass Tunnels (SBTs) convey sediment around reservoirs, increasing reservoir lifespan by greatly reducing reservoir sedimentation and, thereby, mitigating consequent loss of reservoir water-storage capacity. To keep SBTs small and economical in cross-section, SBTs convey supercritical flows. Consequently, SBTs convey supercritical flows with large sediment loads, typically containing high concentrations of coarse sediment particles that can cause abrasion of SBT liners. Especially vulnerable are SBT reaches where secondary currents develop, notably SBT bends. The sediment abrasion that occurs along the invert of a bend requires expensive, frequent replacement of the invert’s concrete liner. Consequently, the abrasion rate of inverts and, therefore, bend flow fields are of interest to SBT designers. SBT design variables such as sediment-size distribution, invert-liner type (usually concrete), flow cross-section dimensions, tunnel slope, and bend radius can affect sediment abrasion in an SBT, doing so by influencing the flow field, secondary currents, and patterns of sediment abrasion. This paper focuses on abrasion of SBTs that occurs due to the presence of bends in the tunnel, and how to mitigate that abrasion with a geometric change in the tunnel geometry by focusing on the flow field generating secondary currents associated with free-surface flows along SBT bends and the resulting impact of this on the movement of sediment and banking of an SBT invert to reduce sediment abrasion. This study aimed to replicate the behavior of the sediment moving to the inside bend of the Solis SBT, in Switzerland, with CFD-DEM modeling, and to evaluate the potential for influencing the behavior of the sediment to more evenly distribute the sediment across the invert, which reduces the rate of abrasion in the tunnel bend. The findings of this paper suggest that a small amount of transverse invert banking (~2%) may spread the abrasion more evenly in the Solis SBT. This finding should be further investigated with physical modeling and may only be relevant to other tunnels with similar hydraulic properties and geometries.