Numerical model of sediment transport in sediment bypass tunnels: influence of transverse slope in tunnel bend

Abstract

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 super-critical flows. Consequently, SBTs convey super-critical flows with large sediment loads, typically containing high concentrations of coarse particles of sediment 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 flow field, secondary currents, and patterns of sediment abrasion. This study focuses on sediment abrasion of SBTs:1. The flow field generating secondary currents associated with free-surface flows along SBT bends; and 2. Banking of an SBT invert to reduce sediment abrasion. The concept of invert-banking was proposed in personal communications with Dr. Ismail Albayrak of the Federal Institute of Technology (ETH), Zurich, Switzerland. The concept was floated during a SBT site inspection in April 2024. The problem of sediment abrasion is a problem for hydraulic structures in mountainous regions such as Switzerland and parts of the United States (e.g., Muller-Hagermann et al. 2020; Melesse et al. 2023). The present study uses the Computational Fluid Dynamics (CFD) code OpenFOAM to create a numerical model of an existing SBT for which hydraulic-model and field data and observations exist. The numerical model was used with the solver interFoam, and the renormalization group (RNG) k-ε turbulence flow assumption, the volume of fluid (VOF) method, and a Discrete Element Model (DEM) coupling. Of focal interest in the modeling was the pattern of secondary flow in a bend whose invert had variable transverse sloping. The prototype bend selected for this study is Switzerland's Solis SBT. The pattern of secondary flow in the bend affected the distribution of sediment across the bend's invert and, therefore, the sediment abrasion experienced by the bend. The Solis SBT, part of the sediment control system used for Solis Reservoir, was chosen for this study because of data and observations availability. Built in 2012, the invert of a bend in the Solis SBT has experienced severe abrasion owing to sediment. This study recommends a small amount of banking in the Solis SBT and other tunnels with similar hydraulic properties. Even a 1% to 2% slope appears to have a substantial effect in distributing the sediment evenly.