Downstream morphological effects of SBT releases: 1D numerical study and preliminary LiDAR data analysis

Abstract

Sediment bypass tunnels (SBTs) are built with the twofold aim of reducing reservoir sedimentation and restoring sediment and water regimes in the downstream reach. The conveyance of water and sediment through SBTs has the potential to cause downstream morphodynamic changes, which have so far been poorly investigated. The main goal of this work is to quantify downstream morphological effects of SBT releases. This will be achieved conducting a numerical study on an idealized but still realistic situation and a field data analysis. First, assuming that the SBT transport capacity is larger than the one of the upstream river reach, we identify realistic SBT-release scenarios in terms of water and sediment discharges being released from the SBT and the dam to the downstream reach. Second, we carry out a numerical study for the quantification of the impacts of SBT operations in terms of bed level and surface sediment grain size changes. Third, we analyze the effect of the SBT releases that occurred in 2014 and 2016 at the Albula River, through a comparison of data collected during two LiDAR surveys carried out after SBT releases. Numerical results show that i) the smaller the water discharge released from the SBT, the steeper the riverbed becomes; ii) if the SBT is delivering sediment and water to the downstream reach, the riverbed grain size distribution (GSD) tends to be close to unarmored conditions, with slight changes for different release conditions. Considering a reduced SBT-efficiency (i.e. only a part of the sediment being transported in the upstream reach is entering the SBT), the described trends do not change but the equilibrium riverbed slope becomes smaller, with some release conditions causing the downstream slope to be smaller than the upstream one. Finally, from the comparison of cross-sections obtained from the points measured with two LiDAR surveys, we observe a significant depositional trend in the river reach downstream of the Solis SBT (Canton Grisons, Switzerland), as can be inferred from the numerical simulations.