Assessment of sediment accumulation in la Baells water reservoir (Catalan Pre-Pyrenees, Spain)

Water reservoirs play a crucial role in providing important services, but they are facing increasing vulnerability due to the impacts of climate change. Changes in precipitation patterns are affecting the quantity and quality of water in the reservoirs. Sedimentation, which is one of the main impacts, reduces the reservoir's water storage capacity. Water reservoirs act as man-made sediment traps, with the amount of sediments trapped being directly proportional to the size of the water body. Most of the sediments produced in the watershed are accumulated at the bottom of the reservoir, leading to a decrease in water storage capacity. In the context of the SED4BUD project, we are analyzing the sediment stored in the Baells reservoir located in the Catalan Pre-Pyrenees region of Spain. The reservoir, which has a capacity of 109.4 hm3, was constructed in the 1960s. We have conducted a survey of the uppermost 20% area of the reservoir, which is approximately 0.62 km2 in size, that has emerged during the 2023 drought. The purpose of the survey is to measure the amount of material that has accumulated since the last bathymetry conducted in 2001. Additionally, we aim to classify sediment fractions into coarse and fine categories to calibrate a sediment transport model. To achieve this, we have employed four observational techniques: UAV photogrammetry, boreholes, stratigraphic description, and electric resistivity tomography (ERT). According to the UAV photogrammetry analysis, it has been observed that sediment accumulation of up to four meters is present in the surrounding areas of the active river channel. The ERT profiles reveal that the majority of the material in the top four meters of soil is fine, with sandy layers present in some areas. In the deeper layers, coarse material can be observed, and the basement is estimated to be between 15 and 20 meters deep. The analysis of the boreholes and stratigraphic description aligns with the findings from the geophysics and the UAV in terms of sediment accumulation and granulometry. However, it was not possible to apply these techniques in the active channel area due to inaccessibility. This area, where the highest sediment accumulations were detected by the UAV, remains unexplored due to limited accessibility. Boreholes and stratigraphic descriptions are precise but punctual measures, while UAV and ERT offer the spatial component. UAV is useful for surface measurements, and tomography provides precision in depth.