A USLE-based model with modified LS-factor combined with sediment delivery module for Alpine basins

Sedimentation in reservoirs constitutes a major issue worldwide, especially when hydropower, and hence profit, is involved. In this study, the sedimentation rates, due to soil erosion and sediment yields, are analyzed in the reservoir of the Rio di Pusteria hydropower dam, in South Tyrol (Italian Alps). The focus is on a five-year period, between two consecutive sediment flushing operations occurred in June 2014 and May 2019. Based on the difference between the pre- and post-flushing bathymetries, the five-year aggregated sediment yield was determined. The soil erosion from the basin surface and the sediment budget in the reservoir are calculated by means of a gridded Universal Soil Loss Equation (USLE), combined with a Sediment Delivery Ratio (SDR) module. The topographic factor is improved by the use of a fine-scale DEM and the slope length factor is adjusted to the Alpine terrain by means of a fitting threshold. The rainfall erosivity factor is determined in two different ways, one representing the precipitation regime of the study area and one the specific rainfall conditions of the study period. The two different applications result to calculated reservoir sedimentation rates of 439,279.2 tons and 589,520.5 tons with deviations from the measured value of 3.1% and 23.2%, respectively, while the sediment delivery ratio to the reservoir was found to be 12.5%. The calculations are also compared with results of the application of the conventional six-factor USLE. The comparison between calculations and measurements shows that the methods used can be efficiently applied in an Alpine basin for the estimation of reservoir sedimentation rates.