Spöl River e-flood impact on the Inn River suspended sediment regime

Abstract

Damming has led worldwide to a reduction in the hydrological variability in rivers. Especially in Alpine streams, hydroelectric production has resulted in an altered hydrological regime that also impedes many eco-morphological dynamics and ecosystem functions. Experimental releases or artificial floods are one of the options river managers use to reduce these negative effects. Since 2000 the Swiss National Park, in collaboration with the Engadiner Kraftwerke (EKW) hydropower company, has led a yearly project for recreating an experimental/environmental flood (e-flood) to re-establish the river Spöl dynamics and ecosystem. The effects of the e-flood, although positive for the Spöl river ecosystem, have not been investigated thoroughly along the river Inn, which the Spöl river joins as a major tributary. Here, we focus on the suspended sediment conveyed to the Inn River and on its routing to the Austrian border. The objective of the present study is to understand how much and how far fine sediment released by the e-flood is transported along the Inn and to identify possible sinks for deposition. In the present study we combine the Suspended Sediment Concentration (SSC) data collected with the Riverkin sensors during the 2021 and 2023 e-flood, the continuous hydro-sediment monitoring data by BAFU-FOEN and the EKW discharge measures along their infrastructure network. These hydro-sediment data are integrated with the channel cross sections surveyed in 2016 (BAFU) and used to build a 1D fixed-bed BASEMENT v4 model to solve the shallow water equations coupled with advection-diffusion of suspended sediments. The numerical model reproduces flow along the river Inn from S-Chanf to Martina, with data-based assumption on the tributary inflows, allowing flood and SSC routing along the 56 kilometre long reach. The results of the numerical model unravel the complex interaction between the riverbed morphology, the EKW infrastructures and the suspended sediments fed from the Spöl river. The results of the SSC routing along the Inn allowed also to estimate the relative contribution of the Spöl e-floods to the river Inn suspended sediment regime.