Analyzing Geomorphic Changes Following Successive Floods in a Partially Regulated Alpine River
Maha Sheikh, Virginia Ruiz-Villanueva
Universität Bern
Floods are forever known to reshape rivers by triggering processes such as localized erosion and deposition, shifts in bar locations (Rusnák & Lehotské, 2014), channel relocation through avulsions, and channel widening (Ruiz-Villanueva et al., 2023). Although these morphological changes have the potential to improve aquatic and riparian ecosystems by promoting and preserving a variety of heterogeneous habitats (Stanford et al., 2005), they may also have long-term negative onsequences. For instance, lateral channel adjustments can result in the destruction of flood protection systems, property, and infrastructure in the vicinity (Environmental Agency, 2018). Furthermore, a river's conveyance capacity may be altered by flood-driven morphological changes, which could enhance flood risk (Todd-Burley et al., 2021). Therefore, interpreting fluvial dynamics and assessing related risks and hazards require an understanding of how rivers react to flooding. However, it is still very difficult to predict these changes with accuracy.
This study seeks to investigate and quantify the primary geomorphological changes induced by floods in the partially regulated alpine Spöl River, located in Graubünden. The river's flow is controlled by two dams: Punt dal Gall and Ova Spin. Within the framework of a river restoration initiative, controlled flood releases are conducted once or twice per year from the dams, offering a distinctive opportunity to examine geomorphological changes in a natural laboratory context (Consoli et al., 2022). The lower reach of Spöl River, which runs downstream from the Ova Spin dam to its confluence with the Inn River, is the focal point of this study. Inflows and silt inputs from uncontrolled tributaries also have an impact on the Spöl in this area. We employed high-resolution digital surface models (DSMs) derived via structure from motion based on drone-acquired data to assess the geomorphic changes during a series of floods, in terms of sediment erosion and deposition and morphological alterations. The geomorphic alterations between the digital surface models gathered prior to and following floods since 2018 were examined using the Geomorphic Change Detection (GCD) 7.5.0 standalone program. Both the area and volume of the Spöl River were impacted by notable morphological changes. About 24% and 18% of the entire region suffered a positive elevation shift (deposition), whereas 16% and 35% underwent a negative elevation change (erosion), according to statistics from 2023 and 2021, respectively. In contrast to the downstream region, which showed a broad pattern of deposition, the upstream portion of the reach showed significant erosion.
The current study offers a unique opportunity to investigate and measure changes in the geomorphology resulting from a series of consecutive floods. While this study emphasizes striking a balance between restoration goals, infrastructure preservation, and flood resilience, future research will focus on prediction models.
