Author(s): Virginia Ruiz-Villanueva; Janbert Aarnink; Javier Gibaja; Bryce Finch; Marceline Vuaridel

Linked Author(s): Virginia Ruiz-Villanueva, Janbert Aarnink

Keywords: Environmental Flows; River Restoration; Experimental Floods; Ecohydrology; Instream large wood

Abstract: The construction of dams significantly alters rivers´ natural regimes, by changing flow conditions, reducing the sediment transport capacity, and altering the vegetation and instream wood dynamics. Environmental flows (e-flows) are often based just on the flow regime, they rarely consider the sediment, and the instream wood is completely overlooked. The instream wood – e.g., downed trees, trunks, branches and rootwads- sustains the physical and ecological integrity of the river. By interacting with the flow and sediments, wood enhances river physical complexity and improves habitat diversity, and thus biodiversity. On the other hand, large quantities of wood transported during e-flows can be a concern, as it may pose a danger to infrastructures such bridges. Integrating flow-sediment and wood regimes in the design of e-flows is thus of great importance, particularly in forested rivers with unregulated tributaries and /or active hillslopes affected by mass movements processes, that may still supply large quantities of sediment and wood to the main channel. This is the case of the Spöl River in the Swiss Alps. The Spöl river is dammed but undergoes a restoration program based on the release of annual e-flows since the year 2000. Despite the e-flows, the river is facing a significant aggradation and other associated processes, such as bank erosion, channel widening, or vegetation die back. Instream wood is supplied to the river from the forested slopes affected by snow avalanches, landslides, debris flows and windstorms, the riverbanks and vegetated islands. In 2018, we enhanced the monitoring framework to include observations of sediment and instream wood transport. Before and after flows, sediment was tagged, and grain size was measured at different locations and wood stored within the river was tagged, measured and georeferenced. We surveyed topographical cross sections and flew a drone. Moreover, during the e-flows a video camera was installed at a bridge. Preliminary results showed that only a small fraction of the stored instream wood was mobilized by the e-flows, but part of it travelled relatively long distances reaching the confluence with the Inn River. The instream wood mainly moved during the beginning and the peak of the flows, with very much less transport afterwards, and two following flows revealed that most of the wood was effectively entrained during the first flow, even if the second had a higher discharge. Knowing the effect of the e-flows on sediment and instream wood transport will be key to understand their effectiveness and to identify a suitable flow, sediment and instream wood regimes that will better sustain the structure and function of the Spöl river. This study is key for the design of future river restoration in the Spöl, but also for the management of regulated mountain rivers in general.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221000

Year: 2022