Combined Flow Abstraction and Climate Change Impacts on an Aggrading Alpine River

Abstract

Recent climatic warming and associated glacial retreat may have a large impact on sediment release and transfer in Alpine river basins. Concurrently, the sediment transport capacity of many European Alpine streams is affected by hydropower exploitation, notably where flow is abstracted but the sediment supply downstream is maintained. Here, we investigate the combined effects of climate change and flow abstraction on morphodynamics and sediment transfer in the Borgne River, Switzerland. From photogrammetrically derived historical Digital Elevation Models (DEMs), we find considerable net aggradation of the braided river bed (up to 5 m) since the onset of flow abstraction in 1963. Reaches responded through bed level steepening which was strongest in the upper most reach. Widespread aggradation however did not commence until the onset of glacier retreat in the late 1980s and the dry and warm years of the early 1990s. Upstream flow intake data shows that this aggradation coincided with an increase in sediment supply, although aggradation accounts for no more than 25% of supplied material. The remainder was transferred through the studied reaches. Estimations of bed load transport capacity indicate that flow abstraction reduces transport capacity by 1–2 orders of magnitude. While residual transport rates vary with morphological evolution, they are in the same order of magnitude as the sediment supply rates, which is why significant transport remains. However, the reduction in transport capacity makes the system more sensitive to short‐term (annual) changes in climate‐driven hydrological variability and climate‐induced changes in intake management and sediment delivery rates.

Morphological Response of an Alpine Braided Reach to Sediment‐Laden Flow Events

“…Over the whole period of investigation, June–September 2015, net river bed degradation occurred, which corresponds to the trend for the period 2010–2014 (Bakker et al, ). Erosion was observed in June–July and in the second half of August (Figure ), both periods that experienced larger‐magnitude floods.…”

“… Note . Gradient and surface grain size are based on Figure 2 in Bakker et al () which provides a longitudinal profile of the reach.…”

Disentangling human impact from natural controls of sediment dynamics in an Alpine catchment

“…The construction of a major hydropower dam, the Grande Dixence was completed in Val d'Hérémence in 1961. The water intakes are equipped with sediment traps for both fine-and coarse-grained material, from which sediment is flushed down the river at frequencies up to several times per day (Lane et al, 2017;Bakker et al, 2018). Water is supplied not only from the~45 km 2 large catchment of the lake itself, but also from the neighbouring valleys through a network of 100 km of transfer tunnels and pumping stations (Margot et al, 1992;Lane et al, 2014;Bakker et al, 2018;Anghileri et al, 2018aAnghileri et al, , 2018b.…”

“…Its water and sediment feeds major European fluvial networks such as the Rhine, Rhône, Po, and Danube Rivers. This is particularly the case for the Swiss Rhône valley, where water fluxes have been heavily managed and regulated particularly through the construction of hydropower dams and flow intake systems Bakker et al, 2018). In parallel, mountainous environments are directly impacted by humans through land use, for example through deforestation and reforestation, and river management (e.g.…”

“…Here, we focus on the catchment of the Borgne, a tributary of the Rhône River, where dynamic landscape responses to water abstraction have already been documented (Reynard et al, 2012;Lane et al, 2014Lane et al, , 2017Micheletti et al, 2015;Lambiel et al, 2016;Micheletti and Lane, 2016;Bakker et al, 2018). The main goals of this study are (1) to trace the current fine-grained sediment flux in the catchment through a geochemical fingerprinting approach and (2) to assess the sensitivity of sediment production and transfer processes to anthropogenic disturbances.…”