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

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Bakker M., Costa A., Silva T., Stutenbecker L., Girardclos S., Loizeau J.L., Molnar P., Schlunegger F., Lane S.,
Project SEDFATE:Sediment fate in a changing watershed during the Anthropocene
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Original article (peer-reviewed)

Journal Water Resources Research
Volume (Issue) 54
Page(s) 223 - 242
Title of proceedings Water Resources Research
DOI 10.1002/2017wr021775

Abstract

Recent climatic warming and associated glacial retreat may have a large impact on sedimentrelease and transfer in Alpine river basins. Concurrently, the sediment transport capacity of many EuropeanAlpine streams is affected by hydropower exploitation, notably where flow is abstracted but the sedimentsupply downstream is maintained. Here, we investigate the combined effects of climate change and flowabstraction on morphodynamics and sediment transfer in the Borgne River, Switzerland. From photogram-metrically derived historical Digital Elevation Models (DEMs), we find considerable net aggradation of thebraided river bed (up to 5 m) since the onset of flow abstraction in 1963. Reaches responded through bedlevel steepening which was strongest in the upper most reach. Widespread aggradation however did notcommence until the onset of glacier retreat in the late 1980s and the dry and warm years of the early1990s. Upstream flow intake data shows that this aggradation coincided with an increase in sediment sup-ply, although aggradation accounts for no more than 25% of supplied material. The remainder was trans-ferred through the studied reaches. Estimations of bed load transport capacity indicate that flowabstraction reduces transport capacity by 1–2 orders of magnitude. While residual transport rates vary withmorphological evolution, they are in the same order of magnitude as the sediment supply rates, which iswhy significant transport remains. However, the reduction in transport capacity makes the system moresensitive to short-term (annual) changes in climate-driven hydrological variability and climate-inducedchanges in intake management and sediment delivery rates.
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