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Ecosystem impacts of Alpine water intakes for hydropower: the challenge of sediment management

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The natural flow hydrological characteristics (such as the magnitude, frequency, duration, timing, and rate of change of discharge) of Alpine streams, dominated by snowmelt and glacier melt, have been established for many years. More recently, the ecosystems that they sustain have been described and explained. However, natural Alpine flow regimes may be strongly modified by hydroelectric power production, which impacts upon both river discharge and sediment transfer, and hence on downstream flora and fauna. The impacts of barrages or dams have been well studied. However, there is a second type of flow regulation, associated with flow abstraction at intakes where the water is transferred laterally, either to another valley for storage, or at altitude within the same valley for eventual release downstream. Like barrages, such intakes also trap sediment, but because they are much smaller, they fill more frequently and so need to be flushed regularly. Downstream, while the flow regime is substantially modified, the delivery of sediment (notably coarser fractions) remains. The ecosystem impacts of such systems have been rarely considered. Through reviewing the state of our knowledge of Alpine ecosystems, we outline the key research questions that will need to be addressed in order to modify intake management so as to reduce downstream ecological impacts. Simply redesigning river flows to address sediment management will be ineffective because such redesign cannot restore a natural sediment regime and other approaches are likely to be required if stream ecology in such systems is to be improved. WIREs Water 2016, 3:41–61. doi: 10.1002/wat2.1124 This article is categorized under: Water and Life > Stresses and Pressures on Ecosystems
Modeled values of the critical discharge for median grain sizes from 0.06 to 0.16 m, 84th percentiles set always to be 0.04 m greater than the median, and shown for a range of slopes (plotted on the first y axis). Critical discharges are calculated based upon the method of Nitsche et al. which corrects for greater energy losses in mountain streams with macroform‐bed roughness. Superimposed are frequency distributions of discharge for the Haut Glacier d'Arolla (data from Figure , plotted here on second y axis) calculated for May 1 to September 30 for the natural flow and the residual flow after abstraction.
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The natural flow hydrograph for a glacial catchment (Haut Glacier d'Arolla, data for a location at 2505 m a.s.l.) and the flow recorded downstream, produced by events designed to empty the intake of sediment. The data were provided by Grande Dixence SA for 1989 as an example.
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Characteristic invertebrate taxa distributions in glacial stream according to Brittain and Milner (p. 1572).
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Example of flow hydrographs for a nival catchment (Vallon de Nant, data for a location at 1350 m a.s.l.) and a glacial catchment (Haut Glacier d'Arolla, data for a location at 2505 m a.s.l.). The Haut Glacier d'Arolla data were provided by Grande Dixence SA (data from 1989 to illustrate and support Table ).
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Example of the Bas Glacier d'Arolla water intake (Valais, Switzerland) and a classic flow intake design (Ref ). The water is caught and diverted to be used for the hydroelectric production; only a residual discharge is released to the river downstream (even sometimes none); the sediments are managed by two traps: the first gravel trap holds the larger sediments by coarse grills whereas the sand trap holds finer sediment with a few water; the respective doors can be manually or automatically opened when the storage traps are full or to let the water flow.
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