Restoring morphodynamics downstream from Alpine dams: Development of a geomorphological version of the serial discontinuity concept

S.N Lane,T Gaillet,L Goldenschue

doi:10.1016/j.geomorph.2022.108131

Abstract

There is well-established evidence that dams disconnect upstream to downstream sediment flux in rivers and that this may have negative impact on downstream ecosystems. For this reason, the development of environmental flows now includes sediment supply and transport whether through reconnecting upstream supplied sediment to a river downstream of a dam, eco-morphogenic flows to rework the stream bed, or artificial sediment supply. However, especially in Alpine systems, there may be unregulated tributaries that are able to deliver gravel and coarser sediment naturally to compensate for the effects of dam-related sediment disconnection. To represent these effects we propose a geomorphic form of the serial discontinuity concept and apply it to two hydropower dam-impacted Alpine streams in the Swiss Pennine Alps. Conceptually, the relative position of a dam influences the degree of coarse sediment disconnection as well as the rate of coarse sediment recovery, especially as many Alpine valleys have strong down-valley gradients in tributary sediment delivery. In both case-studies, there was rapid recovery in likely coarse sediment delivery downstream of the dams. By following geomorphic response of the rivers to eco-morphogenic flow trials, proposed as a solution to perceived dam-driven coarse-sediment disconnection, we confirmed that both rivers are likely to have more than sufficient natural coarse sediment supply and unregulated floods. Natural coarse sediment supply is rarely considered in the management of Alpine streams impacted by hydropower but it needs to be evaluated through a geomorphological assessment, considering both the geomorphic context of the river reaches downstream of the dam and the geomorphic attributes of the basin in which the dam is found.

Highlights

As dams disrupt downstream fluxes of both water and sediment, they can have a major impact on river morphodynamics downstream, the exact impacts will depend on: the dam and its operation; where it is built within a river basin; and characteristics of the river itself (Sherrard and Erskine, 1991; Church, 1995; Kondolf et al, 1996; Rollet et al, 2014)
The results show that geomorphological context is crucial in determining whether the coarse sediment disconnection caused by dams and other hydropower operations can be mitigated via natural coarse sediment supply
In both cases a Mann-Whitney U test confirmed that the modelled values of each tributary (Et) were significantly higher at p < 0.001 for tributaries with observed coarse sediment delivery to the main stem or lake than those with no observed coarse sediment delivery

Summary

Introduction

As dams disrupt downstream fluxes of both water and sediment, they can have a major impact on river morphodynamics downstream, the exact impacts will depend on: the dam and its operation; where it is built within a river basin; and characteristics of the river itself (Sherrard and Erskine, 1991; Church, 1995; Kondolf et al, 1996; Rollet et al, 2014). Coarsening, coupled to the reduced magnitude and frequency of sediment transporting flows, leads to more stable stream beds and biogeochemical processes that reinforce the problems of colmation (Zhang et al, 2011; Gartner et al, 2012; Wharton et al, 2017), the process of fine sediment accumulation in the surface layers of river-bed sediment, and the associated biological and chemical processes that can follow It is not surprising, that dams have been reported as having a negative impact on downstream ecosystems in a wide variety of river settings (Ligon et al, 1995; Camargo and Voelz, 1998; Ogbeibu and Oribhabor, 2002; Lessard and Hayes, 2003; Wright et al, 2008; Jansen et al, 2020). Dam operations such as periodic flushing of fine sediment accumulated behind the dam, if not carefully designed, may exacerbate these negative impacts (e.g. Brooker and Hemsworth, 1978; Cushman, 1985; Moog, 1993; Lauters et al, 1996; Céréghino and Lavandier, 1998; Smokorowski et al, 2011; Schülting et al, 2016, 2019; Bruno et al, 2019; Gabbud et al, 2019a, 2019b)

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