Effects of Hydropeaking on Bed Mobility: Evidence from a Pyrenean River

Abstract

Hydropower production involves significant impacts on the dynamics and continuity of river systems. In this paper we analyse the effects of hydropeaks on river-bed particle mobility along a 2-km river channel. For this, a total of four study reaches were stablished: one considered a control reach (no impact by hydropeaking) and three impacted (upstream and downstream from the confluence of tributaries). Mobility related to three hydrological scenarios considered representative of the entire flow conditions in the control and impacted reaches was investigated. Results indicate that sediment availability and dynamics proved different in the control reach to those observed downstream in reaches daily affected by hydropeaks. In the absence of large floods capable of resetting the system from a sedimentary point-of-view, only the role of tributaries during small flow events reduces the effects of hydropeaks on river-bed particles’ availability and mobility. The effects of a hydropeaked regime are not observed for the whole spectrum of grain-sizes present in the river-bed. While the structural large elements (i.e., boulders) in the channel do not move, sand and fine gravel stored in patches of the bed are constantly entrained, transported and depleted whereas, in between, medium and large gravel are progressively winnowed. Our results point out that hydropeaked flows, which are generally not considered as disturbances in geomorphic terms, initiate frequent episodes of (selected) bed mobility and, consequently, the river-bed becomes depleted of fine sediments from patches and progressively lacks other fractions such as medium gravels, all of which are highly relevant from the ecological point of view.

Downstream changes of particle entrainment in a hydropeaked river

“…Hence, we conclude that in the Upper Cinca, the maximum hydropeak Q from the Laspuña HP plant is capable of mobilising the finer size fractions of the bed, a condition termed partial transport (Wilcock and McArdell, 1997). This was also observed in the Upper Noguera Pallaresa, ⎯ 29 ⎯ another of the many Pyrenean rivers affected by hydropeaking, where sand and fine gravel, stored in patches and medium and coarse pebbles, are winnowed by hydropeaked flows (Vericat et al, 2020) until natural floods supply fresh fine sediment from upstream.…”

“…In our case, fine sediment is supplied from upstream during floods, which annually can exceed Q of 200 m 3 s −1 , a discharge that can mobilise up to D50 of the bed-material grain-size distribution. Finally, despite peak Q associated with hydropeaking in the Upper Cinca having a low impact on the coarse surface layer, as recently described by Vericat et al (2020) in the Noguera Pallaresa River, the condition of partial mobility that is attained (i) in reaches close to the HP plant, (ii) in reaches eventually showing finer grain sizes (e.g.…”

“…Therefore, the importance of adding empirical evidence and model simulations to better understating the magnitude of the effects of this changing flows on river structure. Natural floods carry bedload from upstream, a process that usually mitigates the deficit of fine fractions in the bed after a sustained period of hydropeaking, favouring the reappearance of patches of fine sediments (as recently described in Vericat et al, 2020). As stated, and despite numerous studies analysing the environmental effects of hydropeaking (see early works by Cushman, 1985;Moog, 1993;Cereghino et al, 2002), there is still a lack of studies examining their effects on river sedimentary dynamics and, specifically, on bed-material entrainment (see Aigner et al, 2017 andVericat et al 2020 as some few recent examples).…”

“…The impact of hydropeaks on sedimentary process, particularly bed-material entrainment, has not been extensively studied yet (e.g. Vericat et al, 2020). Paucity of experimental data is likely the most important reason for this, and the lack of data and their transferability preclude generalization of results in comprehensive assessment of the impacts of hydropeaks on river functioning.…”

Hydraulic classification of hydropeaking stages in a river reach

“…Rapidly varying discharge is associated with variable bottom shear stress and water velocities. An increase in bottom shear stress might increase the amount of suspended sediment in the reach, which could deplete the downstream reach of fine sediments (Vericat, Ville, Palau-Ibars, & Batalla, 2020) and add stress to local benthic fauna (Salmaso, Servanzi, Crosa, Quadroni, & Espa, 2021). With rapid flow fluctuations, riverbank stability might get compromised (Mohammed-Ali, Mendoza, & Holmes, 2020).…”