Eco‐hydraulic modelling of the interactions between hydropeaking and river morphology

Abstract

Hydropeaking related to hydropower operations produces adverse ecological effects that depend on its interaction with the channel morphology. A first quantitative attempt is proposed to investigate the eco-hydraulic response of different river morphologies to hydropeaking waves based on a two-dimensional hydraulic modelling approach. Physical habitat diversity, macroinvertebrate drift and fish stranding, all relevant for hydropeaking, are quantitatively investigated with reference to realistic hydro-morphological conditions of regulated alpine streams. Habitat diversity and fish stranding have the strongest dependence on channel morphology and show nearly opposite behaviours with increasing morphological complexity. Braided reaches are the most resilient to hydropeaking offering the highest habitat diversity and very limited base-to-peak variation of macroinvertebrate drift, while alternate bars are extremely sensitive environments to drift and offer safer regions from stranding. Transitional morphologies between single-thread and multi-thread offer the best eco-hydraulic trade-offs. The method allows quantifying to which extent same eco-hydraulic targets can be achieved by either morphological restorations or base flow increases: in transitional morphologies, identical reduction in reach-averaged stranding risk might be obtained either by halving the channel width or by a threefold base flow increase; analogously, the same improvement in macroinvertebrate-fed areas can be achieved in a channel with alternating bars either by a threefold base flow increase or by increasing 2.5 times the channel width. Such quantification of the eco-hydraulic effectiveness of complementary management strategies offers a powerful tool to support design of restoration measures in hydropeaking rivers.

Integrated analysis of flow, form, and function for river management and design testing

“…These results indicate that restoring or designing a pool-riffle dominated stream network to provide interspersed plane bed reaches may support higher overall ecosystem integrity by promoting distinct and complementary functions in different locations during biologically significant periods. Such findings support the emerging recognition of spatial and temporal heterogeneity as fundamental characteristics of fluvial systems and the need for a flexible framework within which natural processes, such as sediment transport and nutrient dynamics, can occur (Clarke, Bruce-Burgess, & Wharton, 2003;Gostner, Parasiewicz, et al, 2013;Vanzo et al, 2016;Escobar-Arias & Pasternack, 2010).…”

“…First, a set of synthetic river corridor DTMs is generated to represent channel types from an existing channel classification (Section 2.1), and a set of hydrologic scenarios is selected for evaluation (Section 2.2). Next, a 2D hydrodynamic model (sedimentation and river hydraulics‐two dimensional [SRH‐2D]; Lai, ) is used to simulate ecologically relevant hydraulic parameters (ERHPs, sensu Vanzo et al, ) for each flow–form scenario (Sections 2.3 and 2.4). Finally, spatio‐temporal patterns in ERHPs are used to evaluate the performance or occurrence of a suite of ecosystem functions (Section 2.5) under each scenario.…”

“…Rivers are complex, dynamic systems that support many natural ecosystem functions, including hydrogeomorphic processes and the creation and maintenance of aquatic and riparian habitat (Doyle, Stanley, Strayer, Jacobson, & Schmidt, 2005). The extent and timing of these functions is largely controlled by the interplay of flow, described by streamflow magnitude, timing, duration, frequency, and rate-of-change (Poff et al, 1997), and form, described by the shape and composition of the river corridor (Pasternack, Bounrisavong, & Parikh, 2008;Small, Doyle, Fuller, & Manners, 2008;Vanzo, Zolezzi, & Siviglia, 2016;Wohl et al, 2015;Worthington, Brewer, Farless, Grabowski, & Gregory, 2014). In spite of this complex interplay, most environmental river management studies evaluate the role of either flow or form without regard for these interactions.…”

“…However, this and similar studies (Brown & Pasternack, 2008;Gostner, Parasiewicz, & Schleiss, 2013;Price, Humphries, Gawne, Thoms, & Richardson, 2013) are site specific, limiting their applicability to the range of flow and form settings exhibited by a given hydroscape, each combination supporting distinct ecosystem functions. Vanzo et al (2016) offer an exception in their evaluation of ecohydraulic responses to hydropeaking over a spectrum of existing and proposed flows and forms. See Section S1 for more details on the flow-form-function conceptual framework.…”

The effects of hydropeaking on riverine plants: a review

“…Hydropeaking regimes downstream of hydropower plants are characterized by a sequence of peak events with several hydrological phases (Bruder et al ., ), ( i ) low base discharge, when no electricity is generated; ( ii ) rapid changes in discharge, when electricity generation is increased or decreased, and ( iii ) high peak discharge during periods of maximal electricity generation. Operating rules determine the duration and frequency of these phases, as well as the magnitude and rate of change of discharge in each phase, which all in turn lead to changes in hydraulic parameters such as water level, flow velocity, water turbulence, and bed shear stress (Shen & Diplas, ), as well as changes in river morphology (Vanzo, Zolezzi & Siviglia, ), water quality (Pulg et al ., ) and water temperature (Zolezzi et al ., ), both downstream and upstream of the hydropower facilities (Fig. ).…”