Hydrological characterization of hydropeaks in mountain rivers (examples from Southern Pyrenees)

Streamflow in the Ésera and Upper Cinca Rivers (South Central Pyrenees) experiences alterations in the form of rapid pulses, commonly referred to as hydropeaks, associated with highly variable on-demand hydropower production from flowing or stored water. A hydrological characterization of the hydropeaks in these two rivers was carried out to examine their distinct hydropeaking pattern during hydrologically contrasted years. The Ésera demonstrated a hydropeaking regime characterized by a high frequency and magnitude, while the Upper Cinca presented a regime with higher frequency but of lesser magnitude. Additionally, this study classified hydropeaks following the shapes proposed in previous studies and assessed their temporal distribution (frequency), timing, and magnitude. Results indicated that hydropeaks were different in shape and magnitude in both rivers; a Front Step pulse shape dominated in the Ésera whereas a Rectangle pulse shape was the most frequent in the Cinca. The Front Step shape had the highest ramping rates during the rising limb of the hydrograph and the shortest time to the maximum flow; whereas the Rectangle shape had higher duration and a larger volume of water released. Hydropeaks in the Ésera were less frequent (one per day) than in the Cinca (two per day). The river Ésera presented, in general, high-magnitude hydropeaks, with double volume and duration, and three times larger up-ramping rates and time between flow pulses. Overall, hydropeak shapes in the Ésera varied depending on the hydrological year and the season, while in the Cinca, shape distribution followed the same temporal pattern regardless of year and season; thus, hydropeaking in the Ésera appeared to be much more dependent on the annual hydrology (whether the year is wet or dry) than in the Cinca. Hydropeaking regimes were characterized based on three categories of variables: magnitude (flow-based or hydrological variables), timing (duration), and frequency (occurrence). According to the variability of these variables, a total of three statistically significant groups were obtained in the Ésera, while two groups were identified in the Cinca. Hydropeaks can be statistically classified by their hydrological characteristics that in turn may have a direct effect on sediment entrainment and transport with implications to the ecological functioning of rivers. The different shape of the hydropeaks, however, will control the attenuation of the flows downstream and, consequently, the maintenance of the river's competence and capacity. This methodology provides a useful quantification of hydropeaking regimes to assess the potential impacts on physical and biological aspects of river systems.     

Introducing HyPeak: An international network on hydropeaking research,practice, and policy

An increase in the demand for renewable energy is driving hydropower development and its integration with variable renewable energy sources. When hydropower is produced flexibly from hydropower plants, it causes rapid and frequent artificial flow fluctuations in rivers, a phenomenon known as hydropeaking. Hydropeaking and associated hydrological alterations cause multiple impacts on riverine habitats with cascading effects on ecosystem functioning and structure. Given the significance of its ecological and socio-economic implications, mitigation of hydropeaking requires an inter- and transdisciplinary approach. An interdisciplinary network called HyPeak has been conceived to enrich international research initiatives and support hydropower planning and policy. HyPeak has been founded based on exchange and networking activities linking scientists from several countries where hydropeaking has been widespread for decades and numerous studies dedicated to the topic have been carried out. HyPeak aims to integrate members from other countries and continents in which hydropower production plays a relevant role, and grow to be a reference group that provides expert advice on the topic to policy-makers, as well as researchers, stakeholders, and practitioners in the field of hydropeaking.     

Increased hydropower production and hydropeaking mitigation along the Upper Inn River (Tyrol,Austria) with a combination of buffer reservoirs,diversion hydropower plants and retention basins

Research on hydropeaking in Austria started in the 1990s and the implementation of the WFD stipulated large research projects since the year 2009. The first research projects supported the process understanding and in a second investigation step, measures were evaluated. The mountains in the region of Tyrol create large heads and thus support the production of flexible energy. In this region, TIWAG is operating 9 large (>10 MW) and 27 small (<10 MW) hydropower plants, with an installed capacity of about 1,550 MW and a mean annual production of 3,000 GWh. As the governmental energy strategy foresees an extension of the hydropower production in Austria, suitable options were selected in 2004 by TIWAG and the water management framework plan for Western Tyrol was developed. This strategic planning instrument proposes five large power plants, with a generation of 1,800 GWh/year of renewable energy, which enables to reach the WFD targets, because the concept includes hydropeaking mitigation by combining buffer reservoirs (impoundments), diversion hydropower plants, and retention basins. We present our promising concept of hydropeaking mitigation and exemplify this based on the GKI, a hydropeaking diversion hydropower plants (HPP) at the Swiss/Austrian border as well as the Silz hydropeaking retention basin, with a volume of about 300,000 m³. As the presented case studies are the largest measures for hydropeaking mitigation being currently implemented in Central Europe, they have pilot-character. Thus, ongoing research and monitoring programs are expected to improve the knowledge about hydropeaking mitigation.     

Effects of hydropeaking on benthic invertebrate community composition in two central Norwegian rivers

Hydropower regulations can have dramatic impacts on river ecological communities. The operation of hydropower stations is related to power demands, but their releases in the receiving water body causes sudden changes in flow, which in turn affect the biota. The effects of such flow variations on benthic invertebrates is not fully understood. Here, we studied the effects of duration and intensity of hydropeaking on benthic invertebrates in two rivers over a 3.5‐year period. We used both quantitative (Surber) and semiquantitative (kick samples) sampling methods to compare the ramping zone with the permanently water covered zone downstream of the hydropower plant, and with corresponding unaffected upstream areas. The ramping zone had a different invertebrate community composition and lower benthic density than other areas, especially after hydropeaking. Mayflies and chironomids were most negatively affected by hydropeaking and oligochaetes largely unaffected. Chironomids and the mayfly Baetis rhodani were able to recolonize the ramping zone and almost reach densities similar to deeper areas within 48 days following hydropeaking. The relative abundance of filter feeders tended to increase and gatherers/collectors tended to decrease from the ramping zone towards the deep, permanently water covered areas. In corresponding areas upstream of the power plant, the relative abundance of different functional feeding groups was the same in the mid‐channel and shore sites. Our study shows that hydropeaking has clear impacts on the functional structure of benthic invertebrates below the power plants. The ecological impact of hydropeaking on invertebrate communities should thus be taken into account, for example, by reducing the amplitude and duration of flow fluctuations.     

Hydraulic classification of hydropeaking stages in a river reach

Hydropower is an important tool in the struggle for low-emission power production. In the Nordic countries, hydropower operating conditions are expected to change and work more in conjunction with intermittent power production. This in turn might increase the amount of hydropeaking events in the reaches downstream of hydropower plants. The current work investigates the influence of highly flexible, high-frequency hydropeaking on the hydrodynamics in the downstream reach. By quantifying four different dynamic stages in the study reach, the influence of the hydropeaking frequencies was investigated in the bypass reach of the Stornorrfors hydropower plant in the river Umeälven in northern Sweden. The hydrodynamics in the study reach were numerically modelled using the open source solver Delft3D. Eight different highly flexible future hydropeaking scenarios, varying from 12 to 60 flow changes per day, were considered. A method for identifying four hydropeaking stages—dewatering, dynamic, alternating and uniform —was introduced. The hydropeaking frequency directly decided the stage in most of the study reach. Furthermore, a Fourier analysis showed a significant difference between the stages and their corresponding power spectra. The classification of stages put forward in this work provides a novel, simple method to investigate the hydrodynamics due to hydropeaking in a river reach.     

Hydropeaking impact on a riparian ground beetle community

The Common Meuse reach is strongly influenced by the operation of a hydropower plant at the upstream weir of Lixhe, especially during periods of low flow. Ecologically based in‐stream flow requirements and fluctuation thresholds have already been determined for this reach by reconstruction of the natural discharge course from historic and actual reference conditions. Nevertheless, more evidence from the present biota at risk has been demanded. This study therefore attempts to define boundary conditions for the low flow regime from the analysis of riparian ground beetles in this river reach. To achieve this, reference conditions for the hydroregime aspects of flow variation were determined. Then, using the habitat templet approach, the hydropeaking pressure was related to biological quality elements. Finally, after detecting the impact on the specific gravel bar ground beetles, thresholds and boundary conditions were determined for the hydropeaking pressure in peak velocity. Copyright © 2007 John Wiley & Sons, Ltd.     

PeakTrace: Routing of hydropeaking waves using multiple hydrographs—A novel approach

Hydropeaking is known for its adverse impacts on river ecosystems. However, the implementation of mitigation measures is still largely pending due to conflicting priorities of ecology and economics, which require scenario building to assess trade-offs. Therefore, widely applicable and standardized tools are needed to analyze hydropeaking hydrology in affected rivers to expedite mitigation efforts. Here, we present a novel empirical approach—PeakTrace—that can (a) detect and follow source-specific hydropeaking waves in the downstream direction by using multiple hydrographs and (b) describe how to flow metrics of hydropeaking waves change along a river's course. In detail, PeakTrace first identifies associated flow events and then models translation and retention processes between neighboring hydrographs. Finally, the models can be combined to establish a non-linear hydropower plant-specific model. We demonstrate the PeakTrace method's usability in 16 Austrian case studies. The results underline the high performance of PeakTrace, describing the longitudinal development of flow metrics with high model accuracy up to 25 km or more. Ecologically-relevant metrics, such as rate of change or amplitude, decrease with distance from the hydropower outlet regarding down-ramping events; the same pattern can be observed for up-ramping events too, except for the rate of change for which an intensity increase may be observed, probably due to slope and the roughness difference between base flow and peak flow. Overall, this paper underlines the usability of PeakTrace as a basis to assess hydropower plant-specific hydro-ecological impacts and evaluate hydropeaking mitigation measures, especially by incorporating critical flow thresholds of river biota and life stages.     

Response of Fish Communities to Hydrological and Morphological Alterations in Hydropeaking Rivers of Austria

Climate change asks for the reduction in the consumption of fossil‐based fuels and an increased share of non‐regulated renewable energy sources, such as solar and wind power. In order to back up a larger share of these intermittent sources, ‘battery services’ are needed, currently provided only in large scale by hydropower, leading to more rapid and frequent changes in flows (hydropeaking) in the downstream rivers. Increased knowledge about the ecosystem response to such operations and design of cost‐effective measures is needed. We analysed the response of fish communities to hydropeaking (frequency, magnitude, ramping rate and timing) and the interaction with the habitat conditions in Austrian rivers. An index of biotic integrity (Fish Index Austria) was used to compare river sections with varying degrees of flow fluctuations under near‐natural and channelized habitat conditions. The results showed that habitat conditions, peak frequency (number of peaks per year), ramping rate (water level variation) and interaction between habitat and ramping rate explained most of the variation of the Fish Index Austria. In addition, peaking during the night seems to harm fish more than peaking during the day. Fish communities in hyporhithral and epipotamal types of rivers are more affected by hydropeaking than those in metarhithral type of rivers. The results support the findings of other studies that fish stranding caused by ramping rates >15 cm h^?1 are likely to be the main cause of fish community degradation when occurring more often than 20 times a year. While the ecological status degrades with increasing ramping rate in nature‐like rivers, fish communities are heavily degraded in channelized rivers regardless of the ramping rate. The mitigation of hydropeaking, therefore, requires an integrative approach considering the combined effects of hydrological and morphological alterations on fish. © 2014 The Authors. River Research and Applications published by John Wiley & Sons, Ltd.     

Classification of hydropeaking impacts on Atlantic salmon populations in regulated rivers

This article proposes and demonstrates a new classification system of fish population level effects of hydropeaking operations in rivers. The classification of impacts is developed along two axes; first, the hydromorphological effect axis assesses the ecohydraulic alterations in rivers introduced by rapid and frequent variations in flow and water level, second the vulnerability axis assesses the site-specific vulnerability of the fish population. Finally, the population level impact is classified into four classes from small to very large by combining the two axes. The system was tested in four rivers in Norway exposed to hydropeaking, and they displayed a range of outcomes from small to very large impacts on the salmon populations. The river with a relatively high base flow and ramping restrictions scored better than rivers with the lower base flow or limited ramping restrictions, indicating that hydropeaking effects can be mitigated while maintaining high hydropower flexibility. Most effect factors could easily be calculated from timeseries of discharge and water level, whereas the use of hydraulic models to estimate potential stranding areas may require more work. The vulnerability factors are mainly qualitative and depend more heavily on expert judgments and are thus more uncertain. The system was deemed suitable for the purpose of supporting management decisions for rivers exposed to hydropeaking operations. It evaluates the severity of the additional pressures due to hydropeaking operations and proved useful to identify mitigating measures. While the system was developed for Atlantic salmon river systems, it could be adapted to other species or systems.     

Nervous habitat patches: The effect of hydropeaking on habitat dynamics

Alteration in the river flow regime due to intermittent hydropower production (i.e., hydropeaking) leads to biodiversity loss and ecosystem degradation worldwide. Due to the increasing shear of volatile green energy (i.e., wind and solar), hydropeaking frequency is deemed to increase in the coming decades. However, our mechanistic understanding of how the frequency of repeated hydropeaking (i.e., series of multiple events) affects ecological processes is still limited. Here, we reflect on the impacts of altered flow frequency and relative duration on the persistency of aquatic habitats. We focus on the habitats at patch-scale, being this the scale representing what organisms perceive when interacting with their environment. With a showcase we explore a temporally explicit approach to quantify altered habitat dynamics at patch-scale due to hydropeaking. We then review how changes in habitat dynamics and persistency may affect ecological processes. Our findings suggest that (i) a time-series approach allows to account for the inherent multi-event nature of hydropeaking; (ii) hydropeaking can increase the dynamics of single habitat patches by at least one order of magnitude if compared to unregulated rivers; (iii) altered habitat dynamics at the patch scale can affect the survival of more sessile species and life cycle stages (e.g., invertebrates) or the energy budget of mobile species and life cycle stages (e.g., adult fish). However, the ecological significance and potential environmental thresholds of patch-scale dynamics and persistency are still poorly investigated and need further attention. Moreover, methods for the aggregation of habitat dynamics and persistency from the patch to the reach-scale are not available yet.     

Past hydraulics influence microhabitat selection by invertebrates and fish in hydropeaking rivers

Hydropeaking hydropower plants are the main source of renewable energy, meeting sub-daily peaks in electricity demand. They induce rapid artificial flow variations, highly variable velocities, drift, and stranding risks for aquatic organisms. In hydropeaking reaches, microhabitat selection likely depends on both present and past hydraulics (flow velocity and water depth); this study aims to assess their relative impact. For this purpose, we used observations of fish abundance in 1,180 microhabitats (507 sampled by electrofishing, 673 by snorkeling) and of invertebrate abundance in 36 microhabitats (hyporheic and benthic) in a medium-sized hydropeaking river. We described past hydraulics of microhabitats over the 15 days preceding sampling, using a 2D hydrodynamic model, by identifying microhabitats dewatering (drying during >10 hr) or with high-velocity conditions (>1.3 m s⁻¹ during >10 hr). Invertebrates guilds (defined based on their selection of present hydraulics in rivers without hydropeaking) responded significantly to past hydraulics, with abundances 3.5–15.3 times lower in dewatering habitats. Selection for present hydraulics by invertebrates was different from that observed in rivers without hydropeaking. For more mobile fish, responses were weaker and different, with a “bank” guild selecting dewatering microhabitats and, secondarily, a “midstream” guild avoiding them. Selection of present hydraulics by fish was similar to that observed in rivers without hydropeaking. Overall, past hydraulics influenced microhabitat selection, with stronger effects on invertebrates and stronger effects of dewatering than of high past velocities. However, high past velocities force fish to move and invertebrates to experience a large range of velocity.     

A probability-based model to quantify the impact of hydropeaking on habitat suitability in rivers

A negative effect of hydropower on river environment includes rapid changes in flow and habitat conditions. Any sudden flow change could force fish to move towards a refuge area in a short period of time, causing serious disturbances in the life cycle of the fish. A probability-based model was developed to quantify the impact of hydropeaking on habitat suitability for two fish species, brown trout (Salamo trutta) and Grayling (Thymallus thymallus). The model used habitat preference curves, river velocity and depth to develop the suitability maps. The suitability maps reveal that habitat suitability deteriorates as flow increases in the studied part of the river. The probability model showed that, on average, suitability indices are higher for adult grayling than juvenile trout in hydropeaking events in this part of the river. The method developed shows the potential to be used in river management and the evaluation of hydropeaking impacts in river systems affected by hydropower.     

Hydropeaking in small hydropower in Norway—Compliance with license conditions?

Hydropeaking involves rapid changes in water flow, causing negative effects on aquatic organisms. To reduce these effects, hydropower licenses contain restrictions on hydropeaking. The owners of a hydropower plant are obliged to follow the requirements in the license. This study was conducted to understand the effect of environmental policies in small hydro licenses. We studied the occurrence of hydropeaking by analysing 256 high‐head, small Norwegian hydropower plants in relation to a set of characteristics that define each plant. We selected plants that were online during 2005–2014 and chose 2015 as a year for hourly production patterns. We counted the number of starts, that is, the moment when the hydro facility starts to produce energy, as an indicator of hydropeaking. Among the analysed plants, 75.8% had restrictions to practice hydropeaking. Production typically showed daily, weekly, and annual cycles though shapes varied depending on turbine type and location. The number of starts was independent on having or not having a license to practice hydropeaking. The average duration of periods with production and the average production in these periods were higher for plants without restrictions. The number of starts varied widely among plants. The number of starts was lower for installed Pelton turbines than for Francis and Kaplan turbines, which was in agreement with the characteristics of the turbines. Overall, these results show that hydropeaking restrictions given in the licenses are rarely followed. The high number of starts may be explained by failure in the grid system, improper maintenance, and financial interests. Their relative importance should be subject to further investigation.     

Multi‐scale Approach to Hydrological Classification Provides Insight to Flow Structure in Altered River System

Rivers are hierarchical systems exhibiting processes and patterns across spatial and temporal scales principally driven by changes in flow. Hydrological indices estimated with mean or median daily flow data (i.e. daily scale) may be insensitive to anthropogenic alteration that imparts sub‐daily variation to flow. Therefore, indices developed at multiple temporal resolutions may provide additional insight into the presence of flow patterns masked by traditional techniques. We characterized the flow regime along the longitudinal gradient of the Platte River, a large Great Plains USA river, using hydrological indices derived with mean daily and sub‐daily flow data and a combination of multivariate statistical techniques. Three unique flow units were evident using daily scale flow data, whereas six unique flow units were evident at the sub‐daily scale. Flow units at both scales were not static, but rather the presence and extent of flow units across the riverscape depended on climate, tributary inflows and human influence. Anthropogenic alteration including hydropeaking was evident at the sub‐daily scale but not at the daily scale. The full complement of flow structure within regulated rivers, therefore, may not be captured using mean or median daily discharge values alone. Inductive river classification studies may benefit from assessing hydrological indices at multiple scales, particularly when investigating river systems with anthropogenic modification such as hydropeaking. Copyright © 2016 John Wiley & Sons, Ltd.     

Response by fish assemblages to an environmental flow release in a temperate coastal Australian river: A paired catchment analysis

Defining appropriate environmental flow regimes and criteria for the use of environmental water allocations requires experimental data on the ecological impacts of flow regime change and responses to environmental water allocation. Fish assemblages in one regulated and one unregulated tributary paired in each of two sub‐catchments of the Hunter River, coastal New South Wales, Australia, were sampled monthly between August 2006 and June 2007. It was predicted that altered flow regime due to flow regulation would reduce species richness and abundance of native fish, and assemblage composition would differ between paired regulated and unregulated tributaries. Despite significant changes in richness, abundance and assemblage composition through time, differences between regulated and unregulated tributaries were not consistent. In February 2007, an environmental flow release (‘artificial flood’) of 1400 ML was experimentally released down the regulated tributary of one of the two catchments over 6 days. The flow release resulted in no significant changes in fish species abundances or assemblage composition when compared to nearby unregulated and regulated tributaries. Flow regulation in this region has reduced flow variability and eliminated natural low‐flow periods, although large floods occurred at similar frequencies between regulated–unregulated tributaries prior to and during 2006–2007, resulting in only moderate changes to regulated flow regimes. Barriers to dispersal within catchments also compound the effects of flow regulation, and findings from this study indicate that the location of migratory barriers potentially confounded detection of the effects of flow regime change. Further experimental comparisons of fish assemblages in regulated rivers will refine river‐specific response thresholds to flow regime change and facilitate the sustainable use of water in coastal rivers. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of hydropeaking on drift,stranding and community composition of macroinvertebrates: A field experimental approach in three regulated Swiss rivers

Hydropeaking operation leads to fluctuations in wetted area between base and peak flow and increases discharge-related hydraulic forces (e.g. flow velocity). These processes promote macroinvertebrate drift and stranding, often affecting benthic abundance and biomass. Our field experimental study—conducted in three hydropeaking-regulated Swiss rivers—aimed to quantify (a) the short-term effects of the combined increase in flow amplitude and up-ramping rate based on macroinvertebrate drift and stranding, as well as (b) long-term effects based on the established community composition. Hydropeaking led to increased macroinvertebrate drift compared to base flow and to unaffected residual flow reaches. Moreover, stranding of macroinvertebrates was positively related to drift, especially during the up-ramping phase. Flow velocity and up-ramping rate were identified as major determinants for macroinvertebrate drift, while flow ratio and down-ramping rate for stranding. Particularly high sensitivity towards hydropeaking was found for Limnephilidae, whereas Heptageniidae seemed to be resistant in respect to short- and long-term hydropeaking effects. In the long-term, hydropeaking did not considerably reduce benthic density of most taxa, especially of some highly resistant and resilient taxa such as Chironomidae and Baetidae, which dominated the community composition even though they showed comparably high drift and stranding responses. Therefore, we argue that high drift and/or stranding, especially of individual-rich taxa, does not necessarily indicate strong hydropeaking sensitivity. Finally, our results demonstrate the necessity to consider the differences in river-specific morphological complexity and hydropeaking intensity, since these factors strongly influence the community composition and short-term drift and stranding response of macroinvertebrates to hydropower pressure.     

THE DUAL NATURE OF HYDROPEAKING RIVERS: IS ECOPEAKING POSSIBLE?

Philosophically, the natural flow regime concept is tremendously appealing; however, its application can be challenging for many biologists without the expertise or resources to handle such approaches on their own. This is particularly true on hydropeaking rivers, where incorporating natural flow is sometimes challenging. Additional challenges include our limited understanding of how individual flow components relate to geomorphic and ecological processes. Supplementary to environmental flow approaches is understanding that many hydropeaking rivers are ecologically two different rivers in one: the low flow and high peaking flow. Taxa that require a narrow range of water velocities or cannot withstand rapid changes in discharge would likely be eliminated or competitively disadvantaged under such harsh environmental conditions. As the low and peak flows diverge, the two rivers become increasingly different ecologically, and there will likely be fewer taxa that can withstand such abiotic variability. Deviations from a natural flow regime may result in new constraints on certain fishes and invertebrates, but this does not necessarily mean a loss of productive fish habitat. Viewing hydropeaking rivers as two rivers in one and the risks associated with high to low flow ratios may serve as a more practical and useful perspective towards maintaining altered yet productive rivers while representing a step towards improving the management river ecosystems. © Her Majesty the Queen in Right of Canada 2013.     

Hydropeaking effects on movement patterns of brown trout (Salmo trutta L.)

Radiotelemetry was used to investigate seasonal movement and home range of brown trout Salmo trutta (size range 188–420 mm fork length, N = 30) in two reaches of the Noguera Pallaresa River (Ebro Basin, north‐east Spain) subjected to different flow regulation schemes. NP‐1 reach is a bypassed section with near natural flow conditions, whereas the downstream reach NP‐2 is subjected to daily pulsed flow discharge (i.e., hydropeaking) from an upstream hydropower station. Significant differences in home range size (95% kernel estimates) and seasonal movement pattern between study reaches were found. Mean home range size was (μ ± SE) 112.1 ± 11.5 m in the bypassed reach NP‐1 and increased significantly in the hydropeaking reach NP‐2 up to 237.9 ± 37.2 m. There was a large individual variability in fish home range size within reaches. Most of the seasonal differences in fish movement among reaches were associated with the spawning season. Pulsed discharge events in NP‐2 during daytime in summer (lasting about 3 hr and increasing water flow from 1 to 20 m³/s) did not cause significant displacements in either upstream or downstream direction during the duration of the event. Our results highlight the importance of habitat connectivity in hydropeaking streams due to the need of brown trout to move large distances among complementary habitats, necessary to complete their life cycle, compared with unregulated or more stable streams.