Operation of Complex Hydropower Schemes and its Impact on the Flow Regime in the Downstream River System under Changing Scenarios

Hydropower is the world’s most important renewable electricity source. More than 40% of European hydroelectric energy is produced in Alpine countries. High-head storage hydropower plants (HPP) contribute significantly to peak energy production as well as electricity grid regulation. Future plant management is faced with several challenges concerning modified availability of water resources due to climate change as well as new economic constraints associated with legal, political and electricity market issues. HPP operation results in unsteady water release to the downstream river system. Hydropeaking is the primary factor of flow regime alteration, impacting the river ecosystem. Even when the biological response to hydropeaking is not fully understood, the recently adapted law on water protection prescribes its mitigation in Switzerland. In this research project, a novel integrative approach to model and assess the impact of the operation of a complex hydropower scheme on the downstream river system is developed. It contains (1) a precipitation-runoff model extended for long-term simulations of glacierized Alpine catchment areas, (2) an operation tool for high-head storage HPP, (3) flow regime generation with cost estimation of hydropeaking mitigation measures and (4) a habitat model of reference river morphologies for a target species. The upper Aare River (Hasliaare) in Switzerland is an Alpine stream, affected by hydropeaking from a complex hydropower scheme with several storage volumes and power houses. Since the 1930s, seasonal water transfer from summer to winter and the amplitude and frequency of daily peak discharge have been continuously increased. Furthermore, the dynamic braided river network with various mesohabitats gave way to a mainly monotonous channel. Although diversity of species and biomass of aquatic biota have drastically decreased, the potential of redevelopment remains. Investigations to improve the river morphology and the flow regime are under discussion. The upper Aare River catchment is therefore an appropriate case study for analysis of the interactions between climatic, hydrological, hydraulic, economic as well as ecological parameters. The simulation of runoff in Alpine catchment areas is essential for optimal hydropower exploitation under normal flow conditions, but also for the analysis of flood events. The semi-distributed conceptual modeling approach Routing System contains a reservoir-based precipitation-runoff transformation model (GSM-SOCONT), extended by dynamic glacier simulation tool. Spatial precipitation and temperature distributions are taken into account for simulating the relevant hydrological processes, such as glacier melt, snowpack constitution and melt, soil infiltration and runoff. The model development, calibration and validation are illustrated for the 2005 flood event, where the flood reduction capacity of the HPP is discussed, as well as future long-term runoff estimations. Climate change scenarios, based on a reference climate period, take into account intra-annual temperature and precipitation variations as well as their long-term tendencies. Runoff series of daily resolution are produced by hourly updating of the meteorological, glaciological and hydrological parameters. An almost complete deglacierization of the upper Aare River basin is simulated for the late 21st century. The resulting reduction of glacier melt in summer and earlier snowmelt in spring change the runoff regime from glacio-nival to nival. The implemented heuristic hydropower modeling tool in Routing System allows simulation of the operating mode of complex HPP. Within the case study of the upper Aare River catchment and despite the complexity of the HPP network, the influence of climate change, electricity market issues, plant enhancements as well as hydropeaking constraints is simulated and assessed. Despite the reduction of future runoff, increased flexibility due to new turbine and pumped-storage capacities allows compensation, especially in the case of volatile electricity prices, and could even partially restore the natural flow regime. Several operational and construction measures to reduce hydropeaking are implemented in the model. Resulting flow regimes as well as the related costs are defined. Operational constraints, such as limitation of turbine discharge, increase of residual flow or limited drawdown range, generate relatively high costs compared to their environmental effectiveness. Better ecological and economic response is achieved by construction measures, such as flow deviation systems or compensation basins installed downstream of the power house outflow where the water is temporarily stored and then released to the river by a guided system. The simulated flow regimes are rated by a river specific habitat model for representative morphologies and three life stages of the target species brown trout (Salmo trutta fario). This is based on results from a 2D hydrodynamic model and in situ investigations undertaken in the framework of a joint project of EAWAG. Steady and dynamic indicators quantify fish habitat suitability and allow comparison through economic indices of the implemented mitigation measures. For the Hasliaare River, investments for mitigation of hydropeaking are only justified by morphological improvements. The developed approach is useful for the enhancement of complex storage hydropower schemes regarding mitigation of altered flow regimes. Despite several uncertainties, it allows operators, authorities and researchers to define and rate the impact of HPP operation on the river network, to ecologically and economically assess mitigation measures and thus to address hydropeaking in a straightforward manner.

Operation of complex hydropower schemes and its impact on the flow regime in the downstream river system under changing scenarios

Martin Peter Bieri

EPFL - LCH2012

Mitigation measures for fish habitat improvement in Alpine rivers affected by hydropower operations

Martin Peter Bieri, Émilie Person, Armin Julius Peter, Anton Schleiss

In mountainous areas, high-head-storage hydropower plants produce peak load energy. The resulting unsteady water release to rivers, called hydropeaking, alters the natural flow regime. Mitigating the adverse impacts of hydropeaking on aquatic ecosystems has become a crucial step in recent water policies. We developed a novel economic-ecological diagnostic and intervention method to assess hydropeaking mitigation measures for fish habitat improvement. This method was applied to an Alpine river downstream of a complex storage hydropower scheme. The approach comprises (1) a hydropower operation model of flow regime generation and cost estimates for different mitigation measures, (2) a two-dimensional hydrodynamic model to simulate the flow conditions in representative river reaches and (3) a dynamic fish habitat simulation tool to assess the sub-daily changes in habitat conditions of three brown trout (Salmo trutta fario) life stages (adult, spawning and young-of-the-year). Simulations showed that operational measures such as limiting maximum turbine discharge, increasing residual flow and limiting drawdown range incur high costs in relation to their ecological effectiveness. Compensation basins and powerhouse outflow deviation achieved the best cost–benefit ratio. Hydropeaking impact was strongly dependent on river morphology. Monotonous river reaches exhibited low habitat suitability for peak discharge, whereas a braided morphology provided high in-stream structure and thus suitable habitat for unsteady flow conditions. The interdisciplinary approach to economic and habitat rating informs decision makers regarding the effectiveness of measures implemented to mitigate the environmental impacts associated with fluctuating hydropower operations. Copyright © 2013 John Wiley & Sons, Ltd.

John Wiley and Sons2014

Impact of hydropeaking on fish and their habitat

Émilie Person

High-head storage hydropower plants are an important renewable source of energy in alpine areas. Kinetic energy released from water, which is stored in reservoir and diverted through turbines, produces electricity. During daily peaks of energy demand, the powerhouse outflow is released in the downstream river, creating artificial flow fluctuation, so-called hydropeaking. This alters the natural flow regime of rivers and has a negative effect on ecosystems and biodiversity. As a result, water discharge, temperature, fine particle load and other abiotic factors are changed. Consequently, river organisms and their habitat are impacted. Resulting from an increased pressure for atmospheric carbon mitigation, hydropower production is expected to increase in the future (e.g. from storage powerplants). In Switzerland this trend is further enhanced by the recent governmental decision to phase out nuclear power production. Thought, the revision of the Swiss water protection act shows the growing awareness to protect natural ecosystems downstream of hydropower facilities. However, there is a strong need for research in this field due to the lack of knowledge on the adverse impacts stemming from hydropower production. This study is part of the interdisciplinary research project “Sustainable use of hydropower – innovative measures to reduce hydropeaking problematic” and it focuses on the impact of hydropeaking on fish and their habitat. Brown trout is used as a target species and important steps in their life cycle are studied. Three target stages of brown trout development were selected: adults, spawning and young-of-the-year. Each different life stage has specific habitat requirements. The latter can be used for identifying potential landscape filters constraining fish population renewal. Landscape filters are determined by the joint influence of river morphology and discharge regimes, such as hydropeaking. In this work, two rivers, with different morphological characteristics, are studied, namely the Vorderrhein and the Hasliaare Rivers. Both rivers show a hydropeaking regime, are situated in alpine areas and have a comparable hydrological regime. The hydrology of the two rivers is characterized by low discharge in winter and high discharge in summer due to snowmelting. Fish species composition is similar and strongly dominated by brown trout. The Vorderrhein River is one of the few natural and morphologically intact rivers found in Switzerland, which allows to isolate the effect of hydropeaking from other potential human-induced stressors. In contrast, the Hasliaare River has been strongly channelized in the past century. Thus, the Hasliaare River system was chosen to investigate the joint effect of hydropeaking and river channelization. In the Vorderrhein River, the seasonal impact of hydropeaking on adult brown trout habitat was modeled using the CASiMiR fish module. Therefore, different critical seasons are defined. Furthermore, the natural reproduction success was assessed and brown trout reproduction and rearing habitat were modeled. Habitat preference of spawning and young-of-the-year are established with specific Habitat Suitability Curves. The habitat model was adapted to the hydropeaking problem and indices measuring habitat dynamics were developed. Moreover, the transferability of Habitat Suitability Curves in habitat models was discussed. In addition, young-of-the-year density as well as egg to hatch survival were monitored. The results show that hydropower operations have an effect on brown trout habitat, whereat spawning and young-of-the-year life stages are more impacted than adults. The impact is seasonal and aggravated in winter. The natural river morphology provides suitable habitat areas at both peak and off-peak discharges. Although these suitable habitat areas are dewatered almost entirely or displaced on a daily basis. In the Hasliaare River system, the joint effect of hydropeaking and channelization on young-of-the-year, lake and stream resident spawning brown trout were studied. Steady and dynamic habitat conditions were evaluated and the habitat was modeled for three different degraded morphologies. Specific preference curves for each investigated life stage were developed. Moreover, the reproduction success was monitored by egg to hatching survival experiments and young-of-the-year density surveys. The results show that channelization aggravates the impact of hydropeaking as no young-of-the-year or spawning habitat is present at peak flow. In addition, egg development was found to be impaired. Therefore, the density of young-of-the-year individuals was negligible in the hydropeaking section. The habitat model shows that in a channelized river such as the Hasliaare River, suitable habitat conditions for fish are restrained at peak flow by the riverbed width. Finally, a tool for evaluating scenarios for mitigating the impact of hydropeaking on the downstream ecosystem was developed. The novel economic-ecological diagnostic and intervention method takes into account financial as well as environmental outcomes of hydropeaking mitigation measures for fish habitat improvement. The approach comprises (1) a hydropower operation model of flow regime generation and cost estimates for different mitigation measures, (2) a 2D hydrodynamic model to simulate the flow conditions in representative river reaches, and (3) a dynamic fish habitat simulation tool to assess the sub-daily changes in fish habitat conditions. This modeling approach gives the possibility to estimate true benefits of rehabilitation measures. The intervention diagnostic method was tested on the Hasliaare River. The developed tools and knowledge will help implement scientifically-based solutions for a sustainable hydropower management. The study may help in supporting the application of river restoration projects at existing and newly developed hydropower facilities in alpine areas.

EPFL - LCH2013