Steffen Schweizer

A conceptual framework for hydropeaking mitigation.

Hydropower plants are an important source of renewable energy. In the near future, high-head storage hydropower plants will gain further importance as a key element of large-scale electricity production systems. However, these power plants can cause hydropeaking which is characterized by intense unnatural discharge fluctuations in downstream river reaches. Consequences on environmental conditions in these sections are diverse and include changes to the hydrology, hydraulics and sediment regime on very short time scales. These altered conditions affect river ecosystems and biota, for instance due to drift and stranding of fishes and invertebrates. Several structural and operational measures exist to mitigate hydropeaking and the adverse effects on ecosystems, but estimating and predicting their ecological benefit remains challenging. We developed a conceptual framework to support the ecological evaluation of hydropeaking mitigation measures based on current mitigation projects in Switzerland and the scientific literature. We refined this framework with an international panel of hydropeaking experts. The framework is based on a set of indicators, which covers all hydrological phases of hydropeaking and the most important affected abiotic and biotic processes. Effects of mitigation measures on these indicators can be predicted quantitatively using prediction tools such as discharge scenarios and numerical habitat models. Our framework allows a comparison of hydropeaking effects among alternative mitigation measures, to the pre-mitigation situation, and to reference river sections. We further identified key issues that should be addressed to increase the efficiency of current and future projects. They include the spatial and temporal context of mitigation projects, the interactions of river morphology with hydropeaking effects, and the role of appropriate monitoring to evaluate the success of mitigation projects.

Evaluation of mitigation measures to reduce hydropeaking impacts on river ecosystems – a case study from the Swiss Alps

New Swiss legislation obligates hydropower plant owners to reduce detrimental impacts on rivers ecosystems caused by hydropeaking. We used a case study in the Swiss Alps (hydropower company Kraftwerke Oberhasli AG) to develop an efficient and successful procedure for the ecological evaluation of such impacts, and to predict the effects of possible mitigation measures. We evaluated the following scenarios using 12 biotic and abiotic indicators: the pre-mitigation scenario (i.e. current state), the future scenario with increased turbine capacity but without mitigation measures, and future scenarios with increased turbine capacity and four alternative mitigation measures. The evaluation was based on representative hydrographs and quantitative or qualitative prediction of the indicators. Despite uncertainties in the ecological responses and the future operation mode of the hydropower plant, the procedure allowed the most appropriate mitigation measure to be identified. This measure combines a basin and a cavern at a total retention volume of 80,000m3, allowing for substantial dampening in the flow falling and ramping rates and in turn considerable reduction in stranding risk for juvenile trout and in macroinvertebrate drift. In general, this retention volume had the greatest predicted ecological benefit and can also, to some extent, compensate for possible modifications in the hydropower operation regime in the future, e.g. due to climate change, changes in the energy market, and changes in river morphology. Furthermore, it also allows for more specific seasonal regulations of retention volume during ecologically sensitive periods (e.g. fish spawning seasons). Overall experience gained from our case study is expected to support other hydropeaking mitigation projects.

Hydropeaking mitigation project on a multi-purpose hydro-scheme on Valsura River in South Tyrol/Italy.

A hydropeaking mitigation project on Valsura River in the Italians Alps is described. The project is of particular interest due to several aspects. First of all, the Valsura torrent has unique morphological braiding characteristics, which are unique in the reach of Adige valley between Merano and Bolzano, and has a good reproduction potential for fish, especially in the terminal stretch along a biotope before its confluence with Adige River. Moreover, the Valsura hydropower cascade, which overall consists of six high-head hydropower plants, has an exceptional economic importance for the local hydropower industry. Lastly, the last HPP on the cascade is a multipurpose plant, so that interesting interactions between hydropeaking mitigation, irrigation supply and peak energy production are considered. The project started from a hydrological and a limnological measuring campaign and from an energetic, hydraulic and legislative framework analysis. The ecological findings are combined into a deficit analysis, founding the basis for the definition of a hydrological target state, which points to achieve a good natural reproduction for brown trout in the hydropeaked stretch, fulfilling at the same time the human safety conditions. Finally, mitigation Measures are described that at the same time comply with the following manifold aspects: a. maintenance of the requested target limits for fish reproduction; b. maintenance of the water release for the agricultural irrigation; c. enhancement of the flexibility of the hydropower plants operation; d. reduction of the risk for local population. The paper compares operational and constructive mitigation measures and shows that constructive hydropeaking mitigation measures, for the present case study, can combine the positive effects of ecological improvement with higher safety standards and more flexible energy production.

Projekte und Praxis

Im Rahmen der Entleerung des Stausees Raterichsboden (Berner Oberland, Schweiz) wurden verschiedene gewasserokologische Schutzmasnahmen realisiert. Fur den Erhalt des Seeforellenjahrgangs 2014/15 entwickelte die Fachstelle Okologie der Kraftwerke Oberhasli AG ein mobiles fischschonendes Leitsystem fur aufsteigendeWandersalmoniden, um die laichbereiten Seeforellen in ein Nebengewasser umzuleiten. Im Praxistest konnte sowohl die okologische Funktionsfahigkeit als auch die Hochwassersicherheit nachgewiesen werden. Das Leitsystem konnte auch fur andere fischokologische Fragestellungen adaptiert werden.

Fischlifte und Fischschleusensysteme

Die Herstellung der okologischen Durchgangigkeit war Auflage der Genehmigungsbehorde fur den Weiterbetrieb der Kraftwerke Hollenstein und Pulling am Schwarzen Regen im Bayerischen Wald. Der Betreiber, die Kraftwerk am Hollenstein AG, entwickelte fur die Talsperre Hollenstein eine neuartige Druckkammerfischschleuse mit energetischer Nutzung. Dadurch brachte man Okologie und Okonomie in Einklang. Diese Fisch aufstiegs anlage eignet sich fur Talsperren und Wehrkraftwerke mit groser Fallhohe und schwankenden Oberwasserpegel wie z. B. bei Hochwasserruckhaltebecken. Die Ergebnisse des Mo ni to ring werden in einem gesonderten Artikel in diesem Heft der Fachzeitschrift Was ser Wirtschaft vorgestellt.

Schwall und Sunk — ein kurzer Überblick

Im vorliegenden Artikel wird versucht, den aktuellen Wissenstand zu den Auswirkungen von Schwall und Sunk auf die aquatische Gemeinschaft zusammenzufassen. Künstliche Pegelschwankungen verursachen kurzund langfristige Auswirkungen auf die Fließgewässerökologie. Auf Basis von Indikatoren lassen sich diese Auswirkungen bis zu einem gewissen Grad abschätzen und daraus möglichst e ziente Sanierungsmaßnahmen ableiten. Abschließend wird der Stand der Schwallsanierung in der Schweiz kurz beschrieben.