Author(s): Lennart Schonfelder; Tor Haakon Bakken; Knut Alfredsen

Linked Author(s): Lennart Hagen Schönfelder, Knut Alfredsen

Keywords: No Keywords

Abstract: The Norwegian hydropower system is complex, in many cases several reservoirs and power plants are interconnected, sometimes across catchment boundaries. A common plant layout is comprised of a reservoir and an underground penstock leading to a downstream located power plant, often fairly distant from the reservoir. Release flows from dams are in many cases constant for extended periods, and a large share of the original discharge in summer and fall is not available in bypassed sections between the reservoir and the hydropower outlet. As a result, the flows in affected river reaches are characterized by reduced water quantity and flow dynamics. However, flow characteristics change along a longitudinal gradient due to tributaries, and water quantity increases and flow dynamics are altered. Low flow conditions and the lack of freshets in extended river segments can be a major obstacle for successful migration and spawning of salmonids. In other parts of the river system affected by the regulation, such as downstream the outlet of the hydropower plants, the change of the natural flow variations can introduce reduction in environmental qualities, even though the annual mean flow is unchanged from before the regulation. In those cases, the hydropower plant is used for hydro-peaking operations, the receiving waters can experience more rapid and frequent changes in water fl ow and water level than in natural hydrological conditions. All these changes might pose stress to the ecosystem depending on the degree of change and the affected ecosystem. The objective of this study was to quantify the deviation from natural flow condi tions using the numerical precipitation-runoff model HYPE. The multi-basin model was forced by gridded precipitation data synthesized from rainfall gauges and weather radar and gridded temperature time series. Two variations of the set-up were realized: the first used historical discharge data of unregulated catchments for calibration and validation. A parameter transfer based on land use and soil properties to ungauged catchments enabled the generation of reference conditions for river segments in the whole model domain. In the second model set-up, we added a module for hydropower operation. The module uses storage capacity and historical release flows of reservoirs. To assess the impact of hydropower production, we used a set of Indicators for Hydrological Alteration (IHA) for river segments. The comparison of natural vs. regulated conditions in regulated rivers helps water managers to classify alterations and detect ecological bottlenecks. Furthermore, recommendations for dynamic flow releases based on the natural system can be deducted. Preliminary results show that runoff regimes of bypassed river sections are generally more altered than those of rivers that are affected by hydropower in a more indirect way, e. g. river sections that are downstream of the hydropower plant outlet, where the timing of water flow is altered and total flow quantity remains natural. They further reveal that low discharges are underestimated by the model and calibration strategies specifically for low flow conditions are needed.

DOI:

Year: 2018