Dynamics of Dewatering and Flooding during Hydropeaking

Abstract

Hydropeaking in rivers is likely to become more frequent in a context of increasing demand for renewable energy due to the storage potential of hydropower systems. Increased peaking operation in hydropower plants will lead to more frequent and more rapid flow changes and to the dewatering and flooding of rivers. General knowledge regarding the environmental effects of hydropeaking exists, but further research is needed to fully understand the consequences of such fluctuations on an individual case basis for the development of targeted mitigation strategies. The present thesis aims to fill some knowledge gaps by focusing on the physical processes occurring during wetting and drying and their consequences for ecology. The approach is to investigate the small-scale biological and physical processes occurring during hydropeaking and to integrate the findings with alternative hydropower production opportunities at the larger scale.

At the micro-scale, the hyporheic zone is altered by hydropeaking with respect to changing flow rates, water levels, durations and temperatures, with differences being captured during individual events, seasons and flooding and dewatering processes. Such physical processes play an important role on Atlantic salmon embryo survival, which was unexpectedly high in the ramping zone but was still lower in comparison to permanently wet areas of the hydropeaked Lundesokna River. With no water quality issues and very low inputs of fine sediments, exposure to dry conditions due to production stops and below-zero air temperatures were the main drivers of mortality in the ramping zone. Survival was extensively explained by the extent of subsurface influx, keeping the eggs from dry and frost conditions. Findings were supported by experiments carried out in the seasonally regulated river Suldalslågen, with a long winter drawdown.

At the meso-scale, specific guidelines for accurately predicting potential stranding areas were established by analysing four different river morphologies in the Lundesokna River. The optimal geometry effort (number of cross sections) was found to not necessarily occur at the maximum but varied depending on sinuosity and channel complexity. In the permanently wet area, a cost-effective methodology to predict dynamic mesohabitats was developed by using existing HMU classification methods. Although further development is needed, this method showed promising results for the prediction of dynamic mesohabitats at the river scale.

The above findings were finally integrated through a linking methodology, using the findings on ecological processes at the micro-scale and on physical habitat at the meso-scales to establish alternative hydropower management options at the catchment scale. The connections within this framework were successfully established through a one-dimensional hydraulic model and a hydropower operation program, providing a method to integrate detailed knowledge of environmental processes into alternative hydropower operational planning.