A modelling framework to support environmental design in regulated rivers
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Despite the fact that environmental design and mitigation measures in regulated rivers have been widely applied in the last decades, yet it is still a challenge to find a method that supports a sustainable balance between the societal needs (e.g. electricity production, recreation) and fulfil the ecological requirements (e.g. Good Ecological Status or Good Ecological Potential defined in the Water Framework Directive). This thesis aims to fill some of the remaining gaps of knowledge related to the lack of quantitative data and uncertainties associated with the implementation of mitigation measures. The following detailed objectives were defined aiming to use modelling strategies to find a sustainable balance between wild salmonids, energy production and stakeholder interests in regulated rivers. This approach was closely related to the environmental design strategy developed in the CEDREN project (Forseth and Harby, 2014): Objective 1. To identify the main hydrological impacts from hydropower and, to provide a hydrological classification to support future mitigation measure decisions at a national scale. Objective 2. To assess environmental impacts from hydropower flow regulation and testpotential mitigation measures by applying an integrated method. Objective 3. To combine remote sensing techniques with hydraulic models to identify impacts,plan and evaluate the success of instream mitigation and restoration measures. The thesis first provides an overview of hydrological impacts from hydropower alteration at a national scale in Norway. As an initial investigation, a large-scale assessment of impacts was undertaken. Paper I contains a hydrological classification across Norway based on 48 hydrological indicators. Results show typical impacts from different hydropower systems and regulation strategies. Approaches for reducing the number of hydrological indices show that the main classification of rivers can be achieved with a reduced set of variables. Results also showed that the degree of regulation can be used as a first indicator of expected impacts. To further study impacts and relevant mitigations from hydropower alterations, an integrated method was developed to combine various modelling and assessment tools to study the tradeoffs between the production of Atlantic salmon (Salmo salar) smolts and the production of hydropower. Paper II describes a study carried out in the river Mandalselva to assess how different combinations of minimum flow and habitat measures influenced both Atlantic salmon production and hydroelectric power production. The main finding was that it is possible to achieve similar production of smolts combining a reduced environmental flow release with habitat measures as obtained with the current proposed environmental flow regime. The study also showed how the use of a modelling-based methodology to define a targeted environmental flow can be utilized to successfully balance the sometimes conflicting requirements of both an effective management of a salmon population and hydropower production. The integrated method described in paper II was applied in the river Ljungan in Sweden in Paper III to identify and better understand bottlenecks in an Atlantic salmon population and provide the basis for mitigation measures to overcome these effects. The main findings in Paper III show that low winter and summer flow are possible bottlenecks, with the highest negative population effects from low winter flows, particularly if dewatering of spawning areas are occurring. Based on the findings from Paper III, the potential of using modelling tools to investigate habitat measures was undertaken. Paper IV evaluates the success of mitigation measures using changes in physical variables and habitat suitability as an index of quality. Bathymetric data derived from LiDAR was used to build a high-resolution two-dimensional model of the existing river morphology, which is a foundation for the hydraulic simulation of changes to the river structure. The main findings show that the instream measures meet the target for what they were designed for and should therefore have a potential to be effective mitigation measures. Modelling and precise mapping of mitigation measures also supports the decision making within the stakeholder's group. The advent of high precision remote sensed river bathymetry opens possibilities to study effects of changes in flow on a fine scale. Paper V uses detailed LiDAR based river morphology together with 2D hydraulic modelling to analyse the impact of hydro peaking in the river Storåne in Norway. The main finding showed that the Storåne river is impacted by hydropeaking, with both dried areas and dewatering rate characterized as large impacted under the current operation procedures. In addition, an alternative scenario operation and its cost, which can reduce the impact of the peaking operation, was suggested. The modelling based approach applied in this study, showed that the fine resolution grid provides new opportunities in assessing effects of hydropower regulations on the ecosystem. The main findings from the papers presented in this thesis demonstrated that it is possible to use modelling techniques to support hydrological related mitigation measures, and to find a balance between salmon production and energy production as well as to include stakeholders' interests. In addition, modelling techniques together, with high resolution remote sensing data, gave the opportunity to identify impacts, plan and to evaluate the success of instream mitigation and restoration measures.