Combining water supply and flood mitigation requirements in multi-purpose reservoir optimization

Optimising the planning and management of reservoir systems by integrating multi-purpose objectives has been the subject of extensive research work since the 1990s, especially when the multiple benefits of reservoir system operation (e.g., supplying demand systems) and the reduction of natural risks (e.g., flood mitigation) are to be combined. The issue is frequently simplified by considering the second requirement (flood mitigation) as a pre-defined constraint working on system operation optimisation and, therefore, modelling for the first requirement (design and management of multi-purpose reservoirs for water supply). The European Flood Directive 2007/60/CE requires that flood-risk evaluation should be in agreement with a cost-benefit analysis and a rational decision-making tool must be used for optimising the flood mitigation system. The study background underlines the importance of the water depth-damage functions as one of the elements of a decision-making tool designed to evaluate the economic damage expected in flood-prone areas. The assessment and evaluation of potential flood damages in quantitative terms need to reconsider existing flood risk management plans to endorse their approach to reducing potentially damaged areas. This requirement has to be considered in areas affected by heavy flood events, such as Mediterranean areas. In the proposed combined approach WARGI decision support system (DSS) simulates the management of multireservoirs and multi-user systems, considering the resource priorities and users’ preferences in the light of hydrological deficiencies and, consequently, water scarcity conditions. A link between the WARGI simulation and hydrologic and hydraulic models defining the vulnerability of flood-prone areas has been constructed in order to verify flood damage reductions and their economic evaluation. The link is used to quantify cost-benefit relations to obtain a rational decision-making optimisation tool. A significant test case has been developed by applying the proposed methodology on the island of Sardinia (Italy). In this region several reservoirs have been built mainly for water supply and hydropower generation. The Regional Flood Mitigation Plan requires the assessment of the impacts of possible changes in reservoir management and the construction of new infrastructures for flood damage reduction. The River Coghinas has been investigated with regard to the interactions of flood mitigation requirements and upstream reservoir management. The river is the pilot basin for the Sardinian Regional Administration’s development of the flood risk management plans required by EU and national legislation. In this pilot basin regulation reservoirs are located in the upstream river, and in the downstream floodplain area residential, touristic, commercial, industrial and agricultural areas and different types of roads and infrastructures are located.