Low-hanging fruits in large-scale fluvial landscaping measures: trade-offs between flood hazard, costs, stakeholders and biodiversity

Abstract

Abstract. Adapting densely populated deltas to the combined impacts of climate change and socioeconomic developments presents a major challenge for their sustainable development in the 21st century. Decisions for the adaptations require an overview of cost and benefits and the number of stakeholders involved, which can be used in stakeholder discussions. Therefore, we investigated the balance between multi-faceted costs and benefits of common landscaping measures to compensate for changes in discharge and sea level rise on the basis of relevant, but inexhaustive, quantitative variables for physical, ecological and societal costs and benefits. We modelled the largest delta distributary of the Rhine River with adaptation scenarios driven by (1) the choice of seven measures, (2) the areas owned by the two largest stakeholders (LS) versus all stakeholders (AS), and (3) the ecological or hydraulic design principle. We evaluated measures by their efficiency in flood hazard reduction, potential biodiversity, number of stakeholders as a proxy to governance complexity, and measure implementation cost. We found that only floodplain lowering over the whole study area can offset the altered hydrodynamic boundary conditions; for all other measures, additional dike raising is required. LS areas comprise low hanging fruits for water level lowering due to the governance simplicity and hydraulic efficiency. Measures implemented in LS areas are 3 to 74 % more efficient than in AS areas. Clear trade-offs were revealed between evaluation parameters, but no single measure represented the optimal combination on all aspects. The multidimensional evaluation space provides a frame for the co-creation of adaptation paths for climate-proofing deltas.