The consideration of recent extreme events in flood statistics implies an increase of design flood peaks and discharge loads. With the focus on the 75 km long Tyrolean Inn River reach downstream the regional capital city Innsbruck, the harmonization of the 100-year flood peak and comprehensive 2d-hydrodynamic modelling simulations indicate the need for an extension of the existing flood protection measures. Lateral protection and object protection measures represent the only feasible option due to the confined areal conditions. However, an increase of the channel capacities would worsen the situation for downstream areas demonstrably. In order to counter this impact, it is planned to build several large controlled flood retention basins situated along the Inn River at the valley floor between Innsbruck and the border to Germany. The retention basin “Voldöpp” as one of these flood polders features a maximum capacity of 1.7 million m³ and a maximum design water depth of 3.6 m. According to current planning the inlet structure consists of four uniform weir fields with two gates each. Aims of the presented experimental and numerical analyses are the investigation of the flow characteristics in close range of the inlet structure, the weir capacity and a possible weir control management. Hydraulic model tests are accomplished at the scale 1:35 according Froude similarity and numerical modelling is done with the software FLOW-3D. Preliminary modelling results confirmed the functionality of the inlet structure and pointed out the need of further tests concerning the potential impacts of intense sediment transport and woody debris.
<p>The flood event of August 2005 at the river Inn and the actual hazard zone mapping indicated a need for flood protection in the Lower Inn Valley in Tyrol (Austria). To ensure an appropriate protection of settlements, industrial areas and infrastructure up to a return period of 100 years, extensive planning activities for protection and compensation measurements were initiated. In accordance with the Austrian Water Law, a so-called water association was established to bundle decision-making, planning and financing of all protection measurements. The members of the water association are the affected communities and the local operators of infrastructure.<br>The majority of the cost of flood protection is carried by the federal and state government in Austria. The remaining cost in the Lower Inn Valley has to be divided between the members of the water association.<br>In order to guarantee a reasonable and fair distribution of costs, as well as voting rights, a specific approach for cost sharing was developed. The allocation of costs and voting rights is based on the protected area and the length of the protected riverbank in the communities. The areas were categorized by actual land use and by the intensity of an expected flood event and weighted respectively in the assessment. Further, the costs for communities were reduced for contributing compensatory measures, such as flood retention space. Operators of infrastructure contribute a fixed share.<br>In addition to the technical aspects of flood risk management and socio-economic aspects, an essential component in the decision-making is the cost and vote distribution of the affected communities, which thus has far-reaching influence on the entire project.</p><p>In this contribution we will detail the fair and specific approach for cost and vote sharing and provide an overview over the process that led to the foundation of the water association. We will report difficulties and the consequences of the chosen approach and give an outlook on the future of the project.</p>
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