Abstract. Flood detention areas serve the primary purpose of controlled water storage during large flood events in order to decrease the flood risk downstream along the river. These areas are often used for agricultural production. While various damage estimation methods exist for urban areas, there are only a few, most often simpler approaches for loss estimation in rural areas. The loss assessment can provide an estimate of the financial provisions required for the farmers' compensation (e.g., in the context of cost-benefit analyses of detention measures). Flood risk is a combination of potential damage and probability of flooding. Losses in agricultural areas exhibit a strong seasonal pattern, and the flooding probability also has a seasonal variation. In the present study, flood risk is assessed for a planned detention area alongside the Elbe River in Germany based on two loss and probability estimation approaches of different time frames, namely a monthly and an annual approach. The results show that the overall potential damage in the proposed detention area amounts to approximately 40 000 € a−1, with approximately equal losses for each of the main land uses, agriculture and road infrastructure. A sensitivity analysis showed that the probability of flooding (i.e., the frequency of operation of the detention area) has the largest impact on the overall flood risk.

Abstract:A flood emergency storage area (polder) is used to reduce the flood peak in the main river and hence, protect downstream areas from being inundated. In this study, the effectiveness of a proposed flood emergency storage area at the middle Elbe River, Germany in reducing the flood peaks is investigated using hydrodynamic modelling. The flow to the polders is controlled by adjustable gates. The extreme flood event of August 2002 is used for the study. A fully hydrodynamic 1D model and a coupled 1D-2D model are applied to simulate the flooding and emptying processes in the polders and flow in the Elbe River. The results obtained from the 1D and 1D-2D models are compared with respect to the peak water level reductions in the Elbe River and flow processes in the polders during their filling and emptying. The computational time, storage space requirements and modelling effort for the two models are also compared. It is concluded that a 1D model may be used to study the water level and discharge reductions in the main river while a 1D-2D model may be used when the study of flow dynamics in the polder is of particular interest. Further, a detailed sensitivity analysis of the 1D and 1D-2D models is carried out with respect to Manning's n values, DEMs of different resolutions, number of cross-sections used and the gate opening time as well as gate opening/closing duration.

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Emergency storage areas can be an effective structural flood protection measure. By their controlled flooding the risk of inundation for downstream areas with higher vulnerability can be reduced.In the present study, the flooding and emptying process of a proposed storage area at the Middle Elbe River is simulated. The storage area has a maximum capacity of 40 million m 3 and is divided into two polder basins. It is designed for the attenuation of extreme floods of 100 years or more return period.A one-dimensional hydrodynamic model is set up for a 20 km reach of the Elbe River, wherein the storage area is schematised by two storage cells each representing one polder basin. Flow between the storage cells and the Elbe River is controlled by adjustable gates, which operate based on the pre-defined conditions.Four flood scenarios which differ in flood magnitude and hydrograph shape are simulated. The scenarios are derived from analyses of a 70 years discharge record. Furthermore, for each flood scenario two gate control strategies are investigated.The results show that during large floods the utilization of the storage area with controlled gate operations significantly reduces the Elbe River peak discharges. However, the magnitude of the attenuation depends on the steepness of the flood hydrograph and the applied control strategy with well-timed gate operations.Lower discharge values recorded at the downstream gauge Wittenberg are due to several dike failures that occurred between the two gauges during the flood in 2002.

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