The reason for this project was the flood disaster in august 2002 along the river Elbe. This event is classified as the highest ever calendared flood of the river Elbe in the Czech Republic and Saxony. In the federal states downstream of Saxony the flood dimension became more alleviate due to the opening of weirs, the use of the Lower Havel polders as an additional retention area as well as dike failures along the Elbe itself. The controlled flooding of the Lower Havel area during the flood disaster in 2002 clearly showed its particular importance for the reduction of extreme flood events for downstream areas. Starting with a detailed analysis of the Elbe flood in 2002, the management of the weir group Quitzöbel (inlet to the river Havel) as well as the flooding of the polders the project mainly focused on the optimization of the flooding system. As a result a concept for the controlled polder flooding has been developed. This concept did assist the contractors (the State Authority for Agriculture and Environment (LUA) Brandenburg and the State Authority for flood protection and water management (LHW)) with the development of a new weir operation manual.
Fig. 1: Model of the Lower Havel area
For this surface water model the software MIKE11 (DHI software) has been used to create a 1D hydrodynamic model, which integrates the Elbe from Tangermünde to Wittenberge, the Havel from Albertsheim to Gnevsdorf, several tributaries to the Havel as well as the 7 Havel polders, each of them divided in a number of sub-catchments. The Lower Havel area is characterized by close interaction between surface water and groundwater. This has to be considered also for the polder flooding. The groundwater storage offers an additional retention area and is therefore increasing the flood reduction along the downstream part of the Elbe. In former projects this influence could never be accurately quantified. Depending on the aquifer storability, the infiltration resistance and the conductivity of the topsoil the share of groundwater storage in the whole polder volume is changing from polder to polder. To analyze the interaction between the ground- and the flooded polder water a coupled surface and groundwater model was developed. The groundwater model has been created with FEFLOW (WASY software). The MIKE11 and the 3D FEFLOW models were coupled using the interface IFMMIKE11 (WASY software). The present interface has been extended to meet the special requirements of this project.
With this modelling system it was found that during the flooding of 2002 a maximum discharge of 25 m³/s and in total about 10 Mio. m³ was infiltrated into the groundwater. This amounts about 6.5 % of the total inflow into the Havel area during the same period. Furthermore, the system showed that using the optimized flooding scheme an additional reduction of the maximum water table in Wittenberge of 25 - 30 cm (compared to the reduction achieved by the real flooding in 2002) could have been achieved. Opening one additional polder (polder no. 6 has not been flooded in 2002) even 41 cm of additional reduction would have been possible. Besides for the 2002 flood the system was optimized for even more extreme hydrological conditions. These scenarios represent a more voluminous wave, a higher wave, a comparable wave but with higher initial water levels along the river Havel and combinations of these three. It was found that also under these extreme conditions, the flooding of the polders can contribute to a significant flood reduction in the downstream part of the river Elbe.