Presentations 2016

Balancing of interests in polder dewatering: A starring role for an integrated groundwater-surface water model

DHI-WASY GmbH

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The Odra river floodplains at the German/Polish border have been dewatered and used for farming since the 18th century. An extensive system of hundreds of drainage ditches, a dike along the Odra river, and numerous pumping stations is used and maintained till date, ensuring to keep groundwater levels low enough for farming. At the same time, the remaining wetlands and pristine riparian forests in the area are valued increasingly from a conservation perspective, energy and maintenance costs become more important, and regarding the downstream flood risk precipitation is to be retained in the area for as long as possible. To facilitate the balancing of interests between the different stakeholders, such as farmers, different government authorities, and conservation groups, a coupled groundwater/surface water model has been set up for one of the polders. The integrated model developed in FEFLOW (groundwater) and MIKE 11 (surface water) considers the surface-water network with all control structures and pumping stations as well as the groundwater system. The model has been calibrated in detail for a transient period. Target groundwater levels have been defined, considering both a spatial and a temporal component. For example, farm land requires a high depth-to-groundwater especially in summer, while for wetlands water levels close to the ground surface during the bird-breeding period (until end of April/early May) are most crucial. The current drainage system operation has been evaluated based on these target groundwater levels, showing large deviations from an ideal situation. On basis of this reference scenario, an optimization of the management of the drainage system has been conducted, with the goals of (2) maximum water retention in the area and (2) matching the target levels as closely as possible. The adjustments to the system included different water levels at weirs, construction of new weirs and removal of existing ones, construction of new and abandoning of existing drainage ditches, modification of ditch-clearance plans and change of pumping station operation.Overall, a scenario could be developed that had the potential to significantly improve the situation. Not all goals, however, could be fulfilled in parallel. In order to use local knowledge as much as possible and to finally achieve a compromise that is acceptable to all different groups, the entire project was accompanied by a comprehensive stakeholder-participation process. Nearly all of the proposed measures have finally been accepted – not the least because the modeling tool, which was trusted by all parties involved, provided nonbiased information about benefits and losses for each party in each of the scenarios discussed.