GIS-based identification of effective dike relocations: considering the synergy between nature conservation and flood risk reduction

Fluvial floods in recent years (e.g., 2002 and 2013) have caused high financial losses in Germany. Floodplains are under extensive use and dikes disconnect two thirds of natural flood retention areas from rivers. Moreover, floodplains serve as hotspots of biodiversity and floodplain habitats as well as ecosystems are categorized as endangered. Additionally, the goal of the German National Strategy on Biodiversity in increasing retention areas along rivers by at least 10% by 2020 failed. In summary, urgent actions need to be taken to reduce flood risk on the one hand, and increase floodplain area for ecological improvement – often synergies are not considered. Dike Relocation (DR) or levee setback is considered as nature-based flood protection measure whereby flood water levels can be lowered by reconnecting floodplain areas to rivers and improving nature conservation. Although DRs are being implemented already, an integrated and systematic approach is needed to consider the synergies between fields, nature conservation and flood protection.

Using dike lines, Basic European Assets Map (BEAM), Natura 2000 protected sites, and EU Copernicus land use map, a GIS-based method was developed. Four criteria were considered to evaluate effective DR; (1) The narrowness of flood channels, (2) flood-exposed assets and population, (3) floodplain habitats, and (4) urban land use and infrastructure behind the dikes. Narrow width in flood channels (between dike lines) were identified as bottlenecks. Using BEAM, flood-exposed population and asset values were calculated upstream of the identified bottleneck. The area behind dikes was searched for Special Areas of Conservation (SACs) with typical floodplain habitats. By ranking and rescaling, indices were provided for each criterion. The indices were combined with equal weights to reach DR effectiveness index. The share of urban land use and transport infrastructure was calculated behind the dikes, and DRs were grouped based on the potential of socio-economic conflicting interests.

The developed method was applied to the German part of the river Elbe. Along the 195 km river reach between Tangermuende and Geesthacht, 29 critical bottlenecks were identified. Because of high urban land use and existing transport infrastructure behind the dikes, no DR is possible at 13 of those bottlenecks. As an example of recommended DRs, the highest effectiveness index was reached for a 72% width contraction and flood-exposed assets of 16 million Euro/km² with high share of habitat area behind the dikes (93%). The results were confirmed by a comparison of this approach with the German Federal Institute of Hydrology (BfG) 2D hydraulic analysis of bottlenecks at the Lower Middle Elbe.

The GIS-based method can be used especially in the initial phase of decision making instead of time-consuming hydraulic models. Hereby, priority is given to DRs with higher synergy and low socioeconomic restrictions. Application of freely available data makes the method transferable to other European countries.