In the Rhineland-Palatinate section, the Lahn is fitted into a narrow bed and often has little room to spread out at higher water levels in such a way that people and towns are not affected by floods.

In order to ease the flood situation, it is planned in the course of the LIFE project LiLa-Living Lahn to go in search of additional retention areas on the tributaries, such as the lower Aar, and thus defuse the flood risk for the city of Diez and the neighbouring communities.

The study on flood prevention on the lower Aar on the Rhineland-Palatinate side is intended to contribute to this. As part of the study, areas on the lower Aar and on the Palmbach, a tributary of the Aar, were examined for their retention capacity and possible impact on flooding.

The Study

The study covers ten selected potential retention areas, eight of them on the Aar and two on the Palmbach, which were examined for their retention capacity. In technical terminology, retention capacity describes a volume of water that is distributed and retained in the area when water bodies overflow. In flood protection, this process is referred to as natural retention and has the effect of slowing down the flood wave and reducing its intensity.

After a close examination of the selected areas, three of them proved to be suitable from a hydraulic point of view.

The three preferred areas are located on the Aar River and can mathematically absorb a water volume of about 19,000 m³ in the event of a flood. Based on the water level of the design flood (HQ 100), a reduction of the water level by about 11 cm would be theoretically possible.

For the implementation of the study results or reactivation of the natural retention areas, further checks are required with regard to the current use and availability of the required areas so that retention can take place largely unhindered and without damage. Furthermore, the aim is to combine the reactivation of the retention areas with the measures for watercourse development that are being developed within the framework of the planned watercourse development concept for the Aar (Action A 11).