Upgrade of the Perlenbach Dam in Germany - A multi-purpose sustainability project

Perlenbach dam is located within the catchment of the Rur River close to the city of Monschau in West Germany at the border to Belgium. The main purpose of the reservoir is water supply, but it also serves the minimum flow requirements, recreation and hydropower. During the devasting flood incident in July 2021 in western Germany the reservoir faced inflows exceeding a HQ(10.000) event. Thanks to the conservative design of the existing spillway major harm did not occur. Contrary to the flood event, the 2018 and 2022 dry periods revealed that the capacity of the reservoir is not sufficient to cover such extreme droughts. The water supply association Perlenbach was forced to purchase water from other suppliers of the region. The existing scheme is a reservoir volume of only 750.000 m(3) which represents only 1.7 % of the annual mean inflow. The increase of the reservoir volumes shows positive effects not only onto the water supply security but also the energy production and minimum flow periods. Additionally, the reservoir could contribute to flood retention since the Perlenbach comprises about one third of the Rur catchment at its mouth. Particularly in summer the reservoir could contribute essentially to flood control downstream. The existing dam is an earth-rockfill dam with an asphaltic surface sealing and a height of 18 m. The upgrade requires the heightening of the existing dam by approx. eight meters. All dam facilities need to be adjusted, the spillway has to be completely reconstructed and existing sediments need to be removed from the reservoir. Upstream of the reservoir valuable fauna flora habitats are located which are affected by the future reservoir. This impact is a critical aspect and compensation is strongly required. An early participation of all stakeholders is anticipated in order to find a realizable solution.