New management plan for the Ennepetalsperre dam in force since mid-June 2025

The new management plan for the Ennepetalsperre dam has been in force since mid-June 2025. This was drawn up by the Ruhrverband in consultation with the drinking water supplier AVU Gevelsberg and approved by the Arnsberg district government. Among other things, the management plan precisely regulates the minimum volume of water that must be released from the reservoir to the tailwater. For this purpose, the reservoir of the Ennepetal Dam is divided into horizontal areas, the so-called management lamellae. The lower the filling level in the dam, the lower the management lamella used and the lower the prescribed minimum volume of water that must be released to the tailwater. The minimum discharge quantity also varies according to the season, i.e. it is lower in the summer months than in winter.

The previously valid management plan dates back to 2003, but due to exceptional droughts in 2018, 2019, 2020, 2022 and the current year 2025, it was necessary to deviate from this with the approval of the Arnsberg district government in order to safeguard the water supply. In each case, a lower management lamella was utilised ahead of schedule, i.e. a lower discharge was made to the tailwater than would actually have been the case with the current reservoir content and at the corresponding time of year according to the management plan. This served the purpose of conserving the water reserves in the dam and increasing the safety of the drinking water supply.

New management plan takes into account seasonal fluctuations in the reservoir content

As climate change means that both long dry periods and heavy rainfall events and subsequent flooding must be expected more frequently in future, the Ruhrverband has drawn up a new management plan, which was approved by the Arnsberg district government in June 2025 with the involvement of the relevant authorities.

The centrepiece of the management plan is the introduction of varying lamella limits, which not only take into account the reservoir capacity, but also the time of year. Due to seasonal fluctuations, it makes a big difference whether the reservoir capacity of eight million cubic metres, which previously represented the first lamella limit for the permitted reduction of the minimum water release all year round, occurs in spring or autumn.

While eight million cubic metres of reservoir content in September and October is normal and does not require a reduction in the minimum discharge, the same eight million cubic metres in April is an exceptionally low reservoir content that has only occurred four times in the 75 years since records began. In order to take this difference into account, the lamella limits in the new management plan are higher in spring than in autumn, so that in future it will be possible to react more flexibly to reservoir levels that are unusually low for the respective season.

In addition, an inflow regulation has been included in the new management plan, which states that the discharge from the Ennepetal dam may be reduced to 100 litres per second as soon as the average inflow to the dam is less than 100 litres per second for three consecutive days. The change in the lamella limits and the additional inflow regulation will not only increase the safety of the drinking water supply, it will also make the water flow in the lower reaches of the dam more dynamic and more natural than before.

Flood protection area: previously voluntary, now mandatory in certain situations

In addition, the adjustments to the management plan allow the first-time introduction of a so-called situational flood protection area amounting to ten per cent of the total reservoir volume of the Ennepetal Dam. In this context, situational means that the Ruhrverband must provide the flood protection area in good time if the occurrence of a flood event is foreseeable in the near future based on corresponding weather forecasts and may otherwise impound it to increase the water supply. This regulation allows for more flexible and therefore more climate-resilient management than would be the case with a permanently defined flood protection area.

In recent years, the Ruhrverband has also created free reservoir capacities in good time before foreseeable flood events and used these to minimise flood peaks by retaining large quantities of water in the reservoir and delaying the release of water after the rainfall has subsided. However, this has so far been done on a voluntary basis. The effect that this situation-adapted dam management can have was impressively demonstrated during the extreme flood event in July 2021: if the Ruhrverband had not created flood protection space in the Ennepetal Dam in the days before and dammed it during the flood, the highest discharge at the Hagen-Haspe gauge downstream would not have been 150 cubic metres per second, but well over 200 cubic metres per second, and much larger areas below the dam would have been flooded.

Accompanying monitoring and planned ecological measures

The new management plan will be accompanied in the coming years by comprehensive monitoring, which the Ruhrverband and the district government have agreed to in order to gain insights into any positive or negative effects of the more dynamic and more natural dam management on the water quality and ecology in the lower reaches of the dam and the protected flora-fauna-habitat areas (FFH areas) there. In addition to the general chemical-physical parameters, the biological quality components "macrozoobenthos" and "fish fauna" will also be analysed using electrofishing by qualified fishing teams. In addition, the Ruhrverband is planning to plant riparian trees along the banks of the Ennepe in the Burg district of Ennepetal next autumn so that the watercourse there is better shaded in future and can heat up less.

The background:

The Ennepetalsperre was built at the beginning of the 20th century and its quarry stone wall was raised by around ten metres shortly after completion. Its total volume is 12.6 million cubic metres. The main tributary is the Ennepe, which flows into the Volme in Hagen. The Ruhrverband manages the drinking water reservoir, from which AVU Gevelsberg extracts between seven and ten million cubic metres of raw water every year and processes it into drinking water. In this way, around 145,000 people in the southern Ennepe-Ruhr district are supplied with drinking water from the Ennepetalsperre.