The Möhne Reservoir used to be one of the largest reser-voirs in Europe. It was put into operation in 1913, aftermerely five years of construction work. It was financedand constructed by the then Ruhrtalsperrenverein (RuhrReservoir Association), which was transformed to an entityunder public law in 1913 and merged with the Ruhrverband in 1990. The Möhne Reservoir used to be the backbone of the Ruhr area’s water supply and still accounts for 28 percent of total storage capacity, thus making an essential contri-bution to controlling the River Ruhr’s runoff.It was engineered by the then Regierungsbaumeister (state architect) Ernst Link. Franz Brantzky, a leadingRhenish architect of his time, won the competition for architectural design of the dam. Parts of the Möhne Reservoir’s dam are today listed as a historic monument.The masonry dam with its slightly arched shape is madeof quarry stones. It is 650 metres long, up to 40 metreshigh and can hold up to 134.5 million cubic metres ofwater.In the dam’s upstream area, i.e. where the rivers Möhneand Heve join the reservoir, two upstream basins were constructed. Their water level is kept at a constant levelindependently of the main reservoir, thus reducing theinflow of sediment and nutrients to the main reservoirand improving the living conditions for flora and faunain the inflow areas. The Möhne Reservoir was put into operation in 1913, after five years of construction work. The main power station (bottom left) was destroyed by an air raid in 1943.
The destruction of the Möhne Reservoir
During the night from 16 to 17 May 1943, an air raid ofthe British armed forces, using a bouncing bomb especiallydesigned for this purpose, destroyed a section of the reser-voir’s dam about 77 metres long and 23 metres high. Within a time span of merely five hours, approximately110 million cubic metres of water streamed from thereservoir, creating a flood wave of up to seven metres high that devastated the Möhne valley. More than 1,200 people perished, and heavy damage occurred across the Ruhrvalley all the way down to where the Ruhr flows into the Rhine. The main power station and compensating reservoir were destroyed as well. Despite wartime difficulties, the gap in the dam was repaired over a period of only five months, quickly making the reservoir available again for water management purposes. After the war, the main power station and compensating reservoir were reconstructed. The new main power station was placed on the Southern side of the compensating reservoir.
Operating the Möhne Reservoir
In principle, water output from the Möhne Reservoirvaries according to the amount of water needed for water management in the River Ruhr area. At the Reservoir Control Centre in Essen, stormwater levels, water levels, reservoir and runoff volumes are being monitored around the clock to determine the water output from the reservoirs and maintain the River Ruhr’s legally binding minimum water levels. Based on these values, water output is then controlled by the reservoir’s movable shutters. These movable parts are subject to technical inspections atregular intervals. In order to preserve the site, it is essential to carry out constant maintenance work and repair even minor damages. Moreover, the areas most frequented by visitors (such as the dam, surrounding paths and shore areas) mustbe maintained, and the condition of the reservoir’s entirestructure must be assessed. As part of a comprehensive measuring programme (e.g. a seepage control system), specific parameters for assessing the site’s structural and operational safety have been established and are measured at regular intervals. In addition, exact measurements of the masonry dam’s surface area are carried out at larger intervals, in order to be able to recognise possible changes at an early stage.The reservoir’s water quality is subject to regular controls by the men and women at the Ruhrverband’s laboratory, based on biological and chemical parameters and carried out at different depths.As a result of the manifold tasks connected with opera-ting a reservoir, we employ specialists with a variety ofdifferent job profiles.
The technology of the Möhne Reservoir
The Möhne Reservoir consists of a gravity dam built inaccordance with the principles developed by ProfessorOtto Intze, a pioneer of German dam engineering. Putsimply, its cross section has a triangular shape, commen-surate with water pressure. A triangular wedge made of clay and loam, named “Intze wedge“ after its inventor, is placed in front of the dam’s lower third to provide additio-nal protection in the area with the highest water pressure. The dam’s arched shape is designed to prevent pressureresulting from temperature changes from affecting the massive construction. During normal operation, the water passes through the main power station’s water conduit and from there to the compensating reservoir. At the main power station, the water’s energy is transformed into electricity, supplyingapproximately 3,000 households with environmentally friendly and climate neutral energy. The dam’s bottomoutlets are used only in times of extraordinarily highinflow or a blackout of the main power station. Thereare two bottom outlet units with two independentoutlets each. Each outlet has three shutter elements:two consecutive bottom gates inside the gate towers,and a third gate in the gate houses. The outer gatehouses are equipped with circular valves, and the inner gate houses with flat side valves. In contrast to the other shutters, water output through the circular valves can be adjusted infinitely.If the reservoir is full and the water inflow from the River Möhne exceeds the bottom outlets’ capacity, the surplus water is evacuated via the flood spillways. At the top ofthe masonry dam, 105 openings over a total length of 262.5 metres serve as flood spillways. The dam’s down-stream side has a rough surface with protruding rocks to effect partial energy dissipation of the overflowing water. The remainder of the energy is dissipated in the compensa-ting reservoir, which serves as a stilling basin. A controlled amount of water runs off every day from the compensating reservoir via the secondary power stationto the lower Möhne River. Larger runoffs are possible using a flap gate located at the secondary power station.
The restoration of the masonry dam and operational facilities
After decades of operating the reservoir, it became appa-rent that the masonry dam was subject to ever increasingseepage, as a result of aging, weathering and war damage.Similar alterations were observed underneath the masonrydam as well. In 1970, a comprehensive inspection clearly showed that major restoration measures were necessary.Work began by driving a maintenance, drainage andinspection tunnel into the transition zone between themasonry structure and the soil. The tunnel was blasted while normal reservoir operations continued. From thetunnel and the dam’s crest, the dam’s structure and the soil beneath were then proofed, thus reducing seepageto a non-hazardous level. In the process, inspection and drainage boreholes were drilled, allowing for regular monitoring of seepage.In the 1990s, the outlet units, which were then 80 years old, were replaced, and the gate towers were proofed.The bottom outlets were replaced entirely. In addition,a comprehensive restoration of the masonry dam’s damaged, weathered downstream surface (approx. 20,000 square metres) was carried out.
Nature around the Möhne Reservoir
Many areas around the reservoir are protected underEuropean and national law. The Möhne Reservoir’s entirewater surface serves as a resting and breeding habitatfor migratory birds and has therefore been listed as abird sanctuary. Almost the entire Heve river arm has been listed as a nature reserve, and the Heve upstream reservoir is subject to special protection as a European nature heritage site.Compared with other forms of vegetation, forests are best adapted to absorb high precipitation levels, thushelping to reduce flooding damage. Moreover, theyreduce ground erosion, and groundwater infiltrationcleans the stormwater. As a result, the majority of theforests adjacent to the Möhne Reservoir are a propertyof the Ruhrverband. They are managed in harmony withnature by our own personnel, in order to guarantee maximum protection of the Reservoir.As the water level of reservoirs fluctuates over prolongedperiods of time, shore vegetation is limited. The unprotec ted, steep embankments are eroded by wave action. To prevent this, more than half of the Möhne Reservoir’s shore surface has been fortified with stone fillings, pavings or shore walls. Preferably, today such fortifications are constructed in accordance with soil bioengineering prin-ciples. Due to its particular characteristics, the Möhne Reservoir lacks, or has a limited, shore and underwater vegetation and therefore offers unfavourable conditions for fishingbiology. As a result of these conditions, natural development of a balanced fish stock is impossible.The reservoir’s fish stock is therefore managed by professional fishermen. They control the stock development ofindividual species, reducing mass stock if needed and breeding the required stock of predatory fish for the MöhneReservoir at their own fish breeding station in Möhnesee-Körbecke.