Wahnbach Dam, Dam in Siegburg, Germany

The Wahnbach Dam is an earth-fill embankment dam near Siegburg-Seligenthal that impounds the Wahnbach stream to form a drinking-water reservoir about 5.8 km long. Its zoned body of greywacke and greywacke slate, some 1,020,000 m³ of rock, has a water-side asphalt-concrete surface seal and a walkable inspection gallery at its foot.

The Wahnbachtalsperrenverband, founded on 12 June 1953, built the dam between January 1955 and June 1956; the reservoir was impounded from 20 December 1956 and the waterworks began operating on 28 April 1958 under Minister-President Fritz Steinhoff. Around 1,020,000 m³ of greywacke from Pinn was heaped up, and the water right granted on 12 November 1955 allows 28.1 million m³ to be taken yearly.

The reservoir drowned the Wahnbach valley: the Lüttersmühle mill, the inn "Wahntaler Schweiz", the farms Hillenbach and Petershof and the Herkenrather Mühle disappeared, and their residents were resettled. Remains of the Wahnbachtalstraße still show at the lake's north and south ends. The Herkenrather Mühle building served the Wahnbachtalsperrenverband for years as a test facility for phosphorus elimination.

The reservoir stretches about 5.8 km southwest from Neunkirchen-Seelscheid to Siegburg-Seligenthal, with many small bays along its shore. The dam stands at the southern end; the intake tower and the Seligenthal pumping station sit at its foot. The former Wahnbach valley road now lies submerged, and remnants of it survive at the north and south ends of the lake.

The dam's core is a zoned earth-and-rock fill: about 1,020,000 m³ of greywacke and greywacke slate were tipped between January 1955 and June 1956, reaching up to 8,600 m³ a day. The material came from a quarry inside the later reservoir area near the village of Pinn. The upstream face, laid at a slope of 1:1.6, carries a two-layer asphalt concrete seal 4.5 cm thick per layer, with a drainage layer beneath that feeds the inspection gallery and reveals any leakage.