On the location of the former Svatojánské proudy (St. John’s Rapids), both loved and dreaded by boaters, on the 91.61 river kilometre of the Vltava, stands the straight, concrete, gravitational, 260 metres long at the crown, and 67.5 metres high above its base dam of the Slapy Waterworks, built during the years 1949 to 1954. The dam has 4 overflow spans, each 15 metres wide, and the total capacity of the overflow is 3,000 m³ of water per second.
The lake, which was created when the dam was built, is 44 km long, has a water surface area of 1,392 ha, and the volume of retained water is approximately 270 million m³. Although the lake (and its environs) is a sought-after recreational location, the primary purposes of the Slapy Waterworks are the generation of electricity, improving water flow on the Lower Vltava – Elbe sailing route, the provision of potable and service water, water for industry, the protection of the area below the dam from flooding, boating, and also the abovementioned recreation.
Interesting and unique in design is the placement of the hydroelectric power station, including the machine rooms and distribution points, at the base of the dam directly under the overflow spans. The hydroelectric power station is equipped with three Kaplan turbines, each of which has an output of 48 MW (at a water flow rate of 108 m³ per second).
Although when designing the Slapy dam the construction of a boat lift for vessels with displacements up to 300 tonnes was planned (the dam is not equipped with a lock), utilising the by-pass tunnel on the right bank, the equipment was never completed (as at 2009). Small boats, up to a displacement of 4 tonnes, are transported between the water levels of the river above and below the dam on special trailers pulled by tractors.
Weather forecast from Yr, delivered by the Norwegian Meteorological Institute and the NRK