The Oker Dam (german: Okertalsperre) is a

dam A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...

in the

Harz mountains The Harz () is a highland area in northern Germany. It has the highest elevations for that region, and its rugged terrain extends across parts of Lower Saxony, Saxony-Anhalt, and Thuringia. The name ''Harz'' derives from the Middle High German ...

Lower Saxony Lower Saxony (german: Niedersachsen ; nds, Neddersassen; stq, Läichsaksen) is a German state (') in northwestern Germany. It is the second-largest state by land area, with , and fourth-largest in population (8 million in 2021) among the 16 ...

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

. It is fed by the River

Oker The Oker is a river in Lower Saxony, Germany, that has historically formed an important political boundary. It is a left tributary of the River Aller, in length and runs in a generally northerly direction. Origin and meaning of the name The ...

Technology

The

is in the shape of an arch (see

arch-gravity dam An arch-gravity dam or arched dam is a dam with the characteristics of both an arch dam and a gravity dam. It is a dam that curves upstream in a narrowing curve that directs most of the water pressure against the canyon rock walls, providing the ...

). It is 75 m high, 260 m long, can impound up to 47 million m³ of

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

and provides

hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water to Electricity generation, produce electricity or to power machines. This is achieved by energy transformation, converting the Pot ...

flood protection Flood control methods are used to reduce or prevent the detrimental effects of flood waters."Flood Control", MSN Encarta, 2008 (see below: Further reading). Flood relief methods are used to reduce the effects of flood waters or high water level ...

and water regulation at times of

low water Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ca ...

. It is also used indirectly for the production of

drinking water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...

. Its average annual discharge is 75 million m³. The

hydro-electric power station Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined an ...

Romkerhalle Romkerhall (also frequently called ''Romkerhalle'') is a popular tourist destination on the River Oker in the Harz Mountains of Germany. There is a public car park here as well as a hotel and restaurant opposite the Romkerhall Waterfall. Romkerh ...

is fed by the Oker Reservoir. File:Okertalsperre_staumauer_talseite.jpg, Dam, downstream side File:Staumauer okertalsperre.jpg, Downstream face of the dam File:Okertalsperre_Hauptstaumauer_UEberlaeufe.jpg, Spillways on the dam File:Okertalsperre02.jpg, Table with technical data on the Oker Dam

History and location

The construction of the Oker Dam was begun by Hr. Press in the years 1938 to 1942 and then completed between 1949 and 1956. Its completion became pressing following the devastating floods in

Wolfenbüttel Wolfenbüttel (; nds, Wulfenbüddel) is a town in Lower Saxony, Germany, the administrative capital of Wolfenbüttel District. It is best known as the location of the internationally renowned Herzog August Library and for having the largest c ...

and Brunswick in the winter of 1946/47 when there were heavy falls of snow. Before work started the small hamlet of Schulenberg in the valley had to be abandoned; it was rebuilt above the reservoir. The Oker Reservoir lies alongside the B 498 federal road, that follows the course of the River Oker between

Goslar Goslar (; Eastphalian: ''Goslär'') is a historic town in Lower Saxony, Germany. It is the administrative centre of the district of Goslar and located on the northwestern slopes of the Harz mountain range. The Old Town of Goslar and the Mines ...

and Altenau. On the east bank is a forest road, that is out of bounds to private vehicles. File:Okertalsperre1.jpg, Main dam under construction File:Okertalsperre2.jpg, Main dam under construction File:Okertalsperre 2.jpg, Main dam under construction File:Okertalsperre 3.jpg File:Okerstausee_von_Schulenberg.jpg, Oker Reservoir: view from Schulenberg to the Upper Harz With its many side arms the Oker Reservoir resembles the much larger Bigge Reservoir in the

Sauerland The Sauerland () is a rural, hilly area spreading across most of the south-eastern part of North Rhine-Westphalia, in parts heavily forested and, apart from the major valleys, sparsely inhabited. The Sauerland is the largest tourist region in ...

in west Germany. For the same reason it is also known as the "'' Vierwaldstättersee'' of the Harz".

Tourism

From the beginning of March to the beginning of January a passenger boat plies the reservoir ferrying tourists (only at weekends during the off-peak seasons). The large tour on the ''MS Aquamarin'' takes approximately 1.5 hours and can be started from all landing stages around the reservoir.HARZ mit Kindern. Die 400 besten Touren & Ausflüge rund ums Jahr. Peter Meyer Verlag. 5. Aufl. (2018) ISBN 978-3-89859-463-9 This boat service is the highest (above

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...

in Lower Saxony. In addition, during the summer months, numerous, private, unmotorised water craft grace the waters of the reservoir. Swimming and diving are also permitted, apart from certain out-of-bounds areas such as the zone near the dam wall. In winter, when the ice is not thick enough to support weight,

ice swimming Winter swimming is the activity of swimming during the winter season, typically in outdoor locations (open water swimming) or in unheated pools or lidos. In colder countries, it may be synonymous with ice swimming, when the water is frozen ov ...

takes place. There is a circular route, using roads and paths, around the Oker Reservoir which is accessible to unmotorised vehicles or walkers. Between the

pre-dam A forebay is an artificial pool of water in front of a larger body of water. The larger body of water may be natural or man-made.< ...

and the primary dam there is a fully metalled circular path, which is used by

cyclists Cycling, also, when on a two-wheeled bicycle, called bicycling or biking, is the use of Bicycle, cycles for transport, recreation, Physical exercise, exercise or sport. People engaged in cycling are referred to as "cyclists", "bicyclists", ...

inline skaters Inline skates are a type of roller skate used for inline skating. Unlike quad skates, which have two front and two rear wheels, inline skates typically have two to five wheels arranged in a single line. Some, especially those for recreation, ha ...

and

roller skiers Roller skiing is an off-snow equivalent to cross-country skiing. Roller skis have wheels on their ends and are used on a hard surface, to emulate .Bryhn, Rolf and Knut Are Tvedt (eds.): ''Kunnskapsforlagets idrettsleksikon''. (Norwegian Encyclop ...

. Only the path between the pre-dam and Altenau, which is barred to motor vehicles, is not metalled, but is surfaced with gravel and stone. On the B 498 in summer there is heavy motorcycle traffic (on the northern approach to

Torfhaus Torfhaus is a village in the borough of the mining town of Altenau in the Harz mountains of Germany and lies at a height of about . It is the highest settlement in Lower Saxony. This small settlement consists mainly of restaurants, youth hostels, ...

). There are numerous restaurants and cafés along the road. In very calm conditions (no wind or precipitation and no man-made disturbances) a very flat surface forms on the reservoir which mirrors clearly the surrounding landscape. File:Okertalsperre8.jpg, Passenger boat on the reservoir; Weißwasser Bridge in the background File:Okertalsperre Harz Germany.jpg, The Oker Reservoir in calm weather File:Okertalsperre01.jpg, The reservoir at low water File:Okertalsperre_Oker_Niedrigwasser.jpg, The Oker in the Oker Reservoir at low water File:Okerstausee_Niedrigwasser.jpg, Side branch of the Oker Reservoir at low water File:Okerstausee_alte_Bruecke.jpg, Occasionally flooded bridge on the Oker Reservoir at low water

Facilities belonging to the Oker Dam

Pre-Dam

The pre-dam (''Vorsperre'') of the Oker Reservoir is of the

gravity dam A gravity dam is a dam constructed from concrete or stone masonry and designed to hold back water by using only the weight of the material and its resistance against the foundation to oppose the horizontal pressure of water pushing against it. ...

type, with a height of 20.0 metres. The pre-dam can impound up to 520,000 m³ of water and its primary purpose is to control the water level and cleanliness of the Oker Reservoir. The dam site was designed as a

retention basin A retention basin, sometimes called a wet pond, wet detention basin, or stormwater management pond (SWMP), is an artificial pond with vegetation around the perimeter and a permanent pool of water in its design. It is used to manage stormwater r ...

(max. 15 m of water at the pre-dam) in order to create a

wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The ...

in the Kellwasser valley. Only when there is a very high level of water in the Oker Reservoir are both reservoirs at the same height. The water level in the pre-dam lake can even rise a few decimetres higher. File:Okertalsperre_Vorstaumauer_Vorstauseeseite.jpg, Pre-dam, retention basin side File:Okertalsperre_vorstaumauer.jpg, Pre-dam, Oker Reservoir side File:Okertalsperre_Vorstaumauer_Talseite_Niedrigwasser.jpg, Pre-dam, Oker Reservoir side at low water

Romkerhalle Hydro-electric Power Station

The

is fed via a pipe from the Oker Reservoir and has a capacity when completed of 4.14 MW. It was opened in 1956 and is equipped with a single

Francis turbine The Francis turbine is a type of water turbine. It is an inward-flow reaction turbine that combines radial and axial flow concepts. Francis turbines are the most common water turbine in use today, and can achieve over 95% efficiency. The proces ...

. It has a descent of 80 m, the nominal discharge is 6.5 m³ per second and it generates 12.5 million kWh per year. The power station is operated as a

peaking power plant Peaking power plants, also known as peaker plants, and occasionally just "peakers", are power plants that generally run only when there is a high demand, known as peak demand, for electricity. Because they supply power only occasionally, the power ...

working for about 8 hours per day. It discharges directly into the river bed of the Oker. At this point the

whitewater Whitewater forms in a rapid context, in particular, when a river's gradient changes enough to generate so much turbulence that air is trapped within the water. This forms an unstable current that froths, making the water appear opaque and ...

section of the Oker begins and is especially popular with

canoeists A canoe is a lightweight narrow watercraft, water vessel, typically pointed at both ends and open on top, propelled by one or more seated or kneeling paddlers facing the direction of travel and using a single-bladed paddle. In British Englis ...

. Large and sometimes abrupt changes in water level on the Oker result from the unregulated flow of the power station from here as far as the stilling basin of the Oker Reservoir. The Romkerhalle power station is also called the Oker Hydro-Electric Power Station (''Wasserkraftwerk Oker''). File:Wasserkraftwerk_Romkerhalle.jpg, Romkerhalle Hydro-electric Power Station

Oker-Grane Tunnel

The 7.4 km long Oker-Grane Tunnel, with its intake near Romkerhalle and outlet on the eastern shore of the

Grane Reservoir Grane may: Places *Grane, Nordland, a municipality in Nordland county, Norway *Grane Church, a church in Grane, Norway *Grâne, Drôme, a commune in Drôme, France *Grane, an old name for Kuwait *Grane (river), a river in Lower Saxony, Germany *Gra ...

, conducts water from the Oker Reservoir (power station or bottom outlet) into the Grane Reservoir where it can be purified for use as drinking water. The Grane Reservoir itself only has a relatively small catchment area and is reliant on other tributaries in order to be able to supply drinking water. The Oker-Grane Tunnel was built in the period from 1968 to 1970. File:Oker-Grane-Stollen_Zugang_Romkerhalle.jpg, Oker-Grane tunnel, intake at Romkerhalle

Stilling basin

The stilling basin lies about 3 km below the dam of the Oker Reservoir and about 2 km below the Romkerhalle power station. It is furnished with a gravity dam which has a height of 18 metres. The stilling basin can store up to 204,000 m³ of water and its main purpose is to regulate the water level downstream. There is no power station immediately below the dam wall, however it services several smaller power station on the Oker as far as the northern edge of the Harz. Like the Oker Reservoir, in calm weather conditions, it forms a very smooth water surface which reflects the surrounding countryside very clearly. Because of the sheltered location of the stilling basin, this phenomenon occurs very frequently. File:Okertalsperre_Ausgleichsbecken_Staumauer_Seeseite.jpg, Stilling basin dam, upstream side File:Okertalsperre_Ausgleichsbecken_Staumauer_Talseite.jpg, Stilling basin dam, downstream side File:Okertalsperre_Ausgleichsbecken_Prallkoerper.jpg, Stilling basin dam, deflector (''Prallkörper'')

Sewage

In order not to pollute the water in the reservoir unnecessarily,

sewage Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system. Sewage consists of wastewater discharged from residenc ...

from

Altenau Altenau () is a town and a former municipality in the district of Goslar, in Lower Saxony, Germany. Since 1 January 2015 it is part of the town Clausthal-Zellerfeld. It is situated in the middle of the Harz mountains, between Clausthal-Zellerf ...

and Schulenberg is led away in a separate pipe around the Oker Reservoir to the Goslar-Oker

sewage farm Sewage farms use sewage for irrigation and fertilizing agricultural land. The practice is common in warm, arid climates where irrigation is valuable while sources of fresh water are scarce. Suspended solids may be converted to humus by microbes an ...

. File:Okertalsperre_abwasserleitung_schacht.jpg, Sewage drain, access shaft between dam and Romkerhalle

References

Sources

External links

Harz waterworks
(with current dam data)

* {{Authority control Dams in Lower Saxony Dams in the Harz Dams completed in 1956