The Oderteich is an historic

reservoir A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation. Reservoirs are created by controlling a watercourse that drains an existing body of wa ...

about seven kilometres northeast of

Sankt Andreasberg Sankt Andreasberg () is a former independent city in the Goslar (district), district of Goslar, in Lower Saxony, Germany. Since 1 November 2011, it has formed part of the town Braunlage. It is situated in the Harz, approximately west of Braunlage ...

in the

Upper Harz The Upper Harz (, ) is the northwestern and higher part of the Harz mountain range in Germany. The exact boundaries of this geographical region may be defined differently depending on the context. In its traditional sense, the term Upper Harz cover ...

in central Germany. It was built by miners from St. Andreasberg in the years 1715 to 1722 and, today, is an important component of the

water supply network A water supply network or water supply system is a system of engineered hydrologic and hydraulic components that provide water supply. A water supply system typically includes the following: # A drainage basin (see water purification – sour ...

known as the Upper Harz Water Regale (''Oberharzer Wasserregal''). Moreover, for 170 years, from the time it was completed to the end of the 19th century, the Oderteich had the largest dam in

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

.''Der Oderteich'' at www.harzlife.de.
Accessed on 16 Nov 09. The

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, aqua ...

lies at a height of by the B 242 federal highway, about a kilometre west of its intersection with the B 4.

Purpose

The Oderteich was built in order to be able to deliver a reliable supply of water to the

water wheel A water wheel is a machine for converting the kinetic energy of flowing or falling water into useful forms of power, often in a watermill. A water wheel consists of a large wheel (usually constructed from wood or metal), with numerous b ...

s of the St. Andreasberg mine via the Rehberg Ditch, even during times of drought. Its capacity of 1.7 million cubic metres was, far and away, the largest of all the Upper Harz reservoirs at that time and was enough to cope with periods of low precipitation of up to three months. Today the water of the Oderteich still drives several

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

stations within its own valley, at the

Samson Pit The Samson Pit or Samson Mine () is an historic silver mine in Sankt Andreasberg in the Upper Harz region of central Germany.Sperrlutter valley. The dam also serves to prevent the regular flooding that happened each year in spring before the dam was built.

History

In 1703 the new Rehberg Ditch was completed, diverting water from the river

Oder The Oder ( ; Czech and ) is a river in Central Europe. It is Poland's second-longest river and third-longest within its borders after the Vistula and its largest tributary the Warta. The Oder rises in the Czech Republic and flows through wes ...

and bringing it to St. Andreasberg. This was initially a great improvement in the supply of water power for the mines there, but the Oder could still not deliver enough water during a period of extended drought. So the call for a water reservoir quickly became shrill. A glance at the Upper Harz 'ponds' (''Teiche'') near

Clausthal-Zellerfeld Clausthal-Zellerfeld () is a town in Lower Saxony, Germany. It is located in the southwestern part of the Harz mountains. Its population is approximately 15,000. The town hosts the Clausthal University of Technology. The health resort is locate ...

and Hahnenklee suggested a solution. At that time there must have been 50 to 60 small reservoirs in operation. Their design, however, could not simply be copied, because there was nowhere in the vicinity of the Oderteich to cut grass sods like those used to seal the dams of the Upper Harz reservoirs. In the meantime, the Rehberg Ditch, which had initially only been made of wooden channels (''Holzgefludern''), was being 'walled'; in other words the wooden conduits were being replaced by a considerably more durable artificial ditch made of earth embankments and

dry stone Dry stone, sometimes called drystack or, in Scotland, drystane, is a building method by which structures are constructed from stones without any mortar to bind them together. A certain amount of binding is obtained through the use of carefully ...

walls. In doing so it was realised that the sand resulting from the weathering of granite, so-called ''granitgrus'', if it was thoroughly tramped down, produced a very good seal. Oderteich Winter

In 1712, the deputy chief miner (''Vizebergmeister''), Caspar Dannenberger, wrote two letters to the Clausthal Mining Office, in which he proposed the construction of the Oderteich dam, sealed with granitic sand, as well as two embankments made of granite blocks. This proposal was duly taken up, but Caspar Dannenberger, the father of the Oderteich dams, did not survive to see it completed. He died on 23 April 1713. Planning was carried out in 1714 and, on 14 September, King George Louis authorised the construction of the Oderteich dam in a ''most gracious special decree'' and made the available the sum estimated for the project: a total of 3048

thaler A thaler or taler ( ; , previously spelled ) is one of the large silver coins minted in the states and territories of the Holy Roman Empire and the Habsburg monarchy during the Early Modern period. A ''thaler'' size silver coin has a diameter o ...

s and 27 gl. The construction phase actually started in early 1715. Initially a core trench was excavated into which the dam was embedded. The building material for this was excavated from the land earmarked for the future reservoir. Granite blocks needed for the up- and downstream sides of the dam were cut from small quarries; this also provided a supply of granitic sand. The mining of material for the dam enabled the future reservoir capacity to be further enlarged. The work took until 1722. Because excess water could neither be drained through the relatively small bottom outlet during the construction period nor be impounded, a

spillway A spillway is a structure used to provide the controlled release of water downstream from a dam or levee, typically into the riverbed of the dammed river itself. In the United Kingdom, they may be known as overflow channels. Spillways ensure tha ...

had to be continuously available whilst the dam was being built. This spillway had to be laid higher and higher as the dam grew in size. Shortly after work on the dam had started there were discussions about raising it even higher than originally planned. In 1717 it was agreed to increase its height from the initial seven '' Lachter'' to nine ''Lachter'' (roughly equating to just under 18 metres). This resulted in an increase in the dam volume of around 55 percent and a doubling of the reservoir capacity to 1.67 million cubic metres. The cost of construction rose considerably during the build phase, not just as a result of the increase in dam height, but also other factors: in the end the Oderteich dam cost around 11,700 Reichstalers, almost four times the originally estimated and approved cost. Because the structure was so convincing, however, subsequent requests and approvals for the additional extra funding were not a problem. The three chief miners (''

Bergmeister A ''Bergmeister'' () was a mining, mine manager or foreman in German-speaking Europe who, along with the ''Bergvogt'', was one of the officials serving on a mining court (''Berggericht''). There were ''Bergmeisters'' in every mining district (Germa ...

'') and excavation engineers (''Grabensteiger'') were rewarded in 1724 with bonuses ranging from 12 to 100 Talers. The Sankt Andreasberg mine ground to a standstill in 1913. From then on the water of the Oderteich was used to generate electricity instead. It was now routed to Sankt Andreasberg itself, where it was used in several power stations and especially the Samson Pit. The water is still used for this purpose today and guarantees the economic operation and maintenance of the Oderteich and Rehberg Ditch. In the late 1920s the Harzwasserwerke planned a further significant increase in the height of the Oderteich Dam. The dam and the geology of the surrounding area were thoroughly examined. Digging exposed the seal of ''granitgrus'' and it was confirmed that this could not easily be worked with clay picks. These plans were later shelved for reasons that are not clear, although it is likely that the project was not seen as economical.

Design

The dam on the Oderteich differs considerably from the other dams in the Upper Harz Water Regale. The structure is clearly higher and the reservoir capacity just under three times the volume of the largest ponds around Clausthal-Zellerfeld and Hahnenklee. In addition completely different construction materials were employed.

Dam structure

In the middle of the dam is a core, up to 11.5 metres thick, made of firmly tamped down, granitic sand. Left and right of that apparently 'normal' dam fill material was used. The up- and downstream sides were given cyclopean stonework walls made of large blocks of granite with a slope of 1:0.5. On the downstream side the dam measures about 19 m from its base. The dam wall has proved to be a very durable structure and, like the entire reservoir, is still virtually in its original condition. Fundamentally it is not quite clear whether the Oderteich dam structure is an earth-fill dam or a

masonry dam Masonry dams are dams made out of masonrymainly stone and brick, sometimes joined with mortar. They are either the gravity or the arch-gravity type. The largest masonry dam in the World is Nagarjuna Sagar Dam, Andhra Pradesh and Telangana, in ...

. It is probably a combination of the two.

Outlet

The tendency for the Oderteich to use particularly long-lasting materials is best demonstrated in the design of the bottom outlet. At the lowest point in the valley, a shaft was let into the dam, that had a cross-section of about 1.10 × 1.20 m. This shaft is edged with large, hewn granite blocks and descends to the natural floor of the valley. From the bottom of the shaft runs a tunnel, 0.75 metres wide and 0.90 metres high into the reservoir. This tunnel enables the water in the reservoir to equalise with the water level in the shaft. From the bottom of the shaft, two wooden chutes (''Holzgerenne'') lead to the downstream side of the dam base. These chutes, made of oak, have a right-angled cross-section of about 25 × 25 cm and feature a valve on the bottom of the shaft that operates in a similar way to the type of bathplug that is closed from below. This valve can be pushed up or pulled shut using a rod operated from the control hut or ''Striegelhaus'' positioned above the shaft. The oak chute is designed in such a way that, even when the valve is shut, it remains submerged in water and so does not rot. This explains why the oak chutes fitted in 1715 are still being used. The entire bottom outlet design is known in the Upper Harz as a ''Striegel''.

Spillway

Every reservoir needs a

which ensures that even under conditions of exceptionally high water, the water does not rise to an uncontrollably high level, possibly even pouring over the dam crest. On the Oderteich the spillway is located at the left hand (eastern) end of the dam. In the reservoir in front of the outlet are several stone columns, like

standing stones A menhir (; from Brittonic languages: ''maen'' or ''men'', "stone" and ''hir'' or ''hîr'', "long"), standing stone, orthostat, or lith is a large upright stone, emplaced in the ground by humans, typically dating from the European middle Br ...

in appearance, about 2.50 metres high and made of granite. They serve to keep ice floes away from the spillway, because these could jam the channel. The ''old'' outlet is clearly identifiable by a weir, which was built in 1895 by the Royal Central Smithy in Clausthal ''Königlichen Centralschmiede Clausthal''. This weir enables the maximum storage level to be raised by a metre or so. Originally the spillway ran for almost a 100 metres in a straight line southwards before the water dropped into the valley. This course is still visible in the terrain to the experienced eye. But when towards the end of the construction phase in there were still not enough stones to complete the dam wall, a steep, 80-metre-long ''Große Ausflut'' ("large spillway") was blasted out of the rock roughly 60 metres below the weir at right angles to the original spillway. Perhaps it was hoped that this would increase the hydraulic capacity of the outlet. The channel carved out of the rock is very impressive especially on the spillway. To begin with the spillway was not large enough: in December 1760 water overspilled the dam during a period of exceptional high water. Damage was limited, which said something, especially about the stable design of the dam. In response to this incident the dam was raised by about a metre, which has probably merely compensated for the subsidence of the dam since that time. In 1886/87, however it appears that the conclusion was reached that the capacity of the spillway needed to be increased. To achieve this another spillway was built a few metres east of the existing one. It had an inlet breadth of 12 metres and its crest was about a metre higher than that of the old, main spillway. This outlet crosses under the B 242 parallel to the main one and merges with the main spillway shortly before the high-water marker and its adjoining chute (''Schussrinne''). This additional spillway enables the discharge capacity during times of flood to be increased by about 50%.

Coffer dam in the reservoir

When the Oderteich is fully emptied the remains of

coffer dam A cofferdam is an enclosure built within a body of water to allow the enclosed area to be pumped out or drained. This pumping creates a dry working environment so that the work can be carried out safely. Cofferdams are commonly used for constru ...

can be seen about 200 metres above the main dam. This was built in 1898 when the Oderteich Reservoir was empty in order to reduce the water flow whilst repairs were carried out to the bottom outlet. The coffer dam must have been demolished immediately after this repair had been completed.

Operation

Capacity

As a result of the use of water for electricity generation the Rehberg Ditch drew 200 to 300 litres of water per second continuously from the Oderteich. When the inflow of water is less than its outflow, the water level in the reservoir drops. If there are several months of drought in succession, the reservoir can become empty. This happens about every five to ten years, the last occasions being in 1991, 1999 and 2003. The surcharge capacity of the Oderteich is very small. Its reservoir can only store 14% of its annual discharge. This results in unusually high fluctuations in the water level of the reservoir, which can even change from being largely empty to overflowing within a few hours. This explains why the Oderteich overflows several times a year.

Water quality

There is no

fishway A fish ladder, also known as a fishway, fish pass, fish steps, or fish cannon, is a structure on or around artificial and natural barriers (such as dams, locks and waterfalls) to facilitate diadromous fishes' natural migration as well as mov ...

at the Oderteich. The relatively acidic condition of the water is probably not a good habitat for fish. Most of the water comes from areas of

raised bog Raised bogs, also called ombrotrophic bogs, are acidic, wet habitats that are poor in mineral salts and are home to flora and fauna that can cope with such extreme conditions. Raised bogs, unlike fens, are exclusively fed by precipitation (ombro ...

and is rich in

humic acid Humic substances (HS) are colored relatively recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The color of humic substances varies from bright yellow to light or dark brown lead ...

. Humic acid also causes its brown coloration. In addition, it reduces

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...

, something which causes conspicuous white horses such as occur in the turbulence at the intakes and outlets of the reservoir.

Largest dam in Germany

The Oderteich is frequently reported as the "oldest dam and reservoir in Germany".''Oderteich'' at www.oberharz.de.
Retrieved on 16 Nov 09. This accolade is disputed, however, because in the early 18th century in the area of Clausthal-Zellerfeld, Buntenbock and Hahnenklee there was already a large number of 'reservoirs' (''Talsperren'') as defined by the German industrial norm, DIN 19700-11. What is true is that from its completion in 1722 to the end of the 19th century, a period of 170 years, it had the largest dam in Germany. Not until the Eschbach Dam in the

Bergisches Land The Bergisches Land (, ) is a low mountain range in the state of North Rhine-Westphalia, Germany, east of the Rhine and south of the Ruhr. The landscape is shaped by forests, meadows, rivers and creeks and contains over twenty artificial lakes ...

was finished in 1891 with a height of 23 m was there a higher dam than the Oderteich.

Recreation

Waterside footpath

In the middle of the 1990s, the operators of the Oderteich, the Harzwasserwerke, laid out a waterside footpath (the ''WasserWanderWeg'') as an

educational trail An educational trail (or sometimes educational path), nature trail or nature walk is a specially developed hiking trail or footpath that runs through the countryside, along which there are marked stations or stops next to points of natural, techn ...

. This runs along the dam crest to the two spillways, past the two additional basins and via the overspill marker then downhill alongside the ''Großer Ausflut'' and its chute to the downstream foot of the dam, from where there is an impressive view of the stonework on this side of the dam and where the bottom outlet and start of the Rehberg Ditch can be seen. Information boards along the path explain the various structures.

Other recreational uses

The Oderteich lies in the centre of the

Harz National Park Harz National Park is a nature reserve in the Germany, German federal states of Lower Saxony and Saxony-Anhalt. It comprises portions of the western Harz mountain range, extending from Herzberg am Harz, Herzberg and Bad Lauterberg at the southern ...

. Swimming is allowed in the southern third of the reservoir and in the vicinity of the dam. The northern area is barred to swimmers in the summer by a chain and is meant to be exclusively reserved as a

biotope A biotope is an area of uniform environmental conditions providing a living place for a specific assemblage of flora (plants), plants and fauna (animals), animals. ''Biotope'' is almost synonymous with the term habitat (ecology), "habitat", which ...

. There is a roughly 4.5 km long circular walk around the reservoir, for which good walking boots are recommended. This path is routed along board walks in places because of the terrain which resembles that of a

. Winter sports, such as skating, on the reservoir in winter are not recommended, because the large variations in the water level make the formation of a stable ice surface difficult, especially near the shores.

Dam keeper's building

Until the 1960s there was a dam keeper's house at the western end of the dam, which was used latterly as a restaurant. It was demolished when it was deemed unnecessary to have a dam keeper permanently presence on the site. The foundations of the old building can be made out behind the present day National Park bus shelter. In addition there is the former MTV Goslar ski lodge, built at the end of the 1940s and now partly hidden and which can also be booked today as accommodation for self-catering groups.

References

Sources

* * * *

External links

Information from the town of St. Andreasberg

Graphic with cross-section through the Oderteich dam

{{Authority control Dams in the Harz Upper Harz Water Regale Sankt Andreasberg Reservoirs in Lower Saxony ROderteich