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Reservoirs

Reservoirs

A reservoir is a large natural or man-made lake used for collecting and storing water when it is in plentiful supply for people to use at a later date when there is less rain. For over 5,000 years, people have built reservoirs to store the water they need to live.

Few large cities today could survive periods of drought without their reservoirs, even where most of their water is obtained by direct abstraction from a river, reservoirs can still be needed to ensure that water is available during drought periods so that too much water isn’t taken from the environment.

There are two main types of reservoir:

Direct supply reservoirs - store water and supply it straight to a water treatment works.

River regulating reservoirs - store water during rainy periods and release extra water into rivers when needed so that it can be taken out further downstream for treatment.

We have two large reservoirs of the first type in the southern part of the operations at Hanningfield and Abberton. They are pumped storage reservoirs, which means water is pumped from the rivers Chelmer, Blackwater and Stour to fill them, rather than simply relying on rainfall from their limited catchment area. They were both formed by damming shallow river valleys, and consequently are much less deep compared to reservoirs in hilly districts. Hanningfield contains about 25,000 megalitres of water and we have just completed a £150 million enlargement scheme to Abberton Reservoir to increase its capacity to 41,000 megalitres, securing the water supply to our customers for the future.

An example here in the north is Derwent Reservoir which was formed by an earth dam across the River Derwent, a tributary of the River Tyne and forms the county boundary between Northumberland and County Durham. Water from the reservoir flows through 2.2 miles of twin 41 inch diameter pipeline to Mosswood Water Treatment Works.

The reservoir collects the water running off 27,200 acres of catchment. The average rainfall of the catchment is 37.5 inches (953mm). The reservoir provides a reliable daily yield of 138.6 megalitres. The Reservoir is 3.5 miles (5.6 km) long and covers an area of 1,000 acres. The maximum depth of the water is 30m at the dam, and when full the reservoir will hold 50,000 megalitres.

Dams

In order to keep water in a reservoir an embankment dam is built at one end forming an enormous impenetrable wall across a valley. It has a sloping face, which is usually covered with grass to help it blend in with its surroundings. Seen in cross-section, a dam looks like an enormous triangle of clay, earth and rock. Both sides slope outwards from the top. The dam is thickest at its base to withstand the pressure of the body of water it holds back. The side in contact with the water is covered in concrete slabs which prevents erosion of the dam by waves or general water action within the reservoir. There is a solid core of clay, which resists the action of any water, which seeps into the dam from the outer more permeable wall.

 

There is usually a spillway to the side of the dam, which acts as an overflow when the reservoir becomes too full. Dams often have a road across the top to give access to the surface of the dam and to either shore. Although impenetrable, dams constantly feed water into the original valley so that the river continues to run. In this way farming and other communities have enough water for their needs regardless of the season. The dam can also reduce or remove the risk of flooding by storing excess rainfall. Water can be released from the reservoir into rivers by special pipes that go through the dam. This is known as the Compensation Flow, which gives riparian owners downstream of the dam access to water that has been taken away by blocking the old river valley.

Water towers and boreholes

Water Towers are more common in the south east where the land is quite flat. Their purpose is to store cleaned water which is pumped from the water treatment works overnight. 

They are usually built on surrounding higher ground and gravity feed on demand, water to the lower lying communities they serve. 

This is an environmental friendly method of supply as it reduces the amount of power used to otherwise pump water to homes in certain geographical areas.

Water is also available from boreholes, and is particularly common in the Suffolk area. Boreholes are drilled to gain access to aquifers, these are underground rivers, or for instance layers of chalk which absorb water. 

Water drawn in this way is of excellent quality, and once pumped to the surface goes through fewer cleaning processes at the water treatment works before arriving at your tap.

Kielder transfer scheme

How the scheme works

The Kielder Water Scheme is a regional transfer system designed to allow water from Kielder Water in the North West to be released into the Rivers Tyne, Derwent, Wear and Tees. This water is used to maintain minimum flow levels at times of low natural rainfall and allows additional flows to be realised for both domestic and industrial abstraction.

The water from Kielder Water begins its journey down the river North Tyne, released from the dam into the river via the valve control tower.

Depending upon the flow rate, it usually takes about 24 hours to travel the 58 kilometres (36 miles) from the dam to the Riding Mill Pumping Station. From here, water either flows on down the river to Tyneside, or it can be pumped up to the controlling level at Letch House. Here it enters a short three mile tunnel, connected by a vertical shaft to Airy Holm Reservoir, from there water flows by gravity along a concrete lined tunnel. This carries it beneath the Durham fells to discharge outlets on the river Derwent at the base of Derwent dam (with an additional direct pipeline to Moss Wood treatment works), the River Wear at Frosterley and the River Tees at Eggleston. Discharge into each river is by cascade spillway which encourages reoxygenation of the water which may have been retained in the tunnel for a long period of time. It can then flow on to extraction points further downstream.

Downloadable content

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    Water and so much more at Derwent

    Thinking outside the Reservoir: The way we look after the land around our reservoirs reaps more benefits than you might imagine. Explore our new interactive PDF to find out more.

    PDF3.22mb

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    Reservoirs

    Fact sheet.

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