Reservoirs

A reservoir is a large natural or man-made lake used for collecting and storing water when it is plentiful for people to use at a future date. For over 5000 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.

There are two large reservoirs of the first type at Hanningfield and Abberton, and both contain about 25,000 Megalitres.  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.

The dam at Hanningfield is in the Guinness book of records, being the longest dam in the UK.  Plans are underway to increase the capacity of Abberton to 40,000 megalitres by raising the  bank height all the way round.

Derwent reservoir is 40 years old this year! 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.

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.