Reservoir

A reservoir (/ˈrɛzərvwɑːr/; from French réservoir [ʁezɛʁvwaʁ]) is an enlarged lake behind a dam, usually built to store fresh water, often doubling for hydroelectric power generation.

The valley sides act as natural walls, with the dam located at the narrowest practical point to provide strength and the lowest cost of construction.

[citation needed] Construction of a dammed reservoir will usually require the river to be diverted during part of the build, often through a temporary tunnel or by-pass channel.

Sometimes in such reservoirs, the new top water level exceeds the watershed height on one or more of the feeder streams such as at Llyn Clywedog in Mid Wales.

Saemanguem in South Korea, Marina Barrage in Singapore, Qingcaosha in China, and Plover Cove in Hong Kong are a few such coastal reservoirs.

The use of bank-side reservoirs also allows water abstraction to be stopped for some time, for instance when the river is unacceptably polluted or when flow conditions are very low due to drought.

[11] Many service reservoirs are constructed as water towers, often as elevated structures on concrete pillars where the landscape is relatively flat.

[15] This type of infrastructure has sparked an opposition movement in France, with numerous disputes and, for some projects, protests, especially in the former Poitou-Charentes region where violent demonstrations took place in 2022 and 2023.

They argue that these reservoirs lead to a loss in both quantity and quality of water necessary for maintaining ecological balance and pose a risk of increasing severity and duration of droughts due to climate change.

They believe that these reservoirs should be accompanied by a territorial project that unites all water stakeholders with the goal of preserving and enhancing natural environments.

[19] The artificial Bhojsagar lake in present-day Madhya Pradesh state of India, constructed in the 11th century, covered 650 square kilometres (250 sq mi).

[21][20] In Sri Lanka, large reservoirs were created by ancient Sinhalese kings in order to store water for irrigation.

An initiative is underway to retrofit more dams as a good use of existing infrastructure to provide many smaller communities with a reliable source of energy.

If done with sufficient lead time, the major storm will not fill the reservoir and areas downstream will not experience damaging flows.

Accurate weather forecasts are essential so that dam operators can correctly plan drawdowns prior to a high rainfall event.

Examples of highly managed reservoirs are Burrendong Dam in Australia and Bala Lake (Llyn Tegid) in North Wales.

Occasionally reservoirs can be managed to retain water during high rainfall events to prevent or reduce downstream flooding.

Dead or inactive storage refers to water in a reservoir that cannot be drained by gravity through a dam's outlet works, spillway, or power plant intake and can only be pumped out.

Dead storage allows sediments to settle, which improves water quality and also creates an area for fish during low levels.

Active or live storage is the portion of the reservoir that can be used for flood control, power production, navigation, and downstream releases.

[33](see also List of dam failures) A notable case of reservoirs being used as an instrument of war involved the British Royal Air Force Dambusters raid on Germany in World War II (codenamed "Operation Chastise"[34]), in which three German reservoir dams were selected to be breached in order to damage German infrastructure and manufacturing and power capabilities deriving from the Ruhr and Eder rivers.

The economic and social impact was derived from the enormous volumes of previously stored water that swept down the valleys, wreaking destruction.

Other impacts on the natural environment and social and cultural effects can be more difficult to assess and to weigh in the balance but identification and quantification of these issues is now commonly required in major construction projects in the developed world[36] Naturally occurring lakes receive organic sediments which decay in an anaerobic environment releasing methane and carbon dioxide.

[37] As a human-made reservoir fills, existing plants are submerged and during the years it takes for this matter to decay, will give off considerably more greenhouse gases than lakes do.

[39] Another study published in the Global Biogeochemical Cycles also found that newly flooded reservoirs released more carbon dioxide and methane than the pre-flooded landscape, noting that forest lands, wetlands, and preexisting water features all released differing amounts of carbon dioxide and methane both pre- and post-flooding.

[41] The Tucuruí Dam in Brazil (completed in 1984) had only 0.4 times the impact on global warming than would generating the same power from fossil fuels.

[39] A two-year study of carbon dioxide and methane releases in Canada concluded that while the hydroelectric reservoirs there do emit greenhouse gases, it is on a much smaller scale than thermal power plants of similar capacity.

After a reservoir's initial formation, there is a large increase in the production of toxic methylmercury (MeHg) via microbial methylation in flooded soils and peat.

For this very reason, worldwide 80 million people (figure is as of 2009, from the Edexcel GCSE Geography textbook) have had to be forcibly relocated due to dam construction.

Upland reservoirs tend to have a much shorter residence time than natural lakes and this can lead to more rapid cycling of nutrients through the water body so that they are more quickly lost to the system.