More than 850.000 listed dams modify approximately 60% of all rivers and streams worldwide in order to provide us with some of our basic needs. About 50.000 out of these are large dams  . Large dams are primarily defined by a minimum structural dam height above foundation of 15 meters and/ or a storage volume greater than 3*106 m3   . In the past, dams were mainly built to serve only one of the following purposes: Flood protection, navigation/recreation, water supply, irrigation. Due to a steadily increasing demand for these provided services and their spatially and temporally overlaps, as well as set sustainability goals, construction of multipurpose dams has been favored in recent years. They fulfill a number of purposes with a single facility. According to WCD and ICOLD more than one third of the World’s large dams are multipurpose dams. Among those, 6% have hydroelectric generation capacities  .
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Contrasted to a single-purpose project that serves only one purpose, a multipurpose project is designed for irrigation, power, flood control, municipal and industrial, recreation, and fish and wildlife benefits, in any combinations of two or more . A distinction is made between primary and secondary uses. It is the primary purpose that initiates planning and construction and dictates financing arrangements of a multipurpose dam. Secondary uses, which were initially planned or attributed as additional functions after planning and construction to make a dam more profitable, have less priority regarding operational management of the reservoir. Since the population receives several domestic and economic benefits from a single investment, a multipurpose dam is a very important and cost effective project for developing countries.
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Based on the assumption that hydroelectric power generation is a dam’s primary use, a variety of economic benefits can be yield by adding other uses. The more compatible these different uses are, the easier becomes the management of a reservoir . Hence it is important to understand a project’s purposes and their relative compatibility.
Water demand for irrigation is defined as a consumptive use and is characterized by seasonal variation. It depends on precipitation and cropping patterns. In order to meet high demands when rainfall is low, reservoirs store runoff during times of high rainfall and low demand.
A distinction is made between storage reservoirs, which have sufficient storage capacity to regulate stream flows, and run-off-the-river projects, whose storage capacity is relatively limited. Hydroelectric power projects are operated as base load stations or as peaking stations. Seasonally -and to a smaller extent daily and hourly- variations determine the water demand of power generation. In general, reservoirs volumes are full to be able to face the next dry season and generate as much power as possible. Hydropower typically generates maximum output from maximum storage levels. Water passing through the turbines can be used for consumptive uses (e.g. irrigation) downstream.
Municipal and Industrial Water Use
Compared to the water demand of irrigation and power generation, required water volume for municipal and industrial uses are relatively constant throughout the year. Peak demand is observed in summer. A continuous increase in water demand over a longer period of time occurs, if population grows rapidly and/or industries expand.
Storage reservoirs are often designed to maintain a sufficient channel flow downstream to make the stream navigable. Required water volume, depending upon type and volume of traffic in a waterway, shows a marked seasonal variation.
In general, publicly recreational benefits, such as boating, swimming and fishing, are only secondary uses. Large and rapid fluctuations in reservoirs’ water levels or fluctuations in downstream releases usually restrict recreational activities.
Water Quality Control
Dam construction significantly affects both water quality and aquatic environment by reducing channel flow downstream and transforming a lotic into a lentic ecosystem. In order to mitigate negative impacts, the maintenance of adequate flows downstream of a barrier has to be guaranteed.
In order to minimize the possibility that peaks from floods in different tributaries arrive coincidently at the same time in the main-stem of a river and cause severe damage, large dams are used to store all or a portion of flood waters, and release these controlled and slowly over time. Creating a waiting volume by releases in anticipation of floods is needed to cope with water volumes during seasons of heavy rainfall. Hence, forecast of inflows into the reservoir plays a vital role in dam operation.
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According to the Agence Française de Développement and the World Bank (2009) , the “development of large multipurpose storage facilities (often combined with hydropower generation) is necessary for mitigating the economic effects of hydro-climatic variability, for ensuring reliable water supply, and for using available water” in Africa. If multipurpose projects are implemented in a basin-wide context, they generally result in optimal water development and maximize economic returns on investments. Some economic benefits which result from uses added to hydropower projects are listed below:
Water Supply & Sanitation
Case studies showed that water supply from dams, built for other uses, emerges over time, if it was not planned from the beginning. Access to water results in an improvement of water supply and sanitation which in turn results in economical savings because expenditures on health care, water treatment and inefficient land management are reduced. The WHO (2004)  figured out that every US$ 1 invested in this sector generates US$ 3 to 34.
Integrated Agriculture makes up to 70% of the World’s water withdrawals and secures about 40% of worldwide food production . Hence, additionally permanent sources of water for irrigation increase net farmer income, employment opportunities, gross national agricultural income, crop diversity and share of high-yielding cash crops and minimize dependency on rain-fed water sources .
If a number of dams are located strategically within a river basin, flood control is effective and efficient and profitable catchment management is enabled. Due to mitigated floods, crop losses are avoided, agricultural subsidies are used more efficient as land is not flooded and stability of land markets is increased because floods do not cause severe shortage in agricultural land . Furthermore, it was observed that “the provision of flood control facilities reduced the incidence of flood and encouraged farmers to intensify production” . Benefits of flood mitigation are valued based on reduced or avoided damages .
Making rivers navigable represents an effective alternative to transportation on roads and railways. Shipping is comparatively cost-efficient because number of barges decreases as load per barge increases and it minimizes petroleum dependency due to fuel saving .
Like natural lakes, man-made reservoirs are attractive to people as recreational areas. Employment opportunities and small businesses are created for/by local community as well as environmentally sustainable site management is initiated to maintain touristic interest and additional source of income .
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Climate Change Adaption
Against the background of climate change, multipurpose dams, combined with political and institutional strategies, offer important solutions to climate friendly energy resource development and risk provision management, especially in vulnerable regions. Although effects of climate change on a local level are imprecise from a certain degree on, a decrease in river flows and an increase in major river floods are predicted on a regional/global scale . Adding other uses to hydroelectric dams, offer, among others, the following solutions to climate change adaptation needs: An increase in water storage capacity contributes to improving and securing access to water - for municipal and industrial uses, as well as for irrigation - in regions, such as Southern Africa, which are likely to experience compounded water stress. Basin-wide management and an additional dam’s waiting volume help to control floods in regions like Southeast Asia which are likely to face future extreme precipitation events. Hence, multipurpose dams are contribution to mitigating and minimizing risks, caused by climate change, associated with crop yields, water supply and energy systems, housing and extreme events.
World Commission on Dams (WCD) Report & Hydropower Sustainability Assessment Protocol
Triggered by protest against, controversial discussions about and withdrawals of financial support of large hydropower projects in the 1990s, the WCD was founded in 1997 to review dam projects and give recommendations for improving the sustainability of hydropower projects. The seven following strategic priorities for project planning are suggested in the WCD Report (2000): (1) gaining public acceptance, (2) comprehensive options assessment, (3) addressing existing dams, (4) sustaining rivers and livelihoods, (5) recognizing entitlements and sharing benefits, (6) ensuring compliance, and (7) sharing rivers for peace, development and security .
The Hydropower Sustanability Assessment Protocol (http://www.hydrosustainability.org/Protocol.aspx) can be seen as advancement of the ambitious recommendations of the WCD, and was developed between 2008 and 2011 by a multistakeholder forum (NGOs, governments, commercial and development banks, hydropower operators and developers, and hydropower consultants, contractors and suppliers). It has been designed to be applied to projects and facilities at all stages of development worldwide. The Protocol comprises five documents for the sustainability assessment of hydropower projects at all stages of development– a Background document and four assessment tools for the different stages of the project life cycle: Early Stage, Preparation, Implementation (= construction), and Operation . A recent report  rates the Protocol as the “most practical and effective tool currently available for measuring […] the degree of respect for WCD guidelines and general good practice of individual projects […].”
Compliance to the WCD Report and the Hydropower Sustainability Assessment Protocol during a multipurpose project’s life cycle contributes to sustainable development of dams, ensures that dams provide development benefits without worsening the overall situation and helps to avoid conflicts known form the 1990s.
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Sharing water amongst competing users is a major challenge. There are three types of conflicts , regarding reservoir operation, which may arise from dams serving more than one purpose. These are:
Conflict in Space
These types of conflict occur if the available volume of the reservoir has to be divided for storing water for divergent objectives. A dam of sufficient height can be built and storage space can be clearly allocated for each purpose, if geological and topographic features of the dam site and the funds available for the project permit. However, this may not be an economical proposition.
Conflict in space arises, for example, if a reservoir is used for flood protection and conservation. Conservational purposes require the reservoir as full as possible at the end of the filling period. Flood protection in contrast needs empty storage volume to maximize retention of incoming floods. To prevent severe damages, flood control is given priority.
Conflict in Time
If water use patterns vary depending on the purpose, this kind of conflict appears. Since water releases might be optimal for one purpose, but not for another, prioritization is required.
An example for conflict in time is observed at dam sites where the uses are irrigation and water supply. While the water volume needed for industrial and domestic water supply is almost constant throughout the year, irrigation varies seasonally. A certain water level should be maintained to secure water supply and satisfy irrigation purposes during dry seasons.
Conflict in Discharge Purpose
At the time water is released, problems may arise, if the release of water is required for more than one consumptive use at the same time, and the amount stored is not sufficient for all of them .
Conflicts of discharge are experienced if a reservoir’s purposes are power generation and consumptive uses. Releases for the two purposes may vary considerably during the course of the day. Small conservation pools downstream of the powerhouse are used to mitigate the fluctuations in releases to meet varying power demands and securing water volume for consumptive uses.
Most multi-purpose dams are funded by governments. They are often supported by international donors. Attracting private investors for financing multipurpose projects is difficult. Firstly, achieving a sensible balance between the interests of the private investor, the consumer and the host government is complicated and often results in complicated and potentially vulnerable contract structure . Secondly, low creditworthy of various users causes multipurpose projects often lacking financial viability, making it unattractive for private investors . Since single purpose dams secure higher returns on investment, they are promoted as more economically attractive.
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Serre-Ponçon Reservoir, France
The Serre-Ponçon dam gathers the waters of the Durance and the Ubaye rivers. Energy generation is the primary purpose of the reservoir. Its second purpose is flood control. Other purposes include recreation and water supply. To meet demands of all types of use, a complex operating plan was developed:
- July – September: potential 200 Mio. m³ of water are available to cope with irrigation and water supply needs
- Farmers are encouraged to save water by a modern financing systems for water use reduction;win-win-situation: lower expenditures for farmers, better seasonal use for hydropower with saved water volume and environmental benefits
- Basin sediment strategy
- Long term registration of recreation projects to ensure use-compatibility.
Arthurs Lake, Australia
The primary purpose of Arthurs Lake is energy generation. Trout fisheries, irrigation and recreation were added as other uses. In order to secure needed water volumes for the different uses, stakeholder agreed on the following:
Water supply agreement: Tasmanian Irrigation pays Hydro Tasmania a fee for taken water that equals an estimated value of revenue that would have been made, if that water was used for power production
- 1 July – 30 November, 1 May – 30 June: winter ‘dam fill’ price
- 1 December – 30 April: ‘direct take’ price, irrigate crops
- Annual price: charges for water that could be taken at any time throughout the coming financial year
Agreement on a minimum lake level between Hydro Tasmania, Tasmanian Irrigation & Inland Fisheries Service:
- 1 June: 948 m
- 1 November: 949 m
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- ↑ http://www.bund.net/nc/presse/pressemitteilungen/detail/artikel/weltwassertag-2014-energiehunger-bedroht-weltweit-fluesse/
- ↑ 2.0 2.1 2.2 2.3 2.4 World Commission on Dams (WCD) (2000): Dam and Development – A New Framework for Decision-Making. (Earthscan Publications Ltd) London.
- ↑ http://www.britishdams.org/student_zone/documents/BDS_Fact%20Sheets_version_low%2 0Res _Web.pdf
- ↑ 4.0 4.1 http://www.icold-cigb.org/GB/World_register/general_synthesis.asp
- ↑ Lempérère, F. & R. Lafitte (2006): The role of dams in the XXI Century to achieve a sustainable development target. In: Barga et al. (Ed.): Dams and Reservoirs, Societies and Environment in the 21st Century (2). (Taylor & Francis), London.
- ↑ 6.0 6.1 International Commission on Large Dams (ICOLD) (2007): Dams and the World’s Water. (ICOLD), Paris.
- ↑ 7.0 7.1 7.2 7.3 7.4 7.5 Nile Basin Initiative (NBI) (2008): Review of Hydropower Multipurpose Project Coordination Regimes – Best Practice Compendium. Online: http://nileis.nilebasin.org/system/files/NBI_%20Best%20Practice%20Compendium_Final.pdf
- ↑ 8.0 8.1 Government of India (2005): Real Time Integrated Operation of Reservoirs. Online: http://wrd.bih.nic.in/guidelines/RealReservoirs.pdf
- ↑ 9.0 9.1 9.2 Dankova, R., Ueda, S., Subramanian, A., Yu, W., Mody, J & Alavian,V. (2009): Water Resources: A Common Interest. In: Foster, V. & C. Briceño-Garmendia (Ed.): Africa’s Infrastructure: A Time for Transformation, p. 217-285. (A copublication of the Agence Française de Développement and the World Bank).
- ↑ Sanctuary, M., Tropp, H. & A. Berntell (2004): Making Water a part of Economic Development. The Economic Benefits of Improved Water Management and Services (A copublication of the WHO and the SIWI). Online: http://www.who.int/water_sanitation_health /watandmacrtoc.pdf?ua=1
- ↑ http://www.weltagrarbericht.de/themen-des-weltagrarberichts/wasser.html
- ↑ 12.0 12.1 Aylward, B., Berkhoff, J., Green, C., Gutman, P., Lagman, A., Manion, M., Markandya, A., McKenney, B., Naudascher-Jankowski, K., Oud, B., Penman, A., Porter, S., Rajapakse, C., Southgate, D., & R. Unsworth (2001): Financial, Economic and Distributional Analysis, Thematic Review III.1 (prepared as an input to the World Commission on Dams). Online: http://www.centre-cired.fr/IMG/pdf/F5_Fin_Ec_Dist_WCD_.pdf
- ↑ Intergovernmental Panel on Climate Change (IPCC) (2013): Climate Change 2013 – The physical Science Basis. Online: http://www.ipcc.ch/publications_and_data/publications_and _data_reports.shtml
- ↑ International Hydropower Association (2010): Sustainability Assessment Protocol. Online: http://www.hydrosustainability.org/IHAHydro4Life/media/PDFs/Protocol/hydropower-sust ain ability-assessment-protocol_web.pdf
- ↑ Skinner, J. & L. Haas (2014): Watered down? A review of social and environmental safeguards for large dam projects. Natural Resources 28. (IIED), London.
- ↑ 16.0 16.1 Branche, E. (2015): “Sharing the water uses of multipurpose hydropower reservoirs: the SHARE concept” -Main document – Final. Online: https://www.hydropower.org/sites/default /files/publications-docs/Multipurpose%20water%20uses%20of%20hydropower%20reservoirs.pdf (published within the Electricité de France (EDF) and the World Water Council (WWC) partnership).
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