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Chapter: Civil - Water Resources and Irrigation Engineering - Reservoir Planning and Management

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Multipurpose reservoirs

Water supply, Flood control, Soil erosion, Environmental management, Hydroelectric power generation, Navigation, Recreation, Irrigation

Multipurpose reservoirs

 

Water supply

 

Flood control

 

Soil erosion

 

Environmental management

 

Hydroelectric power generation

 

Navigation

 

Recreation

 

Irrigation

 

ü        The multipurpose nature of these facilities dictates that the agencies which manage them are responsible for balancing competing demands.

 

ü        For example, managers responsible for hydroelectric power generation often want to keep lake levels as high as possible, since the water stored in the reservoir serves as a kind of "fuel" for their generators.

 

ü        However, managers responsible for flood control often want to keep lake levels as low as possible to provide the maximum amount of storage capacity for rainwater runoff.

 

Water supply

 

ü        Water supply is the provision of water by public utilities, commercial organizations, community endeavors or by individuals, usually via a system of pumps and pipes.

 

ü        Irrigation is covered separately.

 

Flood control

 

ü        Floods are caused by many factors: heavy rainfall, highly accelerated snowmelt, severe winds over water, unusual high tides, tsunamis, or failure of

 

dams, levees, retention ponds, or other structures that retained the water.

 

ü        Flooding can be exacerbated by increased amounts of impervious surface or by other natural hazards such as wildfires, which reduce the supply of vegetation that can absorb rainfall.

 

ü        Periodic floods occur on many rivers, forming a surrounding region known as the flood plain.

 

ü        During times of rain, some of the water is retained in ponds or soil, some is absorbed by grass and vegetation, some evaporates, and the rest travels over the land as surface runoff.

 

ü        Floods occur when ponds, lakes, riverbeds, soil, and vegetation cannot absorb all the water. Water then runs off the land in quantities that cannot be carried within stream channels or retained in natural ponds, lakes, and man-made reservoirs.

 

ü        About 30 percent of all precipitation becomes runoff and that amount might be increased by water from melting snow.

 

ü        River flooding is often caused by heavy rain, sometimes increased by melting snow.

ü          A flood that rises rapidly, with little or no advance warning, is called a flash flood.

 

ü        Flash usually result from intense rainfall over a relatively small area, or if the area was already saturated from previous precipitation.

 

Soil erosion

 

ü        In geomorphology and geology, erosion refers to the actions of exogamic processes (such as water flow or wind) which remove soil and rock from one location on the Earth's crust, then transport it to another location where it is deposited.

 

ü          Eroded sediment may be transported just a few millimeters, or for thousands of

 

kilometers.

 

ü        While erosion is a natural process, human activities have increased by 10-40 times the rate at which erosion is occurring globally.

 

ü          Excessive (or accelerated) erosion causes both 'on-site' and 'off-site' problems.

 

ü        On-site impacts include decreases in agricultural and (on natural

 

landscapes) ecological collapse, both because of loss of the nutrient-rich upper soil layers. In some cases, the eventual end result is desertification.

 

ü        Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses.

 

ü        Water and wind erosion are now the two primary causes of land degradation; combined, they are responsible for about 84% of the global extent of degraded, making excessive erosion one of the most significant environmental problems world-wide.

 

ü        Intensive agriculture, deforestation, roads, anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion.

 

ü        However, there are many remediation practices that can curtail or limit erosion of vulnerable soils.

 

Environmental resource management

 

 

ü        Environmental resource management is the management of the interaction and impact of human societies on the environment.

 

ü          It is not, as the phrase might suggest, the management of the environment itself.

 

ü        Environmental resources management aims to ensure that ecosystem services are protected and maintained for future human generations, and also maintain ecosystem integrity through considering ethical, economic, and scientific (ecological) variables.

 

ü        Environmental resource management tries to identify factors affected by conflicts that rise between meeting needs and protecting resources.

 

ü          It is thus linked to protection and sustainability.

 

Hydroelectricity

 

ü        Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water.

 

ü        It is the most widely used form of renewable, accounting for 16 percent of global electricity generation '3,427 terawatt-hours of electricity production in 2010, and is expected to increase about 3.1% each year for the next 25 years.

 

ü        Hydropower is produced in 150 countries, with the Asia-Pacific region generating 32 percent of global hydropower in 2010.

 

ü        China is the largest hydroelectricity producer, with 721 terawatt-hours of production in 2010, representing around 17 percent of domestic electricity use.

 

ü        There are now four hydroelectricity plants larger than 10 GW: the Three Gorges Dam and Xiluodu Dam in China, Itapúa Dam across the Brazil/Paraguay border, and Guru Dam in Venezuela.[1]

 

ü        The cost of hydroelectricity is relatively low, making it a competitive source of renewable electricity.

 

ü     The average cost of electricity from a hydro plant larger than 10 megawatts is 3 to 5 U.S. cents per kilowatt-hour. It is also a flexible source of electricity since the amount produced by the plant can be changed up or down very quickly to adapt to changing energy demands.

 

ü        However, damming interrupts the flow of rivers and can harm local ecosystems, and building large dams and reservoirs often involves displacing people and wildlife.

 

ü        Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse

 

gas carbon dioxide (CO

 

2) than fossil fuel powered energy plants.

 

Navigation

 

ü        Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.

 

ü        The field of navigation includes four general categories: land navigation, marine navigation, aeronautic navigation, and space navigation.

ü          It is also the term of art used for the specialized knowledge used

 

by navigators to perform navigation tasks.

 

ü        All navigational techniques involve locating the navigator's position compared to known locations or patterns.

 

ü        Navigation, in a broader sense, can refer to any skill or study that involves the determination of position and direction.

ü          In this sense, navigation includes orienteering and pedestrian navigation.

 

ü          For information about different navigation strategies that people use,

 

visit human navigation.

 

Irrigation

ü        Irrigation is the artificial application of water to the land or soil.

 

ü          It is used to assist in the growing of agricultural crops, maintenance

 

of landscapes, and re-vegetation of disturbed soils in dry areas and during periods of inadequate rainfall.

ü        Additionally, irrigation also has a few other uses in crop production, which include protecting plants against frost, suppressing weed growth in grain fields and preventing soil consolidation.

 

ü        In contrast, agriculture that relies only on direct rainfall is referred to as rain-fed or dry land farming.

 

ü        Irrigation systems are also used for dust suppression, disposal of sewage, and in mining.

 

ü        Irrigation is often studied together with drainage, which is the natural or artificial removal of surface and sub-surface water from a given area.

 

ü        Irrigation has been a central feature of agriculture for over 5000 years, and was the basis of the economy and society of numerous societies, ranging from Asia to Arizona.

 

Channel improvement.

 

At the diversion structure, a headwork regulates the flow into a canal.

 

This canal, which takes its supplies directly from the river, is called the main canal and usually direct irrigation from the waters of this canal is not carried out.

 

This acts as a feeder channel to the branch canals, or branches.

 

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Branch canals generally carry a discharge higher than 5 m /s and acts as feeder

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channel for major distributaries which, in turn carry 0.25 to 5 m /s of discharge.

 

The major distributaries either feed the water courses or the minor distributaries,

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which generally carry discharge less than 0.25 m /s.

 

Though irrigation canals may be constructed in natural or compacted earth, these suffer from certain disadvantages, like the following

 

             Maximum   velocity   limited   to   prevent

             Seepage   of   water   into   the   ground

             Possibility   of   vegetation   growth   in

 

         Possibility of bank failure,iesofburrowing either animals

 

All these reasons lead to adoption of lining of canals, though the cost may be prohibitive.

 

Hence, before suggesting a possible lining for a canal, it is necessary to evaluate the cost vis-à-vis the savings due to reduction in water loss through seepage.

 

Apart from avoiding all the disadvantages of an unlined canal, a lined canal also has the advantage of giving low resistance and thus reducing the frictional loss and maintaining the energy and water surface slopes for the canal as less as possible. This is advantageous as it means that the canal slope may also be smaller, to maintain the same discharge than for a canal with higher friction loss. A smaller canal slope means a larger command area.

 

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