RESERVOIRS

A reservoir – artificial lake or dam is used to store water.

Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.

The term reservoir may also be used to describe underground reservoirs such as an oil or water well.

Five thousand years ago, the craters of extinct volcanoes in Arabia were used as reservoirs by farmers for their irrigation water.

Dry climate and water scarcity in India led to early development of water management techniques, including the building of a

reservoir at Girnar in 3000 BC. Artificial lakes dating to the 5th century BC have been found in ancient

Greece. An artificial lake in present-day Madhya Pradesh province of India,

constructed in the 11th century, covered 650 square metres (7,000 sq ft).

In Sri Lanka large reservoirs have been created by ancient Sinhalese kings in order to save the water for irrigation.

The famous Sri Lankan king Parākramabāhu I of Sri Lanka stated " do not let a drop of water seep into the ocean without benefiting

mankind ". He created the reservoir named Parakrama Samudra (sea of King Parakrama), which has astonished archaeologists.

Types of ReservoirsValley dammed reservoir – A dam constructed in a valley relies on the natural topography to provide most of the basin of the reservoir. Dams are typically located at a narrow part of a valley downstream of a natural basin. The valleysides act as natural walls with the dam located at the narrowest practical point to provide strength and the lowest practical cost of construction. In many reservoir construction projects people have to be moved and re- housed, historical artifacts moved or rare environments relocated.Where the topography is poorly suited to a single large reservoir, a number of smaller reservoirs may be constructed in a chain

Contour planfor a

reservoir

Lake Vyrnwy Reservoir. The dam spans the Vyrnwy Valley and was the first large stone dam built in the United Kingdom.

Bank-side reservoirWhere water is taken from a river of variable quality or quantity, bank-

side reservoirs may be constructed to store the water pumped or siphoned from the river.

Such reservoirs are usually built partly by excavation and partly by the construction of a complete encircling bund or embankment which may exceed 6 km in circumference.

Both the floor of the reservoir and the bund must have an impermeable lining or core

Service reservoirService reservoirs store fully treated potable water close to the point

of distribution. Many service reservoirs are constructed as water towers, often as

elevated structures on concrete pillars where the landscape is relatively flat.

Other service reservoirs are entirely underground, especially in more hilly or mountainous country.

Service reservoirs perform several functions including ensuring sufficient head of water in the water distribution system and providing hydraulic capacitance in the system to even out peak demand from consumers enabling the treatment plant to run at optimum efficiency. Large service reservoirs can also be managed to so that energy costs in

pumping are reduced by concentrating refilling activity at times of day when power costs are low.

Storage works are constructed to serve many purposes, such as forIrrigation Domestic uses

Industrial uses Hydro electric power

Navigation Flood control

Land reclamation in low lying areas Debris control

Aquaculture Preservation of aquatic life

Recreation

Depending on the purposes, the reservoir may be classified asStorage or conservation reservoirs

Flood control reservoirsDistribution reservoirs

Multipurpose reservoirs

Storage or conservation reservoirs – are constructed to store water received through excess rainfall and is released gradually when

it is needed for the following purposes - irrigation, hydroelectricity, domestic, industrial etc.

Flood control reservoirs – storage of water received through excess rains causing floods.

Then after gradually released when the flood rate decreases. This kind of storage avoids or decreases the intensity of the damage and loss due to floods.

Distribution reservoir – is a small storage reservoir used for water supply in a city.

Accounts for varying rate of water supply during the day. Such distribution permits pumping plants and water treatment works etc., to operate at constant rate.

Multipurpose reservoir – serves more than one purpose.

Selection of a site for a Reservoir – Depends on the factors like

Geological condition of the catchment area – less infiltration and maximum runoff preferred.

Leakage should be minimum – i.e through rocks – preferred rocks area shales, slates, schists, gneisses and crystalline rocks such

granites etc.

Suitable dam site – water tight rock base, percolation below the dam should be minimum. Cost of dam is often a controlling factor.

Narrow valley opening – the reservoir basin should have a narrow opening in the valley to have a shortest length of the dam

Cost – cost of real estate for the reservoir, road, rail road, re location of dwellings etc., should as low as possible.

HYDROLOGICAL INVESTIGATIONS

It is an important aspect of the reservoir planning.

The capacity of the irrigation canals and the installed capacity of the power houses depends on the available supplies from the reservoir.

Investigations are divided into two heads.

1.Study of run off pattern at the proposed dam site and to determine the storage capacity to meet the given demand

2.Determination of the hydrograph of the worst flood, to determine the spillway capacity and design.

Topography – should have adequate capacity without submerging excessive land and villages and other properties

Capacity of the reservoir – depth of the reservoir must be as deep as possible to hold more water within a short area. Shallow reservoirs occupy more land and involves high costs of acquiring land.Less evaporation loss when compared to wide reservoirsLess likelihood of weed growth. Desilting can be done at longer intervals which is recurring

expenditure and maintenace compared to shallow reservoirs

Suitability of the water in reservoir – should be free from natural pollutants such as mineral deposts especially from the radioactive mineral deposits. Otherwise the water enriched in such elements as U, Th and other objectionable elements as Na, F, K and Nitrates, Carbonates will cause health hazards and cannot be used even for irrigation purposes.

Zones of storage in ReservoirNormal pool level – Maximum level of water in the reservoir during

operation conditions. It corresponds to the level of spillway crest, or to the top of the spillway crest gates.

Maximum pool level – corresponds to the water level during the design flood.

Minimum pool level – the lowest level of water in the reservoir where water is to be drawn under ordinary operating conditions

The different zones areUseful storage – the water stored between the normal pool level and the minimum pool level.Surcharge storage – the water stored between the normal pool level and the maximum level

corresponding to a floodDead storage – the water below the minimum pool level not useful under normal operating

conditions.Bank storage and Valley storage – referred to the water stored in the pervious formation of the

river banks and the soil above it. It depends on the geological conditions of the river banks. The bank storage effectively increases the capacity of the reservoir above that

indicated by the elevation-storage curve

Reservoir storage is divided into four zones, or pools. These include, from top to bottom, the flood-control zone, conservation zone,

buffer zone and inactive zone. The conservation and buffer pools, together, constitute the reservoir's active

storage. Ensure that the flood-control zone is always kept vacant, i.e., the volume of

water in the reservoir cannot exceed the top of the conservation pool.

Reservoir YieldIt is also storage capacity and yield.Yield – it is the amount of water that can be supplied from the reservoir

in a specified interval time. The interval of time is dependent on the size of the reservoir from a day for small reservoir to a year for large conservation reservoirs.Ex – if 35,000 cubic meters of water is supplied from a reservoir in one year, its yield is 35,000 cu.m/yr or 3.5 hectare meters/yr.

Safe Yield or Firm Yield – Maximum quantity of water that can be guaranteed during a critical dry period

Secondary Yield – the amount of water available in excess of safe yield during high flood periods.

Average Yield – the arithmatic average of the first and the secondary yield over a long period of time is called average yield.

Flood Hydrograph of Inflow shows inflow of several years.

A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow. The rate of flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).

Mass Inflow CurveA mass inflow curve is a plot between the cumulative inflow in the reservoir with time.

The reservoir capacity corresponding to a specified yield is determined with the help of mass inflow curve and the demand curve.

A mass inflow curve continuously rises as it shows accumulated inflow.

If there is no inflow during certain period, the mass inflow curve will be horizontal during that period.

The mass inflow curve will rise very sharply during the period of high flood.

The curve will rise sharply during the period of high flood.

The steepness of the curve indicates the rate of inflow.

Demand CurveIt is a plot between accumulated demand with time. Uniform demand leads to a straight line. Demand curve indicates variable demand rate over a time.

Estimation Of Reservoir CapacityReservoir capacity may be estimated by using mass inflow curve.A mass inflow curve is prepared over the years with cumulative inflow of water. A mass demand curve is also prepared on the same scale.The tangent drawn at the apices of mass inflow curve A1, A2, A3, …is the Mass Demand Curve

E1D1, E2D2, E3D3 etc., are between the tangent and the mass inflow curve.The vertical intercepts indicate the volume by which inflow falls short of demand.C1D1 – net flowC1E1 – demandE1D1 – to be provided from the reservoirE1D1, E2D2, E3D3 etc. – required reservoir capacity

It should be noted that the vertical distances between successive tangents represents water wasted ove the spillway.

Reservoir Sedimentation

Silting – Sedimentation in the reservoir is known as silting.

silting is the accumulation of the transported material/debris/rock debris/silt or mud in the reservoir.

Many of the reservoirs have become extinct due to bad maintenance with regard to desilting from time to time.

Silting in the reservoir depends on the amount of load carried by the river or stream, weathering and erosional capacity, transportation over a period of time.

The other factors are – Nature of the soil in the catchment areaTopography of the catchment areavegetation cover andintensity of rainfall.

Important factors – nature of soil at the catchment areaThe sediment transported by the river - divided into two heads.1.Bed load – the bed load is dragged along the bed of the stream. 2.Suspended load – the load is kept in suspension because of the vertical component of the eddies formed due to friction of flowing water against the bed. The bed load is generally smaller. It is only 10-15% of the suspended load. The coarser particles settle down near the dam in the dead storage due to reduced velocity.

Density currents – defined as a gravity flow of fluid under another fluid of approximately equal density. In the reservoirs the water stored is usually clear and the inflow is generally muddy. The two fluids have different densities and the heavy turbid water flows along the channel bottom towards the dam under the influence of the gravity. This is known as density current. The rate of silting in case of reservoirs reduces if the density currents are vented by proper location and operation of outlet and sluice gates.

Measurement of sediment load – sediment sample to be collected at regular intervals of ime and at different depths of the reservoir. The sample are to be dried and the amount of sediment material is calculated. It is calculated as ppm or weight percentage.

Resevoir life – It is dependent on the maintenance of the reservoir and the amount of siltattion taking place at every flood season. A dead storage in the reservoir is provided for silting the fine materials. The water in the dead storage is

unutilized. If proper care is not taken in the maintenance of the reservoir i.e ., desilting then the effective storage or live storage will be affected leading to failure of the reservoir or reducing the effective or live storage of the reservoir.

Trap efficiency (η) – This is a measure of reservoir sedimentation. The investigations show that the trap efficiency is a function of the ratio of reservoir capacity to the total inflow

The rate of silting is much higher in the initial stages of the inflow into the reservoir, then the rate of silting will reduces as silting goes on. Therefore the complete filling of reservoir may take quite a long period.with regards to the small reservoir on a large stream

A small reservoir on a large dam having a small capacity has a large inflow rates.

It has a small capacity/inflow ratio.

The trap efficiency of such reservoirs is extremely small and the stream passes most of its inflow quickly and the finer sediments are discharged

without getting time to settle.

On the contrary A large reservoir on a small stream with less inflow rates has higher capacity/inflow ratio.

Such reservoirs has a greater trap efficiency.

Such reservoirs retain water for several years and permit almost complete deposition of the sediment.

Life of a reservoir

1. Capacity of reservoir – is calculated from the study of inflow rates, leakage factors, type of rocks and structure, topography, drainage basin analysis and trap efficiency etc.

2. Assume 10 % trap efficiency for reduced capacity due to silting.

3. For the interval of 10 % capacity, average trap efficiency is calculated by taking the average of trap efficiency (η).

4. Sediment inflow rate is determined by drying the sediments from the water samples.

5. The total annual sediment transported is multiplied by the trap efficiency and is converted into hectare-metre (volumetric) units for one year.

6. Volume interval (10 % of the capacity) is divided by the sediment deposited in order to get number of years to fill this volume interval of 10 % capacity.

7. This procedure is repeated further at 80 %, 70 %, 60 %,….. 20 % of the capcity. The total life of the reservoir = total number of years required to fill each of the volume intervals.

Reservoir Sediment Control

1.Proper selection of reservoir site

2.Control of sediment Inflow

3.Proper designing and reservoir planning

4.Control of sediment deposit in the reservoir

5.Removal of sediment deposit

6.Erosion control in the catchment area