Unit - 1

Reservoirs

Q1) What do you mean by reservoir?

A1)

• Reservoir can be defined as "It is either a natural pool of water (lake) or an artificial storage of surface water created by construction of a dam-wall, to be used for multipurpose like, irrigation hydel power generation, industrial uses etc."
• The surface flow of any non- perennial river or a stream always have a variation in the amount of water.
• During the rainy season, in India, all the rivers are over flooded and they create a danger to the fields and settlements along the banks, of such rivers : while in summer they have very little surface flow (some of the rivers and streams are totally dry, in some patches.
• So, in case of surface irrigation, based on the direct, lifting of water from the river, has no water to meet the demand. So it is always better to construct a dam across the river or stream, and create an artificial lake, storage or a reservoir to hold the water during the rainy season and supply it during dry summer season or throughout the year as per the requirements of the crops.
• In case of perennial rivers, in north-India, like Yamuna, Ganga, Brahmaputra, and their tributaries have ugh water to be used for irrigation, but during the flooding (twice in a year) they are over flooded, creating danger of floods.
• To control floods to conserve soil, vegetation and to reduce the danger to the human settlements along the banks; the dams are constructed to create artificial storage or reservoir to hold the extra amount of water flooding in the river, during floods; this water in the reservoir is used during the drought years to support the cultivation.
• So, the very purpose of reservoir is to hold the water, either to be used for the dry period or to control the excess flow causing floods.

Q2) Explain engineering survey in detail.

A2)

Engineering surveys

• Under these surveys, the site of the reservoir, and other associated work is surveyed and a contoured plan (showing the relief of the area) is prepared.
• From this plan only, the storage capacity of the reservoir and the water spread area, at various levels (elevations) is calculated as shown below.
• The water spread area of the reservoir at any elevation is determined directly by measuring the area covered by a contour of the elevation, by the help of the planimeter.

Fig 1: Elevation area and elevation capacity curve

• This change in the water spread areas at different height can be plotted on a graph capacity on 'x' axis and elevation on y axis. As shown in Fig.
• In the line graph, Maximum pool level, Normal pool level, and minimum pool level is shown.
• By taking the contour areas at equal interval and by summing up various formulae, the storage capacity of any reservoir can be calculated. Following the formulae used for such calculations.
• If 'V' is storage volume and A₁, A₂, A3 A, are the areas covered by the successive contours, with an interval of 'h'. Three different formulae can be sued for this purpose.

a)     Trapezoidal formula

V = (A1 +A2)

b)    Cone formula

V = (A1 +A2+

• By using any above mentioned formulae the storage volumes at various elevation are calculated and are plotted in the above mentioned elevation capacity curves. Shown is Fig.
• These maps also indicate, the area and the property, which would go under water when, the reservoir is filled upto a certain elevation. This helps to calculate the compensation to be paid to the farmers and other owner of the land and the time, before they should vacate the area to be submersed under the water of the reservoir.

Q3) What is geological survey?

A3)

Geological surveys

These survey are necessary to confirm the following:

(i) Suitability of foundation for the darn.

(ii) Water tightness of the reservoir basin.

(iii) Location and estimation of the store quarry, to be used for the construction work (the cost of transport is added to the total cost of the reservoir)

• On the basis the type and height of the proposed dam, the exploration of the sub surface is carried out, to calculate, the depth of the over burden to be excavated and removed for the laying une foundation, the natural of rock its formation and the extent of fault zone (if the rock has such fault)
• In case of earthen dams, or low dams such sub-surface exploration, gives information about the soil properties at various depths on the basis of the information collected helps in devising a suitable programme B of foundation treatment (say by grouting) before the actual construction is taken up.
• These geological surveys also help to generate the information about proportion of cavities or pores in the rocks. Which is the basic cause of escape of water through such rocks in the adjacent valley, if the proportion of such pores is less it is treated but if it is wide spread, the site of the reservoir is changed.
• For the construction of dam and other works like embankment work, diversion hand work, canal regulation work etc, a huge quantity of building material is required. The availability of material and the distance of such location from the dam site, also is an important criteria to select a site.
• So, the information collected by through such geological field surveys help to calculate the stability and usability of the reservoir.

Q4) What is hydrological survey?

A4)

Hydrological surveys

The whole of the planning of any reservoir is based on the estimates of the total quantity of water to be available in the stream, from season to another season i.e. the total volume of water during summer and rain season and the yearly average availability of water. Through the hydrological surveys investigations are made, two types of

(i) Study of runoff pattern of the stream, at the proposed dam site: This helps to determine the storage capacity of the expected reservoir.

(ii) To study the hydrograph of worst flood:

• This helps to determine the methods to take care of the excess water from the reservoir like spill ways gates etc.
• So, in short, engineering surveys help to locate the site, the geological surveys help to determine the strength of the foundation and expected losses through the rock pores and the hydrological surveys help to calculate to total amount of water, available in the stream and the danger level of worst floods, etc.
• On the basis of the information collected by there surveys the selection of the dam site is made; for the actual select of the dam site following factors are considered.

Q5) Explain storage reservoir in detail.

A5)

Storage reservoirs

• It is constructed to store water which flows during the high flow, during rainy season and the one is used, during the dry summer season.
• It can serve the purpose of irrigation, hydel power generation, domestic and industrial water supply. In same cases, these reservoirs also help to control the floods, to safe guard the fields and the settlements along the banks of the river; on the down-stream side of the river.
• So on the basis of the requirements, there reservoir can be single purposed or multi purposed.

Q6) Explain flood control reservoir in detail.

A6)

Flood control reservoirs

• To reduce the danger and the damage due to floods, the Flood control Reservoirs are constructed. So they are called as Flood mitigation Flood protection Reservoirs. Generally these reservoirs are of single purposed
• Type of Reservoirs i.e. to control floods.

These reservoirs hold, some of the flood water, during the high flow rate, which may cause damage, in the downstream areas.

Fig 2: Effects of flood control reservoir on hydrograph

• The water then is released very gradually after the floods are over. The advantage of flood control reservoir can be seen in Fig.  where in the line graph A B C indicates the Natural hydrograph of a river with the flood-control reservoir while line graph A B C shows he moderated hydrograph due to the construction of flood control Reservoir.
• The natural flood discharge is indicated by point 'B' the peak point of the flood while indicates the reduced peak point of flood.
• The maximum flood discharge from the river is reduced from Q₁ 10 Q₂ and the effect of flooding is also delayed on the time scale (on x axis) and the discharge is shown on y axis.

The flood control reservoirs are of two types namely:

(1)Retarding Reservoirs

(ii) Detention Basins

Q7) What do you mean by retarding reservoir?

A7)

Retarding reservoirs

It has outlets and spillway, which not controlled by valves or Gates. The discharge capacity of these cutlets and spillway is kept such that the total out flow, through then, never exceeds, the safe capacity of the stream, in the down-stream, during the highest foods, It has following merits.

(i) As there are no valves or gates to the outlets and spillways, there is chance to create problem, due to human errors in the process of operation i.e. release of water.

(ii) The cost of gates and valves and also the maintenance cost is saved.

This method has the following demerit:

• When it is constructed on a number of tributaries, which join the main river; the automatic control can result into modified floods which flow in to the main stream causing flood situation so the vary purpose of the flood arrival is not served.
• To avoid this situation, the location of such reservoirs may be selected in such way, the all the flood waves, joining the main river at the same time, can be avoided. The best location of such reservoir should be immediately above the city, (or the region to be protected) to control floods.

Q8) Explain detention basin in detail.

A8)

Detention Basins

• In case of this type, the outlets and spillways have been provided which are controlled by gates and valves. So this type has more flexibility of the operation. The addition discharge of water is totally controlled by the valves and gates.
• The reservoir can be emptied very quickly through the gates and valves, to create space for the subsequent floods. As the water from different tributaries joins the main stream, to avoid the flood conditions,

The gates and valves can be operated at different timings.

Q9) Explain distribution reservoir in detail.

A9)

Distribution Reservoirs

• It is a small storage reservoir which is used to make domestic water supply to the urban areas. They to meet the varying demands of the consumers, at different period of a day.
• The water is pumped into the reservoir, at a constant rate and it is supplied to the consumers when the water demand is reduced than the pumping rate, the water is stored in the reservoir and case of higher water den d than the pumping rate of water, this extra demanded water, is supplied through the storage tank.
• So the rate of pumping is kept constant, and due to storage it can meet the higher demand of water, which may be more than the rate of in flow.

Q10) Explain mass curve with diagram.

A10)

Mass curve

• By using the available stream flow record, for a number of consecutive years, the mass curve is prepared, on the base of the flow hydrograph.
• The span of the period to be considered, must have a critical or driest period of the region (which would) indicate the lower most figures of the stream inflows look at the Fig.  which has expressed a mass curve for period of six years. i.e. from the year 2000 to 2006. The time period in years in given an x axis and the accumulated flow is given on y axis.
• In this Fig.  in the downward side, the demand curve has been drawn (for the comparision). It indicate the variation in the rate of demand. If the rate of demand is constant, the demand curve will be a straight line (with no curve) the demand curve is always on rise as the population rises, the consumption of domestic purpose, irrigation purpose, industrial purpose, hydel power generation purpose always increases.
• On the A-B curve. G-H, and F-J doted lines are drawn, parallel to the demand curve and are tangent to the high points like, G and of the mass curve. These are the point placed at the beginning of the dry period.
• In the mass curve, maximum vertical intercepts i.e. x₁ y₁, X₂ Y₂ etc. between the tangential lines are drawn to measure the mass curve. These vertical intercepts i.e. x, y, or x₂ y₂ indicate, the volume by which the total inflow of the stream falls short of the demand and so it is necessary to make supply form the reservoir storages.
• Let us assume that the reservoir to be fall at point G (in the Fig. 5.4.1) for a period from G to , (in the year between 2001 and 2002) there is a total inflow in the stream which is represented by Y, Z, and at the same time the total demand is represented by X, Z, (leaving a gap of volume of water, shown by X, Y₁). This is needed to be met by using reservoir's storage.

Fig 3: A mass curve to determine the required capacity of the reservoir

• The largest of the maximum vertical intercepts 1.e. X, Y₁, X₂ Y₂ etc; represent the capacity of the reservoir, required to satisfy the given demand. Whatever has been calculate in the 'Net storage' required to meet the demand, so if there is any type of evaporation or percolation loss in the storage of the reservoir, it must be compensated by using storage capacity.
• The vertical distance between two tangentional lines, such as GH and FJ, represents the extra amount of water which spills over from the reservoir, through the spillway. It goes as a waste in the dam stream direction. This happens because between height of H and F, the reservoir remains full and so all the inflow in excess of demand will be sent out through the spill way to the downstream side.
• If these tangential lines which are drawn parallel to the demand curve, are extended further, they should intersect' the mass curve e.g at H or J, so the reservoir which was full at G and F will be filled soon, but these extended lines do not intersect the mass curve it will indicate that the reservoir will not be filled again. If the reservoir is very large, the time interval between the points G and H; F and J etc. may be of several years.

Q11) What is yield of reservoir?

A11)

Yield of reservoir

• When any type of storage reservoir is to be designed. It is of almost importance to analyse the relationship between the capacity of the reservoir and the yield of the reservoir.
• Yield can be defined as, "It is the actual amount of water that can be supplied in a given period of time from the storage reservoir".
• Depending upon the size of the reservoir the given period of time i.e. the time interval varies from, may be a day to a couple of months or may be to a couple of years. i.e. small sized reservoirs have lesser time interval than the larger sized reservoirs. The yields also vary from a year to another. It depends upon the inflow of a water to the reservoir.

Types of yield

The yield of any reservoir can be classified as.

A: Safe yield or firm yield

B: Secondary yield.

C: Average yield.

A: Safe yield or firm yield

• It can be defined as "It is the maximum quantity of water that can be supplied, during the critical dry period i.e. worst dry period".
• The critical or worst period is the time (in days or months), when the natural inflow of the stream is lowest, on record. In same period of time, this dry period may occur, with the lower yield than the safe or firm yield. Which is calculated on the basis of the past record of the inflow of the natural stream.

B: Secondary yield

• It can be defined as, "It is the excess amount of water available than the safe yield during the period of high flows".

C: Average yield

• It can be defined as "It the arithmetic average of the yield i.e. average of the safe and secondary yield; over a long period of time".

The Yield and Storage Capacity of Reservoir

• Both, the yield and storage capacity are interdependent factors. The storage capacity of a reservoir depends upon, the inflow to the reservoir. The relationship among the yield, inflow and storage can be expressed as below.
• Inflow-yield = change in the storage. so, it is very easy to understand that when the inflow is greater than the yield (supplies made through the reservoir), the storage in the reservoir will increase and on the other hand, if the inflow is lesser than the yield, the storage will reduced.
• The value of yield which can be adopted for the design of a reservoir is called as 'designed yield'.
• The designed yield is kept as level that, it should meet the consumers demand still it will not affect the storage capacity.
• The value of designed yield depends upon the urgent need of the water to be supplied or on the amount of risk involved in it; when the actual yield is lesser than the designed yield. i.e. in case of the reservoir designed for the domest water supply must be planned on the basis of safe yield in case of a reservoir for irrigation purpose, should be planned with the designed yield value atleast 20 % greater than the safe yield.
• On the same basis, the power (electricity) commitments to the actual domestic users must be based on the firm basis and it should not exceed the power which can be produced with the safe yield. Unless there is an alternate arrangement of thermal or any other type of power is available to support the Hydel power.
• In case of the demands from the large scale industries for the hydel power, it should be developed form secondary yield, as and when it is available.

Q12) Explain life of reservoir in detail.

A12)

Life of reservoir

Introduction

The useful life of a reservoir can be calculated by following the steps given below,

Step 1: Calculate the capacity efficiency of the reservoir (based on the capacity and the average annual inflow.)

Step 2: Calculate the trap efficiency of the reservoir by the equation given above

Step 3: Divide the total capacity, for some specific period (in %) e.g. 10% and 90% assume this 10% capacity has been reduced due to the deposition the sediments. Now, calculate the trap efficiency for the remaining 90% of the original capacity of that reservoir and the inflow.

Step 4: Get the mean of these two trap efficiencies; which can be assumed as the average value to the trap efficiency for the 10% of the capacity of the reservoir (which is filled totally with the sediments.)

Step 5: Calculate the average annual sediments, transported by the stream, or a channel and multiply this figure by the mean trap efficiency.

Step 6: Convert this quantity of the sediments, into weights (cubic contents) i.e. volume in hectre meters (ha.m)

Step 7: The 10% of the capacity of this reservoir is to be divided by the volume of sediments deposited. This will help to calculate the total number of years required, to till this 10% capacity of the reservoir by the inflow of sediments.

Step 8: The above procedure is to be repeated, to calculate the number of years required to fill next 10% capacity of the reservoir i.e. 80%, 70%, 60%,..... Till the capacity of the reservoir becomes 20%.

The addition of the years, required to till, each of the 10% capacities of the reservoir, will indicate, the useful life of the reservoir.