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Reservoir storage capacity

The total reservoir storage capacity of a distribution reservoir is the summation of:

  • Balancing storage (or equalizing or operating storage);
  • Breakdown storage.
  • Fire storage.

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1. Balancing storage

  • The main and primary function of a distribution reservoir is to meet the fluctuating demand with a constant rate of supply from the treatment plant.
  • The optimum quantity of water required to be stored for this variable demand to provide a constant supply is known as the balancing reserve or balancing storage or equalising storage or the storage capacity of a balancing reservoir.
  • This balancing storage is found using 1) Mass Curve Method 2) Analytical Solution

A) Mass curve method

  • A mass diagram is the plot of accumulated inflow (i.e. supply)or outflow (i.e. demand) versus time.
  • The mass curve of supply (i.e. supply line) is, therefore. first of all, drawn; and is superimposed by the demand curve.
  • The amount of balancing storage can then be easily determined by adding the maximum ordinates between the demand and the supply lines.
  • To construct, such diagrams for a particular water supply project, we have to proceed as follows:

a.From the past records, determine the hourly demand (or draft) for all 24nours for typical days (maximum, average and minimum).

b.Calculate and plot the cumulative demand against time, and thus plot mass curve of demand.

c.Draw the cumulative supply also against the time to plot the supply line or mass curve of supply.

d.Read the storage required, as the sum of the two maximum ordinates between demand and supply line.

e.Repeat the procedure for all the typical days, and determine the maximum storage required for the worst day.

2. Balancing storage

  • Breakdown storage à Emergency storage which takes care of emergencies like failure of pumps, electricity or other mechanical failures, which will cause interruption in the supply.
  • The amount of provision to be made for this factor is very difficult to assess, because it depends upon the frequency and extent of the failures, and also upon the time required for carrying out the repairs.
  • For this reason, a lump sum provision is generally made for this storage.
  • A value of about 25 percent of total storage capacity of the reservoir, or about 1to 2 times of the average hourly supply may be considered as enough provision for accounting this storage, under all normal circumstances.
  • However, if the supply lines of equipment are expected to be out of operation for long times, higher allowances must be made.

3. Fire Storage

  • A very important part of the reservoir storage capacity is the fire storage, which is used in case of emergency.
  • This provision takes care of the requirement of water for extinguishing fires.
  • In case of fires, sufficient amount of water must remain available in the reservoir for throwing it over the fire, in case the fire coincides with pumps or electricity failure at the water works.
  • Fire Demand à1-5 lpcd , and it needs to be discharged by 3 jet streams at rate of 1100 litres/minute.
  • Thus, if a provision for 10 hours of fire fighting per day is desired, the volume of water required to be stored
  • =(3x 1100)x(10×60)litres/day=198x 104 litres/day = 2 million litres/day.
  • The storage for fire fighting, in fact, is dependent upon the chances of fire break-outs in the considered city.
  • According to National Board of Underwriters of America:-

1.For population of 6000-2 lakhs à for 10hrs fire à water required 10,000-50,000 litres/minute.

2.For population <6000 à for 8hrs fire à water required 4000 litres/minute.

3.For population <6000 à for 6hrs fire à water required 2000 litres/minute.

4.For population <6000 à for 4hrs fire à water required 1000 litres/minute.

  • These provisions are too high to be adopted by a poor developing country like India.
  • Under normal conditions in India, we may store about 1 to 4 litres per person per day as the necessary fire storage, depending upon the importance of the city.
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