Design Period of Water Supply Scheme in Environmental Engineering

The water supply works in water supply scheme take into consideration population after the ‘design period’. Now this design period is important as it denotes the population forecast to be done after how many years.

Design Period of Environmental Engineering

Design Period Basic Definition: The number of years for which the various components of the water supply scheme are designed to be sufficient in terms of its capacity (design capacity) is called design period.

If the design period is taken as n years then the scheme should suffice the needs of the population at the end of n years.

Design Period and Population Forecasting

As seen above, the design period takes into consideration the water needs of the population after n years.

Hence, knowing the future population becomes necessary in this case. Now population cannot be just estimated randomly. A scientific approach is taken to forecast the population after completion of the design period.

There are many methods to forecast the population. Certain criteria are to be studied and then the most suitable method of population forecast is adopted.

Importance of Design Period

Design period for water supply scheme is important owing to the following reason:

• The water supply scheme includes the construction of huge structures like dams, reservoirs, treatment works, conveyance systems, and so on. All these structures require a huge capital.
• Another major limitation with these structures is that their capacities cannot be increased in the future without sparing money.

Let us take one example of laying pipelines.

• If we talk about the water supply system, then it will include the laying of pipelines, underground. To accommodate increased population, pipes of larger diameter will be required.
• Besides the cost of replacing the pipelines by larger ones, the whole roads have to be dug disrupting traffic. Again, the water flow will change making the initial water head insufficient.
• This will require the whole pumping system or the overhead water tank to be changed.
• Hence, it is a complete waste of money and other resources to think of increasing the capacity in just a few years of the establishment.

Now, this does not mean design period should be too long.

• Design period should never be longer than the estimated useful life of the water supply work.
• As already said, the construction cost of these structures is too high and unnecessary increase will lead to consumption of huge resources. Even then, these resources will be wasted for life taken more than the life of the structure as the structure itself will fail.

Therefore, all the structures of the water supply scheme are designed by taking into consideration the population of the town after a few years. ‘These few years’ is called design period.

Factors Affecting Design Period in Water Supply

From above point, it is clear that careful consideration is given in choosing the design period for a water supply scheme. The design period which is not too short or too long.

Following are the factors affecting design period:

• Design period should be less than the useful life of the structure. Also, the chances of the structure and its technique becoming obsolete should be considered. For instance, design period of conveyance system should be decided based upon the service life of the material of the pipe used.
• The difficulty and consumption of resources for the future expansion or increase in capacities is also considered. Higher design period is selected if the expansion is difficult and its cost is high.
• Availability of the investment funds for the water supply scheme is also take into account. Lower funds leads to less design period.
• If the rate of interest on money borrowed is less, then again higher design period can be adopted.
• If the rate of population increase of a community is higher, then higher design period can be selected. In this consideration, the population shifts during the course of growth are also considered.

Design Period for Components of Water Supply Scheme in India

Design period for water treatment plant is 15 years.

However, the large projects with huge land acquisition requirements should be designed as per the project requirements. Projects like tunnels, large aqueducts, etc. are such projects.

Design Period for Water Supply Components in India^[1]:

SR NO	Component of Water Supply Scheme	Design Period as per Government of India Manual
1	Storage by dams	50 years
2	Intake works	30 years
3	Pumping:                                                  Pumping house Electric motor & pumps	30 years 15 years
4	Water treatment units	15 years
5	Pipe connections to several treatment units & other small appurtenances	30 years
6	Raw water & clear water conveying units	30 years
7	Clear water reservoirs at head works, balancing tanks & service reservoirs	15 years
8	Distribution system	30 years

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