Subject II -Intermittent to Continuous supplies-Introduction

In many countries Intermittent supply for a couple of hours a day is the norm. Historical information on this subject for Indian towns is probably not easily available, but if available with any reader pl do share.
I have seen a paper re Delhi's water supply according to which Delhi had continuous supplies prior to 1960 and the situation kept on deteriorating after that.
Let us consider the main issues which determine whether we can give continuous supply and then the hours of supply that can be maintained in a particular system.

  Demand

Demand is defined as the quantity of water that meets the requirement and even though more water is available it is not consumed. The  demand of various types like domestic, commercial,industrial etc all vary for different communities for different reasons. Domestic demand depend upon habits of people, climatic conditions, water use facilities available,housing conditions,etc. While I was in Tata Consulting Engineers number of studies were carried out for different cities eg  Ahmedabad, Surat,Mumbai, Delhi, Chennai,  etc etc. An assessment was made of the water demand based on existing and projected housing conditions and the assessed demand in different categories of housing for a particular city. In general these figures were much higher than the supply figures recommended in the Manual on water supply published by Central Public Health Engineering Organisation (CPHEOO) of the Government of India. I am making a general statement and not giving the actual figures, as I do not have these reports now. I am sure some of my collegues or those currently in Tata Consulting Engineers (TCE) can produce an interesting data on demand projected by TCE and compare it with the norms of CPHEOO and publish under this blog.

One of the important components of demand is leakage. If we assess the percentage  leakage  at a point of time in Indian cities we  find that demand  far exceeds the availability of water. Naturally it will be desirable to bring down the leakage demand than to try and make effort to satisfy it.

Genesis of Intermittent Supply

Once it is evident that we do not have sufficient water to meet the demand, system cannot remain charged continuously for 24 hours as water at the source( outlet of treatment or reservoir at the Head works) will be finished and thereafter only air can be supplied. Therefore it becomes necessary to cut short the hours of supply.There are also issues related to the pressure in the system but we will take them up later.
At the moment it will suffice to conclude that if we cannot meet the demand we have to resort to intermittent supplies.  The leakage levels in many of the cities have been found to be of the order of 200 liters per connection per hour against a desirable limit of 10 liters per connection per hour. In fact it will not be out of place to mention that demand for leakage under 24 hour supply system, in Indian Cities in general , will be much higher than the requirement for all other purposes put together. Therefore till the overall demand can be reduced there is no possibility of giving continuous supply.

Stiffness considering embedment and native soil

The soil stiffness is based on the width of the trench, the embedment material and trench wall material, their densities or percent compaction. As explained in an earlier blog

E' = Sc . E'b
 Sc values can be obtained from the above table or the above graph or with the following formula

This enables us to obtain the value of E' the soil stiffness ,duly taking into account the native soil stiffness and embedment or backfilling soil stiffness.This is the most important factor for determining deflection of flexible pipes,particularly those which do not have a high pipe stiffness.
The important finding in the last decade is to consider stiffness of the soil in which the trench is dug and that of the soil in which the pipe is embedded.It is also clear that it may not be worthwhile to increase compaction beyond a point as it is expensive and it may be better to borrow and replace the soil..
We will consider these issues in detail after we deal with live loads and soil loads.
However by requests from many sources we are first moving to deal with the subject of how intermittent supply can be converted into  continuous supply.