Water Supply & Urban Drainage

Water Supply & Urban Drainage Lecture 1 Instructor: Binyam k.

Water Quantity Requirements Introduction While designing water supply scheme for a town it is necessary to determine the total quantity of water required for various purposes. The quantity of water is dependant upon the size of the community and the purpose for which it is needed. Three items need to be determined for calculating the total quantity of water required for a town - Design Period - Design population - Rate of Demand

Design Period Design Period  The number of years for which the designs of the water works have been done is known as Design Period.  It should be neither too short nor too long (Why?)  Most water works are designed for a period of 22-30 years. Factors affecting Design Period 1.Fund available for the completion of the project. if more funds are available the design period shall be large. 2.Rate of interest on loan taken to complete the project. If the interest rate is less, it is good to have more design period.

Design Period 3. Anticipated expansion rate of town. 4. Life time of the pipe and other structural materials used in water supply scheme design period is no case should be more than the life time of components used in water works.

Name of unit Pump house Generators Pipes Reservoirs

Design periods 30 25 30 50

Design population The population to be served by the scheme is estimated using different methods. The water supply scheme are not designed for the present population only but for the population that is expected in the design period. Population Forecasting Methods of population forecasting 1.Arithmetic increase method This method is based on the assumption that population is increasing at a constant rate

Population Forecasting Pn = Po + kn Where Pn= population at n decades n= Decades k= Arithmetic increase This method is used for large cities which have achieved population saturation conditions.

Population Forecasting 2.Geometric Increase Method This method is based on the assumption that percentage growth rate is constant.

Pn = Po (1+ Ig/100)^n Ig= percentage increment per decade This would apply to cities with unlimited scope of expansion.

Population Forecasting 3.Incremental Increase Method Growth rate is assumed to be progressively increasing or decreasing, depending upon whether the average of the incremental increases in the past is positive or negative. The population for a future decade is worked out by adding the mean arithmetic increase to the last known population as in the arithmetic increase method, and to this is added the average of incremental increases, once for first decade, twice for second and so on

Population Forecasting Pn =Po+ n*k+n(n+1)r/2 Where k-average increment per decade (calculation in arithmetic method ) n- Number of decade between project years. r-average incremental increase

Population Forecasting Problem: Predict the population for the years 1981, 1991, 1994, and 2001 from the following census figures of a town by different methods. Year

1901 1911 1921 1931 1941 1951 1961 1971

Population : (thousands

60

)

65

63

72

79

89

97

120

Rate of Demand

1. 2. 3. 4. 5.

Water demand can be classified into the following branches depending upon the type of consumer Domestic water demand public water demand Industrial water demand Fire fighting water demand Unaccounted water demand

Rate of Demand 1.

Domestic water demand This water demand is the amount of water required for domestic purpose such as drinking, cooking, body washing, cloth washing, house washing, washing utensils etc. In developed countries the domestic water demand may be as high as 350 l/cap/day.

Rate of Demand 2. public water demand The water require for schools, hospitals, hotels, public facilities, parks, offices, commercials, military camps etc is called public demand.

Rate of Demand 3. Industrial demand Industrial demand is the required amount of water for different industries. 4. Fire demand Fire demand is treated as a function of population. Empirical formulas used for calculating fire demand are as follows

Rate of Demand a. John R.Freemans formula Q= 1136.5 (p/5+10) where Q= water required in l/s P= population in thousands b. Kuichlings formula Q= 3182 p

Rate of Demand 5. Unaccounted water demands This includes the quantity of due to wastage, losses, thefts, etc While estimating the total water demand allowance should be made for losses and wastage. Generally 15-40% of the total quantity of water is made to compensate for lose.

Rate of Demand Factors Affecting Demand of Water The following are common factors which affect the rate of demand of water. Climatic conditions Standard of living Quality of water System of supply etc

Rate of Demand Average daily demand The average daily demand is taken to be the some domestic demands, public demands, industrial demands, and unaccounted water demand. Maximum day demand The water consumption in a year varies day to day due to varies rezones. Peak hour demand The peak hour demand is the highest demand of any one hour over the maximum day.