Handbook of Civil Engineering Calculations

WATER-SUPPLY SYSTEM SELECTION

Choose the type of water-supply system for a city having a population of 100,000 per-
sons. Indicate which type of system would be suitable for such a city today and 20 years
hence. The city is located in an area of numerous lakes.

Calculation Procedure:

1. Compute the domestic water flow rate in the system
   Use an average annual domestic water consumption of 150 gal per capita day (gcd) (6.6
   mL/s). Hence, domestic water consumption = (150 gal per capita day)( 100,000 persons) =
   15,000,000 gal/day (657.1 L/s). To this domestic flow, the flow required for fire protec-
   tion must be added to determine the total flow required.


2. Compute the required flow rate for fire protection
   Use the relation Qf= 1020(P)
   05
   [1 - 0.01(P)
   05
   ], where Qf= fire flow, gal/min; P = popu-
   lation in thousands. So Qf= 1020(10O)^05 [1 - 0.01 x (10O)^05 ] - 9180, say 9200 gaVmin
   (580.3 L/s).


3. Apply a load factor to the domestic consumption
   To provide for unusual water demands, many design engineers apply a 200 to 250 percent
   load factor to the average hourly consumption that is determined from the average annual
   consumption. Thus, the average daily total consumption determined in step 1 is based on
   an average annual daily demand. Convert the average daily total consumption in step 1 to
   an average hourly consumption by dividing by 24 h, or 15,000,000/24 = 625,000 gal/h
   (657.1 L/s). Next, apply a 200 percent load factor. Or, design hourly demand =
   2.00(625,000) = 1,250,000 gal/h (1314.1 L/s), or 1,250,0007(60 min/h) = 20,850, say
   20,900 gal/min (1318.4 L/s).


4. Compute the total water flow required
   The total water flow required = domestic flow, gal/min + fire flow, gal/min = 20,900 +
   9200 = 30,100 gal/min (1899.0 L/s). If this system were required to supply water to one
   or more industrial plants in addition to the domestic and fire flows, the quantity needed by
   the industrial plants would be added to the total flow computed above.


5. Study the water supplies available
   Table 7 lists the principal sources of domestic water supplies. Wells that are fed by
   groundwater are popular in areas having sandy or porous soils. To determine whether a

TABLE 7. Typical Municipal Water Sources

Source

Groundwater

Surface freshwater (lakes,

rivers, streams, impounding

reservoirs)

Surface saltwater

Collection method

Wells (artesian, ordinary,

galleries)

Pumping or gravity flow

from submerged

intakes, tower intakes,

or surface intakes

Desalting

Remarks

30 to 40 percent of an area's

rainfall becomes

groundwater

Surface supplies are

important in many areas

Wide-scale application under

study at present