Page 319 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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8.4 Location of Storage 279

EXAMPLE 8.7 TOTAL WATER STORAGE VOLUME USING THE TEN-STATES STANDARDS
For a steady gravity supply equal to the maximum daily demand, a 4-h fire supply at 8,000 gpm
3
(0.505 m /s 505 L/s), and no particular hazard to the supply works, find the storage to be pro-
3
vided for a city using an average of 7.5 MGD (0.328 m /s 328 L/s) of water using the Ten-States
Standards.
(Note: This example uses the same parameters as Example 8.6.)
Solution 1 (U.S. Customary System):
Storage volume for domestic consumption average daily consumption (Ten-States Standards):
7.5 MGD
7.5 MG daily

The fire reserve is 8,000 gpm for 4 hours (similar to Example 8.6):
(8,000 gal/min 4 h 60 min/h) 10 6
1.92 MG
Total storage domestic storage fire storage 7.5 1.92 9.42 MG

Solution 2 (SI System):
Storage volume for domestic consumption average daily consumption (Ten-States Standards):
3
0.328 m /s 328 L/s
3
28,387 m daily
3
The fire reserve is 0.505 m /s for 4 hours (similar to Example 8.6):
3
(0.505 m /s)(4 60 60 s)
7,269 m 3
Total storage domestic storage fire storage 28,387 7,269 35,656 m 3

8.4 LOCATION OF STORAGE
In addition to capacity of service storage, location is an important factor in the control
of distribution systems. One MGD (3.785 MLD) of elevated fire reserve, suitably sited
in reference to the area to be protected, is equivalent, for example, to the addition of a
12-in. (300-mm) supply main. The underlying reasoning is that, when drawing this vol-
ume of water in a 4-h fire, flow is provided at a rate of (24 4) 1 MGD 6 MGD or
(24 4)(3.785 MLD) 22.7 MLD. This is the amount of water an 18-in. (450-mm) pipe
can carry at a velocity of less than 5 ft/s (1.5 m/s). Why this must be neighborhood
storage is explained by the high frictional resistance of more than 8% accompanying
such use.
The engineering considerations for deciding the location of water supply storage tanks
or reservoirs are as follows:

1. Consideration should be given to maintaining water quality when locating water
storage facilities.
2. The bottom of ground-level reservoirs and standpipes should be placed at the nor-
mal ground surface and shall be above the 100-year flood level or the highest flood
of record.