Page 273 - Water Engineering Hydraulics, Distribution and Treatment

The fire reserve is 8,000 gpm for 4 h (similar to Example 8.6):
= (8,000 gal∕min × 4h × 60 min∕h)∕10
= 1.92 MG.
Total Storage = domestic storage + fire strage
= 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 6 8.6 Types of Distributing Reservoirs 251
3
= 28,387 m daily.
3
The fire reserve is 0.505 m /s for 4 h (similar to Example 8.6):
3
= (0.505 m ∕s)(4 × 60 × 60 s)
3
= 7,269 m .
Total Storage = domestic storage + fire strage
3
= 28,387 + 7,269 = 35,656 m .
8.4 LOCATION OF STORAGE reservoir. Gravity sewers constructed of water main
quality pipe, pressure tested in place without leakage,
In addition to capacity of service storage, location is an
may be used at distances greater than 20 ft (6 m) but
important factor in the control of distribution systems. For
less than 50 ft (15 m).
example, 1 MGD (3.785 MLD) of elevated fire reserve,
suitably sited in reference to the area to be protected, is 4. The top of a partially buried storage structure shall
not be less than 2 ft (0.6 m) above normal ground
equivalent to the addition of a 12 in. (300 mm) supply
surface. Clearwells constructed under filters may be
main. The underlying reasoning is that, when drawing this
exempted from this requirement when the design pro-
volume of water in a 4-h fire, flow is provided at a rate
vides adequate protection from contamination.
of (24∕4) × 1MGD = 6MGD or (24∕4)(3.785 MLD) =
22.7MLD. 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 8.5 ELEVATION OF STORAGE
high frictional resistance of more than 8% accompanying Storage reservoirs and tanks operate as integral parts of the
such use. system of pumps, pipes, and connected loads. In operation all
The engineering considerations for deciding the location the parts respond to pressure changes as the system follows
of water supply storage tanks or reservoirs are as follows: the diurnal and seasonal demands. Ideally the storage eleva-
1. Consideration should be given to maintaining water tion should be such that the reservoir “floats” on the system,
quality when locating water storage facilities. neither emptying nor standing continuously full. In systems
2. The bottom of ground-level reservoirs and standpipes with inadequate pipes or pumps, or having a storage reservoir
should be placed at the normal ground surface and that is too high, the hydraulic gradient may at times of peak
shall be above the 100-year flood level or the highest demand fall below the bottom of the reservoir. When this
flood of record. occurs, the full load falls on the pumps, and system pressures
deteriorate suddenly.
3. If the bottom elevation of a storage reservoir must
be below normal ground surface, it shall be placed
above the groundwater table. At least 50% of the 8.6 TYPES OF DISTRIBUTING RESERVOIRS
water depth should be above grade. Sewers, drains,
standing water, and similar sources of possible con- Where topography and geology permit, service reser-
tamination must be kept at least 50 ft (15 m) from the voirs are formed by impoundage, balanced excavation and