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8.3 Service Storage 277

storage. In the afternoon, the supply exceeds the demand by 0.86 MG by 6 p.m., but this
excess is required to furnish water from storage between 6 p.m. and midnight. Therefore,
total storage equals 0.84 0.86 1.70 MG, or (1.7 MG/5 MG) 100 34% of the
day’s consumption.

Solution 2 (SI System):
(a) For steady supply during 24 h, the draft plotted in Fig. 8.6a exceeds the demand by 0.40 MG
3
(1514 m ) by 6 a.m. If this excess is stored, it is used up by 11 a.m. In the afternoon, the de-
3
mand exceeds the supply by 0.42 MG (1,590 m ) by 6 p.m. and must be drawn from storage
that is replenished by midnight. Hence, the required storage is the sum of the morning excess
3
and afternoon deficiency, or 0.82 MG (3,104 m ). This storage volume as a percentage of
3
.
3
3
total draft equals [(3,104 m ) (5 3,785 m )] 100 16 4%, where 5 3,785 m 5 MG.
(b) For steady supply during the 12-h period from 6 a.m. to 6 p.m., the draft plotted in Fig. 8.6b
3
exceeds the supply by 0.84 MG (3,180 m ) between midnight and 6 a.m. and must be drawn
3
from storage. In the afternoon, the supply exceeds the demand by 0.86 MG (3,255 m ) by
6 p.m., but this excess is required to furnish water from storage between 6 p.m. and mid-
3
night. Therefore, total storage equals 3,180 3,255 6,435 m , or [(6,435) (5 3,785)]
100 34% of the day’s consumption.
8.3.2 Fire Reserve

Based on IFC recommendations on observed durations of serious conflagrations, it is rec-
ommended that distributing reservoirs be made large enough to supply water for fighting a
serious conflagration for (a) 4 h for fire flows of more than 4,000 gpm (252 L/s), (b) 3 h for
fire flows of 3,000 to 3,750 gpm (189 to 237 L/s), and (c) 2 h for fire flows of 2,750 gpm
(174 L/s) and less (see Table 4.13). The resulting fire reserve may not always be econom-
ically attainable, and design values may have to be adjusted downward to meet local
financial abilities. Changing community patterns, moreover, may make for changing
requirements in the future.

8.3.3 Emergency Reserve
The magnitude of this storage component depends on (a) the danger of interruption of
reservoir inflow by failure of supply works and (b) the time needed to make repairs. If
shutdown of the supply is confined to the time necessary for routine inspections during the
hours of minimum draft, the emergency reserve is sometimes made no more than 25% of
the total storage capacity, that is, the reservoir is assumed to be drawn down by one-fourth
its average depth. If supply lines or equipment are expected to be out of operation for
longer times, higher allowances must be made.

8.3.4 Total Storage
The desirable total amount of storage is equal to the sum of the component requirements.
In each instance, economic considerations dictate the final choice. In pumped supplies,
cost of storage must be balanced against cost of pumping, and attention must be paid to
economies affected by operating pumps more uniformly and restricting pumping to a
portion of the day only. In all supplies, cost of storage must be balanced against cost of