MIL-HDBK-1005/7A
e) Discharge to Storage. Water may run to storage by
gravity or pumping. Provide a blowoff to divert the initial
slug of raw water; it may carry accumulated minerals or
sediment. Provide an intake sump with bar racks of the grating
type in front of the conduit.
f) Storage Reservoirs. Design to conform to
requirements for underground storage in Section 8.
g)
Fencing.
Fence the catchment area.
4.6.5.4
Sanitary Protection. Catchment areas must be lime
washed immediately after construction. Water treatment,
filtration and disinfection are required; (for methods, refer to
Section 6 on Treatment).
4.6.5.5
Data Required. Rainfall data should include monthly
rainfall quantities and where available, the number of days of
rain per month. Catchment characteristics needed are: slope,
roughness, pondage, and discharge capacity.
4.6.5.6
Basis of Design. For each rainstorm, there is a loss
of moisture due to surface detention, pondage, and evaporation.
Rather than analyze by individual storms, proceed on the basis
of the number of days of rain. The loss will range from 0.06
in. per day of rain for steep, smooth catchments free of pondage
to 0.25 in. per day of rain for paved areas having slopes of
0.01 and the roughness of screened surfaces, with some minor
pondage. In this range, choose an appropriate unit loss
allowance. Where data on the number of days of rainfall are
unavailable, allow a gross yield of 0.5 gal/sq. ft of catchment
area per in. of rainfall.
a) Monthly Yield. For each month, multiply the loss
allowance by the number of days of rain. Subtract this product
from the month's rainfall to determine the gross yield.
b) Peak Rate. Check the peak rainfall rates and
resultant flows against the outlet capacity. If a substantial
part of the annual rainfall causes uncollectible spills, make
allowance for this. An appropriate loss allowance for
roof -gutter systems, or others where there are such losses, is
25 percent of the gross yield. Correct each month's value;
rainy season corrections will probably be larger than those in
drier months.
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