MIL-HDBK-1005/7A
4.4.4.3
Design.
Basis of design is as follows:
a) The yield of a particular lens is generally no
higher than half of the recharge rate, and can be much less.
b) The recharge rate may reach half the annual
rainfall where rainfall exceeds 20 in. in a year, but may be as
low as one percent of the rainfall where rains are less than 4
in. per yr. Exact relations will depend on vertical
permeability, vegetative demands, and rate of rainfall and of
losses through runoff.
c) Losses occur even without a draft on such lenses,
through discharge to the sea and vegetative demands.
d) Because of the difference in density between fresh
and seawater, a fresh water lens will extend 40 ft below sea
level for each ft it rises above sea level.
e) The quantity stored at the close of the recharge
season equals the horizontal area of deposit, times the average
thickness of the fresh water lens, times the effective porosity
of the aquifer. The effective porosity in these circumstances
may range from 10 percent in loose sand to 30 percent in coral.
The presence of impermeable layers or of large open channels
greatly reduces the effective storage. On a long-term basis,
not more than half the quantity stored is recoverable between
recharge seasons.
4.5.
Surface Water
4.5.1
Existence . The development of surface sources depends
on hydrologic conditions and geographic features of the area.
Refer to Water Quality and Treatment, Chapter 4 - Source Water
Quality Management, AWWA, Fourth Edition; Soil Surveys prepared
by United States Department of Agricultural, Soil Conservation
Service; United States Geological Services Topographical Maps,
stream flow data, and rainfall data.
4.5.2
Information Required . Use existing data, as far as
practicable, before collecting field information. For the
detailed data required in evaluating surface supplies, refer to
Table 7.
27
Previous Page
