Civil Engineering Reference
In-Depth Information
Filter Pack Thickness
A filter pack is installed in the annular spaces between the screen and the ground.
This filter pack is designed to prevent particles from the ground to enter the well. The
filter pack should be more than 3 inches and less than 8 inches thick.
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The filter pack
gradation should be based on the gradation of the water-bearing strata. The finer the
strata, the finer the filter pack gradation has to be.
Well Screen Diameter
The diameter is selected so that the entrance velocity of the water is less than
0.1 ft / sec.
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The screen diameter can be adjusted after the opening size and length
have been established. Increasing the diameter has less impact on the well capacity
than increasing the screen length. The theoretical increase in yield can be estimated
from:
C
Q
(8-6)
log
R
/
r
where:
C
constant
R
radius of influence, ft
r
radius of screen, ft
For typical situations of R
400 ft, doubling the screen diameter increases the ca-
pacity by only 10 percent.
Recharge
Intentional groundwater recharge can be used to enhance the yield of an aquifer. Sur-
face water, imported water, stormwater, or treated wastewater can be captured and
percolated or injected into the ground. If soil and aquifer conditions permit, and land
costs are reasonable, recharge ponds or spreading basins are generally the most cost-
effective way to get large quantities of water into the ground (Figure 8-22). Injection
wells can also be used, but costs are relatively high for the amount of water recharged.
One of the most effective modes of groundwater recharge is liberal irrigation in
wet years, as long as something other than groundwater is used for irrigation. It is the
best way to get a large land area contributing to groundwater recharge. The recent
trend, however, has been conversion to high-efficiency irrigation systems, which en-
deavor to minimize deep percolation. This is highly desirable in dry years, but severely
inhibits the ability to recharge in wet years. Making matters worse, many high-
efficiency systems require very clean water because of small nozzle and / or emitter
openings, operated over long periods of time. Because surface water is often laden
with silt, algae, or other debris, and deliveries are scheduled for relatively short du-
rations, growers tend to switch from surface water to private wells for irrigation. So
in the conversion, the groundwater supply takes a double hit; new consumptive use
plus lost recharge.
