Page 175 - Water and wastewater engineering

4-30 WATER AND WASTEWATER ENGINEERING

depth of the well is not dependent on the drawdown. However, the drawdown must not lower the
piezometric surface below the top of the aquifer (MSDWA, 1976).

Well Screen Length
The factors that affect the choice of the screen length include: the open area per unit length of
screen, the character of the aquifer, the cost of the screen, the desired yield, and the design ser-
vice life of the well. The optimum length of well screen depends on the thickness of the aquifer,
available drawdown, and stratification of the aquifer. As long a screen as possible should be used
to reduce entrance velocities and the effects of partial penetration of the aquifer. For unconfined
aquifers, optimum specific capacity and yield are generally obtained by screening the lower 30
percent to 50 percent of the aquifer (Walton, 1970). Because the drawdown must be kept above
the top of the screen, longer screens reduce the available drawdown.
In homogeneous artesian aquifers, 70 to 80 percent of the water-bearing sand should be
screened. If the aquifer is less than 10 m thick, 70 percent is satisfactory. Between 10 and 20 m
thick aquifers should be screened about 75 percent of the thickness. Aquifers greater than 20 m
thick should be screened for 80 percent of their depth (Johnson, 1975).
There are some exceptions to this approach. One is to center the well screen between the top
and bottom of the aquifer to make more efficient use of a given length of screen in a uniform arte-
sian aquifer. Another exception is when a portion of the aquifer is not screened because it yields
a poor quality water (Johnson, 1975).
Walton made a study of well failures due to partial clogging of the well walls and screen
openings. He found that, on the average, about one-half of the open area of the screen will be
blocked by aquifer material. Thus, the effective open area of the screen is about 50 percent of
the actual open area. He developed a technique for estimating the screen length taking this into
account (Walton, 1962). The length of screen for a natural pack well may be selected using
Table 4-5 and Walton’s equation:
Q
S (4-11)
L
Av
o
3
where Q flow rate, m /s
2
A o effective open area per meter of screen, m /m
v optimum screen velocity, m/s
TABLE 4-5
Optimum screen entrance velocities
Hydraulic conductivity, m/d Optimum screen entrance velocity, m/s
20 0.010
20 0.015
40 0.020
80 0.030
120 0.040
160 0.045
200 0.050
240 0.055
240 0.060
Source: Walton, 1962.