Page 183 - Water and wastewater engineering

4-38 WATER AND WASTEWATER ENGINEERING

% Retained” by subtracting each value from 100 percent. From the graph, select the
grain sizes to calculate the uniformity coefficient:
.
084
D 40 %
U 283
.
.
D 90 % 0 297
For a corrosive water (from the water analysis given in the problem statement) with a
uniformity coefficient less than 3, the recommended slot size is D 50% . From the graph,
select the 50 percent point on the graph. An acceptable slot number is 20. Note that it is
acceptable to have more percent retained than 50 percent but not less.
g. Screen diameter
Select a screen diameter equal to the casing diameter.
h. Screen length
Screen length is a function of aquifer homogeneity and aquifer depth. Based on Johnson’s
rules of thumb, the aquifer should be screened for 80 percent of its depth. This is a screen
length of
.
.
L (08. )(387 m ) 3096 or 31 m
S
This estimate must be checked using Equation 4-11.
i. Hydraulic conductivity
An estimate of the hydraulic conductivity is required to use Equation 4-11. Hydraulic
conductivity ( K ) and the transmissivity ( T ) are related (from Equation 2-6, T KD ).
Transmissivity was estimated in step c (1) above.
The hydraulic conductivity is

3 2
.
T 2 818 10 m /s 5
K 728. 10 m/s orr 629 m/d
.
.
D 387m
The value of D is the thickness of the sand layer (that is, the artesian aquifer) given in
the well boring log.
j. Check of screen length and entrance velocity
Using Table 4-6 , a slot number of 20 and a well diameter of 15 cm has an effective
2
open area of 0.097 m /m of screen length. From Table 4-5 , for a hydraulic conductiv-
ity of 6.29 m/d, the screen velocity should be 0.010 m/s. The recommended screen
length is

⎛ 1 ⎞
3
(32 m /h ) ⎜ ⎟
S L ⎝ 3 600s/h, ⎠ 916 m
.
.
(0 097 )(0 010m/s. ) )
The selected screen length of 31 m exceeds this recommended length.
Check the entrance velocity: