Location of water bearing soil layer is at 490-570ft.

Let, the static water level at 500’

Average rate of water level declination (per year) = 2’

Design period= 30 years

Drawdown of 15’ while pumping each time

Safety distance of 15’ So length of the casing pipe= (500+30*2+15+15)= 590’

Aquifer depth= (570-490) = 80’ which is more than 50’.

So 80% of the aquifer screening can be made which gives the strainer length of = (80 *

0.80) = 64’

Maximum safe length of the strainer is 100’, so we can choose a strainer of 65’

Selection of strainer size

To retain 90% of the gravel pack materials,

Slot = D10 / 25.4 * 1000; here D10 will be of the gravel pack material

D10 = 0.62 mm

Slot= 0.62/ 25.4 * 1000 = 24.41

So 4-6 inch diameter 30 slot strainer is selected having each opening area of 30/1000 inch

3 to 8 inch diameter envelop of gravel pack material will surround the entire screen

Sample calculation

Yield of well = (area of strainer x flow velocity) / factor of safety

• For 30 slot strainer,

• Strainer area= 15% of strainer surface area

= 0.15 x 3.1416 x Diameter x Strainer length

• Here, Diameter = 6”

Length = 65’

• Assume, Flow velocity = 0.10 fps

Factor of safety = 2.5

• So, Yield of a well = (0.15 x 3.1416 x 6/12 x 65 x 0.1) / 2.5

= 0.612612 x (0.3048)³ x 3600 x 1000

= 62450.06409lph

Design considerations:

• For one pump

• In one day

• 8 hour continuous pumping

As pumping is for 8 hour per day, Yield = 62450.06409x 8

= 499600.5127lpd

Our present water demand is 1495884.5 lpd.

So, No of wells required at present=1495884.5/499600.5127

=2.99

≈ 3 wells

Water demand after 30 years =2322912.1 lpd

So, No of wells required after 30 years=2322912.1/499600.5127

=4.65

≈ 5 wells

So, 2 more wells need to be installed after 30 years.