Accepted Papers - 3.pdf - UNESCO

= Aquifer thickness in ft.
H du = Draw up in ft.
r o = Radius of influence in ft.
r w = Radius of well in ft.
Diminishing rate of recharge :
Based on above discussion it can be said that
the recharge will stop when the effective head H eff
becomes zero. This will happen due to several
reasons – (1) Increasing aquifer hydrostatic pressure
H aq (2)Falling recharge water level H R (3) Increasing
frictional losses due to choking of filter pit media,
screens, rusting of casing pipes, chocking of gravel
pack and formation.
Assuming all other factors remaining same an
important phenomena takes place continuously is
that with the recharging of aquifer the hydrostatic
pressure of the aquifer keeps increasing if there is
no pumping from that aquifer or if the rate of
transportation of water from the recharge point to
the pumping point in the command area is lesser
than the recharge rate. This will cause diminishing
rate of recharge whereby the rate of recharge will
gradually keep falling upto a point where it equals
the rate of transportation of water from the recharge
point to the pumping point. At this point an
equilibrium state will occur and recharge will
continue at this rate if all other conditions remain
constant.
Therefore for a sustainable recharge rate our
site selection must ensure the rate of transportation
of water from the recharge point to the point of use
as close as possible to the recharge rate.
Upper limit of Recharge pressure on aquifer :
The maximum pressure of recharge water being
injected into the aquifer shall be such that it does
not cause fracturing of the formation of the aquifer.
If fracturing occurs then there is usually a severe
loss in hydraulic conductivity because the bedding
planes are disturbed. On the other hand while
recharging into massive consolidated rock formation
fracturing may increase the rate of recharge (ref.
Howard and Fast,1970). The pressure that will cause
fracturing varies widely depending upon the nature
of formation and the designer must determine this
prior to designing the recharge system.
Fracturing pressure ranges from as low as 11.3
k Pa/Meter for poorly consolidated coastal plain
368
sediments to 27.1 k Pa/Meter for crystalline rock.
(Warner and Lehr,1981).
As a general guideline the for most recharge
wells in unconsolidated sediments the recharge
pressure shall be controlled so that the positive head
does not exceed a value equal to 0.2 times the depth
from the ground level to the top of the screen or
filter pack. (Olsthoorn, 1982).
� Design considerations : Following are the
major elements of a recharge well –
• The Filter pit or collection pit as it is popularly
known.
• Well casing
• Well screens
• Filter/Gravel pack
Now we will discuss important design
considerations while designing these elements-
The Filter pit
FUNCTION OF THE FILTER PIT
• Main function is to reduce turbidity of raw
water to reduce chocking of gravel pack
• Keep feeding filtered water to Recharge well
IMPORTANCE OF FILTER PIT
• To sustain the Recharge process
• To maintain the Recharge rate
DESIGN CONSIDERATION
• Filter media should not get clogged
• The System to collect percolated water should
have following features –
1. Large % open area for minimal frictional head
loss.
2. Facility for cleaning or backwash
3. It is preferred to be made from non-corrosive
material because during the period of no recharge
this will be exposed to atmosphere.
DIFFERENT DESIGN FOR FILTER PIT
1. Vertical flow filter pit
2. Horizontal flow filter
1. VERTICAL FLOW FILTER
DESIGN