Design
of groundwater recharge structures
Recharge of aquifers
In places where the withdrawal of water is more than the rate
of recharge an imbalance in the groundwater reserves is created.
Recharging of aquifers are undertaken with the following objectives:
- To maintain or augment natural groundwater as an economic
resource
- To conserve excess surface water underground
- To combat progressive depletion of groundwater levels
- To combat unfavourable salt balance and saline water intrusion
Design of an aquifer recharge system
To achieve the objectives it is imperative to plan out an artificial
recharge scheme in a scientific manner. Thus it is imperative
that proper scientific investigations be carried out for selection
of site for artificial recharge of groundwater.
The proper design will include the
following considerations:
Selection of site: Recharge
structures should be planned out after conducting proper hydro-geological
investigations. Based on the analysis of this data (already
existing or those collected during investigation) it should
be possible to:
n Define the sub-surface
geology.
n Determine the presence
or absence of impermeable layers or lenses that can impede percolation
n Define depths to
water table and groundwater flow directions
n Establish the maximum
rate of recharge that could be achieved at the site.
Source of water used for recharge:
Basically the potential of rainwater harvesting and the quantity
and quality of water available for recharging, have to be assessed.
3. Engineering, construction and costs
4. Operation, maintenance and monitoring
Design of recharge structures and settlement
tank
For designing the optimum capacity of the tank, the following
parameters need to be considered:
1.) Size of the catchment
2.) Intensity of rainfall
3.) Rate of recharge, which depends on the geology of the
site
The capacity of the tank should be enough to retain the runoff
occurring from conditions of peak rainfall intensity. The
rate of recharge in comparison to runoff is a critical factor.
However, since accurate recharge rates are not available without
detailed geo-hydrological studies, the rates have to be assumed.
The capacity of recharge tank is designed to retain runoff
from at least 15 minutes rainfall of peak intensity. (For
Delhi, peak hourly rainfall is 90 mm (based on 25 year frequency)
and 15 minutes peak rainfall is 22.5 mm/hr, say, 25 mm, according
to CGWB norms).
Illustration
For an area of 100 sq. m.,
volume of desilting tank required in Delhi = 100 x 0.025
x 0.85
=
2.125 cu. m. (2,125 litres) |
Design of a recharge trench
The methodology of design of a recharge trench is similar to
that for a settlement tank. The difference is that the water-holding
capacity of a recharge trench is less than its gross volume
because it is filled with porous material. A factor of loose
density of the media (void ratio) has to be applied to the equation.
The void ratio of the filler material varies with the kind of
material used, but for commonly used materials like brickbats,
pebbles and gravel, a void ratio of 0.5 may be assumed.
Using the same method as used for designing a settlement tank:
Assuming a void ratio of 0.5, the required capacity of a recharge
tank
= (100 x 0.025
x 0.85)/0.5
= 4.25 cu. m.
(4,250 litres)
In designing a recharge trench, the length of the trench
is an important factor. Once the required capacity is calculated,
length can be calculated by considering a fixed depth and
width.
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