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).

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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