Recharge through recharge wells can serve two very important purposes: first to control and mitigate flooding and secondly to help ensure rainwater percolates into groundwater. Groundwater recharge can be achieved through multiple “artificial recharge structures” such as recharge wells, direct borewell recharge, check dams (found in the rural context) etc. However this guide will focus on the “recharge well” as it is most appropriate in the urban context, consumes little real estate and can be retrofitted or integrated into real estate development. Further, the recharge well is very suited to the geology of Bengaluru.
The main components of a rainwater harvesting system for groundwater recharge are:
Recharge wells can be retrofitted/integrated into real estate development in multiple ways.For any catchment, the type and number of recharge wells can be determined based on the catchment area, rate of rainwater runoff and infiltration into the soil. Also, topography of the plot should be studied to locate valley points since they get the highest amount of rainwater runoff and hence are ideal locations for recharge wells.However when flooding in valleys is a problem, recharge wells can be integrated from the ridge down to the valley. You can consult an RWH expert for advice on where to situate your wells.
For RWH systems that have both storage and recharge components, refer to this guide on building a direct use RWH (Rain Water Harvesting) system, i.e., the system for storage and direct use of water. If there is overflow from the storage barrel or sump in a Direct Use RWHM, you can channel it for recharge. Overflowing water, along with rainwater falling in your plot, can be channeled for groundwater recharge.
The recharge well takes water down to the permeable zone which is made up of weathered rock. Here the earth itself acts as filter and fine filtration is not required. Filtration is required only to remove larger debris + silt (which will silt up the recharge well). The design of the filter therefore depends on the catchment and what kind of debris get captured from the catchment. In case of gardens / farmland, silt & leaf debris may need to be controlled. In paved areas, solid waste like plastic packets are important forms of debris that need to be controlled. An in-drain filter – normally a grating/concrete jaali – for large debris along with a silt trap can be an effective solution. For recharge wells connected to clean rooftop areas or that capture overflow from tanks / sumps, filtration is not necessary.
For recharge wells connected to clean rooftop areas or overflow from tanks / sumps, filtration is not necessary.
Construction of a recharge well is far simpler than that of a conventional open well. This video explains how a recharge well is built. Recharge wells could be of varying sizes. A typical well of 3 ft diameter and 20 ft depth can hold 4000 litres of water. This water will take about two days to percolate down in sandy soil.
Activities involved in digging a recharge well include:
|3ft diamter and 20ft deep||4000 litres|
|4ft diamter and 25ft deep||8000|
|5ft diamter and 30ft deep||16,000|
|6ft diamter and 40ft deep||32,000 litres|
Recharge wells can be constructed inside stormwater drains. In these structures, the components are similar. In the SWD, before water enters the recharge well, there is a small trench which serves as the de-silting chamber. Then comes a concrete grate to trap solid waste, followed by a bed of stones and a low concrete wall to further filter out silt. The concrete wall, called a check dam, has flow pipes at the top to carry water to the recharge well. The mouths of these pipes are covered with netlon mesh to prevent any remaining solid waste from entering the recharge well. The recharge well should be covered with an RCC slab. In case of too much rain, an overflow pipe on the other side of the recharge well carries away the excess water. Recharge wells inside SWD ensure that hardly any real estate is wasted for the structure.
Traditionally there has been an entire community of well diggers called “mannu waddars”. With the decline of open wells and fall of water tables, they have been losing their livelihoods. However when we construct recharge wells in our cities, we make their skills and knowledge relevant again and they become a critical part of the new “green economy” we need to develop. Watch their skills at work. Here are a few who could help you dig your recharge well.
Since a recharge well is not a standardised product, each well digger will have his own vision of the well, and hence costs are bound to vary. An experienced digger usually quotes so as to cover the cost upto a minimum depth. As of early 2017, digging a recharge well of 3ft diameter and 20ft depth costs Rs 25,000-35,000. The costs then vary depending on the diameter and depth. Construction of one well takes only couple of days.
Costs vary based on these factors: