WATER WISE
Rainwater harvesting and open wells
S. VISHWANATH
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Utilising 30 per cent of rainfall will give more water to Bangalore than what it gets from the Cauvery
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— Photo: Shaju John
Rainwater harvesting is the process of collecting and storing rain for future productive use. One easy method is to recharge the rainwater into the ground using small open wells called recharge wells.
The building bye-law for Bangalore city makes rainwater harvesting mandatory for new buildings. Among other things, it suggests that provision be made for 20 litres of storage or recharge for every square metre of roof area. A recharge well is thus in
conformity with the law and excellent to harvest rain.
Method
How is the recharge well made? A well-digger typically will test the soil as he digs and understanding its property will dig up to six metres in firm soil. A typical recharge well is one metre in diameter. It is then lined with pre-cast concrete rings with perforations to allow water to slip out from the sides. These rings are then gradually lowered into the well to line it from the bottom all the way to the ground level and above. A solid cover of steel or pre-cast concrete rings is placed on the top for security and safety purpose. This is very important.
Recharge capacity
The capacity of such a recharge well of one metre diameter and six metres depth is about 5,000 litres. By filling it with water and observing how long it takes for it to go into the ground we can estimate its recharge capacity. A typical recharge well will allow anywhere between 2,500 litres and 10,000 litres per day.
The rooftop rainwater can then be led into the recharge well through pipes which pass through a small filter. This filtered water is then allowed to hit the sides of the well and fall in.
Depending upon the well size and the recharge capacity anywhere between one lakh litres and 10 lakh litres can be recharged by a single well.
Benefits
Major benefits include a compliance with the law, recharge of the aquifer and prevention of urban flooding. All individual households, apartments, layouts, industries and institutions can construct recharge wells and should appropriately include it as part of the design itself. In many cases the groundwater levels increase almost immediately after a rainfall year and this water is available for supplementary and appropriate use. In many cases borewells too are benefited as the shallow aquifer eventually recharges the secondary porosity from where borewells draw water.
Let us remember that in Bangalore alone, nearly the equivalent of 3,000 million litres per day water falls as rain. On an acre of land this is roughly 36 lakh litres annually and on a 30 x 50 site it is 1,30,000 litres. If collectively as a city we are able to recharge 30 per cent of rain it will be more water than what is being pumped into the city from the Cauvery currently using enormous energy.
Precautions
It may not be possible to build recharge wells in hard rock areas and flood prone or high water table areas. It is also important to maintain quality of the water which we recharge. In no case should anything but pure rainwater be sent in. Recharge wells can become withdrawal wells if the aquifer permits and the water table rises. This water should be given first charge i.e. used first when available. Normal procedures for well maintenance, such as regular cleaning and disinfection annually, are recommended. Removal of silt regularly helps in greater recharge.
Certain areas of the city, especially close to the ridge, are recharge zones and certain areas which are in valleys are discharge zones. Benefits of recharge may not be available equally to all but as a community we collectively will definitely gain.
Open wells have provided water for centuries. The shallow aquifers in our city need to be revived to ensure adequate 24 x 7 provision of water. By integrating rainwater harvesting with groundwater recharge each citizen can contribute to the cause by using water judiciously and ensure its continued availability for our children.
(The author can be contacted at 23641690 or zenrainman@gmail.com)
www.rainwaterclub.org
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