(Note: a.Annual average rainfall of Delhi=611 mm; b. runoff coefficient is assumed as 0.60;Sq. m to be read as square metre)
An analysis done based on the rainfall availability and demand supply gap shows that even 50 per cent of the rainwater harvested could help in bridging the demand supply gap.

Photograph: S Vishwanath

Bangalore receives 970 mm rainfall annually and the number of rainy days is 59.8. Highest amount of rainfall is received during April to November, while the rest of the months receive scanty rainfall. Peak runoff is 50 millimeters per hour. Due to the availability of rainwater throughout the year, water is basically stored in these rainwater harvesting systems and used for non-potable purposes. Water from the rooftops is led into storage structures. First flushing is normally done by providing an extra length of pipe to collect the polluted 2.5 mm of rainfall. Filters are made of sponge and a mixture of sand, gravel and charcoal. After first flushing and filtration water is led into under ground sumps (which are very common in Bangalore) or to a new storage tank.

The overflow from this tank is taken to an open well to recharge the aquifer.The geological formations are predominantly granite and granitic gneiss, with joints and fractures in abundance due to intense chemical weathering of rocks. The depth of weathering varies from 0.2 m to 20 m. This geological set-up offers an immense scope for recharging of ground aquifers.

The undulating terrain with gentle slopes draining into lakes offer an ideal situation for water harvesting. In the urban area of Bangalore waterbodies cover about 5 per cent of land. A study made by the Centre for Ecological Studies and Indian Institute of Sciences revealed that out of 262 lakes in 1960 only 82 exist now of which less than 10 have water.

Notes: Average annual Rainfall = In mm 970; Annual demand-supply gap 49.28 billion litres
Source: A conceptual frame for rainwater harvesting in Bangalore, STEM, 2001

A pilot project was set up in May 2000 covering about 1, 280 sq.m of roof area for the administrative block and the canteen building. With storage capacity of 42,00 litres, the unit collects about 1.05 million litres per year. The system is expected to pay back for itself in five years.

The pilot system has received widespread publicity and is seen as a pioneering model for water harvesting by an industrial unit in Bangalore. The rooftop water harvesting is now being scaled up to cover 3000 sq.m of roof area.
(Source: http://www.rainwaterclub.org/industries.htm)

Source: Proceedings of the workshop on rainwater harvesting , Indore p-67

Structures commonly found in Chennai

Source: Making Water Everybody's business

Source: Making Water Everybody's business

For recharging the well, the rainwater pipe can be connected to the open well to divert the rainwater from the terrace into the well through rainwater downtake pipes. The rainwater falling around the open space surrounding the building can be diverted to the front gate where a gutter is provided for a depth of 457.2 mm and a width of 609 mm with perforated slabs. The rainwater collected in the gutter in front of the entrance is discharged into another recharge well of 914.4 mm diameter and 6.9 m depth, provided nearby through necessary piping arrangements.

To keep this tank free from microbial contamination he has mixed a waterproofing chemical with the cement slurry to give an acrylic-poly-sulphate cement slurry coating. Through simple treatment and later filtering through 'Aqua -Guards' a commercially available water purifying product that kill germs, the rainwater is used for drinking and kitchen purposes. Each filling of the tank can sustain the drinking and kitchen needs for about 2 months. In a year the tank gets filled four to five times. Sometimes, the tank gets filled up more than once in a week. During these times, the excess water is diverted to recharge the groundwater aquifer. Ramani had to invest Rs. 8,000.00 in this project which was implemented in 1994.

A series of investments were made by Chennai Metrowater from 1991 onwards to harvest floodwaters along the course of the rivers bordering the city. Three checkdams were constructed at Valliyur, Jagnathapuram and Melsembedu in 1991, 1992 and 1995 respectively. Moreover the government agencies are involved in implementing rainwater harvesting structures in public places like parks, roads, fly overs, and storm drains. Thus every possible catchment is being utilised for water harvesting. As of now 400 buildings, 216 schools and 56 parks owned by the corporation of Chennai has the rainwater harvesting systems installed in it.

PIX: Fly over.

There are about 12 rainwater pipes are drained out through 3 recharge trenches and a recharge well. The recharge trench is 10 m in length, 1.0 m wide and 2.0 m deep. The bottom of the trench is filled with 60 cm of pebbles followed by coarse sand upto 1.20 m. Three unlined recharge bores of 150 mm diameter are drilled inside the trench each to a depth of 4.0 m feet and filled up with pebbles. Three of the rainwater pipes are connected to each trench.

At present, Aizawl has more than 10,000 rainwater harvesting tanks in individual houses which have been constructed by the residents at their own expense or with state government assistance. In a pollution-free state like Mizoram where major industries are yet to come, rainwater is free from undesirable chemicals and is of potable quality.