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Rainwater Harvesting in Urban Areas

Much of the water supply comes from rivers or lakes and the constant quest of engineers is to seek these ‘perennial’ sources. Of interest is the growing ‘Ecological footprint’ for water. As local sources dry out, become polluted or are simply insufficient the city marches farther and farther for its water. For example, the city of Bangalore, used to meet its requirement from lakes and tanks within the city’s boundary till 1896, then it moved to a source 25km away. Proving insufficient the next source was identified as a reservoir 35km away in 1936. This too proved insufficient and the current source is a river 95km away and about half a km below the city’s elevation. Likewise, the city of Chennai (population seven million) has as one of its water sources, a lake about 235km away. The chase for a source is on and the sustainability a big question mark.

Bangalore developed a master plan for rainwater harvesting for its entire Comprehensive Development Plan area of 1279sqkm. The report suggested that up to 25% of the city’s requirement by 2011 cold be met through rainwater harvesting and in the optimistic scenario that 592.90 million litres per day equivalent could be harvested in the city. Chennai again in South India has made rainwater harvesting compulsory for all buildings in the city both old and new and claims that 98% buildings have complied with the requirement which seeks to whether store rainwater or to recharge it to the underground aquifer.

Domestic rooftop rainwater harvesting

Rooftops, paved areas and unpaved areas and the entire city itself are therefore to be managed as a water provision area. Rainwater harvesting would include collecting and storing rainwater runoff from rooftops – roof water harvesting and from the ground – runoff harvesting and from channel flow – flood water harvesting. The process of rainwater harvesting would encompass catching rainwater, directing it to an appropriate location, filtering it if required and storing it for use. Storage could be in tanks, sumps, ponds or lakes. Wherever appropriate and conditions permitting recharge of ground water would also qualify as storage. Harvested water could be used immediately as a first choice thus saving city level supplies or ground water for a future date. The water could be stored for later use during days of water shortage.

Typical Structure

Designing a rooftop rainwater harvesting system would mean taking the following steps. Sloping the roof appropriately, preferably towards the direction of storage and recharge. Designing gutters and/or down pipes depending on site rainfall characteristics and roof characteristics. Putting in place a first rain separator to divert and manage the first 2.5mm of rain. Filtering the water to remove solids and organic materials, storing the filtered water in appropriate devices and recharging the ground water through open wells, bore wells or percolation pits.

Cost of water

In the Indian context, urban water is heavily subsidized. As the water connections are usually to households who are in the higher income group, in effect subsidies travel to the rich. The poor generally have to make do with public stand posts and public hand pumps. The true cost of water is when we release it back to nature in the same quality situation as it was appropriated. If not in the same condition it should be released back to nature within the carrying capacity of the soil or water system and geographical area to which used water is released. This rarely happens. Thus the true cost of water is when it accommodates capital cost, operations and maintenance cost, opportunity cost, economic and environmental externality. While this is rarely done at the city level, it is possible for individual houses, apartments, institution buildings and industries to be proactive and accommodate this process. While this may add to short term cost in the long term it ensures sustainability.

Strategies

A water audit statement would need to be prepared for each consuming unit where water is used. This would include identifying such parameters as water demand, stratified to all classes of requirements like potable water, water for process, de-mineralized water, landscaping/garden water and water for toilet flushing and sources and quantum of supply including city level supply from open wells bore well supply and private tankers. Supply of water from rainfall can then be calculated using various areas of the buildings including rooftop area, paved area and unpaved area.

Determination of incident rainfall is calculated by obtaining average rainfall data for 30 years. Month wise data averages increase accuracy. By using a factor of loss total monthly supply is determined on each of the surfaces. Then a decision is made whether to store the water or to recharge the water so collected from rainfall. The process of rainwater usually works best as a combination of storage and recharge. Some rules of thumb provide interesting information on rainwater harvesting potential. In Bangalore, for example with an average rainfall of 970mm over the last 10 years, a 100 square metre roof area of any building would have 97,000 litres of rainwater incident on it in a year and of which 87,000 litres would be harvestable. For a family of five this harvested rain could provide nearly 50 litres of water per person per day. Similarly on an acre of land, 39 lakh litres of rainwater is incident. It is for the designer to put in place strategies to ensure maximum benefit from this free gift of nature.

Much of the water supply comes from rivers or lakes and the constant quest of engineers is to seek these ‘perennial’ sources. Of interest is the growing ‘Ecological footprint’ for water. As local sources dry out, become polluted or are simply insufficient the city marches farther and farther for its water. For example, the city of Bangalore, used to meet its requirement from lakes and tanks within the city’s boundary till 1896, then it moved to a source 25km away. Proving insufficient the next source was identified as a reservoir 35km away in 1936. This too proved insufficient and the…

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