Source : The Hindu Date : 27.06.2009
Dealing with storm water
| A source control strategy will look at ‘retention’ and ‘detention’ methods |
Consider an undeveloped plot of land in a city. A fairly level site with no construction on it yet. When it rains on this plot, depending on the intensity and volume of rainfall, this is what is most likely to happen:
Ten per cent of it will runoff as storm water, about 90 per cent of it will be absorbed by the soil. But only about 10 per cent of this 90 per cent will infiltrate and percolate to become groundwater, 80 per cent of it will most likely come up through the soil and either evaporate or evapo-transpirate, being taken up by the roots of plants and trees and sent out into the atmosphere through the leaves.
Now consider the same site with construction on it. Ninety per cent of the rain falling on it will run off as storm water, about five per cent will evaporate or evapo-transpirate and about five per cent will infiltrate and percolate into the ground.
This 90 per cent run-off now races to low lying areas, overwhelms the drainage system and causes urban flooding.
The double whammy comes from the fact that urban areas are prone to increased rainfall in volume and intensity because of the heat island effect, where the crust of the city such as roads, pavements and buildings absorb heat and radiate it into the atmosphere, causing temperatures to go up and for the rain clouds to fall more and faster.
Solutions
A source control strategy will look at two major methods called retention and detention. Retention is to store storm water. Since most storm water comes from individual sites, these sites can act responsibly to retain or detain the water. Retaining is a more permanent affair while detention means a temporary holding of the water before its release into the drainage network.
Retention can be as simple as storing rooftop rainwater in a drum. By placing a rain barrel of 5,000 litres capacity and leading their 200 sq.m. rooftop rainwater to the barrel, the residents can not only ensure that rainfall does not go out of the house but also collect additional water ‘straight from the sky’ for their use. Retention can also be taking the rooftop rainwater and storing it in sumps.
Use of wells
Detention is a slightly different affair. This is easily done by making recharge wells and leading the rooftop rainwater into them. They will gulp 6,000 to 10,000 litres in a day and ensure that the water goes to the aquifer than to the storm water drain.
Some of the water may emerge as base flows and some make up the underground water table. Typical recharge wells in the Bangalore context are three feet in diameter and 20 feet deep.
Detaining structures can also be placed in the storm water drain or adjacent to it, thus picking up and recharging storm water.
Rainwater harvesting rules help facilitate storm water management, provided they are designed and implemented with care. Cities which seek to manage water harmoniously can avoid floods and augment their water supply through these retaining and detaining structures. The choice is ours and we must make wise decisions now.
