Storm Water Drains

An article that I wrote which i thought would be relevant for this discussion 

Storm-water management– The new paradigm 

Consider an undeveloped plot of land in a city. A fairly level empty 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. 10 % of it will runoff as storm-water, about 90% of it will be absorbed by the soil but only about 10% will infiltrate and percolate to become groundwater, 80 % 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. The situation will dramatically change. Of the rain falling on the site 90 % will run-off as storm-water, about 5 % will evaporate or evapo-transpirate and about 5 % will infiltrate and percolate into the ground. This 90 % run-off now races to low lying areas quickly, overwhelms the drainage system and causes urban flooding with disastrous consequence to life and property.

The double whammy comes from the fact that urban areas are also 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. Retain 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 as this house on Hayes road shows. By placing a Rain Barrel of 5000 litres capacity and leading their 200 square metre rooftop rainwater to the barrel they have not only managed to ensure that a 30 mm rainfall does not go out of the house but also that they have additional water ‘straight from the sky’ for their use. Retention can also be taking the rooftop rainwater and storing it in sump tanks.

http://www.youtube.com/watch?v=Dz_VGaPr610&feature=channel_page

 Detentions are slightly different affairs. Detaining waters is easily done by making recharge wells and leading the rooftop rainwater into them. They will gulp close to 6000 litres 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 but some may completely go to make up the underground water table. Typical recharge wells in the Bangalore context are 3 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.

With simple steps from individual homes and apartments to either store rainwater or to recharge it , it should be possible to manage urban floodings. It is my opinion that a combination of simple technologies would be better than a foreign technology which is centralized in approach

Rainwater harvesting rules help facilitates storm water management, provided they are designed and implemented with care. Cities which seek to manage water harmoniously can avoid floods and augment there water supply through these retaining and detaining structures instead of investing huge sums of money to build large storm water drains, difficult to maintain and wasting a precious resource. The choice is ours and we must make wise decisions now. 

Dear Mr Chari

I for one believe that even public works such as cleaning up and maintenance of lakes, water supply, sewage disposal, etc (and, certainly public bus transport services and power supply), need to be outsourced to private contractors/ players, as compared to their being undertaken directly by government agencies like LDA, BWSSB, etc. The government agencies should restrict themselves to being the facilitators and regulators. And, to ensure that the works are carried out properly, only reputed players should be enlisted, with decent returns on their investments being provided for under the contract.

Now, many of these activities may not be financially self-sustainable, and they need not be too. In such cases, the expenses whatever need to be met through state/ municipal budget provisions. The problem arises when the babu's try to convert these entities into profit centres, applying business principles picked up from some management courses they attend, but which have very little relevance to their kind of work. I have elaborated more on it         here

A typical example of how not to go about things is the Nagavara lake "privatisation" (Hebbal lake maintenance deal with Oberoi's is a lot different - I'll elaborate on that elsewhere). And, a typical example of how to go about things the proper way perhaps is the JUSCO (Mysore) contract.

So, coming to the subject at hand, a good scheme should always be welcome even if it is slightly costlier, since the cost the city is paying otherwise is huge, and in very many ways. But, the schemes have to be relevant.

Now, just treating the water in the lake without figuring out proper solutions to the various causes of contamination all along the kms of stretches of storm-water drains, is just a lot more money down the drain.

That is the challenge before you.

Muralidhar Rao