Urban floods in BengaluruSeptember 6, 2008
The problem is in the planning
|Water, water everywhere…planners, institutions and individuals can take several steps to mitigate the physical and economic impact of urban flooding, says S. VISHWANATH|
— Photo: K. Murali Kumar
The deluge: The state of affairs in an upmarket villa after a lake breached on the Whitefield-Hoskote Road.
A series of flooding events across Bengaluru has brought into sharp focus the need for better management of rain. Though nothing on the scale of the Kosi floods yet, it has caused severe economic and physical damage to the city and left many psychologically scarred. The coming of the rains is looked at with trepidation and newer areas of the city seem to be affected every time it rains.
Several interesting facts emerge around urban floods. In Bengaluru, it is clear that it is rain which causes the floods unlike, say, a city like Patna where rain could cause the Ganga to swell and flood the city. Surprisingly, recent evidence suggests that it rains more in the city and slightly downwind than in the regional rural periphery.
This is according to a study by NASA scientist Marshal Shepherd. The urban heat island effect, where cities are warmer than their surroundings and which causes the build-up of rain clouds on the city; pollution, which allows rain to coalesce around dust and oil particles; and the wind-break effect of cities, which causes the clouds to discharge on the cities, all seem to contribute to this phenomena. Bengaluru needs to prepare for more rain than average and higher intensity rains at the same time.
Cities also increase runoff as more and more soft agricultural and fallow areas get built upon or paved. From a small well-mulched site, hardly 10 per cent of the rain falling will runoff as storm water. However, build a house on the same site and pretty much 90 per cent of the rain falling will runoff as storm water. Buildings increase runoff tremendously in the Bengaluru context and the storm water drains have to cope with this increase.
Solid waste management is crucial to flood management since most of the uncollected garbage will end up in the lowest area, usually the storm water drains, choking them and reducing their ability to carry storm water out.
Tanks and lakes which collected surplus water and recharged the groundwater or dissipated it slowly are on the decline. These are built up, like the ISRO headquarters built on an old tank bed. They then become prone to flooding or transfer the flood problem downstream.
The network of tanks and the valleys and drains connecting them are in a bad state of management with encroachments on several of them. With no institutional approach to manage the tanks and the valleys, little is done except during the flood event itself to ensure that the channels flow freely and that the tanks are not encroached upon. Traditional storm water management techniques simply collect the rain water and funnel it across the city downstream. Newer methods combine traditional approaches with new ones such as Sustainable Drainage Systems (SUDS). It employs a range of natural processes to purify urban runoff. Removal of sediment, bio-filtration, biodegradation and water uptake by plants all help to remove pollutants. Vulnerability maps of areas prone to flooding need to be prepared for citizens to become aware of the choice they make for where they live.
Even as the Government is working towards making RWH mandatory in the coming days for the city, the system has one of the best potentials to replenish ground water, improve its quality, provide supplementary water for domestic requirements and mitigate flooding. If every building in Bengaluru can store or recharge 60 mm of rain in a single day it should be possible to mitigate the effect of almost every flood except the rare. This means that a 100 square metre roof area will need to store or recharge 6,000 litres of water. Zones with the best possible recharge and zones with the best possible storage need to be identified in the city and steps taken to encourage people to go in for rainwater harvesting.
A recharge well of 3 feet diameter and about 20 feet depth can send in up to 12,000 litres of water into the ground in a single day, provided lithological conditions are favourable. The city needs many such recharge wells in the catchment area of critical flood zones to detain flood waters and top up the aquifers instead of surface flow flooding.
At the broader scale, tanks and lakes need to be networked and managed as retention and detention structures. With rainfall prediction accuracy being developed, tanks have to be linked to catchments and kept ready to hold the maximum water to dampen peak storm events. A deslited tank in Bengaluru can recharge up to 11 mm of water every day while an undesilted one can recharge hardly 1 mm. Desilted tanks can recharge aquifers quickly, lower the surface water levels and be in a position to function as flood mitigators. Full tanks are not good at dampening floods.
In Europe, urban flood research has been driven by insurance companies who want to understand risks associated with floods and plan premiums accordingly. This sector has yet to mature in India but taking flood insurance is a wise step especially if your car has been found floating in the basement after a rain. Good advice comes from ICICI-Lombard on its website on what to do after a flood. It starts by saying that you should not return home till the authorities declare it safe to go back. Then the steps recommended are: turn off electricity and gas, make sure the water and food you consume are safe, stay healthy, call your insurance agent, take photographs and videographs of the damage caused and finally take care of yourself and family. Wise words, indeed, and this is water wisdom when it relates to urban floods.
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