Homes and gardens

How a city can tackle floods

Cleaning lakes and drains, adopting rainwater harvesting, adhering to building bylaws are a must. By S. Vishwanath

The heavy rains of August 14 and 15 will stay in the memory of Bengalureans for some time. Floods, waterlogging, sewage flowing into homes are not things that people will forget fast. There is much blaming to happen on the woeful state of our city infrastructure but there are lessons to be learnt too.

The data generation on rainfall has improved by leaps and bounds. The Karnataka State Natural Disaster Monitoring Centre (www.ksndmc.org) has an extensive set of weather monitoring stations, currently numbering 82, which provide increasingly granular data on rainfall across the city. The recent rain for example was concentrated in the south central part of the city while the north had very less rainfall. The city is not homogeneous when it comes to rainfall for intensity, duration and total rainfall.

Is there a pattern to this? Are there areas which receive heavier rainfall? Are there areas which are more vulnerable to rainfall? All these are interesting questions which can be answered robustly by flood modelling. This is currently being carried out by the IISc and KSNDMC together, one understands. In the mean-time here is what citizens, communities, wards and the city can do to tackle the problem.

Citizens: Do not build on old tank beds, buffer zones of lakes and rajakaluves and generally low lying areas prone to flooding. Understand that a built plot can increase the runoff from site on to the storm-water drain by factors of 6. A 150 mm rain falling on a 150 square metre site can send 22,500 litres of water outside the plot. Add up all the sites in a neighbourhood and that is a flood waiting to happen.

Follow and exceed the bye-laws for rainwater harvesting. If the law mandates harvesting 20 mm of rain falling on a site, design to hold in sump tanks or percolate in recharge wells at least 60 mm of rain, if not 100 mm That means a 100 square metre of roof area should provide for 6,000 litre or 10,000 litre of storage. In case of a recharge well, the dimensions should be at least 3 feet in diameter and 20 to 30 feet deep with a grill on top for safety and security. Allowing all the site rainwater can recharge the aquifers and prevent flooding

Communities: Identify and map flood prone areas in your layout, apartment complex or neighbourhood. Make sure the drains are cleaned regularly and there is no dumping of garbage blocking and choking drains.

Keep emergency pumps ready with a plan to bail out water and the place to put the water into. Have the contact numbers of the local Civil Defence and BBMP authority responsible ready.

Identify the nearest lake or waterbody and make sure that the bund is robust and will withhold full levels. Make sure that the overflow weirs and rajakaluves are cleared and ready to drain storm water out.

Ward level: With the help of the Corporator, identify flood prone zones and develop plans to ensure drainage. Regular desilting of storm drains and removal should be insisted upon and pushed especially during the months of August, September and October when heavy rains are expected.

The connections between roads and stormwater drains are weak points which tend to get choked. Good design should be ensured here. Wherever possible, recharge wells should be made within the storm-water drain to percolate water into the aquifer. Regular removal of leaves and a good solid waste collection system avoiding the dumping of any waste in drains should be in place during the monsoon.

City level: A flood response cell with adequate maps of flood prone areas should be in place to quickly provide rescue and relief efforts.

Prediction using rainfall and storm water modelling and transfer of information to citizens and ward level teams should become part of the management system.

The global concept being tried out is called ‘sponge cities’ where the hard crust of urbanisation is converted into a sponge concept with aquifers, wetlands and lakes being included to detain and retain storm water and convert foods into a water asset stored for later use.

Water wisdom lies in transforming a problem into a resource. Urban floods are wake-up calls to treat rain with respect.

zenrainman@gmail.com

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Printable version | Feb 20, 2020 5:57:26 PM | https://www.thehindu.com/life-and-style/homes-and-gardens/how-a-city-can-tackle-floods/article19518102.ece

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