Climate change: Measuring flood risk

Gautam Pradhan
Nov 3, 2022

September 2022 saw some really heavy rains in Bengaluru, India’s most important tech city.

Many took to Twitter to share graphic images of their homes and vehicles going under water.

We prepared a flood risk map for Greater Bengaluru that was shared on Twitter for public awareness.

The impact of floods depends on amount of rainfall, the shape of the land (topology), the drainage capacity and the affected people/assets.

The following plots are simple ones based only on topology.

Darker blue = greater risk

The high-level view

Overall, the water drains north to south in 2 channels east and west of the city. Within the city, there is broadly westward or eastward flow into one of these 2 channels. But depending on where you live, the flow can be in any direction.

South-east Bengaluru — Outer ring road/Sarjapur road

This area is a peculiar case. A couple of decades ago, most of this area was either wetlands or farms. It is now almost completely built up.

Checkout the difference posted by Arati Kumar-Rao.

Back to the risk map (below): The flow is locally northward from Sarjapur road and crosses the pink line (Outer Ring Road or ORR). The water then accumulates and has to move eastward, again crossing the ORR!

The storm water flow channels are all choked because of relentless concreting. This turns the ORR into a dam that keeps the water where it is.

East Bengaluru (Varthur, Gunjur and Sarjapur)

The junction where you turn into Varthur from Whitefield is a high risk area. There are several spots of higher risk (red circles) south of this in Gunjur and on Sarjapur road.

South Bengaluru (BTM Layout, HSR Layout, Koramangala)

A bit east of Bellandur, you have areas like Koramangala and BTM layout which are also heavily built up with high population densities along flood channels.

North Bengaluru and the Airport

Moving north you can see how Manyata Tech Park is basically built on a stream! Channels flowing north-south and west-east meet here, before flowing further east.

An example of the flooding at this location is here.

The Kempegowda International Airport is also built without allowing for proper rainwater flow.

The new runway and the road into the airport (red) are blocking some natural flow of water (blue arrows).

Challenging some narratives

There was an effort to build a narrative around “not all places” and showing pristine unflooded areas in parts of the city.

They were not wrong but this is misleading. People forget that it rains everywhere but water drains off and all the suffering is heaped on the unfortunate few.

If you look at the high-level map, the central parts of Bengaluru are on a ridge and all the water flows ultimately towards the suburbs which then get overwhelmed.

Earthmetry got the opportunity to write a piece for News9 and speak to the The News Minute.

Try it out

DM me on Twitter or comment here if you would like to see your city or region examined for flood risks. We are evaluating if we can turn this into a useful product at a reasonable price and all feedback is welcome.

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