The rains that battered Bengaluru over the last few days have hit the headlines, as the city still struggles to solve the crisis brought on by severe inundation in many areas. Visuals have emerged of several gated communities and arterial roads being completely flooded, with many residents being forced to evacuate their homes on tractors. Further, the flood situation has also cut off access to some areas, making it difficult for residents to source essential items such as food and drinking water.
The flooding of Bengaluru, which is considered the ‘Silicon Valley of India’, has put the focus on poor city planning and encroachments on lakes, stormwater drains and land through which rainwater naturally flows. To understand the various factors behind the inundation, TNM spoke to Gautham Pradhan, who runs Earthmetry, a company that helps climate policy researchers get access to data. Based on data sourced from the United States Geological Survey (USGS), Earthmetry has put together a flood risk map of Bengaluru, taking into account the topology, natural flow of water in the city, rapid development, and other factors.
Indiscriminate construction is a problem in many parts of Bengaluru and many other cities in India. What makes east Bengaluru different from other such areas is the topography. The central core parts of Bengaluru are on a ridge from where water usually drains away and doesn’t accumulate. All the rainfall east of Majestic transport hub flows generally toward the east and south-east as you can see in the map. For example, the run-off from Ulsooru passes through Domlur, Koramangala, Agara and ends up at Bellandur lake. At Bellandur we also see a local northward flow joining in from the Harlur/Sarjapur road direction and that crosses the ORR to join the Bellandur lake. This then crosses the ORR again to the north near Marathahalli and joins Varthur lake and then flows out of the city. At every successive point you have water accumulating from all previous points. So the people at Varthur have to deal with the run-off that comes all the way from the centre of the city!
The flatter land from Bellandur onward and heavy construction and encroachments means the water keeps accumulating here in various areas. Historically these were wetlands that could absorb a lot of run-off.
Flood risk map for Greater Bengaluru ️
— Gautam Pradhan (@Quillambar) September 5, 2022
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
1/n pic.twitter.com/pwLgRb0YSf
If most land is concreted, there is less scope for local soil to absorb water. Rainwater harvesting and storage can mitigate some of this. If not, the water then runs-off into the streets and into lakes and other low lying areas.
At this point the drainage capacity is the driving force. If the storm drains are blocked and the connectors between the lakes are narrowed and encroached, the low lying areas keep accumulating water. It is as if a series of unintended dams suddenly appeared! Eventually some lakes get filled up and breach or overflow their embankments leading to severe flooding.
Analysing flood risk and vulnerability requires data on rainfall events, topography, storage and drainage capacities and land-use. The source we used for the current analysis was purely topographic information. Currently most specialists rely on data opened up by the US or European Government agencies.
But the Union government has started an initiative to open up geospatial data through the Bhuvan other portals. This is a new development and we are still learning about what is available through this.
I think we need to find ways to have local bodies work with domain and data specialists to make more data driven decisions. I think at this point a useful collaboration can be initiated with available data sources. The process has to be iterative. We can first work with what data is available and drive action. But over time local bodies should invest in creating locally important data and opening it up. For example, they should track the storage in the lakes, flow between the lakes, the storm water drain maps and so on.
The technical solution is to add more storage and drainage capacity. At this point, action on storage would be limited due to a lack of space for large reservoirs. But even implementing rain water harvesting in buildings where we add storage sumps and tanks can add some capacity. Improving drainage capacity would mean hard actions like removing encroachments and reducing corruption. These actions will be politically challenging. How to move forward in such a situation is not my area of expertise but I will offer my thoughts subject to validation by experts.
Imagine you newly arrived in Bengaluru and are looking for houses in Munnekollala or Bellandur. All you see around you are buildings. You are not going to look at a topographic map! You will pick a house in your budget and live there. So blaming immigrants doesn’t make sense.
You could blame the landowners but they were just responding to incentives of the system. Most people in their situation would have done the same. If the system doesn’t change, you can never eliminate such actions. Blaming the companies setting up offices doesn’t make sense as they are Bengaluru’s economic engine.
In the short term, I think the government should compensate those who will be displaced and take over the land needed to widen the drainage channels. In the long term, there should be better monitoring of land-use to prevent future encroachments. Local associations can be empowered to raise these issues and hold officials accountable.
The shortage of land for good housing and commercial space also directly plays into this indiscriminate construction. So we should raise our housing capacity by increasing the FSI (Floor Space Index) and build more vertically while widening roads and improving public transport. Why not have 80-floor IT parks? All the IT offices in Bellandur could be housed in a couple of skyscrapers that also have metro and other transport connectivity.
Find the Topographical Wetness Index of Greater Bengaluru on the Earthmetry website, or click here.