On August 30, a resident of Rainbow Drive, on Bengaluru’s Sarjapur Road, passed away as he could not get to a hospital in time. His neighbourhood had been flooded and its residents marooned for hours, during which even tractors could not reach them.

On September 5, after the heavy overnight rains of up to 90 mm, residents of this area and the larger Sarjapur Road-Outer Ring Road area continued to suffer, with large sections of even main roads inundated.

Rainbow Drive is one of the parts of the city that has had far too many trysts with flooding. As recently as August 3, people had to use tractors to get out of their homes to reach offices and schools. The rain recorded in nearby Bellandur that day was a mere 46.5 mm.

But why are areas like Rainbow Drive so vulnerable to floods?

The entrance to Rainbow Drive. Credit: PTI

Why it floods in Rainbow Drive

KP Singh, a resident of Rainbow Drive explained the context. The layout is spread over 35 acres, developed some 25 years ago on what were once paddy fields. This area, as is evident from the elevation map, is in a valley area, connecting the lakes of Halanayakanahalli and Junnasandra downstream to Saul Kere.

The kaluve or channel carrying the overflow from Junnasandra ends at Rainbow Drive. In addition, the level of the Sarjapur Main Road has increased by over 3 feet, says Singh, while the layout is at the original valley level.

“This means the water reaching the Rainbow Drive exit gate is not flowing fast enough towards Saul Kere,” he said. “As of now, Sarjapur Road storm water drain is not wide enough to carry the rainwater accumulating at the Rainbow Drive gate.”

In the same way, the valley connecting Saul Kere to Bellandur lies on the ORR-Ecospace area, where it was flooded too.

Water expert S Vishwanath explained that the original lake system was designed with kodi kaluves or waste weir overflow channels, to handle overflow from those lakes to Saul Kere, from which in turn water overflowed to the Bellandur-Varthur waste weir channel. (Raja kaluves refer to the channels used for irrigating the command area.)

“We have ignored the kodi kaluves,” said Vishwanath.

Each lake typically had two waste weirs to let the overflow water move downstream. Most of these channels are now defunct, except in the larger lakes like Yelahanka, Bellandur and Jakkur, which continue to have two waste weirs but only one waste weir channel.

How ‘development’ leads to flooding

Before these areas got built up, water would also flow in “sheets” into the lakes and out of the lakes after heavy rains, not just restricted into channels. KP Singh remembers seeing the water just flowing over Sarjapur Road till a decade ago. But now these lakes are completely surrounded by bunds that prevent sheet flow and cause water logging on their sides.

With heavy rainfall, the volume of water or runoff is too high to be limited to flowing in these narrow and sometimes missing kaluves. “Given that the catchment areas are now paved with concrete, there is no percolation into the ground, and the kodi kaluves have to deal with five times more water,” Vishwanath said.

What is the solution?

The flooding of Rainbow Drive is ironic given that it is a pioneer in rainwater harvesting and waste water treatment and one of the few neighbourhoods in Bengaluru’s peripheries that is completely water sustainable.

But their situation is the cumulative result of decades of uninformed decision-making at several levels – from the state to real estate developers to the individual property owners in the entire area.

Priya Ramasubban, a lake activist who has worked on many of the lakes in the neighbourhood, says one can’t just stop with blaming the government, “Residents attach a lot of importance to their property value and immediate amenities over long-term needs,” she said. “Active citizens know which properties have encroached on low lands or watershed/valley areas but they don’t dare challenge the systems lest they find themselves in the crosshairs of neighbours who don’t like them, or to have lost a bit of their property value.”

A sustainable solution requires all parties to acknowledge these issues and look at the entire area holistically. “Without collective action and a willingness to sacrifice, we won’t get anywhere,” Ramasubban said. “So we have to decide, do we want to just suffer a few days of floods and then move on or work to bring lasting change to our neighbourhood that could be painful to make and could result in a lot of work.”

What can be done right now?

Two weeks ago, the Mahadevapura Special Commissioner said Bengaluru municipal corporation would survey the stormwater drains in Rainbow Drive as there were encroachments and properties built on the kaluves. But a piecemeal approach is problematic when the entire lake network and the land topography needs to be analysed, and the drainage network through storm water drains and kaluves need to be relooked at from an environmental and equity perspective.

Singh says there are some immediate action items to prevent their layout from getting flooded in the short term: First, the authorities can open the Halanayakanahalli lake sluice gates whenever necessary. The city municipal corporation must take up construction of storm water drains along Junnasandra Road as well as the western boundary of Rainbow Drive to take the water to the culvert.

The Halanayakanahalli lake’s overflow from the east side of Rainbow Drive should have a proper channel towards the culvert too. Finally, it is essential to ensure desilting of the Junnasandra and Halanayakanahalli lakes, in the summer.

Flooding now ubiquitous

Such challenges are not just Rainbow Drive’s alone.

A recent study by the Karnataka State Natural Disaster Monitoring Centre has found that Bengaluru’s rainfall patterns have been changing and it is an indicator of climate change. Data from the India Meteorological Department and the state natural disaster monitoring centre also show that the annual rainfall has been increasing steadily over the past few decades.

“Bengaluru will need to rework its building design and storm water design codes to deal with the increased rainfall, and also intensity of rainfall,” said Vishwanath S. “We need to design for short bursts of 180 mm per hour intensity and for 200 mm per day rainfall. Roof slopes, rainwater pipes, storage and recharge structures and storm water drains will all need to cater to this new regime of rainfall. We also need to improve our predictive powers and models of rainfall events.”

Meera K is the co-founder of Citizen Matters, the award-winning civic media platform. She also helped initiate Open City, an urban data platform (opencity.in)

The article was first published in Citizen Matters Bengaluru, an online civic media platform supported by Oorvani Foundation.