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The Hindu
The Hindu
Rasheed Kappan

Tackling Bengaluru’s flash floods with Sponge City systems, underground storage tanks

Year after year, flash floods have played havoc with Bengalureans, particularly those in and around low-lying pockets. Erratic rains fuelled by climate change concerns give them no time to prepare, as a month’s rainfall pours down in a few hours. The frequency of these high-intensity events has increased dramatically, demanding urgent but smart solutions.

So why not build massive underground tanks in low-lying areas to let flood waters rush in? Why not let the forces of nature, the push of gravity and human ingenuity combine to let the excess water settle safely, minimizing human suffering? Could this stored water be used later to address the issues linked to a dry season?  

Successful models abroad

Globally, such systems have worked although they demand a huge initial investment. In the German city of Munich, for instance, 13 underground tanks were built to arrest the recurring floods. The tanks were constructed in big vacant plots such as Bengaluru’s civic amenity sites. Each tank had two tiers of 8-metre depth. Flood waters would flow into the tanks to be pumped out later when required.

Flood-prone Tokyo and Pittsburgh too had built such tanks. The one in Tokyo, arguably the world’s largest underground water tank, is larger than a football field. Fifty-nine pillars, each 18m tall, support the tank’s ceiling.

The models look promising. But not many are convinced. Their contention: Why invest in such expensive projects when you can clean up the city’s existing tanks and lakes - many of which are in low-lying areas -, and use them for rainwater storage? This is a mammoth task yet feasible, but the stormwater drains that lead to these water bodies are horribly polluted with sewage and solid waste.

A big storage problem

“Bengaluru has a storage problem, not a water problem,” notes architect Naresh Narasimhan, who had conceptualised K-100, a joint project by the Bruhat Bengaluru Mahanagara Palike (BBMP) and Bangalore Water Supply and Sewerage Board (BWSSB) to transform the Rajakaluves into clean waterways flanked by aesthetic urban spaces. The K-100 leads to Bellandur lake, one of the city’s largest.

For years, the BBMP has been trying to solve the drain capacity problem through a grand remodeling exercise. But even after years, nearly 175km of drain network are today without any flood protection structures. The Palike has now set aside ₹1,200 crore to build concrete walls on either side of the drain. So far, about 686km of the city’s 859km of primary and secondary stormwater drains (Rajakaluves) have been remodeled.

Giant watersheds

Beyond the Rajakaluves, the focus should also be on converting the big water bodies on the city’s outskirts into rainwater storage tanks. Explains Naresh: “There are eight giant watersheds around Bengaluru. We have to pass a law to create a string of pearls around the city. And these have to be protected and water stored there. In times of crisis, this water can be used.”

The watersheds with the lakes already exist: Hoskote lake, Byramangala kere, Bellandur and Varthur lakes, Yele Mallappa Chetty kere, Hesaraghatta lake. “Besides, we should bring back the Arkavathy river and revive the Dakshina Pinakini. There is a mega plan and a micro plan, and it has to be ruthlessly mandated that rainwater is stored, delayed and not released so that the aquifers are not depleted,” says Naresh.

Decadal failures to scale up the city’s rainwater harvesting potential, storage capacities and to streamline its storm water management have left Bengaluru at the mercy of an external water source. Faced with depleting water levels at the Krishna Raja Sagar (KRS) reservoir, BWSSB has now urged the Cauvery Neeravari Nigam Limited (CNNL) to set aside Bengaluru city’s share for the rest of 2023. To cater to the city’s potable needs, it wants sufficient water in both KRS and Kabini reservoirs.

Did Bengaluru not see this coming? The warning signs have been loud and clear for decades, and yet the city’s record of retaining and reusing its precious rainwater has been poor. This is why, former BWSSB Chief Engineer Thippeswamy has been articulating the need to adopt the proven ‘Sponge City’ concept perfected by the Chinese.

Sponge City benefits

The objective is simple: Retain water at its source, slow down the flow, clean water naturally and adapt it at the sink where it accumulates. In this model, rainwater gets soaked in many ways: Rooftop gardens, roads that let water permeate down, ponds, filtration pools, wetlands and public spaces that allow the water to seep underground. In Bengaluru, a few stretches of the smart city roads have earmarked areas for water to seep down. But this needs urgent scaling up.

BBMP, says Thippeswamy, should think beyond flood control and find ways to retain the water. This obviously mandates a coordinated effort with the BWSSB. The ongoing white-topping of roads across the city has already raised serious concerns about concretisation preventing natural infiltration of rainwater to feed the groundwater table.

A file photo of sponge parks being developed by Greater Chennai Corporation. (Source: PRINCE FREDERICK)

Chennai experience

Chennai has taken the lead among Indian cities in adopting the sponge city model. Thippeswamy is convinced that Bengaluru should follow the system, which is now being adopted by cities across the world. The Greater Chennai Corporation has embarked on a grand plan to transform the entire metropolis into a sponge, arresting both floods and groundwater depletion.

Fifty-seven ponds are to be developed as model Sponge City Parks, scaling up across Chennai. Equipped with recharge wells, inlet and outlet pipes, the parks will act as temporary storage points during heavy rains. Permeable pavements in all newly built parks, real-time flood forecasting and spatial decision support system to develop lake and reservoir operation guidance are also part of the plan. Several such parks have already sprung up with walkways, trees and fencing all around.

Efficacy of flood alerts

Bengaluru has all the reasons to transform itself as a sponge city. But it may not happen in a hurry, like all slow-moving projects in the city. Can timely flood warnings offer some respite to the citizens? Based on telemetric weather stations and rain gauges placed at key locations, the Karnataka State Disaster Management Authority (KSDMA) has a system in place to alert the BBMP, besides BWSSB, the police, Bescom and the disaster management teams.

However, the Palike insists that no alert can help during flash floods. The civic agency, instead, identifies flood-prone areas in advance and focuses its attention there. In June 2020, the Karnataka State Natural Disaster Management Centre (KSNDMC) had launched the ‘Bengaluru Megha Sandesha’ mobile app to provide real-time information and alerts on weather, rainfall and flooding in the BBMP jurisdiction. But the app is yet to make much of an impact with poor public awareness levels.

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