How to design a flood-proof city

(Top and above) A view of how a road looked earlier and a redesign after intervention by the Delhi Development Authority. Credit: Oasis Designs Inc

(Above) The blueprint and (below) an aerial view of the K100 gateway project, meant to upgrade stormwater drain from Majestic to Bellandur. The image shows the redesigned drain near KH Double Road and BMTC headquarters in Shantinagar.

The proposed and existing design of a road under the Delhi Development Authority (DDA), redesigned by Oasis Designs Inc.

Street smart and workable ideas are much needed for a city that crumbles after a spell of rain, trapping motorists in flash flood waters that refuse to drain out quickly. Waterlogged woes are a recurring monsoon ritual in Bengaluru — at the heart of it is a hugely compromised stormwater drain network that extends to about 850 km.

Clogged by sewage inflow and dumped solid waste, encroached and narrowed down by multiple interests, its design is hardly able to ferry storm water to the city’s lakes.

Project K100

A design was proposed in 2020 to transform the stormwater drain — the Rajakaluve — into a clean waterway flanked by aesthetically laid out pedestrian walkways and public spaces. Project K100 (Citizens’ Water Way), extending from Majestic to Bellandur Lake, is a pilot to show how a city like Bengaluru’s entire network can be transformed.

For years, flood control has meant desilting the drains a few weeks before the onset of monsoon. To boost water quality and natural infiltration, the K100 project employs natural material and bio-remediation techniques.

“When it rains, storm water mixes with sewage and hardened silt and overflows. We removed the silt, diverted the sewage to the pipe network so when it rains, the drain has enough capacity,” explains architect Naresh Narasimhan, who conceptualised the BBMP-BWSSB project, through the Mod Foundation, an urban action and research institute.

There is now almost no flooding along the K100, which extends to about 36 km of primary and secondary drains with a catchment area of 32 sq km,
he says.

A key design principle is to offer citizens a sense of participation and ownership. Naresh elaborates, “The public walkway extends about 8 km from Double Road to Bellandur. The idea is to bring ‘public’ into public infrastructure. When there is a walkway, people will look after it and not dump waste.”

The green spaces along the drain have a purpose. “We are introducing an Israeli concept called Plants Treating Sewage, which is a constructed wetland for certain sections of the drain,” he adds.

The blueprint

Urban experts suggest an emergency flood management plan activated in four critical phases: mitigation, preparedness, response, and recovery.

The first phase, mitigation, will eliminate flooding risk before it occurs. This includes updating building codes, boosting safety mandates, designating open space uses in flood-prone areas. It also includes hydrologic modelling to estimate an existing and potential flood risk due to climate change. This modelling helps clearly demarcate flood-prone areas through Geographic Information Systems (GIS).

Mitigation projects include floodwalls, impoundments, restoration of wetlands, acquiring flood-prone properties, rainwater harvesting, public outreach and education. Citizens are involved in estimating and understanding the flood risk.

Preparedness is meant to achieve a sense of readiness for any flooding emergency. Plans are tailored to the specific needs of a local area or community, and neighbourhood communities are engaged and involved in mock emergency drills. Emergency warning systems, flood insurance, and stockpiling flood-fighting material are part of the plan. Desilting the drains ahead of the monsoons is a part of preparedness.

The ‘response’ phase will ensure emergency relief, search and rescue to affected people, and the ‘recovery’ phase includes action to bridge the gap between emergency and normalcy. Temporary housing, reconstruction, compensation, counselling and education are a part of this process.

The flood is documented, where after the flood waters recede, high water lines are marked, and citizen experiences are recorded from ground and drone-levels. This is to boost flood prediction accuracy, plan for the future and educate the public.

Sponge City

Former BWSSB chief engineer Thippeswamy draws attention to the Sponge City concept launched in 30 Chinese cities in 2015. The idea was to help cities absorb 80% of their rainwater and reuse at least 70% of it.

The basic idea of a Sponge City is: Distribute and retain water at its source, slow it down as it flows away from the source, clean water naturally, and be adaptive to water at the sink where it accumulates.

Source control, local adaptation, learning from nature, preservation of urban ecological space, restoration of biodiversity, natural infiltration and purification are all part of this design principle. Constructed wetlands, rain gardens, green roofs, grass ditches and ecological parks are the elements that make a Sponge City.

DDA plan

Adopting a similar strategy, the Delhi Development Authority (DDA) proposed a stormwater management plan: Disconnect from the conventional pipe system and use all possible alternate areas for groundwater recharge.

In this model, storm water falls on roads, parking lots, open lands and rooftops, and not directly into the drains. Disconnected from the drain, the water enters the green area and only the groundwater recharge overflow goes into the drain and to the lakes.

The road median collects only the rainwater falling on its own surface area. Since the road camber is towards the green stretches on the side, the water run-off from the entire road flows there. The overflow then either goes into an open land or the existing pipe system.

Unlike the Smart City streets, most Bengaluru roads are designed in a way where rainwater directly enters the drainage pipe leading to the drain. The DDA model allows water to get into the adjacent green area through kerb cuts, where the green area is always lower than the road level.

The model also has a way to prevent silt accumulation: Filtration through rock and vegetated swales, filter strips or buffers, or sand filters. It reduces runoff volumes and the strain on the drains, stopping dirt, oil and grease from entering the drains.

The DDA model shows that impurities settle down on the gravel bed filter installed at regular intervals on the roadside. The impurities are removed periodically, and the filtered water gets into the swale for ground water recharge, while the excess water flows to the drain through an underlay pipe.

Aquifer recharge

Both the Sponge City and DDA designs, says Thippeswamy, emphasise on recharging aquifers so that rainwater does not flood the city.

“The objective of the Sponge City model is to recover storm water either through surface or groundwater, allowing it to seep down. Today, aquifers are drying up as we are only lifting the groundwater and not recharging,” he explains.

To address the flooding of low-lying areas, he suggests underground tanks as a solution. “Munich has 13 such tanks/reservoirs, some 250m x 50m. Water that floods into low-lying areas can be stored here for reuse later and there is no loss due to evaporation,” Thippeswamy adds.