Monsoon flood fury: Not yet battle-ready

Rain water flooded on Oakalipura Underpass in Bengaluru on Friday, 17 May, 2019. Photo by Janardhan B K

Disaster by deluge… Is that what awaits Bengalureans this June? As pre-monsoon showers thoroughly exposed big chinks in the city’s rain preparedness, frantic questions are being raised all across town: Will the monstrous floods return with more force? Will preparedness be any better than the years before?

Encroached, silted and poorly designed, the city’s Storm Water Drains (SWDs) have always offered the civic agencies a readymade alibi: The drains are being redesigned, de-silting will make them work. But this excuse has long overshot its expiry date.

Here’s why: Design flaws need correction on a war-footing. Lazy-paced interventions will just not work when seasonal rains and pre-monsoon showers arrive without notice. Seen across the city, silt taken out from drains is deposited right there for the flood waters to push them back.

So, there were no surprises as the Monday showers had low-lying areas in a predictable mess. BTM Layout residents, for instance, had access to their houses virtually cut-off as all roads lay waterlogged. Following the natural gradient and nowhere else to go, flood waters rushed into apartment basements. Senior citizens, children and pets were trapped.

Caught between pot-holed roads and white-topped, half-finished streets with irregular stretches, motorists turned edgy. As sheets of water enveloped pot holes and man holes alike, they could trust nothing. And, when many roadside trees came crashing down, their nightmare was complete.

Need for speed

The need for speed, then, is a no-brainer. But consider this: To prepare in advance for the monsoons, the Karnataka State Natural Disaster Monitoring Centre (KSNDMC) had identified 201 flood-prone areas in the city. The Bruhat Bengaluru Mahanagara Palike (BBMP) had to act fast to fix the issues.

All locations had to be surveyed to install water sensors in 25 spots. But so far, only three tentative choices have been made. Can the May-end deadline set by Deputy Chief Minister G Parameshwara to complete all preparatory work be met? Palike insiders say this could prove to be a tough task.

Sanctioned Rs 42 crore by the government to desilt 842 kms of SWDs, the Palike claims work on 440 kms have been completed so far. But unless the silt is shifted out from the SWD periphery, the entire exercise could prove futile.

Holistic approach

Desilting without a holistic, city-wide approach centred on the network of drains leading to catchment areas will be largely ineffective, warns urbanist V Ravichander. “Adhoc desilting will not work. You need to design the drains for peak rainfall, dig recharge pits every 30 metres, understanding the road geometry. All this should be done with a catchment view,” he explains.

Over the last decade, the peak rainfall has hovered around 125 mm per hour. “But the old, historical design of our drains were meant for peak rainfall of 75 mm per hour, which has been overshot several times.”

The trigger for urban floods has been obvious for years. The capacity of the city’s primary drains has been greatly reduced due to two main factors: Rampant inflow of domestic and industrial sewage and dumping of construction and demolition debris.

Recharge pits

But desilting alone will not help, reasons Ravichander. When it rains, the drains will have to reduce the speed of water rushing to the low-lying areas. This does not happen now because the base of the drains are concretised. “This is part of the problem. Recharge pits will reduce the flow.”

This is the issue with most primary drains, the rajakaluves, that are naturally aligned with the largest gradient along the city’s major valleys. But, as environmentalist Nagesh Aras points out, the secondary and tertiary drains that feeds these rajakaluves are not aligned that way.

The secondary drains, he says, have very little gradient by design. “Here, the water cannot push the sediments beyond a particular speed. More sediments mean slower speeds. This is why the BBMP has to open the drains every year. The key is to prevent sediments from getting in by installing sediment traps,” Aras explains.

Gradient-based strategy

The lack of a gradient-based strategy in laying secondary and tertiary drains is an invitation for flooding. This is clearly evident along the Outer Ring Road which follows a contour. “The ORR goes up and down due to the undulations in the terrain. But when you align the drains along the same path, water does not go up and down.”

This is precisely why even a light shower floods the ORR along various stretches. Since only the two inner lanes have camber (road sloping on either sides), they remain dry. Motorists tend to get onto these central lanes, triggering heavy congestion.

Topography mapping

On the BBMP’s agenda is a massive redesign of the city’s drain network to correct the flaws. But can this costly exercise be efficient without fully understanding the topography of the city? GIS mapping of the city and sharing this spatial data with academic and research institutions for detailed analysis might throw up lasting solutions to address the recurring floods.

This will not happen in a hurry. For BBMP, the priority at this advanced stage would be to minimise damages. Although desilting work remains largely unfinished, the Palike has set up several smaller control rooms at the subdivision level, each staffed with 10 personnel, a vehicle and equipment.

To escape the monsoon fury, Bengalureans will have to depend a lot on the speed and efficiency of these men and machines, as they await a flood of complaints.

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