The story so far
The Road Infrastructure Department of the Bruhat Bengaluru Mahanagara Palike (BBMP) has readied an audit report of 41 underpasses after a 23-year-old woman died in a flooded underpass at K.R. Circle in the heart of Bengaluru on May 22.
The cab in which she was travelling, along with her family, drowned as the driver misjudged the depth of water and entered the flooded underpass.
The civic body has proposed a slew of measures to avoid such incidents. They include covering roofs using galvanised coloured sheets at the three U-shaped underpasses at K.R. Circle, Sankey Road and near Cauvery Theatre in Bengaluru.
BBMP has started desilting drains, laying humps, and removing mud at vehicle and railway underpasses. The work is monitored by B.S. Prahlad, Chief Engineer (Road Infrastructure), BBMP.
Flooding at Kadubeesanahalli underpass in Bengaluru due to the faulty drainage system following heavy rains in October 2022. | Photo Credit: special arrangement
What are the different types of underpasses and who maintains them
Of the 53 underpasses in Bengaluru, 35 are vehicle underpasses, and 18 are railway underbridges (RUB). There are two types of designs, which are L and U shaped.
While RUBs have been there for many decades in Bengaluru, according to Mr. Prahlad, the first vehicle underpass was constructed in the city in 2001-02 at Mehkri Circle. While most of them are designed in L shape, a few, including the one at K.R. Circle, have a U-shaped design, which is prone to flooding as water accumulates at the lowest point rapidly, with gratings being the only way out.
While the BBMP is using its resources to maintain underpasses, it has also signed annual maintenance contracts to manage underpasses at Nayandahalli, Anandnagar (Hebbal), Cantonment Railway Station, RMV Layout, and Yelahanka railway underpass.
Also, private players, who were given rights to display advertisements, are maintaining underpasses at C.N.R. Road, Kuvempu Circle (towards New BEL Road) and Mehkri Circle.
Why underpasses get frequently flooded
According to experts, design flaws, lack of maintenance, and poor monsoon preparedness are the key reasons for the flooding of underpasses in the fast-growing tech city. Most of the U-shaped structures have flaws in the design, and the reason is BBMP’s below-par engineering skill pool. The civic body also tries to save on construction cost and settles for less-than-perfect design.
Professor M.N. Sreehari, a traffic expert and consultant who inspected K.R. Circle along with a team of officials sent by the High Court of Karnataka after the fatal incident, said the laying of steep approach ramps for vehicles to pass through is the first reason for flooding. The deep slopes on both sides push the water fast to the lowest point, increasing the water level. Water simply accumulates with nowhere to go in the absence of open drains.
Citing the K.R. Circle underpass as an example, he said the rainwater falling on four different roads that meet at the junction, is gravitationally pulled towards the middle point of the underpass. The steep approach on the one end and steep exit on the other increases the gravitational pull, resulting in speedy accumulation of water. This is a design flaw. No amount of short-term measures taken up by the BBMP will help alleviate the problem.
The gushing water carries fallen branches, leaves, mud, and other waste, worsening the situation. The only way out for the water in U-shaped underpasses is gratings as per the existing design. The floating materials block the granting vents, making it impossible for the water to reach the drains.
Then comes the apathy of the BBMP. Before every monsoon, the BBMP must clean the vents and drains once a fortnight to ensure smooth flow of rainwater. In the absence of periodic maintenance, instance of flooding have gone up.
Rainwater that flowed into the underpass near Badrappa Layout in Bengaluru in November 2021. | Photo Credit: SUDHAKARA JAIN
What can be done to avoid this
BBMP engineers are aware of vulnerable underpasses, and must ensure that at least two pumps are placed on standby to pump out water. However, this exercise is usually done only after the water level rises to a dangerous level.
The alignment of the up ramp and the down ramp of most of the U-shaped underpasses is curvy. Mr. Sreehari said the alignment is curved because the ramps are steep. The ramps should be appropriately aligned with the roads. For this to happen, ramps should be flat in design. When the ramps are flat, the speed of water flow reduces drastically, facilitating the drain to receive flood water moderately. This needs more work, and construction costs shoot up, but that’s for the better.
The BBMP has to build open drains, apart from gratings. The capacity of the grating to steer water into the drains is limited. The open drains of reasonable size spread the water immediately, considerably minimising the accumulation of water. Unless these measures are taken, flooding will continue. The BBMP’s plan to cover roofs will only prevent rainwater from falling on the ramps, but the water flowing from the roads will continue to end up in underpasses.
