Where roads are rivers: Decoding, fighting a man-made flood

ByVishwanath S
Sep 07, 2022 10:37 AM IST

It takes a special kind of skill to cause floods, or more appropriately waterlogging, in a city like Bengaluru.

It takes a special kind of skill to cause floods, or more appropriately waterlogging, in a city like Bengaluru.

People pull a car through a water-logged road following torrential rains in Bengaluru, India (REUTERS) PREMIUM
People pull a car through a water-logged road following torrential rains in Bengaluru, India (REUTERS)

A city that is on a ridge line between the Kaveri and Dakshina Pinakini basin.

A city whose landform slopes down its three valleys of the Vrishbhavathi, the Koramangala-Challaghatta and the Hebbal, allowing rainwater to rapidly cascade out.

A city where the people trying to make a living off the land 1,000 years ago actually had to throw earthen berms across valleys to hold on to water in human-made lakes called “Kere” in Kannada and “tanks” in English and who built a 1,000 of them, big and small.

A city where the rains are not unusually high and in the range of 700mm to 1,200mm annually and whose rainfall is spread over eight months from April to November simply has no reason to flood except for human interventions and therefore the floods of Bengaluru are entirely man-made.

What then has changed from a Bangalore of the 1960s and the 1970s to the Bengaluru of the 2020s where waters flow knee-deep on a major ring road and bring traffic to a crawl. Where local water accumulation results in people having to use boats to move out of their residences to the road?

First, there’s the city itself: A huge sprawl that has grown from about a 100 square kilometres to more than 800 square kilometres and more. This sprawl creates an environment of its own, and the resultant urban heat island effect seems to have increased the intensity of rainfall. Climate models as early as 2016 predicted an increase in rainfall by 6% and an increase in intensity of rainfall too.

The Karnataka State Natural Disaster Monitoring Centre (www.ksndmc.org) has set up 99 automatic weather stations across the city, making it one of the best granulated cities in India for detailed weather information gathering. The rainfall patterns measured with these indicate a city of varying rainfall and with a very high intensity of rainfall reaching up to 220mm per hour for a short duration. Clearly our rainwater infrastructure, from the rainwater pipes to the stormwater drains have to be designed for an intensity of at least 180mm per hour.

Then, there’s the hydrology.

A ridge line runs through the city. To the west is the steeply sloping north-to-south flowing Vrishbhavati valley. Here flooding occurs more near storm water drains. To the east of the ridge line is the east-to-west flowing Hebbal valley and Koramangala-Challaghatta valley. Here the flow of water is broadly west to east, the slopes are gentle, the terrain flat, the lakes shallow and wide and here flooding can occur in occupied lake beds, low-lying areas, places where roads and railway lines have obstructed flows and where the storm drains have been encroached. The ridge line bifurcates the floods into different types altogether and solutions will be slightly different and contextual.

From a terrain perspective, the old people who occupied Bengaluru had designed a system of “tanks”. These tanks had a command area usually cultivating paddy. These command areas were usually flat lands with a lot of clay puddling. The tanks were interconnected through the drainage channels running from the overflow weir called the Kodi Kaluve or popularly referred to as the Raja Kaluves. These drainage channels, crucial for stormwater drainage from the city, have no geo-tagged reference map available for citizens to consult. Only available as lines on old revenue maps, they have no legal identification and protection. Many have been encroached either deliberately or unknowingly by developments in the city. There are some drains that still exist and do a fairly decent job. These are prone to garbage dumping and require frequent maintenance. Access remains a challenge.

Compounding this is the wastewater flowing in the drains. This causes drains to be diverted away from lakes to avoid sewage filling the lakes up. However, this also means that much of the storm water too is diverted.

Lakes do not have a controlled outlet when needed. When heavy rains are expected, the water levels need to be lowered to retain and detain the water. Siphons or sluice gates are necessary if lakes have to act as flood-mitigating structures.

Finally, there’s the issue of transport.

The city’s roads and railway lines cut across hydrological drainage lines, have poor cross-drainage systems, are not designed for heavy storm events and end up as dams obstructing water flows, causing waterlogging. Flyovers and the metro, especially the large metro stations, simply dump rainwater onto the roads below. The roads themselves are badly designed with no camber to slope water away into appropriate roadside stormwater drains and if the drains exist, they are not interconnected to the larger drains with adequate slopes. An urgent redesign of our transport systems incorporating impacts on water flows is called for.

So, what next?

Clearly, a multi-pronged approach is needed to at least mitigate, if not eliminate, floods in Bengaluru. An immediate survey and audit of the storm drain network and its mapping and placing in public domain is called for. Similarly, an audit of roads, flyovers and the metro and identification of adjacent storm drain network should be done. Absent and incomplete drain work should be completed on a war footing. Sewage networks leaking into the storm drainage should be identified and completed so that no sewage flows into storm drains. Lakes should have sluice gates installed to lower water levels before anticipated heavy rains and moderate floods.

Individual plots and apartments should be encouraged to adhere to the rainwater by-laws and ensure that at least 60mm of rain is stored or recharged.

The Comptroller and Auditor General (CAG) had come out with an excellent performance audit of the storm water drain system in Bengaluru in 2021. The institutional and governance aspects, the data and mapping aspects, the implementation shortfalls all were pointed out in the report.

The state government must immediately constitute a special committee to implement the recommendations of the audit with a fixed deadline. The impact of non-implementation of a good storm water infrastructure will cost way more than the benefits it will bring.

Vishwanath S is the mind behind Bengaluru-based NGO Rainwater Club, and is a visiting faculty at the School of Development at Azim Premji University

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