Madiwala tank cannot take over two hours of 72.3mm rain

Two hours after downpour, water will flow back, inundating valley and roads

Its capacity severely dented by incessant inflow of untreated sewage, the city’s Madiwala tank will start inundating after two hours for a rainfall of 72.3mm/hour.

Established through an independent study, this means the next downpour will again trigger havoc for everyone in the lake’s catchment and beyond.

The total run-off volume of stormwater at 72mm/hr rainfall is 4,149.89 million litres (ML). Madiwala tank’s capacity to carry stormwater now stands at 3,542 ML with 4.5-metres depth. The implication is clear: Beyond two hours, the water will flow back, inundating the valley, roads and beyond.

If the rainfall intensity is 161mm/hour, the tank will be filled up within one-and-a-half hours before inundation sets in. As environmental architect Mohan Rao explains, with an effective capacity of nearly zero, “the inundation is almost immediate from the direct incidence of rainfall on tank followed by run-off from catchment.”

Ridge formations

The tendency of water to flow back is accelerated by another key factor: Formation of ridges, where land on either side is at a lower level. The Outer Ring Road stretch near the lake is built on the valley enclosed by these ridges. This, says Rao, leads to a greater tendency for water to flow back from the Madiwala tank and spread along the Hosur Road and other key streets.

At the Silk Board Junction, the valley further narrows both from the Adugodi and Madiwala-BTM Layout sides. The flooding often seen at this hyper-congested junction is attributed to this narrowing.

The ridges also come in the way of the natural flow of stormwater from Madiwala to Agara to Bellandur lake and to Varthur lake. The heavy encroachment of the stormwater drain linking Bellandur to Varthur lake severely hampers the flow. Worsening it is the ridge formed by the dumping of debris on the Eastern edges of Bellandur lake.

Seasonal waterbodies

Originally, the Madiwala tank and other lakes were designed as seasonal waterbodies. They would often dry up during summer when stormwater flow stops.

“Today, however, thanks to incessant flow of sewage, they don’t. Hence, their capacity is also compromised,” Rao explains.

Blatant encroachment of wetlands, that would function as emergency sponges sucking in flood water, have only added to the chaos.