On December 4 2023, Chennai plunged into chaos after Cyclone Michaung caused heavy rains and flooding in different parts of the city. The visuals of the Vadakkupattu Lake (Medavakkam Periya Eri) overflowing and breaking a compound wall, with cars in the nearby apartment blocks getting washed away because of heavy inundation, is a sight etched in the memories of residents.

A citizen audit done by a group of residents and experts to investigate the causes of the large-scale inundation has laid bare the negligence of the State Water Resources Department (WRD). According to the detailed audit, the lack of proper infrastructure in and around the lake — specifically the failure of the WRD to provide flood regulators and surplus channels that would have helped in mitigation — was the main reason for the devastation.

What happened on that day

As rains lashed the city, the Medavakkam Periya Eri reached full capacity. Because of incessant rains, the flood surplus breached the tank bund due to the absence of a surplus weir (a concrete structure constructed to dispose of excess water from a lake or a tank). Owing to the force of the water, there was also reverse flow and all this inundated the nearby houses and multi-storied apartments.

The subsequent sheet flooding had disastrous consequences, as the water levels in low-lying areas around the lake rose as high as five feet. The water pressure was so high that it broke the 5-foot compound wall adjoining the E-block of the Puravankara Windermere apartment complex and entered many of the multi-storied apartment buildings in the vicinity. The lake breach caused grave damage to property and cars in the surrounding areas.  

Read more: How Chitlapakkam remained flood-free during the recent Chennai rains 

Observations made during the citizen audit

Medavakkam Periya Eri (Vadakkupattu Lake) is one of the largest tanks in Medavakkam village and had an initial water-spread area spanning over 150 acres. Its size has now shrunk to cover about 80 acres. The Lower Palar division of WRD has jurisdiction over the management of the tank, but unfortunately, it has not been properly maintained.  

The localities impacted by the tank overflow include Vadakkupattu, Thiruvalluvar Street (Medavakkam) and Pallikaranai. The citizen audit done collaboratively by the Medavakkam Rising team and myself (in the capacity of GIS consultant), found that around 20,000 to 25,000 people were affected by the lake breach and the financial impact was approximately ₹50 crore.

We found that the lack of a proper surplus weir arrangement or flood regulator in the Medavakkam Periya Eri, and the surplus channels that were once present, but are now absent, were the major reasons for the uncontrolled discharge of flood surplus and sheet flooding.

Read more: All that’s wrong with stormwater drains in Chennai and how to fix them

The WRD had not been properly overseeing the surplus infrastructure in urban lakes; there was a lack of surplus channels and the authorities had failed to establish ‘macro cut and cover drains’ to correct the missing links of surplus channels, despite several recommendations. This is what led to the flash flooding in the areas inspected during the audit. 

Based on the findings of the citizen audit and GIS maps prepared, one of the key observations made by the audit members was:

Monitoring failure by the WRD — lack of proper engineering monitoring systems, maintenance, manpower and funding failed to maintain urban lakes mainly the Vadakkupattu Lake, which resulted in flooding of the downstream areas. Waterlogging damaged over 1,000 cars and property worth several crores of rupees in Medavakkam and nearby areas.

Urgent need for remedial measures

The citizen audit team also suggested several remedial measures for flood mitigation:

Also read:

• Explainer: Why we have urban floods and how to stop that
• Nature-based solutions: The answer to flooding in Chennai?