Data | Why does Chennai get flooded even after a single spell?

Every rainy season, Chennai is flooded. Some of its streets get inundated even after a single spell. The maps show that as the need for residential units grew, houses were built in low-lying areas and floodplains, leading to stagnation. The loss of a portion of the Pallikaranai marshland added to the crisis. Moreover, many missing links of stormwater drains, even along major roads, led to frequent stagnation.

Flat terrain

Chennai lies in a relatively flatter terrain and is close to sea level in most places. Some localities are in relatively low-lying areas, so the water doesn't get drained quickly, leading to flooded streets

Land of lakes

Before the rampant increase in residential units began, bunds were constructed to store water and irrigate farms, forming thousands of lakes in and around the city, as shown on the map. However, once the farms were sold, the lakes lost their original purpose. The smaller drains that fed these lakes got closed.

Such drains are crucial during floods as they aid the excess flow of water. Also, these lakes can be used as flood buffer and to temporarily store and clean sewage. However, they are not being repurposed for such uses

Rampant urbanisation

Not able to view clearly? Click here to download the map in PDF.

The map shows the built-up area of Chennai and its immediate surroundings by 1990 , by 2000 , by 2010 , and by 2015. It can be observed that until 1990, the built-up area mostly stayed clear of water bodies and rivers. However, the later constructions came up increasingly closer to waterbodies and in low-lying areas, leading to stagnation after a rainy spell.

Near the river

One of the major issues plaguing the city is the failure to protect the floodplains of major rivers from urbanisation. The maps below show the built-up area around a portion of the Adyar in 2007 and 2020. Many residential, commercial and industrial places have mushroomed in the period, which is a cause for concern. Due to the rampant urbanisation, the river's flood-carrying capacity has also been reduced in recent times

Repeated flooding in Adyar

The places marked blue in the map recorded inundation during the 2020 rainfall. The sites near the river get flooded repeatedly every year. This year too, houses located close to the Adyar were among the worst affected

Building on marshland

The Pallikaranai marshland has shrunk in size in recent years due to urbanisation. The satellite photo on the left is from 1990, and the one on the right is from 2018. Many of the major constructions in the marshland are government buildings, apart from residential and commercial sites. Wetlands act as a sponge during a flood. They will trap the excess water and will slowly release it back. Especially in an urban area, such regions are valuable as their massive holding capacity will naturally provide flood control

MISSING LINKS

Lack of stormwater drains

In a study published by the Greater Chennai Corporation in January 2021, covering select zones, it was estimated that about 45 km of stormwater drains missing from the GCC area. Of the 45 km, some links were missing and had to be constructed anew, and the rest had to be demolished and reconstructed . In some parts a mix of both was required. The first graph shows the missing links (in metres) in the zones surveyed

Notable roads and missing links

The second graph shows the missing links in individual roads. For instance, South Boag Road in Teynampet was missing about 1,162 metres of stormwater drain network, and the Cathedral Road was missing 900 metres. Each circle corresponds to a road in a particular zone Missing maps

According to data compiled by ChennaiCares, a civic action group, the Chennai Corporation has not published SWD maps for about 86 of the 200 wards. In Manali, Tiruvottiyur, Sholinganallur and Perungudi, none of the wards had maps.