There is a very easy and slightly unhelpful answer to the question of why Chennai flooded. Namely, the city is by definition built on a flood-plain, and is bound to be flooded in rain as high as we experienced in the recent North-East Monsoon. However, this is true of almost all big cities and urban areas - civilisation grew on the floodplains of rivers and lakes, and Chennai is no exception. This naive answer is unhelpful because it doesn’t answer the more critical question of how to keep the city and its inhabitants safe during heavy rain.
Old vs new: The key to answering this is to see which parts of the city did not flood. Chennai, like many other cities, is not a planned entity. It grew by swallowing pre-existing townships or villages and then filling the gaps. Each of these areas has its own story to tell. Consider areas such as Mylapore, Triplicane, Royapuram, Tiruvotriyur, Tiruvanmiyur and so on, where flooding was minimal. All of these are old settlements that existed, sometimes for hundreds or even thousands of years prior to their being absorbed into the city. All were built on higher ground, some of them have old temple tanks or other water-bodies that take the load, and others like Royapuram have drainage that was planned and built long in the past. All have been continuously inhabited for several centuries.
What about places that did experience major flooding? Take Velachery as an example - this too is an old settlement, finding mention in thousand year old records. However, the original settlement of Velachery was a small village, bearing no resemblance to the massive urban area of today. This new Velachery grew up on the bed of a drained semi-artificial waterbody known in Tamil as an “eri”. Eris (lakes) are created by taking the natural contour of the land and building a bund on the lower end, impounding water above them. In the wet season, this protects the area to the other side of the bund, and in the dry season, it provides a stored supply of water to the same places it protected. Velachery Eri still exists, but is severely reduced in size. Similarly, Kotturpuram of today was built on the bed of the Kottur Eri in the 1970s and 80s. In many parts of the city, we have “Lake-View Roads”, which recall these old water-bodies.
Other areas that were part of the agricultural or pastoral land associated with old settlements, such as Mandaveli and Anna Nagar lie between the old eris and settlement. Often, these would have been classified as “nanjai” or wet-agricultural land, suitable for irrigated cultivation of paddy or sugarcane. The only difference between these lands and the eris is that while the eri holds water throughout the year, the agricultural land becomes inundated by design during the monsoons. One could be converted into the other by simply building a bund at the lower end.
Hydrology of Chennai: Chennai has three major rivers - Adyar, Cooum and Kosasthalaiyar - running through it, along with several other smaller streams such as the Mambalam Drain, Samiyar Madam channel, Virugambakkam Channel, Otteri Nullah and Captain Cotton Canal. Cutting across all these and linking several old backwaters together, is the Buckingham Canal. In the lean season, these watercourses are often completely dry or have a very small base-flow. During an ordinary year, most of the rain during the North-East monsoon flows through them. However, during a surplus or extreme monsoon, these river-courses become something of a formality, and most of the water really courses over-land, from eri to eri, filling one and overflowing into the next. There are often defined channels from eri to eri, which direct most of the flow through a safe and useful path. Finally, here are the four reservoirs - Puzhal, Poondi, Sholavaram and Chembarambakkam. Poondi is a modern reservoir, while the other three are Eris that were enlarged considerably to provide for the city’s water needs.
Over the last 100 years, we have systematically lost several large lakes and the channels running between them to development. The first to go was the Long Tank of Mylapore as it was known, which today is the entirety of T. Nagar. The eastern bund of this lake is still visible in the alignment of Anna Salai and the different sections of Boag Road. In the west, it extended upto and in places beyond the railway track. Lake-View Road in West Mambalam commemorates this water body. Similarly, Nungambakkam Tank which was connected to the Long Tank, is now the Valluvar Kottam area, another locality very prone to flooding.
(to be continued)
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