How & Why did Adayar wreck havoc in Chennai?
It is fascinating, to say the least, when one learns about how the riverbed of Adayar stream heads towards the sea. Google Earth gives a foot-by-foot image and description on Adayar’s journey from its place of origin to the ocean. If the movements of the rivers are studied based on the elevation of their riverbeds from the sea level, then, one can easily understand why Cauvery wouldn’t overflow into Pallipalayam or why River Bhavani wouldn’t drown the villages on its banks. Why then does Adayar alone behave this way?
The birthplace of Adayar is the Chembarambakkam Lake, located about 62 feet above sea level. Most parts of Chennai are located about 25 to 35 feet above sea level. Triplicane is about 30 feet above sea level. Madras Central is about 18 feet above sea level, and Purasawakkam, about 14 feet. The southern part of Velacherry is a mere 14 feet above sea level. Kannagi Nagar in Kottivakkam is at sea level (zero feet). A cursory study of elevation of ground level is more than enough to give a fair idea of places that are likely to flood and stagnate.
From an elevation of 62 feet at Chembarambakkam Lake, the water in Adayar should head to the ocean, at zero level, without any problem. Even if there isn’t sufficient width, basic physics says that water shouldn’t have a problem reaching the ocean without overflowing. But the slope has to be constantly maintained. The slope can be better understood if the riverbed is studied from the sea-level. When the water that flows from Chembarambakkam Lake reaches Chrompet’s West Chennai Outer Ring Road (Bypass), the river bed is 35 feet lower than the sea level.
When the river reaches Meenambakkam, its riverbed is 12 feet lower than the sea level. There is no further descent from this point onwards until it reaches the ocean. The rest of the journey is marked by an increase in the riverbed’s height by 10 to 20 feet and a decrease. This can be measured on Google Earth at each and every point. When excess water is opened into the river, instead of heading straight to the ocean, it stagnates and overflows wherever the riverbed dips.
The riverbed, which is 12 feet below sea-level near the airport, collects water until it is about 35 feet high before it turns north. When it encountered blocks that are more than 35 feet tall, it should have taken a diversion and flooded the airport. After leaving the airport, the riverbed descends to 15 feet near Nandambakkam. At Ekkattuthangal north, the riverbed rises to 30 feet above sea level. From Nandambakkam right up to the Ekkattuthangal bridge (NH45), about 10 feet of water has to accumulate before the river could move further.
After the bridge, when the river reaches Thiru Vi Ka industrial estate, its riverbed dips once again by 12 feet. When it reaches Annai Velankanni School and College, it rises to 28 feet. When water begins to accumulate to 28 feet, it should have been let out on the sides. From this point onwards, until it reaches Anna Salai Bridge, the riverbed once again descends to 12 feet. After Saidapet bridge when the river nears Turnbulls Road, the riverbed increases to 28 feet, and reaches Thideer Nagar. This is where it reaches zero feet above sea level.
From this point onwards, the river merges with the ocean by the stagnate-and-overflow process. Through its entire course, the Adayar River has to repeatedly stagnate and accumulate water of up to 10 to 20 feet and then overflow. This stagnated water easily breaks the banks and the weak protection walls and flows into the residential areas. When the riverbed itself is naturally full of such depressions and elevations, imagine how worse things could get if human beings begin encroaching on it with their cement-and-mortar structures? This is exactly what had happened with the Adayar River.
- Poet Magudeshwaran