Mumbai Rains: Nature Can’t Shoulder All The Blame For The City Going Down Under

Originally published at https://www.moneycontrol.com on September 13, 2017.

Mumbai rains are equally romanticised and dreaded by its citizens. The city witnesses waterlogging every year and Mumbai still lives with haunting memories of July 26, 2005 — the worst flood the city has seen.

On Tuesday, Mumbai saw at least 316mm of rainfall with heavily populated areas of Sion, Bandra, Kurla, Lower Parel, Worli, Ghatkopar submerged in knee-deep water, traffic clogging the streets and train services being disrupted in the city.

Tuesday’s rain was no stranger to citizens, who by now have come to terms with the consequent flooding and inconvenience. However, a crucial question that must be raised is why is Mumbai so vulnerable to flooding during monsoons and heavy rains?

The answers are manifold and plenty — most of which are human-made — from high tides, environmental exploitation, unabated constructions or the overbearing population. Thus, the floods which happen almost every year cannot completely be put on nature’s shoulder.

Mumbai lies in a low-lying saucer area which obstructs rainwater from flowing and stagnates the civic facilities. This accompanied with very heavy downpours of the South-west monsoon winds due to a low-pressure belt above the sea. Moreover, global warming has led to an unnatural rise in the sea level causing high tides to rise, adding to the flood situation in the city.

The city’s planning history also contributes to the flood-prone nature of the city. Mumbai has seen massive housing constructions and industries mounting on its landscape. The waterways that drained out the stagnant water have drastically reduced with encroachments of infrastructure and slums. The linking of seven islands to build the city during the British era furthered after independence to accommodate the growing population of the city.

The most populated city in India has seen the construction of large slum colonies encroaching on the storm water drains. Mumbai’s storm water drainage system is a complicated system of drains, rivers, creeks and ponds. A network of roadside surface drains in the suburbs, underground drains and rivers, canals and about 180 outfalls which drain surface water into the sea and rivers. Most of these outfalls drain water into the sea or the Mahul creek, Mahim creek or Thane creek. Some outfalls drain in the Mithi river which joins the Mahim creek.

The Mithi river is crucial to storm water drainage as it directly separates the main city from its outskirts. The flooding of the Mithi river has a direct implication on Mumbai railways, which is the main mode of transport. The Central Railways, Western Railways, Western Express Highway, Eastern Express Highway & the Harbour Railway Line are all affected. Land encroachment along the banks of Mithi river including hutments, storages, processing industries, workshops, and scrapyards have disrupted the course of the river. Untreated sewage, wastewater, and industrial effluents flow into the river, clogging it and raising the water level during monsoons.

The storm water drainage (SWD) system of the city was built during British Raj in the 1860s and despite the population booming 10 times since then, no government has built advanced drainage to tackle the current situation. The SWD of Mumbai is capable of handling an intensity of 25mm of rainfall per hour at low tide. If it exceeds the 25 mm mark, which often happens and there is a high tide, the city is inundated.

The drains in Mumbai have sloped in a way that it creates a self-cleaning velocity in the rainwater flow, making flood situations very rare. However, most drains are contaminated with garbage, plastic deposits and solid debris slowing down the velocity of rainwater.

The construction of the Bandra-Worli sea link has also contributed to floods as it has narrowed the mouth of Mithi River at Mahim Bay. In a high tide situation, when the water is discharged from the river into the bay, it overflows and surges back, adding to the floods.

The runway for Chhatrapati Shivaji International Airport has also been extended along the course of the Mithi River, forcing the river to change its path. When rainfall exceeds the normal, the water flowing at high pressure takes its normal path entering the city to flood it.