Rapid urbanisation poses flood risk in Thane, finds study

Thane, India is facing increased coastal flooding due to rapid urbanization and extreme weather events, according to a study by Veermata Jijabai Technological Institute. Flood-prone areas include Krantinagar, Rabodi-Koliwada, Vrindavan Society, CIDCO Bridge, Majiwada village, and Chendni Koliwada in Thane East. By 2050, 9.6 sq km of the city could be inundated during annual floods, up from 3.14 sq km in 1995. The study recommends creating holding ponds and pumping stations, flood protection walls, and early flood warning systems, as well as preparing evacuation strategies. Future developments should be situated at least 4.5 metres from the highest tide level.

Mumbai: Rapid urbanisation along with increased frequency of extreme weather events raises serious concerns about coastal flooding in several localities in Thane, a study by the civil and environmental engineering department of Veermata Jijabai Technological Institute (VJTI) suggested.

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The flood-prone areas include CIDCO Bridge, Vrindavan Society, Rabodi-Koliwada, Krantinagar, Majiwada village and Chendni Koliwada in Thane East.

The study, published in the latest iteration of the Journal of Integrated Disaster Risk Management, found that Thane’s built-up area increased by 27.5% between 1995 and 2020. During the same period, open spaces, forests, wetlands and mangroves—which are natural buffers against waterlogging—reduced by 29.5%, 8%, 18.9% and 36.3%. Respectively.

Urbanisation increases imperviousness of the surface and by 2050, as much as 9.6 square km of the city could be inundated during the annual flood, the study suggests. This is a rise from just 3.14 square km in 1995.

Compared to the base year of 1995, the city’s built-up area is projected to increase by 56% by 2050, while open spaces, forests, wetlands and mangrove cover is expected to decline by about 17.0%, 55.6%, 87.4% and 72.1%, respectively.

Built-up area here refers to commercial and residential zones, defence lands, recreational areas, educational institutes, government buildings and lands, and other public and semi-public utilities like town centres and areas of transportation.

Due to the rapid mushrooming of these land-use categories, the flow of water in and around the city’s 17 stormwater drains will continue to rise. The study considered an area of 81.97 square km, of which major drains cover 68.46 square km of catchment area. These drains are designed based on an analysis of rainfall data using the ‘Gumbel distribution’ method, which does not consider the effects of climate change.

Based on India Meteorological Department (IMD) data, the study demonstrates how “increasing urbanisation and the impact of climate change have resulted in increased peak runoff by 31.8%” since 1995. Increased peak runoff at the outlets of these drains is heavily influenced by tides. Of the 17 major drains in the city, eight are below sea level, six are above mean sea level but below the high tide level and only three drains are above high tide level.

“Hence, whenever heavy rainfall coincides with high tide levels, water accumulates in the area surrounding the creek in the city,” the study noted, emphasising on the need to delineate future flood-prone areas along Thane’s coastline based on observed changes in land use and land cover (LULC).” “The city faces repetitive water-logging every year in the creek’s vicinity. Even though future flood disasters cannot be avoided, the extent of their damage can be minimised by creating appropriate countermeasures,” the study said.

Accordingly, the researchers have prepared a ‘flood hazard map’ for the city and suggested beefing up other interventions, including creating holding ponds and pumping stations, flood protection walls, early flood warning systems and preparing evacuation strategies for the future. It also suggests that future developments be situated at a minimum of 4.5 metre from the highest high tide level for the city.