A recent research paper has identified single-day extreme rainfall events as the primary cause of recurring flash floods and waterlogging in the Sylhet region. The study found that climate change has significantly increased the frequency of these heavy monsoon rains in Bangladesh and northeast India.

Officials from the Bangladesh Meteorological Department and Bangladesh Water Development Board have observed this pattern in recent years. For example, on June 17, 2022, a flash flood submerged almost all of Sylhet and Sunamganj districts, triggered by a record rainfall in Cherrapunji, India, which is located just uphill from Sylhet.

On May 29, 634mm of rain in Cherrapunji caused another flash flood, submerging five upazilas in Sylhet the next day. More recently, a flash flood was triggered on Friday after 349mm of rain fell in Cherrapunji over 24 hours.

In Sylhet city, two recent waterlogging incidents were caused by extreme rainfall over a short period. On June 9, 220mm of rain fell within three hours, causing severe waterlogging. On June 13, 186mm of rainfall in six hours submerged many parts of the city.

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The research, conducted by scientists from NASA Goddard Space Flight Centre, North Carolina State University, the Indian Institute of Tropical Meteorology, and Jahangirnagar University, was published in the Journal of the Royal Meteorological Society in February.

The study analysed extreme rainfall events, defined as 150mm or more in a day, from 1950 to 2021.

It found that while overall monsoon rainfall is decreasing, single-day extreme rainfall is increasing. These extreme rain events have become four times more common due to climate change.

Dipak Ranjan Das, executive engineer of BWDB in Sylhet, said, "Single-day extreme rainfall in Meghalaya, located right uphill from the region, is responsible for the recurring flash flood situation. The river system here cannot handle that much water in a short period."

A flood hazard mapping of the Surma River basin in Sylhet city by researchers from Shahjalal University of Science and Technology calculated the maximum peak discharge through the Surma river at Sylhet city as 321.35 cubic metres per second, indicating the river's limited capacity to manage excessive rainwater.

Prof AKM Saiful Islam of the Institute of Water and Flood Management at Bangladesh University of Engineering Technology said, "Haors (wetlands) and rivers are crucial for holding and discharging rainwater from upstream. However, ongoing development projects like road construction are disrupting these haors, prolonging the flood situation."

"Waterbodies and canals in city areas are being filled up, causing waterlogging due to excessive rainfall. Hillock cutting and tree felling are major reasons behind the filled-up waterbodies and canals," he added.

"As single-day rainfall events are increasing, we must act now to protect waterbodies. Otherwise, we will suffer more intensely in the future," he warned.