Why Hanoi flooded severely and water took long to recede

From Monday night Typhoon Kajiki brought heavy and non-stop rain to the capital, and at 7 a.m. on Tuesday the Hanoi Sewerage and Drainage Company reported flooding in more than 80 locations, mostly in the inner city, with some spots such as Duong Dinh Nghe Street being under one meter of water.

People row an inflatable boat on Duong Dinh Nghe Street, which is submerged by over a meter of floodwaters on Aug. 26, 2025. Photo by VnExpress/Duc Dong

By Tuesday evening there were still 26 flooded spots on roads like Vo Chi Cong (UDIC building), Lac Long Quan, Le Duc Tho (My Dinh Square), Duong Dinh Nghe, Phan Van Truong, Tran Binh, Hoa Bang, Phuc Xa, Pham Hung, Thien Hien, Nguyen Chinh, Road 2.5 Den Lu (near Den Lu Lake), Huynh Thuc Khang, Thai Ha, Nguyen Van Troi, Ngoc Lam, Co Linh, Dam Quang Trung, Cau Buou, Yen Xa, Trieu Khuc, Yen Nghia, and Quyet Thang and the Thang Long Boulevard underpass.

The company blamed the widespread and persistent flooding on the huge volume of rain that fell in a short period.

In the 24 hours until noon Tuesday 30 wards and communes recorded 200–300 mm of rain.

Five locations received even more rain: It was 460 mm in Hai Ba Trung, 368 mm in Yen So, 352 mm in Tay Mo, 334 mm in Yen Nghia, and 323 mm in O Cho Dua.

Nguyen Van Huong, head of weather forecasting at the National Center for Hydro-Meteorological Forecasting, said the downpour was caused by multiple weather systems at once.

In addition to Typhoon Kajiki, Hanoi was also in the northern part of an intertropical convergence zone stretching across north-central Vietnam, while a subtropical high-pressure ridge pushed moist winds strongly northward, he said.

"Under the combined effect of the tropical convergence zone and the subtropical ridge's easterly winds blowing strongly at 1,500–5,000 m, the northern delta and midlands experienced heavy rainfall."

The inadequate drainage system was another major reason for the prolonged flooding.

According to the company, the only area with a fairly complete system is the four core former districts of Hoan Kiem, Ba Dinh, Dong Da, and Hai Ba Trung (now all wards of the same name) covering the To Lich River basin.

But even it can only cope with up to 310 mm of rainfall over 48 hours.

With nearly double the rain falling, the drainage system was overwhelmed.

A deliveryman rides his motorbike through a flooded area in Hanoi, Aug. 26, 2025. Photo by VnExpress/Duc Dong

In the Old Quarter, situated in these core districts, the drainage system built in 1954 is outdated.

Rapid urbanization has altered the ground level here, causing localized depressions. Some drainage pipes have deteriorated, leading to flooding on Tong Dan, Dinh Tien Hoang, Phung Hung, Hang Bai–Hai Ba Trung, Lien Tri–Nguyen Gia Thieu, and Phan Boi Chau–Ly Thuong Kiet.

The Old Quarter is filled with restaurants and shops that regularly discharge grease into the drainage system, clogging the pipes.

The city has yet to mandate the use of grease-trap devices at these establishments.

Other drainage basins also have low water discharge capacity due to poor infrastructure.

For example, the Nhue River basin, which comprises the erstwhile districts of Cau Giay, Thanh Xuan, Hoang Mai, Nam Tu Liem, Bac Tu Liem, Ha Dong, and Thanh Tri and a part of Tay Ho, has urbanized rapidly but lacks a proper drainage system, retention lakes and adequately powered pumping stations.

Rainwater mainly drains by gravity, depending on the Nhue River’s water level.

The Long Bien basin (62 sq.km) has also urbanized quickly. Though its infrastructure was planned for pumping water into the Red and Duong Rivers, the main pumping stations have not yet been built, and so water still drains naturally, depending on the Cau Bay–Bac Hung Hai river system's levels.

During the recent rains, water levels in the To Lich, Nhue and Cau Bay–Bac Hung Hai rivers all rose, slowing drainage.

The western and southwestern areas of Hanoi also urbanized rapidly, but infrastructure has not kept pace.

New urban areas such as My Dinh, Nhan Chinh and Cau Giay were built on former farmlands with low elevation.

Heavy concreting has reduced percolation into the ground while drainage systems remain incomplete, leading to flooding.

By 9 p.m. Tuesday streets around the Cau Giay Ward headquarters were still under 30–40 cm of water.

The Hanoi Drainage Company said there has not been adequate investment in pumping stations and retention lakes to meet design capacity in the western and southwestern parts of the city.

Some canals meant for draining urban water have not been handed over for management and remain gravity-based and dependent on the Nhue River's level.

The entire Vo Chi Cong Street, which connects Hanoi downtown with Noi Bai Airport, is flooded throughout Aug. 26, 2025. Photo by VnExpress/Van Ngoc

Waterlogging mainly occurs in areas with rapid urbanization but poor infrastructure, such as along the Thang Long Boulevard frontage road, National Highway 32 and Road 2B.

Other reasons for flooding include shrinking water surfaces due to concreting and less green spaces compared to 10–20 years ago, the company said without citing specific data.

It also pointed to poor public awareness with people still throwing trash into drains and covering sewer mouths to block odors, obstructing drainage.