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Rapid urbanization delivers Da Nang's worst flooding ever

By Nguyen Dong, Gia Chinh   October 16, 2022 | 12:22 am PT
Excessive runoff and a drainage system designed for lower rain volume are blamed for the "unprecedented" flooding of Vietnam's central hub on Friday night.

Under the impacts of Storm Son Ca, which devolved into a tropical depression offshore Friday night, Da Nang received rainfall of up to 795 mm from 7 p.m. Friday to 6 a.m. Saturday. The downpour was particularly heavy for the first six hours with the rainfall gauged at 500 mm.

The downpour, described by Da Nang authorities as "historic" and "unprecedented," inundated the entire city by up to 1.5 m on Friday night. Some parts of the coastal Lien Chieu District recorded flood water at almost 2 m.

Many residents who have lived in Da Nang for 30 years or so said it was the worst flooding that they had ever seen, with floodwaters rising very fast and widespread.

Volunteers help a man wade through waters in Cam Le District of Da Nang, October 14, 2022. Photo by VnExpress/Nguyen Dong

Volunteers help a man wade through waters in Cam Le District of Da Nang, October 14, 2022. Photo by VnExpress/Nguyen Dong

To Van Hung, director of the municipal Department of Natural Resources and Environment, said the heavy rains exceeded the capacity of the city's drainage system, which is built for Da Nang to receive 100-200mm per 24 hours.

The designed capacity is the standard applied by many cities around the world, said Phung Phu Phong, director of the Construction Department.

He argued that though the system was not capable of draining such a high amount of water at once on Friday, it proved effective because the floodwaters had receded almost completely by Saturday morning.

Explaining the heavy flooding in two residential areas in Hai Chau and Thanh Khe districts, Phong said those areas were developed on old facilities and the drainage system was yet to be upgraded.

Pham Thanh Hung, lecturer on hydropower and irrigation at the Da Nang University of Science and Technology, said that apart from the extremely heavy rainfall, an important reason for the severe flooding was the rapid increase in urbanization.

Cars submerge in flood waters on Nguyen Van Linh Street of Thanh Khe District in Da Nang, October 14, 2022. Photo by VnExpress/Nguyen Dong

Cars left in floodwaters on Nguyen Van Linh Street of Thanh Khe District in Da Nang, October 14, 2022. Photo by VnExpress/Nguyen Dong

More concrete buildings going up means more impervious surface area, resulting in excessive runoffs, as there is not enough soil left to absorb the water.

He said urbanization was usually accompanied by an increase in urban population and an increase in the amount of urban waste, increasing the potential risk of garbage clogging the drainage system.

Official data shows the urban land area of Da Nang, considered the third most important in Vietnam after Hanoi and HCMC, jumped from about 7,000 ha in 2005 to 12,502 ha in 2010 and 18,396 ha in 2019, which is nearly four times the 1997 figure.

According to a plan approved last March, Da Nang's urban area will increase to 31,836 ha in 2030 and more than 35,000 ha in 2045, housing a population of 1.79 million and 2.56 million, respectively.

Phong of the Construction Department said it was not possible to reserve more area for ditches and reservoirs or to expand culverts to meet rainfall of 700-800 mm a day, because that would mean the city having to spend a "huge" amount of money.

"If the Hanoi or HCMC receive such a large volume of rain in the same period, it would take a few days for flood waters to recede," he said.

Flooding at Dien Bien Phu and Ha Huy Tap intersection in Da Nang, October 14, 2022. Video by Bao Loc

Nguyen Van Huong, head of the weather forecast department under the National Centre for Hydro-Meteorological Forecasting, said rainfall of 500 mm within six hours was "substantial."

A tropical depression devolved from Son Ca storm combined with a cold spell from the north, an intertropical convergence zone and the northeast monsoon winds to cause such abnormally high rainfall, he said.

The flooding was also worsened Friday night by high tides in the central stretch from Thua Thien Hue to Phu Yen provinces, including Da Nang.

The flooding killed four people and caused blackouts for over 200,000 families.

 
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