VietNamNet Bridge - HCM City has spent tens of trillions of dong to ease flooding, but the historic rainfall on September 26-27 seriously flooded streets as the drainage system could not handle the deluge. 

The Thu Thiem Tunnel across the Sai Gon River, considered the most modern in South East Asia with newly upgraded water drainage systems, also suffered unexpectedly.

HCM City has been warned that they will still see more such floods in the future. 

HCM City has spent tens of trillions of dong to ease flooding, but the historic rainfall on September 26-27 seriously flooded streets as the drainage system could not handle the deluge.

According to the HCM City Anti-flooding Center, the rainfall on September 26 afternoon reached 204.3 mm, far beyond the designed capacity of the water drainage system. 

Lao Dong reporters found that the water drainage systems in HCM City were designed based on the Prime Minister’s Decision No 752 dated in 2001 about the master plan for the sewer system by 2020.

Under the planning, 3rd-class sewers were designed based on the rain frequency of 75.88 mm in three hours;  2nd-class sewers 85.36mm and 1st-class canals 95.91 mm for three hours. The tide peak was designed for +1.32 meters.

As for 2nd-class sewers, they were designed to bear rainfall of up to 137.70mm within 1.30 hours, expected to come once every 100 years. 

Meanwhile, the rain on September 26 had rainfall of 200mm within 1.30 hours. 

The overloading of the water drainage systems and the Thu Thiem Tunnel, as analysts commented, partially show problems in design and calculation.

The Thu Thiem Tunnel, 33 meters in width, 9 meters in height and 1.4 kilometers in length, including the 390 meter section put under the river, at 25 meters under the water level, was designed and implemented by Japanese companies and put into operation in November 2011.

According to Tran Chi Trung, director of the Saigon River Tunnel Management Center, though there were five rainwater storage tanks (500 cubic meters) and 13 pumps (9,000 liters per minute) runnng at full capacity, the tunnel was inundated for one hour.

Five years ago, when the tunnel was put into operation, reporters raised a question if the tunnel would be inundated one day because of heavy rains and high tide. 

The representatives from the Transport Departments and relevant agencies all affirmed that the flooding would not occur because of the water storage tanks and high-capacity pumps.

Chi Nam