Viet Nam

Viet Nam: Report on River bank erosion of Thu Bon River in Quang Nam Province

Situation Report
Originally published
The People's Committee of Quang Nam
The Agriculture and Rural Development Service

Hanoi, 14 November 1999

In early November 1999, due to the effect of northwest monsoon in combination with activities of a tropical depression, there were heavy rains in many places in Quang Nam Province. Mean rainfall from 01 to 03 November 1999 was at 700 to 1000 mm (notably over 1000 mm in some places). The flood water levels on rivers rose rapidly. Upstream, the flood water levels rose at a rate of 1 m per hour. From 02 to 04 November and at early morning on 06 November 1999, there appeared four consecutive peaks of flooding. The highest peak was 0.8 to 1.5 m and surpassed the flood Alarm Level III (0.14 to 0.22 m above the flood water levels in 1998).

Torrential rains over a long period caused deep inundation in many districts such as Dai Loc, Duy Xuyen, Hoi An, Thang Binh, Tam Ky, Nui Thanh, and Que Son. The 1A national Highway was heavily eroded. Many of its portions were eroded for 2 m. Torrential rains caused fierce flash flooding in many mountainous areas of Dai Loc, Tien Phuoc, Nam Giang, Phuoc Son Districts, Many mountains were eroded. Many mountainous transport routes were separated. Notably, large flooding over a long period caused erosion on two sides of the Vu Gia and Thu Bon River banks. There are many places where the erosion penetrated the mainland for 30 to 50 m such as in Duy Hai Commune of Duy Xuyen District, Dai Quang Commune of Dai Loc District, Dien Quang, Dien Hong, Dien Phuong Communes of Dien Ban Districts.

To minimise future damage caused by riverbank erosion, it is necessary to have technical solutions such as to make bank protection, embankments, and unclog water flow at river's mouths. It is also of great importance to build work items at the following vulnerable areas on the Thu Bon and Vu Gia River System:

1. The Vu Gia River(in Dai Loc District):

On the left and right hand side of riverbanks in Dai Hong, Dai Lanh, Dai Dong, Dai Phong, Dai Nghia, and Dai Cuong Communes, the tributary on the Ai Nghia, Quang Hue Rivers, the left bank of the Hoa Dong bridge pier, the right and the left side of the Ai Nghia Bridge. The total length needed to be protected is 15.5 km. Estimated expenditure is VND 45 billion.

2. The Thu Bon River (Duy Xuyen, Dien Ban Districts, Hoi An Town)

Duy Xuyen District

The Cu Ban embankment is 2-km long, Duy Huoc embankment is 1.5-km long, and Duy Nghia-Duy Hai embankment is 2-km long. Estimated expenditure is VND 18.3 billion.

Dien Ban District:

Dien Phuoc, Dien Quang, Cam Dong, Dien Tho embankments are 6.5-km long. Estimated expenditure is VND 21.4 billion.

Hoi An Town

Ngoc Thanh embankment is 700-m long, the left embankment in Hoi An Town is 1.5-km long (the Chua Cau to Cam Nam portion). It is necessary to unclog 1-km of water flow on the Dai River mouth, upgrade the Cam Ha embankment for 1.2 km. Estimated expenditure is VND 1 billion.

The total length needed to be riverbank protected on the left and right hand side of the Thu Bon and Vu Gia Rivers is 29.9 km. Estimated expenditure is VND 119.7 billion.

(Details on locations, volume, and expenditure are summarised on the attached Table).

The embankment, after being constructed, will protect many households, schools, hospitals, and traffic roads; especially the ancient streets in Hoi An Town.

Protecting riverbanks by embankments has proved its effectiveness. Land will not be eroded. Residence and infrastructures will be secured. Over the last few years, the Central Government and the province itself have invested to build a 6-km embankment to protect the Thu Bon River at portions on the Cam Nam, Cam Thanh embankments. The Cam Ha embankment protects the Hoi An ancient town; the Dien Phuong embankment protects the Trans-district transport lines and residences of the Dien Phuong Commune, Dien Ban District. The Tra My, Van Ly, Van La embankments protect infrastructures and residences in Tra My Town, Dien Quang, Dien Dong Communes.

English Translation prepared by the UNDP Project VIE/97/002 - Disaster Management Unit on 22 November 1999.