VU GIA THU BON INFORMATION CENTRE (VIETNAM)
Hydrology
The VGTB river basin is one of the nine largest basins in Vietnam (MARD, 2009). With a drainage area of 10,350 km2, it accounts two entire provinces (Quang Nang and Da Nang) and part of Kon Tum province of Central Vietnam. The VGTB system originates in the Truong Son Mountain Range near the border to Lao P.D.R. Below the mountain range the river and its tributary streams flow through the medium flat terrain of the coastal plain and are cross linked about 36 km upstream from the coast (To, 2006; ICEM, 2008). Their major outflow is at the Thu Bon estuary at the ancient port city of Hoi An. Nevertheless, a complex of channels links the lower reaches of the Vu Gia and Thu Bon to the Han estuary in the city of Da Nang. The low gradients of the coastal plain slow the river as it continues to flow southeasterly towards Hoi An, where it changes direction. The freshwater flowing downstream becomes brackish as it merges with the tidally influenced saltwater of the estuary and flows into Vinh Dien close to Hoi An.
During the flood season the two rivers interact through the Quang Hue and Vinh Dien rivers forming a braided river delta system. This connection is often broken in the dry season (Mai, 2009). The basin is some 100 km wide and 120 km long, and has a wide, flood-prone coastal plain backed by the steep Truong Son Mountain Range that rises to 2600 m (Sheaves et.al, 2008). The amount of surface water flowing in the basin is directly related to precipitation, which varies geographically and seasonally. The majority of rainfall occurs in the wet season (September to December), with 70% of runoff generated during these 3 months. In contrast, during the height of the dry season (February to March) only 12% of runoff is generated (ICEM, 2008; To, 2006). The basin has one of the longest dry seasons in Vietnam with 9 months of the year receiving only 30% of the rainfall, resulting in severe water shortages and problems with saline intrusion at the coast (Kellogg Brown, Root Pty Ltd, 2008).
The Vu Gia Thu Bon Basin accounts for approximately 2.5% of the surface water in Vietnam making it the 5th largest (in volume) basin in the country (MARD, 2009).
Tributaries
The two main rivers of the basin are Vu Gia and Thu Bon, which originate in the mountains with narrow valleys, rapids, steep bank. Middle and lower parts are relatively wide and shallow with a number of islets in the river.
Vu-Gia River
The Vu-Gia River begins its 204 km journey in the north-western Truong Son mountain range. The main stem of the river is formed by the joining of the tributaries Song Cai (Dak Mi) from the south,Bung river from west, A Vuong from the north western direction and Thanh River from the southwest in the Dai Son commune of the Dai Loc district. The Song Cai (Dak Mi) river originates at the high peaks of Ngoc Linh (above 2000 meters) in the Kon Tum province and flows a total distance of approximately 129 km, dropping around 700 m before merging with the main stream. The river flow in the north-south direction and joins the Bung River at Trung Hiep with a total basin area of approximately 1900 km2. Song Bung originates in the high mountains in the northwest and flows in east-west direction with a distance of approximately 132 km until it joins the Cai River at the Dai Son commune in the Dai Loc district. A Voung is one of the main tributaries of the Song Bung River which originates in the western mountainous range of the Lao border and covers an area of 898 km2 with a total length of 84 km. Along with the A Voung tributary; the Song Bung river basin encompasses an area of approximately 2530 km2. The 47 km long west branch (Con River) of Vu Gia starts from high mountains of Dong Giang district and joins the main stream at the Dai Lanh
commune in the Dai Loc district. The total catchment size of this small branch is approximately 627 km2. The Vu Gia River main stream, which flows 204 km in an easterly direction to its outfall at Da Nang at Cam Le. The main stream of the river has an overall drop of approximately 40 m (from Dai Loc to Da Nang at the coast). The Vu Gia River and its tributaries drain an area of 5180 km2. From its headwaters flowing east toward Dai Loc, the valley is narrow and the river flows quickly and is characterized by several swift drops in elevation, before it emerges from the Truong Son mountain range. In this section there is increasing hydropower development and the area belongs to the midlands, with low agricultural development. Below Dai loc, the valley broadens. This region is characterized by pronounced agricultural production and shows high commercial and industrial development. Through this section the river flows more slowly, meandering its way through the valley to Da Nang close to the coast.
Thu Bon River
Thu Bon River originates in the border area between the three provinces Quang Nam, Kon Tum and Quang Ngai at an altitude of about 2,000 m and at first flows from south to north. The river then changes direction at Phuoc Hoi and flows from southwest to northeast. When the river flows further down to Giao Thuy, it changes again direction and flows to the east until it joins the sea at Cua Dai. From upstream to Nong Son the river basin comprises an area of 3150 km2 with a length of 126 km, to Giao Thuy area is 3825 km2 and length is 152 km. Similar to the Vu Gia River, the Thu Bon river also has several tributaries. The main stream of the river is formed by the union of the Tranh River from the north and the Khang river from the south at Hiep Hoa Commune (Hiep Duc district). The southern branch originates at the Trung Giang reservoir and flows 29 km parallel to the coast until it reaches the sea at Hoi An. Truong river sub-basin covers an area of approximately 446 km2. The Tranh river comprises an area of about 44 km2 with a total length of 196 km, whereas for the Khang river the area is 609 km2 with a total length of 57 km. In the downstream part of the basin there is a connection between the two rivers by means of the Quang Hue river. The Quang Hue river transfers water from the Vu Gia River to the Thu Bon River. Vu Gia connects to Thu Bon river by means of the Vinh Dien river which is 16 km from the Quang Hue river. The river transports water from Thu Bon and returns it to the Vu Gia river. In the downstream part, the Vu Gia River also receives waters from Tuy Loan river and Ly Ly river with 309 km2 and 275 km2 of area and 30 km and 38 km in length.
Hydrological Characteristics of the VGTB
Compared to many other river systems, the VGTB basin has quite a large amount of water, with over 2000 mm of rainfall on average in a year. In a natural system flows are variable, but unmanaged, and dependent on precipitation and groundwater base flows. Flow regimes reflect the effect of precipitation on streams which can be seen in the hydrograph of flow volumes over a certain period of time. Flooding occurrences is the cumulative effects of flow from the tributaries; the Vu Gia and Thu Bon Rivers are the two greatest contributors to VGTB system. Generally, the contribution of each tributary is proportional to the area of land it drains—its watershed— but the magnitude of flows is also determined by the geology and soils of the watershed.
References and further reading:
Ministry Of Natural Resources And Environment of Vietnam (Ed.) (2009) Climate Change, Sea Level Rise Scenarios for Vietnam, Hanoi
The European Parliament and the Council of the European Union (6/11/2007) DIRECTIVE 2007/60/EC of the European Parliament and the Council of the European Union of 23 October 2007 on the assessment and management of flood risks, Directive 2007/60/EC. In Official Journal of the European Union (L 288), 27–34.
