Vung Tau - Co Gong dam, Vietnam

Vung Tau - Co Gong dam, Vietnam

Dekens, B.
Meerdink, L.
Meijer, G.J.
Sirks, E.
Van Vliet, R.

Civil Engineering and Geosciences

Hydraulic Engineering

2011-10-12

The area around Ho Chi Minh City (HCMC) faces flooding and salt intrusion problems. Flooding problems are caused by intensive rainfall in the city, high river discharges and high tides on sea. Dr. Hoc, the vice-minister of MARD (Vietnamese Ministry of Agriculture and Rural Development) proposed to construct a hydraulic structure downstream of HCMC. This will solve both the flooding and siltation problem. A possibility is to construct a dam between Vung Tau and Go Cong. This solution has been worked out into a preliminary design, which includes a design for the cross section and an investigation into the closure method. The hydraulic system where the project will be constructed consists of the Saigon - Dong Nai river system and the East Sea, and can be classified as an estuary with a (mainly semi-diurnal) mixed tide. Waves are moderate in the area. A dam will be constructed between Go Cong and the Dong Nai navigation channel. Over the Dong-Nai navigation channel to Vung Tau a bridge is planned, to minimise navigation delays and changes in tidal and salinity range in the Can Gio mangrove forest. To make a closed basin, Can Thanh will be connected with the dam near the Dong Nai navigation channel. Ship locks and discharge sluices will be constructed at the sides of the existing navigation channel in the Soi Rap. The final closure of the dam will take place in the Soi Rap navigation channel. A road on the dam will connect Vung Tau with Go Cong. Because there is no subsoil information available at the location of the dam, the soil layering and parameters are estimated based on information from locations in the vicinity of the dam. Three different soil profiles were drawn up. Measures have to be taken to increase the bearing capacity of the subsoil and decrease settlements during the lifetime of the dam. If thick layers of weak subsoil is present, drainage in combination with pre-loading will be the best solution. In case of a stronger subsoil partial replacement with sand is favourable. To build the dam, the use of local material is a cheap and easy solution. Near Vung Tau a number of areas are present where sand can be obtained. Rock can be found in mountains in the neighbourhood of Vung Tau. Since there is already a stone quarry in this area, it is assumed that the rock is of good quality. By means of a Multi Criteria Analysis it was found that a building a dam with a sand core is the best solution in this project. Degradation or even failure of (elements of) the structure may occur as a result of loadings during the operational phase or in accidental situations. The principal failure mechanisms for the VT-GC dam are checked. Slope, crest and toe protections were designed to prevent failure. Two designs were made for the cross section of the dam: one with an outer slope revetment made of asphalt and one for an outer slope revetment made of rock. The biggest differences in these designs can be found in the crest height and layer thicknesses. The construction of the dam should start with the construction of the bridge and work islands. Next, the discharge sluices and navigation locks will be made in building pits. Then the dam will be constructed. This starts with the foundation and scour protection which will be constructed using barges. Waterborne equipment is preferred because of the easier logistics and larger capacities. The core material, filter layers, underlayer, toe and armour layer will be constructed layer by layer to be able to raise the relatively steep slopes. Where water depths are too low, cranes operating from pontoons or rolling equipment can be used. After completion of the core and revetment, the crest is constructed and the dam can be finished with the construction of the road. The closure of the final gap is a very important aspect within the construction of the dam. A storage area approach is used to determine the system behaviour during closure. High velocities in the final gap make it impossible to close the gap with sand. A bottom protection is necessary in the final gap to prevent a scour hole, caused by high velocities, from developing too close to the dam. The gap is closed with coarser material than sand. A combination of vertical and horizontal closure will be used. First barges dump stones to a level of MSL -5 m. When the water is too shallow, dumping will continue from the sides of the closure gap. The final dam profile in the closure gap will be similar to the regular dam sections. A general estimation of the cost for the total project, based on costs in reference projects, is 3.1 billion US dollar. The Dong Thap Muoi region is a very important agricultural area for the production of rice. Due to the low river discharges and the large influence of the tide, the salt concentrations in the Vam Co rivers are high, causing problems for the farmers. An estimation of the salt intrusion length was made using a predictive steady-state salt intrusion model, according to Savenije. Unfortunately it was not possible to perform salinity measurements in the Vam Co River, so data from three fixed measurement stations was used. With this data, it was possible to fit the model roughly but more data is needed to make the model more accurate. Student report for a Master Project.

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