Assessment of the effect of the reclamation of acid sulphate soil for agricultural development in Vietnam

Vietnam is an agricultural country where rice is the main crop cultivated in the Red river delta in the North and in the Mekong delta in the South. Both deltas refer to lowland. However, the degree of development is clearly different between the two deltas. In the Red river delta, the dike system as well as the canal networks have been constructed. Flood is almost controlled. Canal networks have been excavated to supply water to the fields or to evacuate drainage water out of the areas of interest. Meanwhile, there are no dikes in the Mekong delta. This results in flooding every year with a duration of about two months and causing damages to the local people. Canal networks are poor, 5 m/ha on average. Field canals are rarely found in practice. The other constraint for agricultural development is the problem of acid sulphate soil (ASS). This type of soil accounts for a big portion of the total area of the Mekong delta, 1.6 million ha out of 3.9 million ha. Under the pressure of the increasing population growth and the requirement on rice production attention is given to the development of the Mekong delta. Actual and future problems deals with a flood control and soil and water management on ASS. To contribute to the solution of these problems the Plain of Reeds is studied in this thesis. The study focuses on: - development of criteria for dike design and technical specifications for drainage which can be applied in planning, design and construction; - analysis and assessment of possibilities of removal of toxic elements from the potential acid sulphate soil. To reach the objectives mentioned above an analysis of available data has been carried out and three simulation models have been used: the simulation model for acid sulphate soil (SMASS) to predict the duration of land reclamation and to determine strategies of soil and water management; the simulation model for one dimensional unsteady flow in channel systems (DUFLOW) for flood analysis, canal design and water management; and the geographical information system (GIS) for planning activities. The analysis and simulation results show that submerged dikes designed for a return period (T = 3 years) and open drainage systems are appropriate under conditions of the Mekong delta. To enhance the land reclamation and water management a ground water table at 0.2 m-surface is considered as appropriate. Canal networks should be improved to increase the discharge capacity. Water control structures should be constructed to prevent a backwater flow and salt water intrusion under tidal conditions. In addition the GIS shows that the layout of existing canal networks is reasonable. It is possible to lay-out irrigation and drainage canals separately. The reclamation of acid sulphate soils for agricultural development is possible by proper water management measures. Combination of data analysis with the simulation models can deliver more or less the the required information to enable policy makers and decision makers to take proper decisions. However, a study on dynamics of acid sulphate soil and consequences due to reclamation of acid sulphate soils are necessary and recommended to be carried out