Freshwater is becoming an increasingly scarce and vulnerable resource in some places in the world, like in the Dong Nai River basin, which houses Vietnam's largest population centre Ho Chi Minh City as well as its largest concentration of industrial output. Because of that, proper management of the river basin is becoming extremely important. With the world progressing into the age of information and communication technologies (ICY), the use of computer-based systems may prove helpful for better management of available water resources. Some of these hydroinformatics technologies are being applied here to support design and decision making processes relevant for river basin development. Due to uneven spatio-temporal distribution of rainfall in the catchment area and strong tidal dynamics in the East Sea, the Dong Nai River basin encounters considerable problems such as flooding, drought, salinity intrusion, etc. Various developments in the basin have suffered in terms of water resources availability and environmental degradation. During the rainy season, there is a considerable amount of rainfall and river run-off, easily satisfying water demands, particularly for irrigation. On the other hand, during the dry season, water requirements for irrigation and other use increase significantly while water resources availability is seriously reduced. This conflict between demand and supply of water resources exists in the entire Dong Nai basin area, particularly in the lower region, which consists of a complex branched and looped river networks, strongly affected by tidal intrusion from the East Sea in the dry season. Also, the river branches in this area are the main sources for water supply for municipal and industrial use as well as for irrigation of riparian areas. The main problem occurring here is qty intrusion leading to difficulties for the water supply in the dry season. Optimal utilisation of water resources for various purposes has already been studied extensively. However, due to the complexities and varieties of water resources system, so far there is no universal formula that can be applied to all system, certainly not to a complex branched river network subjected to tidal forcing and containing several upstream reservoirs. Also, present day operation of darns and reservoirs as put forward recently by the World Commission on Dams (WCD, 2000) calls for consideration of Environmental Flows (EFs) to be taken into account when developing optimal reservoir operation. EFs, i.e. the release of water from a dam specifically to ensure the maintenance of downstream aquatic ecosystems and their biodiversity, may require seasonal or annual flow release and/or regular or irregular pulses. In this thesis a hydrodynamic simulation model is developed and calibrated against actual qty measurements. The results are used as constraints for multi-reservoir optimisation of the Dong Nai basin. The numerical simulation model is based on the SOBEK software package of WL Delft Hydraulics, while the MATLAB programming environment is used to develop the optimisation model. The objective is to derive optimum reservoir operation rules in order to reduce salinity intrusion from the East Sea, especially near vital drinking water intakes. The computed results show that salinity intruusion is much better controlled by combined reservoir operation than by separate reservoir management. Computer-based numerical simulation and optimisation methods prove indispensable when undertaking such studies.
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