Estuary is a unique area of important economic, cultural and environ- mental value. It is also a region with very complicated hydrodynamic mechanisms, partly due to the interaction of freshwater and seawater. Much research e®ort has been invested in improving the application of estuary and preventing estuarine environment from any undue damage. With the rapid development of computational resources, mathematical model has become a popular approach used for the investigation of es- tuarine hydrodynamics. The aim of this research study is to set up the numerical models which can be used for investigating the saline-wedge purging process, astronomical tidal circulation and typhoon-induced storm surge in the estuarine regions. Two mathematical models have been developed for this purpose. One objective of this project is to set up a two-dimensional model for exploring the °ushing process of trapped saltwater subject to upstream freshwater turbulent °ow. Most numerical simulations currently ap- plied are based on single-phase models, which are not suitable for the two-phase °ow before the mixture of saltwater and freshwater. The multiphase Eulerian model, a part of commercial code FLUENT6.2, has been applied for the ¯rst time to study this complex mixing inter- action in estuary. The distinguishing characteristic of this model is to treat saltwater and freshwater as two single miscible phases instead of a mixture phase with density variation, and the advantage of using a multiphase approach over a single-phase model is that it can e±ciently and accurately treat both the free water surface and relatively high density excess between two °uids simultaneously. The other objective of this project is to develop a three-dimensional model based on the FVCOM open source code, with the aim to better understand the estuarine hydrodynamics with or without the presence of typhoon. It is found that the original FVCOM code can not re- produce an accurate tidal hydrodynamics in estuary, mainly due to the inaccurate calculation of bottom friction in shallow water. To overcome this di±culty, an improved simulation of the bed friction has been in- corporated into the existing code for estuarine tide. This model has also been developed by including air-pressure gradient term to study the hydrodynamic response to cyclonic typhoon. To include the e®ect of typhoon (wind stress and pressure de¯cit), a symmetrical cyclone model is adopted. However, the typhoon-induced wind ¯eld has been predicted poorly when the typhoon enters the near-shore region. This is because the typhoon quickly loses its symmetrical property in the near-shore region. To overcome this di±culty, an asymmetrical cyclone model is derived on the basis of characteristic isobar. The accuracy of open sea boundary for storm surge model has also been improved by using large scale model. The numerical models have been compared with laboratory experiments or ¯eld observations. The comparison results show a good agreement numerical simulations and physical measurements. It is anticipated that the models developed in this research can make signi¯cant contribution in estuarine application and protection.