Seasonally open tidal inlets usually occur in micro-tidal, wave-dominated coastal environments where strong seasonal variations of streamflow and wave climate are experienced. These inlets are closed to the ocean for a number of months every year due to the formation of a sand bar across the entrance. Because the estuaries connected to these inlets commonly serve as harbours or recreational facilities, the annual inlet closure is undesirable in most cases. Hence, there is increased interest in understanding the processes governing inlet closure and the hydrodynamics and exchange processes of these estuaries. This study was undertaken with the objectives of: (a) identifying the morphodynamic processes governing seasonal inlet closure, and (b) determining the effect of the seasonal closure of the inlet on the hydrodynamic characteristics of the estuary/lagoon connected to the inlet. Wilson Inlet, a typical seasonally open tidal inlet located along the southwestern coast of Australia, was selected as a case study. The inlet is usually closed to the ocean for 6-7 months every year. Field experiments and numerical models were employed to examine inlet morphodynamics and estuarine hydrodynamics. As cross-shore processes, which the field data indicated to be important, have not been included in coastal area models to date, a quasi-3D morphodynamic model, which includes longshore and cross-shore processes, was developed to determine processes governing seasonal inlet closure. Application of the model to Wilson Inlet clearly indicates that onshore sediment transport during summer is the cause of inlet closure. Further applications show that summer streamflow and storm events also exert significant control on inlet closure depending on the intensity and timing of the event. The results of the estuarine hydrodynamic study indicate that the hydrodynamic regime of seasonally open estuaries changes distinctively when the inlet state varies from "open" to "closed."