[Truncated abstract] Coastal lagoons play an important role in the transport of materials between the coastal zone and the ocean. Understanding the dynamics associated with the movement of waters between and within these systems is therefore significant in defining the ecological health of the system. An important sub category of lagoons is Intermittently Closing and Opening Lakes and Lagoons (ICOLLs). These systems lack any significant river inflow; have a restricted sill type inlet and experience intermittent exchange with the ocean, making them susceptible to the retention of nutrients and pollutants from the catchment. The duration and frequency of an opening event may vary from weeks to months between each ICOLL, and inter and intra annually, respectively, and during an opening event, there are appreciable fluctuations in water level (1-3m range) accompanied by large changes in salinity (7 30ppt) within a short timeframe (hours). . . Regardless of the processes complete oceanic flushing was still predicted for each system, by the end of their respective opening events. The modelling work successfully reproduced the spring tidal setup in water level and exchange, using real bathymetry and meteorological forcing and defined the spring tidal set-up as the key predictable process in the exchange of water and salt between the larger ICOLL and the ocean. It was also shown that strong winds had the capacity to influence the magnitude of the exchange. The overall outcomes of this research therefore include the identification of key physical processes associated with the variability of the hydrodynamics within and between ICOLLs, which will aid in the future management of these highly dynamic systems.