Coastal lagoons in Sri Lanka are shallow water bodies connected to the sea through narrow and often restricted inlets. These physical features strongly affect tidal water level variations and water exchange within lagoons. Understanding lagoon water level behaviour is important for studying circulation and environmental conditions. This study presents the development of a one-dimensional numerical modelling framework to simulate tidal water level variations in Sri Lankan coastal lagoons.

The model is based on simplified forms of the continuity and momentum equations, which describe the conservation of mass and momentum along the lagoon channel. The restricted water exchange at the lagoon inlet is represented using a choking model to account for the effects of narrow and shallow entrances. Observed sea level data were used as boundary conditions. The governing equations were solved numerically to obtain spatial and temporal variations of water levels inside the lagoon. The modelling framework was applied to Chilaw Lagoon as a case study.

The numerical results show how tidal water levels propagate inside a shallow lagoon with a restricted inlet. Both temporal and spatial variations of lagoon water levels are illustrated using the model simulations. These results provide an initial understanding of tidal behaviour in shallow lagoons with restricted inlets.

The developed modelling framework provides a useful starting point for future studies. It can be further improved and validated using observed lagoon data and extended to include discharge and salinity transport processes. This approach will support more detailed investigations of water circulation and environmental dynamics in the coastal lagoons of Sri Lanka.