Influence of tide and waves on water renewal in Óbidos Lagoon, Portugal

Abstract

The role of oceanic tide, wind stress, freshwater river inflows, and waves in the long-term circulation and residence time in Obidos Lagoon is investigated using a sensitivity analysis carried out by means of a two-dimensional model. MOHID modeling system coupled to Steady-State Spectral Wave model for simulate Obidos Lagoon circulation were implemented. For residence time calculus, a Lagrangian transport model was used. Tidal forcing is shown to be the dominant forcing, although storm waves must be considered to simulate accurately the long-term circulation. Tidal forcing enhances a spatial distribution in water residence time. Renewal time scales varies from values of 2 days in the near-ocean areas and 3 weeks in the inner areas. Freshwater river inflows decrease the residence time, while waves increase. In heavy rain periods, the water residence time decreases by about 40% in the upper lagoon. When wave forcing is considered, the residence time increases between 10% and 50% depending on lagoon area. The increase in residence time is explained by the sea level rise within lagoon (~1 m above average lagoon sea level) during storm wave periods. Average residence time is 16 days for tidal forcing, 9 days when the rivers are included (wet period), and 18 days when the waves are considered.