Seasonal-scale Transport Patterns in a Multi-inlet Coastal Lagoon

Abstract

A finite-difference numerical model is used to investigate seasonal-scale lagoon-shelf exchanges and transport within Indian River, a multi-inlet lagoon on the Atlantic coast of Florida, U.S.A. Meteorological, hydrologic and hydrographic data quantify forcing by seasonally-varying wind stress, coastal sea level and the net freshwater gain. The along-axis component of the wind stress reverses seasonally. North-westward wind stress stores water in the northern part of the lagoon in summer months, then south-westward wind stress removes it in winter months. The seasonal rise and fall in water level at opposite ends of the lagoon differ by ±0·1–0·2m, primarily as a result of seasonal variations in wind forcing. The rise and fall of coastal sea level forces water into the lagoon during summer months, then draws it out during late fall and winter months. Net freshwater gain forces water out of the inlets during summer months, when rainfall rates are highest, and to a lesser extent during mid winter, when evaporation is relatively low. When forcing is by shelf tides only, simulations suggest that the northern and southern inlets are flood-dominant, while the central inlet is ebb dominant. With the addition of seasonal-scale forcing, the northern inlet becomes flood-dominant from late fall through early spring, then ebb-dominant during the rest of the year. The central and southern inlets are ebb-dominant throughout the year but seasonal variations differ. The central inlet has strongest outflow during summer months, while the southern inlet has strongest outflow during fall and winter months. Convergent and divergent patterns of transport within the lagoon are a complex response to both local and remote forcing. Simulations suggest that transport in the dredged part of a navigation channel opposes transport outside the channel.