Influences of Islands and Shoals on Coastal Water Properties, Flushing Time, and Dispersion Within Western Long Island Sound

Effects of islands and shoals on coastal water temperature and salinity, flushing time, and dispersion were quantified for a study area within the Western Long Island Sound, inshore of the Norwalk Islands. Analysis focused on a summer warming period in July 2015. Observations were used to force a realistic high-resolution hydrodynamic model nested within a larger domain model. Island and shoal effects were isolated through intercomparison of three model runs with islands and shoals (the natural configuration), without the islands, and without islands and shoals. With islands, there was cooler and saltier water along a band immediately inshore of the islands. With shoals, water was warmer and fresher between the coast and islands. The shoals influence occupied a larger area with higher magnitude water property differences than the island influence. The islands influence tidal residual currents by intensifying eddies and creating across-shore exchange through island passes. Islands reduce flushing time and shoals increase flushing time. The retention effect of the shoals dominates over the dispersive influence of the islands for 2 days after dye tracer release, but the effects offset each other for later times.