Pollutants moving with undiluted offshore water can approach a tidal inlet for most part of the tidal cycle. During flooding tide, a wave induced longshore current arriving from the adjacent surf zone is readily entrained into the inlet, whereas an ambient cross current, either driven by tide or sea breeze, tends to bypass the inlet by deflecting seaward at the jetty. During ebb tide, these currents can still operate in strength in the underlayer, bypassing the inlet beneath the buoyant jet or approaching the inlet under a laterally expanding effluent along the adjacent surf zone. The effluent discharging with the jet can undergo strong buoyant expansion into the adjacent coast, forming a partially diluted effluent pool against the shore under a sea breeze. The land breeze and the instability at the density boundary between this nearshore effluent pool and the undiluted offshore water are the two most important factors affecting the eventual dispersion and diffusion of the tidal inlet effluent.
Numerous bays and lagoons along the U. S. and the world's coasts support an increasing amount of recreational, industrial and cultural activities in addition to delicate ecological systems. Should a pollutant from offshore source reach an inlet which connects to a bay, the consequent damage inside the bay could require a prolonged and costly effort for cleanup. The hydrodynamics of tidal inlet affecting possible entrainment from offshore and flushing of bay effluent into adjacent coasts and deep water is not well understood.
A field study was undertaken in order to investigate the interplay among tide, water density distribution, longshore currents, waves and wind associated with a tidal inlet. The study site was East Pass, located at the eastern end of Santa Rosa Island adjacent to Destin, Florida (Figure 1). East Pass connects the Gulf of Mexico with the Choctawhatchee Bay, a hypopycnal lagoon with a surface area of approximately 138 square miles with water depths generally less than 3 m. Numerous small streams which enter the Choctawhatchee Bay add a considerable volume of fresh water, creating distinct density contrasts with the Gulf water.
The main flood and ebb discharges take place adjacent to the east bank where a deep channel about 4 m deep and less than 100 m wide has developed. The East Pass channel is about 3.6 km long and connects with the Gulf through an opening about 300 m wide between two rubble mound jetties (Figure 2). A flow through a 300-meter long weir section on the west jetty is insignificant as compared with the dominant flux through the main opening. A crescent shaped bar exists approximately 400 m off the entrance, where an average depth is about 2.55 m.