Dynamics of currents in a small-scale coastal lagoon, southwestern Taiwan

A portable electromagnetic current meter was used to measure currents in a coastal lagoon southwest of Taiwan, Shipboard measurements were made at ebb and flood stages during neap and spring tides. To retain as much synoptici ty as possible, each round of measurement covering most of the lagoon was m inimized to less than two hours. Observations revealed a stagnation area fo r tidal currents inside the lagoon. Tidal streams entering and leaving the two inlets converge and diverge at this location. Current speeds in the two inlets range from 30 to 50 cm/s during neap tide and up to 60 cm/s during spring ride. A vertically averaged two-dimensional numerical model was used to simulate tidal currents and possible modulations by winds and river run off. Model results indicate tidal dominance over other forcing, The width d ifference between northern and southern entrances determines the strength d ifference between two tidal streams entering and leaving the lagoon. The st rength difference then determines the location of the flow stagnation area. The effect of northerly wind stress pushes the flow stagnant area southwar d during flood but northward during ebb. The effect of large river runoff ( 200 m(3)/s) also modulates the stagnation location, which is moved northwar d during flooding but is hardly displaced during ebbs. The observational an d numerical results also suggest that the flow stagnation may hinder transp ort of waterborne materials between northern and southern reaches.