Wind Domination of Residual Tidal Transport in a Coastal Lagoon

Abstract

A 4-layer, two-dimensional finite difference model is used to quantify residual motion in response to six tidal constituents and windstress. The model is applied to Indian River lagoon, on the Atlantic coast of central Florida. Wind, water level and near-bottom current measurements were made hourly during a 65-day study period in the summer of 1981. Tidal motion in the southern segment of the lagoon is a response to interacting tidal waves moving northward and southward from St. Lucie and Ft. Pierce Inlets, respectively. M2 amplitudes of 12 cm and 31 cm/s were computed from water level and current data collected approximately midway between the inlets. In the absence of wind forcing, the depth-averaged mass transport residual current is 0.9 cm/s to the SSE; the depth-averaged Stokes drift is 0.5 cm/s to the SSE. The model is run with steady wind speeds of -4 to +5 m/s (NNWly headings defined positive) paralleling the axis of the lagoon. For NNWly winds of +2.4 m/s, a northward residual flow first appears in model layer 1. Winds of approximately +2.5 m/s and -2.5 m/s negate and double, respectively, the mass transport residual current calculated for calm conditions. Wind data from the 1981 study period are used in a final simulation. Results show that during relatively quiescent summer conditions wind forcing reversed and dominated the residual transport. The study suggests that wind forcing dominates residual tidal transport in the lagoon in all seasons.