Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 380

the 2D and 3D model configurations were started at 12:00 h on May 10 and ended
at 12:00 on May 15, 1989. The time step was set at 30 s, giving a Courant number
of 3 for depths in the domain. The Smagorinsky eddy viscosity formulation
(Appendix 9.2.A) was used for both 2D and 3D model configurations.
The vertically integrated or 2D currents in GEL under prevailing southwesterly
and northeasterly winds are shown in Figure 9.2.5A and Figure 9.2.6A, respec-
tively. Corresponding wind directions are also shown for clarity. Under southwest-
erly wind conditions (13:00 h, May 12, Figure 9.2.5A), currents are quickly
established near both shores in the shallow areas. A weak return flow is seen in
the basin, oriented toward the south. An almost opposite situation occurs for the
vertically integrated coastal currents under northeasterly winds (16:00 h, May
13, Figure 9.2.6A). However, no return integrated flow is observed in the deep
basin.
The 3D simulation was performed under homogeneous density conditions in
order to avoid additional 3D baroclinic current structures. Horizontal currents at
0.75 and 4 m depths are presented in Figure 9.2.5B and Figure 9.2.5C, respectively,
for prevailing northeasterly winds, and in Figure 9.2.5C and Figure 9.2.6C, respec-
tively, for southeasterly winds. The major difference between the 2D and 3D current
distributions can be seen in the deeper basin. In the 3D simulation, the wind-drift
coastal currents near both shores are well defined in the surface layer, much as in
the 2D case. However, the return flow or gradient currents from the 3D simulation
occurs mainly at depth (Figure 9.2.5C and Figure 9.2.6C), in a direction opposite
to that of the wind and the surface currents (Figure 9.2.5B and Figure 9.2.6B). When
integrating the currents over the water column, opposing currents should cancel, as
observed in the 2D case. In the shallower areas, to the west of the navigation channel,
the drift current structures are almost identical in the 2D and 3D simulations.

9.2.5 C ONCLUSION

The circulation and the transport of ecosystem variables in shallow lagoons are
significantly affected by winds. Wind stress quickly establishes wind-driven currents
in the lagoon shallow areas. Water accumulation at the downwind lagoon end then
creates horizontal pressure gradients that generate a return flow in the form of upwind
gradient currents. Wind-induced currents are further affected by friction at the bottom
and, in wide subtropical lagoons, by Coriolis acceleration. In the presence of topo-
graphic variations, the resulting circulation becomes quite complex and a return flow
can occur anywhere between the coastal currents. The hypothesis put forth in this
study is that gradient currents will occur closer to the bottom in the deeper parts of
the lagoon, away from the wind stress at the surface. This hypothesis was tested for
GEL where a relatively deep (5-6 m) basin exists and where near-bottom current
observations are available. An EOF analysis of low-frequency principal axis current
fluctuations and the wind longitudinal component showed that near-bottom currents
are negatively correlated with the wind direction. A numerical model was used to
simulate vertically integrated currents under prevailing southwesterly wind condi-
tions. Results show that coastal wind-driven currents are quickly established in the