Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 372

9.2 THREE-DIMENSIONAL STRUCTURE OF WIND-DRIVEN

CURRENTS IN COASTAL LAGOONS

Vladimir G. Koutitonsky
9.2.1

I NTRODUCTION

A coastal lagoon is a shallow water body separated from the ocean by a barrier,
usually parallel to the shore, and connected, at least intermittently, to the ocean by
one or more inlets. 1 A lagoon can be choked, restricted, or leaky depending on the
inlet configuration. 2 These geomorphologic features affect hydrodynamic processes
inside the lagoon, namely the flushing time, the circulation, and the mixing of
waters. 3, 4 Depths of most lagoons seldom exceed a few meters so they respond
quickly to forces acting at the air-sea interface and at the lagoon open boundaries.
Forces acting at the lagoon lateral boundaries are tides, nonlocal forcing at the ocean
boundary, and freshwater run-off, when present, at the upstream boundary. Forces
acting at the air-sea interface include heat and water fluxes resulting from changes
in air temperatures, precipitation and evaporation, and wind stress. These forces
accelerate the motion and establish barotropic and baroclinic pressure gradients in
the lagoon and across its inlets. They also modulate the turbulent mixing as well as
the vertical and horizontal density gradients in the lagoon. 5 Bottom friction decel-
erates the motion and plays a significant role in the momentum balance. 6 This study
focuses on the combined effects of wind stress, horizontal barotropic pressure gra-
dients, and bottom friction in coastal lagoons.
Normally, winds blowing along the major axis of a lagoon set up downwind
coastal currents in the shallower regions near both shores. The resulting water pile-
up at the downwind end of the lagoon sets up horizontal (barotropic) pressure gra-
dients that can induce upwind gradient currents elsewhere in the lagoon, between the
coastal currents. This study suggests that wind-driven currents in lagoons are three-
dimensional (3D) in space and that 2D models may not be adequate to describe them.
Such a 3D structure may influence primary production, 7 nutrient fluxes, 8 and the
transport of dissolved and particulate matter in the lagoon. 9,10 Modeling the response
of ecosystems to the above physical processes is discussed in Hearn et al. 6 and in
earlier chapters.
The objective of this study is to demonstrate that wind-induced currents can
have a 3D structure in some coastal lagoons even when their depths are relatively
shallow (~5 m). The hypothesis put forth is that return gradient currents occur in
the deeper layers of the lagoon, away from the surface where motion is still respond-
ing to wind stress. This may have implications for water renewal in the bottom layer,
for the transport of dissolved or particulate matter in the lagoon, and for biogeochem-
ical fluxes at the sediment interface. The hypothesis is tested in Grande-Entrée
Lagoon (Section 9.2.2) where predominant winds are oriented along the lagoon
axis. 11 Low frequency current, sea levels, and wind time series obtained during a
1989 field experiment are analyzed in Section 9.2.3. Section 9.2.4 compares the
results of 2D and 3D model simulations of these wind-driven currents. The numerical
model used in this study is the MIKE3-HD model, 12,13 briefly described in
Appendix 9.2.A. Conclusions are given in Section 9.2.5.