130
E. Wolanski . B. King . S. Spagnol
At the ecosystem level patches are known to exist at all scales in all environments,
and patches have been recognized as important natural processes (Engbert and
Drepper 1994; Hamner 1988; Hassel et al. 1991; Pickett and White 1985; Steele 1978).
Surprisingly, this knowledge has largely been ignored in environmental engineering
models for coastal management.
Our study sites are topographically complex coastal environments. They include
estuaries (Fig. 1), islands and coral reefs (Fig. 2) and mangroves (Fig. 3). We demonstrate that patches are the common rule and not the exception. We argue that engineers, scientists and marine resources managers need to take this process into account.
Specifically they must learn to deal with variability and unpredictability and not with
static or quasi-steady, rigid models. When dealing with particularly sensitive areas,
more caution is needed to deal with the unpredictability of the system.
7.2
Water Currents
Eddies behind islands and headlands are commonly observed in coastal waters (Fig. 4;
see also Wolanski 1994). They generate patchiness and variability along the coast, but
this process has remained largely unstudied. Oceanographers have largely ignored
them, as recently as 1983 Robinson's book on eddies in marine science does not even
mention them.
Water currents are usually one of the primary parameters measured in the field, this
is best done from oceanographic moorings with current meters suspended under a
subsurface buoy on a taut wire or, in shallow waters, within a frame (Anim. 1). In
topographically complex systems, such as around a headland or an island, eddies are a
common feature (Fig. 4). Many current meters are needed to characterize the flow field.
For instance 26 current meters were necessary (Wolanski 1994) to measure the eddy
behind Rattray Island, a 1500 m wide island in 20-30 m depth of water (Anim. 2). All
26 current meters 'Were needed because currents in the eddy region varied rapidly, both
temporally and spatially (Anim. 3). If only a few current meters had been used, as is
common practice in most environmental impact assessment studies, an unrealistically
simple (and incorrec;:t) picture of the flow field would have emerged. The flow field at
Rattray Island develops an eddy, a patch of water which varies in size throughout the
tidal cycle; with currents inside the eddy lagging that of the undisturbed tidal current
outside the eddy (Anim. 3). Other measuring techniques are possible, e.g. the use of
an acoustic Doppler current profiler mounted on a ship criss-crossing the area (Geyer
and Signelll990; Signell and Geyer 1991). This improves spatial resolution but diminishes temporal resolution.
It was only after the Rattray Island data set became available that it was possible to test
if 3-D models were able to reproduce complex coastal flows and to compare the performance of the models against the observations from the 26 current meters (Galloway
et al. 1996). The only other comparison between 3-D models in the literature was the MOMP
numerical experiment for which no field data were available (Roed et al. 1995). Four
models compared by Galloway et al. (1996) were NOAA's Mecca Model, the Princeton
Ocean Model (POM), the Hamburg Oceanography Model (HAMSOM), and the AIMSGHER model. Initially none of the models performed satisfactorily because they all
under-estimated by 30 to 90% the size and st(,ength of the observed eddy (Galloway
E. Wolanski . B. King . S. Spagnol
At the ecosystem level patches are known to exist at all scales in all environments,
and patches have been recognized as important natural processes (Engbert and
Drepper 1994; Hamner 1988; Hassel et al. 1991; Pickett and White 1985; Steele 1978).
Surprisingly, this knowledge has largely been ignored in environmental engineering
models for coastal management.
Our study sites are topographically complex coastal environments. They include
estuaries (Fig. 1), islands and coral reefs (Fig. 2) and mangroves (Fig. 3). We demonstrate that patches are the common rule and not the exception. We argue that engineers, scientists and marine resources managers need to take this process into account.
Specifically they must learn to deal with variability and unpredictability and not with
static or quasi-steady, rigid models. When dealing with particularly sensitive areas,
more caution is needed to deal with the unpredictability of the system.
7.2
Water Currents
Eddies behind islands and headlands are commonly observed in coastal waters (Fig. 4;
see also Wolanski 1994). They generate patchiness and variability along the coast, but
this process has remained largely unstudied. Oceanographers have largely ignored
them, as recently as 1983 Robinson's book on eddies in marine science does not even
mention them.
Water currents are usually one of the primary parameters measured in the field, this
is best done from oceanographic moorings with current meters suspended under a
subsurface buoy on a taut wire or, in shallow waters, within a frame (Anim. 1). In
topographically complex systems, such as around a headland or an island, eddies are a
common feature (Fig. 4). Many current meters are needed to characterize the flow field.
For instance 26 current meters were necessary (Wolanski 1994) to measure the eddy
behind Rattray Island, a 1500 m wide island in 20-30 m depth of water (Anim. 2). All
26 current meters 'Were needed because currents in the eddy region varied rapidly, both
temporally and spatially (Anim. 3). If only a few current meters had been used, as is
common practice in most environmental impact assessment studies, an unrealistically
simple (and incorrec;:t) picture of the flow field would have emerged. The flow field at
Rattray Island develops an eddy, a patch of water which varies in size throughout the
tidal cycle; with currents inside the eddy lagging that of the undisturbed tidal current
outside the eddy (Anim. 3). Other measuring techniques are possible, e.g. the use of
an acoustic Doppler current profiler mounted on a ship criss-crossing the area (Geyer
and Signelll990; Signell and Geyer 1991). This improves spatial resolution but diminishes temporal resolution.
It was only after the Rattray Island data set became available that it was possible to test
if 3-D models were able to reproduce complex coastal flows and to compare the performance of the models against the observations from the 26 current meters (Galloway
et al. 1996). The only other comparison between 3-D models in the literature was the MOMP
numerical experiment for which no field data were available (Roed et al. 1995). Four
models compared by Galloway et al. (1996) were NOAA's Mecca Model, the Princeton
Ocean Model (POM), the Hamburg Oceanography Model (HAMSOM), and the AIMSGHER model. Initially none of the models performed satisfactorily because they all
under-estimated by 30 to 90% the size and st(,ength of the observed eddy (Galloway