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E. Wolanski . B. King . S. Spagnol
Ensemble-averaging is also questionable even in much simpler systems, such as a
branched estuary. A case in point is the Fly river estuary in Papua New Guinea (Fig. 1).
The estuary is about 60 km long, is shallow (typically only a few metres deep at low
tide) and has three major channels. The dominant forcing is the tide, the freshwater
inflow and the wind (Wolanski et al. 1997b). Current data of 2-8 weeks duration at ten
sites were available to verify the model. The model explained at least 90% of the variance at the mooring sites (King and Wolanski 1996a), so it can be used to explore the
dynamics of the system. The flow field is very complex and its understanding requires
computer visualisation (Anim. 8). This animation illustrates clearly the tides propagating from the sea to be dissipated over the shallow bottom. It shows the tides propagating at different speeds and with a different flood-ebb tide asymmetry in each channel. In each channel there are strong lateral and along-channel variations, with zones
of extremely strong flushing next to zones of much weaker currents leading to stagnation zones. The currents are strongly steered by the topography of channels meandering through shoals. At the apex of the delta where the three channels meet, each channel injects momentum and vorticity at different times in the tidal cycle and at different locations. The resulting currents in this area are chaotic with horizontal quasi-turbulent motions at the scale of the channel width.
7.3
Suspended Sediment and Plankton
Visual observations in the muddy coastal zone (Fig. 5) show the presence of patches
of turbid waters in blue waters. These patches can be a few metres to a few hundreds of
metres in diameter, with no clear pattern of distribution. To quantify the dynamics of
the fine sediments, there are a number of field techniques that rely on deploying automated sediment samplers in an oceanographic mooring or on ship-born measurements.
Because of the high costs of automated samplers, an oceanographer can seldom
deploy more than a few samplers at sea, usually obtaining temporal but not spatial data.
Automated samplers are usually very bulky and necessitate large ships for most deployment and recovery (Anim. 9). Even 40 km offshore from the mouth of Fly river in
Papua New Guinea, in shelf water deep enough that resuspension of bottom sediment
does not occur, suspended sediment distribution is extremely patchy. This can be seen
from the factor of 10 difference between daily catches of sediment (Fig. 6).
Suspended sediment concentration can also be measured using optical sensors such
as nephelometers and transmissometers, though marine fouling can limit their use.
Used in turbid topographically complex estuaries, these instruments also record enormous temporal and spatial variability (Wolanski et al. 1998). As a result of this variability even 24 nephelometers as they used may have been insufficient to measure the
net flux of sediment at the mouth of the Fly river.
In very muddy coastal environments a convenient ship-borne sampler is a highfrequency echo-sounder. The data show enormous patchiness, with fluid mud entrained
in suspension in patches at scales of centimetres to metres in an apparent chaotic
manner (Fig. 7).
Another shipboard technique is direct observation of suspended sediment and the
plankton in suspension, using an underwater video camera equipped with a macro-
E. Wolanski . B. King . S. Spagnol
Ensemble-averaging is also questionable even in much simpler systems, such as a
branched estuary. A case in point is the Fly river estuary in Papua New Guinea (Fig. 1).
The estuary is about 60 km long, is shallow (typically only a few metres deep at low
tide) and has three major channels. The dominant forcing is the tide, the freshwater
inflow and the wind (Wolanski et al. 1997b). Current data of 2-8 weeks duration at ten
sites were available to verify the model. The model explained at least 90% of the variance at the mooring sites (King and Wolanski 1996a), so it can be used to explore the
dynamics of the system. The flow field is very complex and its understanding requires
computer visualisation (Anim. 8). This animation illustrates clearly the tides propagating from the sea to be dissipated over the shallow bottom. It shows the tides propagating at different speeds and with a different flood-ebb tide asymmetry in each channel. In each channel there are strong lateral and along-channel variations, with zones
of extremely strong flushing next to zones of much weaker currents leading to stagnation zones. The currents are strongly steered by the topography of channels meandering through shoals. At the apex of the delta where the three channels meet, each channel injects momentum and vorticity at different times in the tidal cycle and at different locations. The resulting currents in this area are chaotic with horizontal quasi-turbulent motions at the scale of the channel width.
7.3
Suspended Sediment and Plankton
Visual observations in the muddy coastal zone (Fig. 5) show the presence of patches
of turbid waters in blue waters. These patches can be a few metres to a few hundreds of
metres in diameter, with no clear pattern of distribution. To quantify the dynamics of
the fine sediments, there are a number of field techniques that rely on deploying automated sediment samplers in an oceanographic mooring or on ship-born measurements.
Because of the high costs of automated samplers, an oceanographer can seldom
deploy more than a few samplers at sea, usually obtaining temporal but not spatial data.
Automated samplers are usually very bulky and necessitate large ships for most deployment and recovery (Anim. 9). Even 40 km offshore from the mouth of Fly river in
Papua New Guinea, in shelf water deep enough that resuspension of bottom sediment
does not occur, suspended sediment distribution is extremely patchy. This can be seen
from the factor of 10 difference between daily catches of sediment (Fig. 6).
Suspended sediment concentration can also be measured using optical sensors such
as nephelometers and transmissometers, though marine fouling can limit their use.
Used in turbid topographically complex estuaries, these instruments also record enormous temporal and spatial variability (Wolanski et al. 1998). As a result of this variability even 24 nephelometers as they used may have been insufficient to measure the
net flux of sediment at the mouth of the Fly river.
In very muddy coastal environments a convenient ship-borne sampler is a highfrequency echo-sounder. The data show enormous patchiness, with fluid mud entrained
in suspension in patches at scales of centimetres to metres in an apparent chaotic
manner (Fig. 7).
Another shipboard technique is direct observation of suspended sediment and the
plankton in suspension, using an underwater video camera equipped with a macro-