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HENCH AND LUETTICH
and offshore due bathymetric asymmetries (unique to
Spatial patterns in the momentum balance terms show
each system). If this is the case, near slack, currents on
the dynamics can vary dramatically over subkilometer
one side of the inlet will change direction before the
distances. High grid resolution revealed small-scale fea-
other and may flow across the inlet (as seen in Fig. 10a).
tures, such as localized flow separation zones, and the
This mechanism is not present in symmetric idealized
dynamical comparison between idealized and natural
inlets and therefore has not been identified previously.
inlets showed the importance of topography. The inlet-
At Beaufort Inlet, water tends to flow from east to west
scale momentum features in our models are in general
across the inlet at the start of flood, and provides a
agreement with the previous results of Imasato (1983)
mechanism for transporting material to the sound west
and Ridderinkhof (1988). However, the transient dy-
of the inlet. The addition of wind forcing or lateral
namical features described above were not identified in
baroclinic pressure gradients may make cross inlet ex-
their results.
change during slack tidal phases particularly effective.
The time-dependent dynamics have direct conse-
However, as the tide gains strength, these cross-inlet
quences on inlet exchange during the transition between
pathways will again be ``walled off '' by the dominant
the two end states. As the tide switches from ebb to
pressure gradientcentrifugal acceleration balance and
flood, the offshore spatially uniform streamwise pres-
sharply diminish the possibility for cross-inlet exchange.
sure gradient acts upon a spatially variable ebb jet flow
Inlet morphology may also play a role in determining
field. Low momentum fluid behind the headland features
the amount of cross channel exchange and the fate of
(sheltered from the ebb jet) switches to flood before the
water entering an inlet. Our model results indicate that
jet, and ``new'' ocean water floods the inlet first, while
the pressure gradientcentrifugal acceleration balance
sound water in the jet continues to spin down under the
sharply diminishes with distance from the headland fea-
``braking'' influence of the adverse pressure gradient.
U
tures. This distance scales with the inertial radius r
As the tide gains strength, secondary circulation may
s / f (Hench et al. 2002). If the bottom is flat and the
induce lateral mixing within the inlet but not across the
geometry is simple (as in the idealized inlet), cross-inlet
inlet centerline because of the ``dynamical wall effect.''
mixing may take place in zones inshore or offshore of
To our knowledge the formation and relaxation of the
the inlet where the ``wall effect'' is dynamically small.
dome of water across an inlet during a tidal cycle has
However, many natural inlets (including Beaufort Inlet)
not been directly observed in nature and this appears to
have channels that bifurcate on the sound side. In these
be a case where model results precede direct measure-
cases channelization may preserve two distinct water
ments. The models indicate that the cross-inlet elevation
masses inside the inlet (see Fig. 12j).
differences are at least several centimeters. Given the
general tendency of numerical models to underpredict
sharp velocity and elevation gradients, this is probably
c. Conclusions
a lower bound of what can be found in nature. If this
An analysis of transient momentum balances has elu-
is true, the dome should be within the detection limits
cidated the dynamics, circulation patterns, and exchange
of standard oceanographic pressure sensors.
mechanisms at shallow barotropic tidal inlets. Circu-
The wall effect may help to explain a number of
lation computed with high-resolution models was used
exchange and transport processes found in nature. For
to directly evaluate the contribution of each term in the
example, in a study of larval ingress at Beaufort Inlet,
momentum equations to the overall momentum balance.
Forward et al. (1999) observed fish larvae concentra-
Transformation of the time-dependent, frictional, fully
tions in the sound that were an order of magnitude great-
nonlinear xy shallow water equations into an sn co-
er on the east side of the inlet than on the west side. If
ordinate system greatly simplifies interpretation of the
the major offshore source of these larvae was east of
dynamics. This set of equations appears to be the sim-
the inlet, the wall effect (and channelization in the
plest dynamics that retains all the essential physics; inlet
sound) would confine them to the east side of the sound
circulation is at least a two-dimensional transient non-
where their fate would be determined by the quality of
linear problem.
nursery habitat in that part of the sound. Similar cross-
The temporal evolution of momentum indicates that
inlet differences in transport may occur for other plank-
inlet momentum balances oscillate between two dynam-
ton, as well as contaminants, nutrients, and suspended
ical states. During the stronger tidal phases, the stream-
sediments.
wise balance is between pressure gradient and stream-
wise acceleration in the sink region, and between
Acknowledgments. We thank Brian Blanton and Crys-
streamwise deceleration and bottom friction in the ebb
tal Fulcher for help setting up and running the models.
jet. Cross-stream balances are between centrifugal ac-
The crew of the NOAA ship Whiting kindly provided
celeration and normal direction pressure gradients. Near
1998 bathymetric data. Discussions with John Bane,
slack, the dynamics are nearly linear with streamwise
Jack Blanton, Cheryl Brown, and comments from three
pressure gradients balancing local acceleration, and nor-
anonymous reviewers significantly improved the man-
mal direction pressure gradients balancing rotary ac-
uscript. Funding was provided by USACOE Coastal In-
cleration.
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