HENCH AND LUETTICH
Transient Tidal Circulation and Momentum Balances at a Shallow Inlet
JAMES L. HENCH
RICHARD A. LUETTICH JR.
Institute of Marine Sciences, University of North Carolina at Chapel Hill, Morehead City, North Carolina
(Manuscript received 18 June 2002, in final form 23 October 2002)
An analysis of transient momentum balances is carried out to elucidate circulation, dynamics, and exchange
mechanisms at shallow barotropic tidal inlets. Circulation is computed using a depth-integrated, fully nonlinear,
time-stepping, finite-element model with variably spaced grids having horizontal resolution down to 50 m.
Velocity and elevation fields from the model are used to directly evaluate the contribution of each term in the
momentum equations to the overall momentum balance. A transformation of the xy momentum terms into an
sn coordinate system is used to simplify the interpretation of the dynamics and provide vivid illustrations of
the forces and resulting accelerations in the flow. The analysis is conducted for an idealized inlet and contrasted
with a highly detailed model of Beaufort Inlet, North Carolina. Results show that momentum balances in the
immediate vicinity of these inlets vary significantly in time and space and oscillate between two dynamical
states. Near maximum ebb or flood, the alongstream momentum balances are dominated by advective acceleration,
pressure gradient, and bottom friction. Cross-stream balances are dominated by centrifugal acceleration and
pressure gradients. Near slack, balances more closely follow linear wave dynamics, with local accelerations
balancing pressure gradients, and (to a lesser degree) Coriolis. Comparisons between the idealized inlet and
Beaufort Inlet show broad similarities in these momentum balances. However, natural inlet geometry and bottom
topography, as well as the tidal transmission characteristics of the sounds behind Beaufort Inlet produce strong
asymmetries. Moreover, momentum balances are highly localized, often with subkilometer length scales. The
dynamics are used to explain the physical mechanisms for inlet exchange. In particular, the results indicate that
the cross-stream dynamics generate a ``wall'' along the length of an inlet during the stronger phases of the tide.
The wall is established by opposing cross-inlet pressure gradients and centrifugal forces, and it poses a significant
barrier to cross-inlet exchange during the stronger phases of the tide but is absent near slack.
Signell 1992). A complete dynamical description of in-
let circulation has proven elusive as the circulation fields
Tidal inlets are important conduits between estuarine
are spatially complex, transient, and nonlinear.
waters and the coastal ocean for the movement of con-
In the earliest work on inlet exchange and kinematics,
taminants and nutrients (e.g., Signell and Butman 1992;
Stommel and Farmer (1952) noted the distinct differ-
Sheng et al. 1996), sediments (e.g., Fenster and Dolan
ence between flood (resembling a potential sink) and
1996), and biota (e.g., Weinstein 1988; Crowder and
ebb (resembling a jet) circulation outside an inlet. These
Werner 1999). Inlets are common coastal features; one
basic flow patterns have subsequently been reproduced
recent study indicates there are 150 inlets along the
in numerical models of idealized inlets (e.g., Kapolnai
United States coast alone (Carr and Kraus 2002). Their
et al. 1996; Wheless and Valle-Levinson 1996). Cir-
total number worldwide is unknown, but they are found
culation models of natural inlets have also been devel-
on all continents and particularly along barrier island
oped (Blanton et al. 1999; Luettich et al. 1999; Brown
sound systems, which compose about 13% of the world
et al. 2000) and have shown that irregular bathymetry
coastline (Cromwell 1973). Many reef passes also pos-
and shoreline geometry can produce highly asymmetric
sess inlet features (e.g., Wolanski et al. 1988). Since
circulation patterns that differ substantially from ide-
inlets are the primary locales for exchange between
alized inlets. Direct observations of circulation in nat-
sounds (also called lagoons or bays) and the coastal
ural inlets also indicate that the spatial and temporal
ocean they are naturally of great interest. However, an
flow structures are more complicated than in idealized
understanding of exchange processes is based implicitly
inlets (e.g., Kjerfve and Proehl 1979; Takasugi et al.
on detailed knowledge of the circulation (Geyer and
1990; Chadwick and Largier 1999a,b; Churchill et al.
1999). Baroclinic effects have also been explored, at
Corresponding author address: James L. Hench, Institute of Ma-
least for idealized inlets (Chao 1990; Kapolnai et al.
rine Sciences, University of North Carolina at Chapel Hill, 3431
1996; Wheless and Valle-Levinson 1996).
Arendell St., Morehead City, NC 28557.
Concurrent with the work on inlet circulation and
2003 American Meteorological Society