US Army Corps of Engineers

Coastal Inlets Research Program



Hench, James, L. 2002. "Circulation and Dynamics at Shallow Tidal Inlets," Ph.D. Dissertation, Department of Marine Sciences, University of North Carolina at Chapel Hill, 107 pages. Chair: Dr. Richard Luettich, Jr.


Circulation and dynamics at shallow tidal inlets are studied using a combination of observations, numerical modeling, and theory. The observational component includes the design, testing and application of a portable and retractable shipboard acoustic Doppler current profiler (ADCP) boom-mount. The boom is specifically adapted for small fiberglass boats. Tests of the boom show that high quality ADCP data can be obtained at boat speeds up to 4.0 m s-1 and that data quality depends on transducer depth.

An observational study of circulation was made at Beaufort Inlet, North Carolina using twelve shipboard ADCP cruises, and a moored current meter. ADCP data were harmonically analyzed and combined on a tidal constituent basis to make detailed mosaics of tidal ellipses and Eulerian residual currents. The observations provide direct confirmation of classical inlet circulation patterns. Comparisons with a barotropic tidal model of the inlet indicate that the major depth-integrated circulation features can be explained with non-linear barotropic tidal dynamics.

An analysis of transient momentum balances was carried out to elucidate circulation, dynamics, and exchange mechanisms at shallow barotropic tidal inlets. Circulation was computed using a depth-integrated, fully nonlinear, time-stepping, finite element model. Model results are used to evaluate each term in the momentum equations. Transformation of the x-y momentum terms into a s-n coordinate system was developed and used to simplify dynamical interpretation. The analysis was conducted for an idealized inlet and a detailed model of Beaufort Inlet. Results show that momentum balances in the immediate vicinity of these inlets vary significantly in time and space and oscillate between two dynamical states. The dynamics are used to explain the physical mechanisms for inlet exchange.

The analysis was extended to inlets with different geometry and forcing. Spatial patterns in momentum reveal that the lateral balances can vary from nearly geostrophic to strongly cyclostrophic. Inlet regions of geostrophic or cyclostrophic balances can be predicted using two dimensionless parameters, the dynamic length and dynamic width. A classification scheme is proposed using and to compare the idealized inlets analyzed here with inlets from 20 previous studies. Four distinct inlet types are identified and discussed.