260 David Prandle
and hence accurate descriptions require more sophisticated measurement techniques and finer resolution 3D numerical models. Recent technology developments in both H.F. Radar and ADCP (Acoustic Doppler Current Profiler) illustrate
these intricacies.
.
The dynamics of storm surges are dependent on the magnitude (depth of depression), spatial scale and speed of travel of the associated meteorological system in
relation to the water depths and extent of the tidal basin. For the North Sea, these
dynamics are similar to those ofthe semi-diumal tides (Heaps, 1967).
Mixing
The conservation of mass equation is:
ac + uac + vac + w ac = l.(K ac) + l.(K ac) + l.(K ac) (B4)
at ax ay az ax xax ay yay az zaz
Short term turbulent fluctuations are not necessarily the primary mechanisms for
horizontal dispersion in shelf seas. Dispersion often arises from the interacting 3-D
variations in phase and amplitude of both the currents and contaminants (though
these variations are themselves highly dependent on both topography and the vertical turbulence levels). Simulation ofmixing processes is much more difficult than
for tidal dynamics since a much wider spectrum of length scales is involved. Simulation of mixing is often de-coupled from simulation of tidal dynamics with
velocities and elevations from the latter stored and used both to specify appropriate
advective transports and to determine appropriate dispersion coefficients (Prandle,
1984). Altematively the tidal dynamics may be specified from observations.
Whatever approach is used it is generally convenient to define separately the various tidal constituents and the residual. Unfortunately the definition of a residual
current in a shallow region with pronounced spatial variability in tidal currents is
complicated.
Several (depth-averaged) definitions are possible, e.g. Eulerian, Lagrangian,
Stokes Drift (Longuet-Higgins 1969). Lane et al. (1997) discuss these definitions
and suggest a framework for relating estimates from models and observations.
and hence accurate descriptions require more sophisticated measurement techniques and finer resolution 3D numerical models. Recent technology developments in both H.F. Radar and ADCP (Acoustic Doppler Current Profiler) illustrate
these intricacies.
.
The dynamics of storm surges are dependent on the magnitude (depth of depression), spatial scale and speed of travel of the associated meteorological system in
relation to the water depths and extent of the tidal basin. For the North Sea, these
dynamics are similar to those ofthe semi-diumal tides (Heaps, 1967).
Mixing
The conservation of mass equation is:
ac + uac + vac + w ac = l.(K ac) + l.(K ac) + l.(K ac) (B4)
at ax ay az ax xax ay yay az zaz
Short term turbulent fluctuations are not necessarily the primary mechanisms for
horizontal dispersion in shelf seas. Dispersion often arises from the interacting 3-D
variations in phase and amplitude of both the currents and contaminants (though
these variations are themselves highly dependent on both topography and the vertical turbulence levels). Simulation ofmixing processes is much more difficult than
for tidal dynamics since a much wider spectrum of length scales is involved. Simulation of mixing is often de-coupled from simulation of tidal dynamics with
velocities and elevations from the latter stored and used both to specify appropriate
advective transports and to determine appropriate dispersion coefficients (Prandle,
1984). Altematively the tidal dynamics may be specified from observations.
Whatever approach is used it is generally convenient to define separately the various tidal constituents and the residual. Unfortunately the definition of a residual
current in a shallow region with pronounced spatial variability in tidal currents is
complicated.
Several (depth-averaged) definitions are possible, e.g. Eulerian, Lagrangian,
Stokes Drift (Longuet-Higgins 1969). Lane et al. (1997) discuss these definitions
and suggest a framework for relating estimates from models and observations.