Abstract:
A series of laboratory experiments were conducted in a glass wave
tank to investigate the propagation of internal gravity waves up a sloping
bottom in a fluid with constant Brunt-Vaisala frequency. Measurements of
the wave motion in the fluid interior were primarily taken with electrical
conductivity probes; measurements in the boundary layer were made with
dye streaks and neutrally buoyant particles. The results indicate that,
outside of the breaking zone, the amplitude and horizontal wave number of
the high-frequency waves increase lineariy with decreasing depth; this is
shown to agree with existing linear, inviscid solutions. A zone of breaking
or runup is induced by these high-frequency waves well upslope. Shadowgraph
observations show that, if the wave characteristics are coincident,
or nearly so, with the bottom slope, the upslope propagation of the low-frequency
waves causes a line of regularly spaced vortices to form along
the slope. Subsequent mixing in the vortex cells creates thin horizontal
laminae that are more homogeneous than the adjacent layers. These laminae
slowly penetrate the fluid interior, creating a step-like vertical density structure.
Available linear theoretical solutions for the velocity in the viscous
boundary layer, determined to be valid for certain experimental conditions,
are used to develop a criterion for incipient motion of bottom sediment induced
by shoaling internal waves. The maximum sediment sizes that can be
placed into motion, according to this criterion, are larger than certain
mean sediment sizes on the continental margin off New England. This suggests
that internal waves might induce initial sediment movement. Speculation
about the geological effects of breaking and vortex instabilities is also
given. These processes, not definitely measured in the field as yet, might
also be conducive to sediment movement.
