PHYSICAL MATHEMATICS SEMINAR

TOPIC:		GENERATION OF SAND BARS AND SEDIMENT/WAVE INTERACTION

SPEAKER:	JIE YU
		Department of Civil and Environmental Engineering
		Massachusetts Institute of Technology

ABSTRACT:

A quantitative theory of the formation of sand bars is derived by
coupling sediment transport and wave hydrodynamics.  Sediment motion is
assumed to be dominated by bedload.  Assuming that the slopes of waves and
bars are comparably gentle, an approximate analysis is worked out. The
result shows that sand bars grow and evolve via a forced diffusion process
rather than arising from some instability mechanism.  The forcing is
directly caused by the non-uniformity (period of half a surface
wavelength) of the wave-induced bottom shear stresses due to the
reflection and the effective diffusivity is induced by the gravity and
modified by the local bed stress.  During the slow growth of sand bars,
incident and reflected waves exchange energy quasi-statically through
Bragg scattering.  A general result we have found for this problem is that
the location of the bar crest relative to the surface node is important to
how the local energy is transfered.  Moreover, the current theory shows
that Bragg reflection is not the primary cause of sand bars, but the main
product of the bars.  Shore reflection is both necessary and sufficient
for the formation of bars.  Quantitative comparison with the existing
experiment is made, which demonstrates a good agreement.

DATE:		TUESDAY, SEPTEMBER 14, 1999

TIME:		2:30 PM

LOCATION:	Building 2, Room 338

Refreshments will be served at 3:30 PM in Building 2, Room 349

					Massachusetts Institute of Technology
					Department of Mathematics
					Cambridge, MA  02139