These near-bottom flows are especially complex in coastal regions where both waves and currents are commonly present. As waves and currents pass over the shallower regions of the continental shelf, the associated fluid movement commonly extends down to the bottom. Due to the "no-slip" boundary condition at the bottom, a thin layer forms in which the velocity gradients are extremely high and viscous effects are important. This region is known as the bottom boundary layer.
The significant velocity gradients and shear in bottom boundary layers impose shear stresses on the bottom which will suspend sediments and lead to the development of bottom bedforms. Boundary layers resulting from wave motion tend to be quite thin due to the short time scales of the oscillatory flow, while boundary layers resulting from currents tend to be thicker due to slow variation of the current. Since steep velocity gradients and high shear stresses are often a result of the short time scales of wave boundary layers, waves are often considered to be primarily responsible for bedform development and sediment transport. When both waves and currents are present, a thin wave boundary layer typically exists within a larger current boundary layer,and additional interactions between the waves and currents must be considered.
The complex interactions between waves, currents and movable sediment
beds motivated my doctoral research at MIT. I began studying bottom
boundary layers in the coastal environment while
completing my M.S. and Ph.D. degrees at MIT with Dr. Ole S.Madsen. This
research has resulted in the publications summarized in the following
list. Another
publication has recently been accepted in JGR.
Mathisen, Paul P., and Madsen, Ole S.,1996, "Waves and Currents Over
a
Rippled Bed: 2. Apparent and Bottom Roughness Experienced by Current in
the Presence of Waves" Journal of Geophysical Research, Vol 101, No
C7, Pp. 16543-16550.
Mathisen, Paul P., and Madsen, Ole S.,1996, "Waves and Currents Over
a
Rippled Bed: 3. Bottom and Apparent Roughness Experienced by
Spectral Waves in the Presence and Absence of a Current"
Journal of Geophysical Research, Accepted for publication in the Journal
of Geophysical research.
Mathisen, Paul P, and Madsen, Ole S., "Bottom Roughness for Combined Wave and Current Boundary Layer Flows Over a Rippled Bed" Sea Grant Report No. 93-27 , 1993.
Madsen, O.S., P.P. Mathisen, and M.M. Rosengaus "Movable Bed Friction Factors for Spectral Waves" Proc. 22nd Int'l Conf. on Coastal Eng'r., ASCE, Vol.1, pp. 420 429 (1991)
Mathisen, P., and O.S. Madsen "Bottom Roughness for Wave and Current Boundary Layer Flows Over a Rippled Bed" Presented at the 1992 Fall Meeting of the American Geophysical Union, December 1993.
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