Funded by:

UH Research Council

UH Seagrant: Program Development

Office of Naval Research: Young Investigator Program (beginning, May 2003)

 

Surface waves begin to generate significant oscillatory motion near the seabed well before they break in the surf zone. The interaction of the wave flow with the bottom results in wave refraction as well as in the dissipation of wave energy. Understanding of the wave boundary layer mechanics is then fundamental to accurate modeling and prediction of nearshore currents that, in turn, affect coastal erosion, pollutant dispersal, and the health of coastal marine biology. The case of wave flow over a rough boundary has typically been treated by considering homogenous bottom roughness, resulting in parameterizations based on a characteristic roughness scale.  In natural settings, however, roughness is rarely homogenous. Coral reefs, for example, are characterized by widely varying scales of roughness. In addition, near boundary flows can significantly affect benthic biota. Residual flows can alter nutrient transport and the associated turbulent stresses may also affect morphology.

 

Coral reef off of Ala Moana on the south shore of Oahu.  Note the occurrence of various scales of roughness.  Individual coral heads in the image have a horizontal dimension of 20-50 cm.

The residual flows observed by Pawlak and MacCready (JGR, 2002) serve as mechanisms for enhanced mass and momentum transfer from the boundary and can thus significantly affect wave dissipation and refraction. This view of the wave boundary layer highlights a significant horizontal variability in wave dissipation, dependent on the local bottom structure.

Study area off Kakaako, on the South Shore of Oahu with preliminary field layout.  The region is approximately 750m by 500m and will be connected to shore via submerged cable. Depths in the study area range from 2 to 20 m.  Location of the Wave Boundary Layer Profiler is shown by the white circle with a black cross.  Pressure sensor locations are indicated by yellow circles. The white dotted line shows the nominal location of sediment transport survey line. 

 

We are developing a 2D wave boundary layer profiler that will obtain a phase-averaged view of the spatial structure in the wave boundary layer.  The spatial velocity field is obtained using a downward-looking ADCP moved along a track in the plane of the wave motion.  The flow field is reconstructed by phase from the various ADCP positions using along-beam velocities. Pilot field deployments of a profiler have been carried out in the summer of 2002 off the South Shore of Oahu.  A fully automated version of the profiler with a shore cable connection will be deployed in 2004.

 

 

Wave Boundary Layer Profiler platform during field deployment on the south shore of Oahu, August, 2002.

 

Further laboratory experiments are planned to explore interactions in the residual flows observed by Pawlak and MacCready, 2002 for more general roughness geometries and examine to the mass and momentum exchange induced by flow over inhomogeneities in the roughness scales. The experiments will characterize the turbulence and flow dissipation over the parameter space and to attempt to reconcile these characterization with existing observations and theory in the limit of homogeneous roughness.

 

 

For more information contact Geno Pawlak