Vertical Structure of Mean Currents and Turbulent Stresses in the Nearshore Boundary Layer


Investigators

John Haines, Guy Gelfenbaum

Funding Source

U.S. Geological Survey, Center for Coastal Geology

Objectives

Our primary interest is in making detailed measurements of the vertical structure of the mean flow and the turbulent stresses associated with the mean flow and surface gravity waves. To accurately model sediment transport in the nearshore environment requires knowledge of the mean-flow profile and the turbulent stresses on the bed. Offshore of wave breaking the boundary shear stress has a mean flow component and a wave-induced component. Under the influence of wave breaking an additional component of the boundary shear stress may result from the advection of turbulence down to the bed from the surface. One of our primary objectives is to determine under what conditions the component of bottom stress due to the breaking waves is important to the total stress. Models of the interaction of the three components of the total boundary shear stress can be tested with the observations made at the field site.

We are also interested in testing models of cross-shore mean flow profiles. This includes attempting to understand the influence of wind-driven coastal set-up as well as the set-up due to breaking waves and the transition between the two. Another objective of the field work in this experiment will be to test instrumentation that is new to the nearshore environment.

Approach

We will deploy three vertical arrays of velocity sensors along a cross-shore transect in the region inside and just outside of the normal surf zone. The primary array will have at least six instruments, each capable of simultaneously and rapidly measuring all three components of velocity in a single sampling volume. These measurements will be used to calculate vertical profiles of mean flow and turbulent stresses through the main part of the water column. This main stack will be deployed in conjunction with pressure sensors returning additional wave data. Also at this main location, we would like to deploy a rotating side-scan sonar to identify small-to-medium scale bed roughness. In addition, we may attempt to characterize the breaking wave field in the vicinity of this main stack using video remote sensing techniques in collaboration with Rob Holman. The two additional arrays will be instrumented with Marsh-McBirney flow meters. The inner-most stack will be located as far inshore as the outer flank of the primary bar. This spread should cover a variety of wave breaking conditions and allow examination of forcing and mean currents in the outer surf zone.



Results

For a list of addresses of the investigators involved in Duck 94 click here.