Spatial Distribution of Wave Breaking and Turbulence


Investigators

Tom C. Lippmann, Edward B. Thornton, Tim P. Stanton, Ming-Yang Su

Funding Source

Office of Naval Research, Coastal Dynamics Program

Naval Research Laboratory

Objectives

Remotely estimate (with video techniques) the spatial distribution of turbulent kinetic energy (TKE) generated at the surface boundary layer associated with wave breaking. Integral to this work is a robust test of the relationship of measured video image intensity time series to TKE production and dissipation. Ground truthing will be done as part of the DUCK94 experiment in conjunction with the sled measurements of Thornton and Stanton inlcuding measurements of bubble production. The surface expression of sea surface luminance, measured with video, will be compared with TKE and dissipation estimates, bubble concentrations, and bubble cloud depths-of-penetration. The decay of bubbles, and their relationship to both turbulence and the surface video signal will be defined. It is hoped that the surface video can be calibrated to give at least an order one approximation to the local turbulence and dissipation of waves.

Remotely estimate the directional distribution of wind waves locally within the surf zone. This work involves the development of stereo video techniques to estimate sea surface elevation in tightly spaced arrays. Stereo analysis satisfies several other needs, including accurately sampling image intensity of the sea surface at locations directly above an in situ gage, and measurement of characteristics of the breaking wave waveform. This objective is approached utilizing two frame-synchronized cameras imaging the same ocean area from two separate vantage points (stereo video). Initial stereo analysis and ground truth (provided by Thornton and Stanton) will be done as part of DUCK94 in preparation for SandyDuck.

Preliminary evaluation of the influence of wave breaking on the vertical distribution in sediment concentration. Video derived estimates of TKE production at the surface will be compared with estimates of suspended sediment concentration within the water column (from sled measurements of Thornton and Stanton).

Approach

Four remote-controlled video cameras will be deployed for DUCK94, two on the FRF tower platform and two on radio mast antennae erected at two locations on the dune crest (at approximately 775m and 875m alongshore in the FRF coordinate system). Two camera views will be generally fixed and overlooking the main instrument arrays. The remaining two camera views will be variable and focussed on the sled (Thornton and Stanton) location. In conjunction with other investigators, a fifth camera may also be located on the tower and oriented to view wave breaking from about 4-8m depths. Analog video will be transmitted over fiber optic cable to a central processing area where the signals will be multiplexed into one video signal, digitized at known ground locations, and recorded for later stereo analysis.

Principal collaboration will be in conjunction with the sled measurements of Thornton and Stanton. Video logistics and analysis will be coordinated with Holman to avoid duplication of effort. We have been approached by several other investigators about collaboration involving video data during the DUCK94 and subsequent SandyDuck experiments.


.

Results

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