David Clark
Assistant Scientist
Applied Ocean Science & Engineering

Ring Of Doom Super EXperiment
The powerful surfzone eddy field is generated breaking waves and transports sediment, pollution, and other suspended material between the shoreline and the inner continental shelf. The eddy field is likely generated by a combination of short-crested breaking waves, sheared currents, and wave groups, but it is unknown how the strength of the eddy field depends on these mechanisms. Understanding how the incident wave field generates eddies will help beach managers predict pollutant and sediment dispersal along the coast. The 5-m diameter "Ring of Doom" vorticity sensor (Fig. 1) was deployed for 4-weeks at Duck, NC, to investigate surfzone vorticity dynamics and vorticity generation by short-crested breaking waves. In addition, 28 collocated pressure and velocity sensors were deployed to measure wave groups and sheared currents.


Fig. 1.  The Ring of Doom vorticity vensor is deployed in the surfzone by helicopter while swimmers wait offshore to secure the sensor to the seabed.




Aerial measurements of fluorescent dye
Dye tracers are easy to put in the water but difficult to measure on spatial and temporal scales appropriate for investigating the evolution of coherent structures in the nearshore. An aircraft mounted hyperspectral imager (Fig. 2) was used to map dye injected into the ebb tidal flow of an ocean inlet (Fig. 3). This method provides much higher spatial and temporal resolution than traditional boat based transects, and the newly developed 3-band algorithm accounts for the variable background turbidity often found in the coastal zone. A 5-km long dye patch is imaged in roughly 90 seconds and the measurement can be repeated every 5 minutes, so that the evolution of structures within the dye patch is resolved. These images are being used to measure the exchange of fluid between the surfzone and the inner shelf.


Fig. 2. Partenavia Observer survey aircraft with belly mounted (a) multispectral, and (b) hyperspectral imagers. A GPS / IMU system is used to orient the camera and results in pixel spatial accuracy of < 2m without ground control points.



Fig. 3. Dye concentration in color, with light gray areas outside the image swath, dark gray areas indicating land, and black regions indicating foam from breaking waves.



The VORTEX experiment made the first field measurements of vorticity about a vertical axis (i.e., a horizontal eddy) generated by individual short-crested breaking waves (Fig. 4), and confirmed Peregrine’s theoretical work from 1998. Vorticity was measured with a 10 m diameter velocity array (Fig. 5) and combined with video data to measure the change in vorticity associated with a breaking event. This pilot experiment found that individual waves generated large amounts of vorticity, and the maximum vorticity was generated near the end of a breaking crest. Energy from 10 seconds period waves was transferred to vortical motions with the longer timescales associated with mixing in the surfzone.

    Fig. 4. Schematic of vorticity generated by a short-crested breaking wave. Positive vorticity is generated near the left-handed end of the wave (using surfing terminology), while negative vorticity is generated near the right-handed end.


  Fig. 5. The field crew gets ready to deploy a 10 m diameter guide for installing velocity sensors.