Observations of Instabilities Along an Intruding Flood Front via sUAS on the James River

Recent field work on the James River, under the auspices of the ONR-USRS project, identified propagating disturbances along the intrusion front that forms on the flood tide. The importance of these disturbances to the overall mixing budget is as yet unquantified. Here we will present initial analysis of these features using optical image sequences collected via sUAS (hexacopter drone). Additionally, in situ data collected from SWIFT drifters is available for comparison.

The included figure shows the flood intrusion front nosing upriver as evident from the white foamy arrowhead feature in the center of the image. Lighter colored river water is on the left and bottom of the image. Darker saltier water is upwards and to the right in the image. The northern terminus of the Monitor-Merrimack Bridge Tunnel is at the top of the image. Along-front propagating disturbances are in the ellipse along the southern flank of the intrusion front. Using image processing tools we will characterize the length scales and propagation speeds of the instabilities. Included in the ~180 minutes of optical video of the intrusion front are approximately 30-40 minutes of video with co-located SWIFT drifters. The SWIFT drifters collected all three velocity components along vertical profiles, wind wave spectra, and turbulent dissipation rates. At times, the SWIFTs were caught in the front and propagated with a vortical motion along with the instabilities, as evident from the videos.