Today was day 2 of drifter releases. Everything went very well with the exception of possibly dinging the WHOI location 55 with our Whaler. We did three releases. The first was during a strong ebb tide. The 2nd was an attempt to capture the transition from ebb to flood. Two drifter track images are shown below. The first image is approximately 15 minutes after the drifters were released. They were released in clumps of 5 at different locations in the navigation channel. This was as the ebb tide was becoming weaker, but still offshore propagation is quite clear.
About an hour later as the tide had switched from ebb to flood, the drifters had turned around in a loop and headed back up the inlet (see image above) The very interesting thing is that all the drifters began coming together onto a single streamline as they converged at the inlet bend near Can #10.
The next release was on a strong flood tide. The 34 drifters were released offshore of the shoal to the south-west. The drifters rapidly crossed the shoal and spread out on the inside of the inlet. Again, the drifters started organizing themselves by where they were released offshore (see image below).
Later on nearly all the drifters have converged on this one particular line. From the boat it was very neat. Like a train of drifters coming at your. You could see them relatively spread out and then they would converge onto the line. Below is a photo the Dennis Darnell took from the SIO Whaler.
Two more days of drifters, then a day for turning around instruments, and then dye.
Today was the first day of the SIO (Feddersen/Guza) drifter releases. We did 3 set of 2hr+ drifter releases with our 35 drifter fleet. The first release began at about 0830 as the tide began to ebb. We released the 35 drifters near channel marker 08 but in slightly different cross-channel positions – see the screenshot image below from our drifter tracking software.
All the drifters flowed out of the inlet into the ocean. The interesting thing is that the path the drifters took – whether the “old” or “new” (dredged) channel – depended strongly on where in the cross-shore the drifters were released at. The drifters released closer to the Onslow Beach side (ie, redish tracks ) went out the old channel, those on the topsail beach side (blue-ish tracks) went out the new channel. The two channels are separated by a shoal that can be near dry sand at low tide. Basically, this is saying that the initial condition matters.
The last drifter release occured offshore of the ebb shoals in about 4 m depth as the tide was flooding. At first the drifters moved along contant bathymetry lines and then began to cross the shoal (see photo below).
Eventually the drifters released the furthest down-shoal (most to the SW at y=-800 m), got pulled into the deep part of the inlet and rapidly advected up inlet. The drifters deployed farther to the mid-point of the shoal meandered around much longer going onshore but also in the +y direction (see photo below).
Eventually all but 3 drifters made it up the inlet and were recovered up-inlet of where the FRF bathymetry survey ended. But the paths were again quite different – as some went along the navigation channel and some in the remnant old channel near the Onslow beach side. Another interesting point is that unlike surfzone drifter dispersion, the 2-particle dispersion appears to be relatively weak. We’ll be exploring that in greater detail.
Airborne operations are underway at RIVET. Below are some photographs of the inlet from the APL-UW test flight on Monday (17:00 – 18:00 local time). Onboard the Cessna 172, we have a dual-beam along-track interferometric synthetic aperture radar, two infrared cameras, and a visible wavelength camera. We will be posting imagery from these sensors as soon as it is available.
In preparation for our quadpod deployement I conducted a quick sidescan survey of the old channel out my pod1 site and the new channel out to my pod2 site. Here is one spot in the newly dredged channel near WHOI (Elgar&Raubenhiemer’s) site 07.
KMZ’s with the complete data set can be downloaded here: