The above movie was made by Matthew Spydell. It shows May 3rd ebb tide drifter trajectories from the RIVET experiment at New River Inlet. Bathymetry is shaded. Land regions are in gray. The bar on top indicates elapsed time (in hours) since the start of the release. For more information on this release look at this blog entry. This movie contains a lot more drifter information though. In the original blog entry the drifters got out of radio contact so we couldn’t track them in real time. But they log their GPS positions onboard the drifter memory. What was interesting (as noted in the original post) is that there was no indication that the drifters going back onshore during the flood tide.
Sorry for the late post, but… Thursday May 17 was the last day of SIO drifter releases. Again weather (rain and potential T-storms) cut the releases short and we only did two releases. Both releases were during ebb tide and were generally similar (thus only the first is examined in detail here) but novel compared to previous releases during ebb tide. We’ll get to the novelty shortly… For both releases all 35 drifters were released upstream of the inlets “S” turn, however, downstream of previous ebb releases which were released near the ICW. We hoped that by spreading them out at this location (where the inlet is wider) we might avoid the mass convergence that we had seen in the past. The mass convergence still occurred!
Fig.1 Tracks of drifters during ebb tide. Notice the convergence through the “S”. During this release, the exit of the drifters was different than previous ebb releases in which drifters were released upstream of the “S”. Notice that some drifters (blue) are exiting the old channel while others (red) are exiting the new channel. Previously, converged drifters exited either the “old” or “new” channel.
Fig 2. Blue drifters quickly exiting the “old” channel and red drifters sampling the inlet width: the last two (21,26) are completely in the “new” channel and the others are spread over the shoal between new and old channels.
Fig. 3. The exit location of the drifter is spread out entirely over the inlet mouth. Many drifters exit through the “old” channel heading north, many over the shoal between “new” and “old”, and 2 exit the “new” channel and head south. This spreading of the exit had not been seen in previous releases. What dictates (tide phase, tide magnitude, wind, waves?) the final drifter location is not known.
May 15 was the 6th day of drifter releases. We did 2 releases in the morning during the ebb tide, 1 during ebb to slack tide, and 1 afternoon release during flood tide. The releases today were shorter in length so that we could collect all 35 drifters more quickly if thunderstorms arrived, which thankfully they didn’t. Below are images of the flood tide release.
Fig. 1: 12 drifters were released in the old channel (blue lines), and 22 were released in the new channel (green, yellow, and red lines). Regardless of the initial release locations, all drifters converge in the inlet. (The coast line (thick white lines) used is only an approximation and clearly inaccurate!)
Fig. 2: The drifters leading the pack are now hugging the south-west bank and have made the first part of the “S” turn. A little further upstream of drifter 32, some drifters were strangely sucked underwater for about 20 s.
Fig. 3: All but a few drifters hugged the south-west bank and the leading drifters have made the second portion of the S turn. Drifters suggest surface convergence as the drifters do not sample the width of the inlet. We allowed the drifters to continue up the inlet until they reached the ICW where they were collected.
Yesterday (May 3rd) was the 3rd day of SIO drifter releases. It was quite interesting. We released again on an ebb tide near channel marker 8. This time we released drifters in pairs (except for the last 3) separated by 1 minute. With 33 drifters it took 17 minutes. The reason for time separation was to see how that might affect dispersion, as we had noticed that drifters release in clumps often stayed together for long periods of time – or that the relative (2-particle) dispersion was weak. This indicated that the decorrelation legnth-scales are long and so we wanted to get some separation. We also released the first 14 on the +y (Onslow) side of the buoy 8 channel marker, and the 2nd group (13 drifters) on the -y (Topsail) side of the channel marker. The idea is to continue to dial in initial condition dependence. The conditions on May 3rd were small waves, no wind, and a glassy ocean.
After the drifters were released, the quickly moved out the inlet, with the first group divided between the “old” and “new” channels, and the 2nd group going out the “new” channel. See the two images below.
The drifters then really started to jet out the main channel and below past the 6 and 8 m depth contour (where our WW1 is located) and also spread out in the alongshore direction.
Eventually all the drifters continued to go offshore and got out of the range of our HQ antennae. The eventually stopped about 2.5 km offshore and sat in a long line along a front between browner and clear water – just like the aerial photo Gordon recently posted. We decided to keep the drifters out there to see if they would come back to the inlet as the tide turned. We had sandwiches and drinks delivered to the guys & gals on the 2 boats and jetski. The wind came up a little in the side-onshore direction and either to wind or tides or shelf processes, the drifters slowly started to march in the +y direction (to the NE). After a few hours the drifters had moed ~ 1.5-2 km in the +y direction (and at x ~ 2.5 km). As the water was strongly flooding in the inlet it was pretty clear these drifters were not coming back so we began collecting them. Guza’s over/under on # of drifters lost was 2. Miraculously, as the drifters were spread out over 3-4 km without any radio contact, we managed to collect ALL of them. It was amazing work by the drifter support crew. Thank you guys. The funny thing is that in the end the drifters stayed organized according to their position when they exited the channel; drifter 15 was at the northern end and 20 was at the southern end. Quite remarkable.
More drifters out on Friday.
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.
The schedule for RIVET dye and drifter releases have been tentatively schedule for:
- May 1, 2, 3, 4: Drifter Release Experiments
- May 5 : Turnaround Instrument Day
- May 6,7,8 : Dye Release Experiments with Airborne Dye Observations
- May 9th: Turnaround
- May 10,11, 12: Dye Release Experiments with Airborne Dye Observations
- May 13th : Turnaround
- May 14, 15, 16, 17: Drifter Release Experiments
- May 18th: Turnaround
- May 19, 20, 21: Dye Release Experiments: no airborne dye observations
Some details on the drifter and dye releases:
- SIO (Feddersen/Guza) have 30 surfzone capable GPS tracked (and transmitting to shore) drifters which will be released on drifter days. The plan is to release on ebb tide and track the drifters for as long as possible – hopefully into the flood tide. Please see earlier post on numerical simulations of drifters at New River Inlet.
- On the dye release days, we will be releasing 20-30 gallons/day of 23% Rhodamine WT dye. This will be tracked at fixed locations (WireWalkers, ADV, and ADCP locations) and via mobile platforms at the surface by jetskis (and plane), and subsurface by Boat and REMUS (Terrill).
In order to get an idea of the offshore extent of the tidal jet, exchange, and overall dispersion, Matt Spydell has taken the NEARCOM (tide+waves) model output provided by Tom Hsu, Fengyan Shi, and Jia-Lin Chen @ UDEL and released numerical drifters inside the inlet at the start of the ebb tide. These drifters are advected by the currents and also randomly dispaced in a random walk (for details see Spydell et al. JGR 2012 in press) with a diffusivity of 0.1 m^2/s. Arguably this is small for the inlet but ….
The top panel shows the tidal elevation. time=0 is 1.5 hr after high tide. The panels below show the numerical drifters as red dots. At t=1min after release, they are tightly clustered near the release location. The drifters then get ejected out the inlet after 1 hour. They spread and eventually some start to get pulled back to the inlet on a flood tide (t =480 min, 8 hrs after release).
It is interesting to compare this to releasing drifters right at high tide (1.5 hours before the release above). Now, the drifter rush all the way up the inlet before turning around on ebb tide and being ejected roughly 6 hours after release.
Note, in this simulations after 8 hrs, some drifters are more than 3 km from shore!