The dye release on Sat 19 May was interesting. 30 gallons of dye were released for 5 hours starting at 0430 (AM!) in approximately 10 m water depth a few meters (3-4 m) above the bed between buoy #6 and #8 in the channel Dye went up the inlet towards the ICW on the flood. Dye that had reached the surface could also be observed to enter the marsh at the start of the S-turn. By the start of the ebb, the marsh was really pink with Rhodamine concentrations of approximately 10 ppb. As sampling occured on the ebb tide, concentrations of the order of 1-2 ppb were observed coming out of the inlet. This had lead some specularion that the marsh stored a signficant fraction of the dye. But that is not the case.
First the dye was released in the lower water column in 10 m depth. Only the top 0.5 m at most fo the water column could make it into the marsh. Most of the dye had to make it’s way up the inlet. Second, dye was input at 5 mL/sec at a concentration of 2e+8 ppb. 1 mL = 1 cm^3 = 1e-6 m^3. So the pumped flux of dye into the inlet is about Q=1000 ppb m^3/s.
To balance this coming out of the inlet, one needs a water velocity V, a depth h, and channel width L, and a concentration D – Assuming a rectangular channel and a uniform velocity and dye concentration. Then the flux is Q = L*h*V*D.
Taking representative values for V=1 m/s, h= 5 m, L=200 m this implies that to balance the input flux one needs….. drum roll…… D = 1 ppb. Which is the order of magnitude that we were measuring coming down the inlet during the duration of the flood.
It will be interesting to see what a more detailed dye mass/volume balance will show.
Today (Sunday May 13th) is instrument turnaround day and general catching up. We also have a report on the dye-bomb release we did on May 11th. In short, it was an amazing dye of observing the ocean. The RIVET community had many assets in the water measuring dye, currents, temperature, and salinity. Here we’ll focus on a few observations to give a quick overview of what happenned during the day.
In contrast to the previous releases where we continuously pumped dye for 1.5-2.5 hours, this time we poured 20 gallons of Rhodamine WT at ~0800 into the inlet 300 m down from the junction with the Inter-coastal waterway. Below you can see the dye getting poured in.
As the tide was ebbing rapidly the dye gets stretched and stirred rapidly. It is quickly well mixed (confirmed with CTD+F) casts and makes its way down the inlet. As it approaches the inlet mouth, the dye has spread all the way across the inlet and (in contrast to the point releases) the dye moves out both “old” and “new” channels.
The conditions on May 11 were very benign. Very small waves and extremely light winds during the entire release. The dye then gets rapidly ejected offshore out of the two channels and the SW near-beach channel.
By shortly after 0900 dye is clear of most of the inlet. CTD+F casts offshore of the main or “new” channel reveal a surface concentrated (< 2 m) dye field of about 20 ppb with no dye at all below. This is associated with a strong halocline. This is in part due to the weak mixing that is induced due to the light winds and lack of waves and whitecapping. See the figure below.
The fresher water of the inlet simply slides on top of the saltier (and slighly colder) oceanic water. Thus forming a “lens” within which the dye is contained. The weak winds likely contribute to the maintenance of this strong water-mass gradient. These type of vertical structure conditions will be a very significant challenge for models to reproduce as a) they may not have enough vertical coverage and more importantly b) the model-induced mixing is often far too strong to maintain such a gradient.
The tide then starts to flood at ~1000, and begins to pull water back from “old channel”
Notice the wispy dye to the SW of the inlet. This is all very much surface concentrated. Often boats and jetskis driving in it left a trail of dye-free water behind. This was confirmed by the CTD casts, which show an even thinner surface layer of dye with lower max dye. Dye stratification is again is influenced largely by a strong halocine, but here there is also a stronger thermocline than in the previous set of casts
As the tide started to seriously flood, the dye patch was broken into two parts. 1) an offshore component that spread itself out from the main channel entrance to the SW and 2) a patch that got sucked back up the inlet. This mostly occured from dye in the “old” channel. As the dye approached the “S”-turn of the inlet, it converged strongly on the ever-present scum line, just as the drifters had (see previous post). See the photo below.
This sort of “front” will also be a challenge for models to reproduce as the lateral (horizontal) eddy viscosities that they have may preclude the formation of such convergence zones in these rapidly changing regions.
The ‘Maple’ group from BIO, Dalhousie and UMaine arrived Tuesday. Our objective is to characterize suspended particles (size distribution and composition) and relate it to ocean color. We have already scouted the river, taken grab samples and water samples for grain size, and have completed a drift station with a LISST and Digital Floc Camera through
the mouth of the inlet. In the next few days we plan to occupy a 13h station near the river mouth (Friday) and perform a transect along the river (Saturday). With the arrival of Emmanuel Boss, in addition to the in situ particle size spectrum, our measurements now include spectral absorption, attenuation and backscattering of both dissolved and
particulate material and hyper-spectral upwelling radiance and irradiance. Our observations will help link the in-water optical properties to remote observations.
Tim Milligan, Brent Law, Gary Bugden (BIO/DAL) and Emmanuel Boss (UMaine)
The dye release yesterday (May 8th) was very interesting. The dye release was initiated shortly after slack tide as the water started ebbing. 30 gallons of Rhodamine WT were released in 2:24 min. As usual the mounted instruments, the jetskis, NPS/RSMAS crew, REMUS, towed ET array, and the Partenavia were all measuring the dye plume. At first, as the tide was still not very low, the dye poured down the main channel and then oozed out over the southern shoal. See the aerial hyperspectral image below (taken at 1339 local time, from Luc Lenain and Nick Statom). Up the inlet near where the dye was released, the surface dye is very patchy because the dye is released about 3 m above the bed in ~9 m water depth. At times it seems like the surface dye patch has a periodic signature. The dye moves offshore of the south-western shoal in jets and squirts.
The nice thing about this image below is that it can be used to quantify Rhodamine dye concentration. As light is measured across a spectrum of wavelengths, information at absorption and emmision wavelengths of dye will be used to calibrate dye concentration against in situ sensors.
Below are some SLR camera images of the plume at similar stages. These are useful for providing context but are nowhere near as quantitative.
Yesterday (May 6th) was the first day of dye releases. Although we had a couple of glitches things overall went very smooth. In the AM we deployed 15 fluorometers (measure dye concentration) at the SIO and WHOI current meter locations. Combined with the velocity measurements, this allows one to make flux measurements of dye out or into the inlet. The NPS/RSMAS crew set up their mooring and catamarans. The SIO-MPL guys got their REMUS into position. The dye pump was calibrated (see image below) for the exact length of tubing to be used. That is a 30 gallon drum of Rhodamine WT!
Dye release began at 1245 at a rate of 21.6 gallons per hour. As the tides are in a spring phase right now the tidal currents were very strong, and in < 10 minutes dye had gotten to the entrance of the main channel. Below is the view from HQ which looks out over the main channel (to the left) and the SW ebb shoal (to the right). It is pink!
After about 1.5 hours, the entire 30 gallon drum had been released and the dye pump shut off. The tidal inlet experiment, then quickly became an inner-shelf experiment. We had jetskis, REMUS, boats with CTD and towed arrays, and an airplane sampling the dye plume as it advected towards Surf City. Below is an aerial hyperspectral image from Luc Lenain of the dye patch (note this is not a final product just a quick and dirty first cut).
One issue that the airborne observations had was the shadows due to the clouds. But the dye patch can clearly be seen in the image. Also note the boat wake.
The NPS boast and our Sally-Ann were also doing CTD casts. Below are some images from the CTD casts in the dye patch. The first is earlier on and measured 7ppb near the surface and a strong dye, T, and salinity -cline at 4-5 m depth. Near the bottom there is no dye.
Sometimes however, dye was maximum in mid water column. See the image below.
The MPL REMUS was also hard at work out there. Below is an 3D(lat, lon, and Z) of offshore plume dye concentration.
Lots of good stuff on day 1. There is a ton of more data collected by the NPS/RSMAS guys and our ski and all the moored and tripod instruments. Day 2 dye release begins at 1230
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.
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.
we are driving up today, predictions will be more timely in the days ahead once we get settled.
cheers! -Patrick & Ad
Below is an image on flooding tide from the model simulation showing current vectors on top of the bathymetry from Apr 16/17. Although the comparison is qualitative, note the difference in how far offshore the ebb tide jets relative to the NEARCOM simulation.