REMUS Observations of rhodamine dye as it advects out of the inlet on May 8, 2012 (CORDC)
First ever blog post for me. The ‘other’ Scripps group has been running the REMUS AUV to characterize inlet dynamics and map the outgoing plume. Before the dye surveys, we ran a series of missions to survey the naturally occuring optical signals leaving the inlet over an outgoing tide. Above are cross-sections of CDOM across 4 different transects. We’ve also run some offshore to onshore transects to characterize how far offshore the plume is advecting and characterize the vertical structure. Should be interesting to compare with the remote sensing imagery. -Eric
After arriving on Monday, our NRL SAR crew flew on Tuesday, Wednesday, and Friday of this week. Apart from some drama before the first flight (resolved with a 300-foot daisy chain of extension cords across the airport tarmac), we’ve had no major hardware issues and have enjoyed excellent SAR collection conditions: decent surface winds to roughen the water, but smooth flying at 2900′. Smooth air is doubly appreciated, since it makes compensating for aircraft motion easier while it keeps me (Sletten) from throwing up.
Our sensor is an X-band SAR that operates in an interleaved along-track/cross-track interferometric mode (ATI/XTI). The ATI imagery provides measurements of the line-of-sight component of the surface velocity (closely related to the line-of-sight component of the surface current), while the XTI imagery is a measure of the surface elevation, i.e., the wave height.
Below is a magnitude image (black and white) of the inlet along with its corresponding, quick-look ATI phase image (color). These data were collected on Wednesday morning at 11:48 AM EDT, approximately one hour after the predicted low (also max ebb) tide. The south side of the inlet can be seen clearly on the left side at the top, while the north side is visible to the right but is fainter. We were flying over the Atlantic looking towards the land during this particular run. The shoaling waves are obvious in the magnitude image, but both the breakers and the strong ebb flow are visible in the velocity (i.e. phase) image. We haven’t done any motion compensation yet, so the land is not uniformly motionless, but the phase calibration is still pretty good thanks to the smooth air. The flow through the inlet appears to be split into a north and a south channel, with a velocity close to 1.5 m/s towards the radar, which is negative using our convention. Note that this is the ATI-SAR velocity, not the current. (The pesky velocity contribution of the waves themselves has not yet been estimated and removed.) The crest velocity of the breakers is high enough to cause the phase to wrap, which is the source of the rapid color changes across these regions.
The XTI quick-look interferograms are still to come. The signal from one of our XTI antennas is a bit anemic, and we hope to track down the reason this weekend. (Well, Steve is hoping to track that down. I’m actually back home in Washington DC.)
Attached is a higher resolution local merged timex from today’s low tide showing detailed bedforms on the western side of the inlet. I’m amazed by the amount of structure. Also, we have posted a first draft web description of Argus at RIVET at cil-www.oce.orst.edu/rivet. It includes description of our geometry set up procedure and the location of a decent 3D benchmark by the OSU house that Tom Lippmann helped us set up.
The image below is an overlay of a SAR intensity image recorded by the APL-UW dual-beam along-track interferometric microASAR system (the microASAR units are developed by Artemis, Inc., http://artemisinc.net). This data was recorded on May 2 at 11:55 EDT during the low tide. A Google Earth version of this image (KMZ file) can be downloaded from http://herschel.apl.washington.edu/darla/RADAR/SAR/SNAPSHOTS/uASAR_05022012_115500CH1_image532376_-9.kmz.
The width of the image is around 3 km. Breaking waves around the inlet are not focused correctly by the SAR processing code because their Doppler shift is non-zero. However, the regions in which active breaking is clear. Patches of increased surface roughness, possibly due to lines of convergence, and also slicks are evident in the inlet and river.
We will be using this data to form interferograms from which we will estimate surface velocity.
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.
Yesterday (May 2nd) we deployed two quadpods in the old and new channel (near the outer most red can channel marker #4) at the locations in the linked kml
These are 7′ tall frames that have the upper ~2 exposed at low tide and are submerged by ~2′ at high tide. They have various downward aimed sensors to measure flow, turbulence, sediment transport and bed morphology.
They are marked with red 6′ tall poles, and additional poles with yellow flashing lights will be added tomorrow or the next day.
We will be conducting REMUS and shipboard ADCP surveys for the next several days.
Many Thanks to Steve Elgar and Danik Forsman for the help with the deployment
This is an example radar image showing crossing wave trains on the shoal that extends from the south side of the inlet. We are analyzing these image sequences to see how well (or not) wave directions can be extracted and bathymetry estimated. Hope to have more soon.
I’m not sure if this is well known yet, but Argus merged images are available on the web at http://cil-www.oce.orst.edu/RIVETmerge. They are posted as they are collected and consist of snap, timex, var, bright and dark images for every 1/2 hour. The upper half of the page is rectifications for the large inlet area at 2.5 m resolution. If you continue down you will see equivalent images for the west side area at 1.0 m resolution. Click on thumbnails to see full res images. Further explanations will be posted soon (after I teach), but a nice example snap from 1100 today is attached below (with luck) – check the small scale features in very shallow water around x=-150, y = -450. Tres cool (and it looks like people are surveying based on dark image tracks).
Here are some photographs of the New River Inlet plume that were taken during the morning low tide on Wednesday, May 2 (low tide was 10:49 EDT) form the APL-UW Cessna 172. These photographs were taken between 10:30 and 11:00 EDT. Multiple lobes of the plume may indicate flow through different channels in the inlet. A secondary plume characterized by a darker color (e.g., forth photograph) was visible during this time, possibly indicating a source of sediment from further upstream.