All posts by mspydell

Last SIO drifter releases: May 17

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.


6th day of drifter releases

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.