Florida Bay Shallow Water Surface Drifter
David S. Bitterman, Ryan. H. Smith, W. Douglas Wilson
NOAA’s Atlantic Oceanographic and Meteorological Laboratory, Miami, FL
Nelson Melo
Cooperative Institute for Marine and Atmospheric Studies, U. of Miami, Miami, FL
Thomas N. Lee
U. of Miami, Rosenstiel School of Marine and Atmospheric Science, Miami, FL
In response to NOAA/COP funded research needs for high resolution Lagrangian analysis of Florida Bay interior basin flow fields, a shallow water drift buoy was designed and developed by AOML and RSMAS in Miami, Florida.
Primary design guidelines required a device capable of operating in water depths of one meter or less for up to a two week period. Small size was a factor both in minimizing windage at the surface and in aiding deployment and recovery efforts. The drifter would also require onboard GPS for accurate positioning. The need for a high resolution record of the drifter’s track required GPS data to be logged onboard for post-recovery upload. However, in attempts to aid users in the drifter’s recovery, a subset of these data would need to be transmitted to the users during deployment; this is handled by an onboard ARGOS transmitter.
Figure 1 - Shallow Water Surface Drifter
The hull is constructed of a 0.125 inch thick lexan, two-piece thermoformed shell in the general shape of a disc. The top is slightly domed and the bottom has molded fins in an attempt to couple the buoy to the water. Unfortunately, in the first Bay drift test the molded fins proved to be insufficient to keep windage to a minimum, so a "window shade" drogue has been incorporated with excellent results to improve coupling to the water (Figure 1).
The drogue is constructed of two crossed, flexible PVC sheets and is approximately 0.75 meters long. The shape of the buoy was analyzed to insure that it would be self-righting if turned over by a wave. When deployed, the hull floats with the top of the dome approximately 3 cm above the waterline. The dimensions of the hull are 33 cm in diameter by 15 cm high and the weight ready for deployment is less than 5 kg.
To derive high resolution currents, the position of the buoy must be determined frequently. A Motorola 12 channel GPS receiver is built into the electronics to collect this data. It computes 3 dimensional position information at pre-selected rates which are then stored in the buoy memory for later retrieval and subsequent data analysis. A relocation ARGOS satellite transmitter system is included in the electronics to recover the buoy at the end of each experiment. While the initial costs are relatively high, it is a proven system and provides unlimited geographical coverage. The ARGOS positions are used to get within the general vicinity of the buoy, and then a handheld ARGOS receiver on the boat is used to locate and retrieve the buoy.
Figure 2 – Florida Bay surface drifter sea test
The operating procedures are very simple and very little training of field personnel is required. Typically, the buoy operating parameters are programmed through a waterproof serial port, and then the buoy is shut down until ready for deployment. Magnets are used to turn it off and on after sealing the hull. The transmissions are checked with the handheld receiver and then it is deployed. After retrieval, the GPS data is uploaded into a computer through the serial port.
Two “sea tests” were done to check the coupling of the drifter with the water. Five drifters with different drogue legs, and co-located patches of colored dye, were deployed in Biscayne Bay in a 23 knot wind. Although all of the drifters moved in the same direction as the dye patch, the drifters with 75 cm drogues had the best coupling with the water (Figure 2). The average difference between drifter speeds and dye patch speed was less than 1 cm/sec.
To date, a total of 68 drifter deployments have been used on eight cruises in Whipray Basin and the NE basin of Florida Bay (where the mean depth is 1.5 m) with excellent results (Figure 3). In the Bay the drifters generally move in the direction of the wind and display circulation patterns influenced by the basin configuration and topography.
Figure 3 – Florida Bay inner basin surface drifter trajectories
Bitterman, David S., NOAA/AOML/PhOD, 4301 Rickenbacker Cswy, Miami, FL 33149, Phone: 305-361-4432, Fax 305-361-4412, , Question 1