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Collecting Ocean-Circulation and Sediment-Transport Data Offshore of Fire Island, New York
Researchers from the U.S. Geological Survey (USGS) Woods Hole Coastal and Marine Science Center (WHCMSC) in Woods Hole, Massachusetts, are collaborating with scientists from Coastal Carolina University and the University of South Carolina to investigate ocean circulation and sediment-transport processes offshore of Fire Island, New York. Although the investigation encompasses the linear extent of Fire Island (approximately 50 kilometers, or 30 miles), one main study area is near the western end of the island where offshore sand ridges extend across the inner continental shelf and connect to the nearshore bar system. These ridges are on the order of 5 to 10 kilometers long and are spaced several kilometers apart, oriented obliquely to the coastline (see map). The USGS mapped these features at high resolution in 2011 to provide detailed bathymetry, subsurface structure, and surface texture. It is hypothesized that the ridge field modifies surface waves from storms, causing a complex wave pattern that reaches the coastline and influences sediment-transport and coastal-erosion patterns.
This study is part of a larger effort in cooperation with the U.S. Army Corps of Engineers (USACE) and the National Park Service (NPS) to study coastal processes on Fire Island. The offshore efforts described here were coordinated with nearshore beach surveys to assess coastal change during winter storms (see NPS news release).
To investigate the ocean circulation and wave-refraction patterns across the offshore sand ridges, several instruments were positioned on the seafloor in lines along the ridge crests and in lines that crossed the crests and troughs (see map). Site 1 at the top of a ridge and site 2 at the bottom of the adjacent trough were each populated with two tripods holding similar instruments. At each of the rest of the sites, we placed a single tripod holding fewer instruments. Surface buoys were deployed at sites 1 through 6 to mark the tripod locations and to serve as platforms for surface measurements.
Measurements and Sensors
The primary objective was to measure current speed and direction, temperature, salinity, surface waves, bottom stress (the force created at the seabed by currents and waves; see related Sound Waves article "How Often Do Sediments on the Seafloor Move?"), and suspended sediment along and across the ridges. This deployment was similar to previous efforts, such as a 2009 deployment off Cape Hatteras, North Carolina. The instruments placed at sites 1 and 2 were designed to provide high-resolution measurements near the seafloor to record events in which seafloor sediment may be resuspended by strong currents associated with storms. Because most of these measurements can be made only in unobstructed flow, 3.5-meter-high flow tripods (flobees) were deployed at sites 1 and 2 to provide measurements at various heights off the seabed and at a high sampling frequency within 3 meters of the seabed. Each flobee included a conductivity/temperature/depth (CTD) sensor, an acoustic Doppler velocimeter to measure current speed and direction at a single point with high accuracy and sampling rate, and a current profiler to measure current velocities over a range of depths. Sediment resuspension was measured by both optical (transmission and backscatter) and acoustic techniques. Pressure was also measured.
Measurements that would interfere with sensors on the flobee were made from a smaller tripod (minipod) deployed alongside the flobee. The minipod measurements included sonar images (overhead views) and sonar profiles (side views) of the seabed surface, as well as current-velocity profiles of current speed and direction over the entire water column. The minipod at site 1 also included a still camera for taking photographs at regular intervals (“time series”) to document seabed changes and a laser backscatter system to measure sediment size.
Surface buoys were deployed at sites 1 through 6 primarily to mark the position of the bottom equipment. Surface salinity was measured at each buoy by Seabird MicroCAT recorders. At site 2, the buoy was fitted with a weather station that measured barometric pressure, wind speed and direction, humidity, air temperature, and solar irradiance.
This deployment was a rare opportunity for the WHCMSC to perform long-term testing of some new methods and equipment. For example, the weather-buoy system is being evaluated as a tentative step toward further development of telemetered (wirelessly transmitted) data. During the 2009 work off Cape Hatteras, we learned that weather measurements from other sources were inadequate and that we needed to measure local atmospheric data using simple, inexpensive, portable systems that could be deployed at sea and on land at critical locations. We are investigating the use of telemetry so that if these measurements are made during a large storm, such as a hurricane, that might put the buoy adrift, the buoy can still be recovered, and, more importantly, much of the data will already be safely in hand. The system deployed off Fire Island was built to WHCMSC specifications by Down East Instrumentation. It was modeled on a Carolinas Regional Coastal Ocean Observing System design that proved itself in the early 2000s as part of the larger east-coast monitoring network.
We are also testing a new technique for inhibiting biofouling (the accumulation of organisms, such as algae or barnacles, on submerged surfaces), which has long been a challenge when making long-term coastal and continental-shelf measurements. As new studies by the WHCMSC bring operations into ever-shallower waters, the need for better anti-fouling techniques becomes more urgent. Older methods that used biocides are a hazard to the environment and to the people who handle the gear. During the 2009 Cape Hatteras deployment and during fieldwork by the USGS Pacific Coastal and Marine Science Center (PCMSC) in Santa Cruz, California, a customizable Zebra-Tech wiper system was successfully tested on the optical backscatter sensor. More wipers were acquired for the Fire Island deployment. Zebra-Tech’s system consists of a brush attached to an actuator that is plugged into a battery pack with a timer. The timer can be set to pass the brush across the face of the sensor at a predetermined interval of minutes to hours. For transmissometers, the WHCMSC had previously developed a method of preventing fouling with biocide-impregnated rings. As an alternative, Ray Davis (WHCMSC) developed a prototype wiper for WET Labs C-Star transmissometers that could be fitted to a Zebra-Tech actuator. The wiper was successfully tested in Buzzards Bay, Massachusetts, as well as off the U.S. west coast (by the PCMSC). Emile Bergeron (WHCMSC) improved the design and made more units for this deployment.
Additional new work includes the testing of new equipment for possible replacement of existing gear. Some of the standard instruments on which the WHCMSC has relied over the past decade are reaching the end of their service life or have been discontinued by their manufacturers, including many of the current-velocity profilers, the acoustic-backscatter profiler, the optical-backscatter sensors, acoustic releases, and the system that logs sonar images and profiles. A newly designed acoustic Doppler profiler Sentinel V model from Teledyne RD Instruments (TRDI) was recently acquired and was deployed alongside the old TRDI Workhorse profiler; it will also be compared to two Nortek AWAC systems deployed at other sites.
Sonar imaging is becoming a standard measurement for the WHCMSC, and the recent deployment will allow comparison of sonar bottom images and profiles from site 1 (at the top of a ridge) and site 2 (in an adjacent trough). Imaging sonars generate a picture of the seabed showing bed morphology (shape) over time and the direction of ripple movement. More accurate information about the height and wave-length of ripples is measured by profiling sonars, which provide a side view, or profile, of the seabed surface. To measure ripple height and wave length over a greater spatial area, the profiling sonar head must be rotated in a horizontal plane using an azimuth drive. The sonar logging system deployed at site 1 has these abilities and thus provides a unique system. Made for the USGS in 2000 and never commercialized, this system is at the end of its service life. To duplicate the measurements at site 2 and test a replacement for the older system, the WHCMSC borrowed an imaging sonar system and acquired an ASL Environmental Sciences IRIS sonar logging system and Imagenex profiling sonar. At the center’s request, ASL added azimuth drive capability to the IRIS to rotate the profiling sonar.
Preparation and Deployment
From mid-November 2011 through mid-January 2012, numerous WHCMSC personnel spent time at the Marine Operations Facility to build the nine tripods and prepare the parts for the six buoy moorings. We would like to thank Brandy Armstrong for keeping track of all the instrumentation, how it was programmed and where it went, frame assembly, and instrument mounting; Sandy Baldwin for instrument assembly, mounting, and photography; Robert Barton for help with anchor assembly; Emile Bergeron for case mounts, wiper brushes, and various other parts and mounts that had to be handmade at the last minute; Phillip Bernard for help with tripod transportation and ship contracting; Dann Blackwood for instrument mounting, camera and SEABOSS preparation, and tripod transportation; Jonathan Borden for just about everything; Soupy Dalyander for stuffing battery cases; Patrick Dickhudt for preparing the LISST suspended-sediment sensor and tripod transportation; Barry Irwin for setting up a bathymetric data and navigation station for the cruise; Jeff List for helping with site selection and experimental design, stuffing battery cases, and mounting instruments; Marinna Martini for project management and instrumentation preparation; Ellyn Montgomery for sonar testing, programming, weather-station-system testing, telemetry and telephone-system wrangling, and other general help; Jeff Obelcz for instrumentation mounting; Chris Sabens for coming in to direct others even when she could use only one hand; Chuck Worley for SEABOSS preparation; and John Warner for serving as Chief Scientist.
All of the equipment was deployed off Fire Island in two round trips from January 23 to 26. Jonathan Borden led Brandy Armstrong, Sandy Baldwin, Jeff List, Marinna Martini, Ellyn Montgomery, Jeff Obelcz, John Warner, and Chuck Worley aboard the research vessel (R/V) Connecticut to deploy the six buoys and nine tripods. Bathymetric survey data produced by the WHCMSC in 2011 and depth readings taken on site were used to finalize the positions. The SEABOSS was used to make visual surveys of the seabed and take sediment samples.
All of the gear was recovered aboard the R/V Connecticut in April 2012, and analysis of the data is underway. We thank the crew of the R/V Connecticut for their support, and the PCMSC, the University of South Carolina, and the Woods Hole Oceanographic Institution for providing additional equipment for the deployment.
in this issue:
Ocean-Circulation and Sediment-Transport Data Offshore of Fire Island
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