Survey Overview and Acquisition

Bathymetry data were collected by the USACE using the Lighter Amphibious Resupply Cargo (LARC) vessel. Forty-one shore-perpendicular transects with 50-meter (m) spacing were measured using a single-beam echosounder. A typical transect along the coast adjacent to the breach began at the foot of the dune and extended offshore to a water depth of approximately 8 m. For transects in the breach channel, the lines were extended from the 8 m isobaths to a point within the breach that the LARC could not safely traverse on the flood shoal. Navigation data were acquired and stored using HYPACK, Inc., and the data were differentially corrected using land-based global positioning system (GPS) stations. Raw datasets were stored digitally and processed systematically using HYPACK version 2013 in conjunction with a custom FORTRAN routine developed by researchers at the USACE FRF and ESRI ArcGIS version 10.2.1.

Vessel

The USACE LARC vessel was transported from Duck, N.C., to Fire Island, N.Y., via tractor trailer to conduct the bathymetry survey (fig. 3). The LARC vessel was used as the survey platform in order to collect continuous topographic and bathymetric data of the breach morphology, including the ebb shoal deposit on the ocean side of the breach. The ebb shoal is difficult to survey using traditional methods because of the shallow water and wave breaking conditions. The LARC is ideal for use in this environment because it is capable of being driven from the beach into the water and continuously collecting data in very shallow water (<1 m) and subaerial environments. The LARC typically surveys on the ground at approximately 8 kilometers per hour and transitions to speeds of about 7 kilometers per hour in the water (fig. 4). For more information on the USACE LARC survey system, see http://www.frf.usace.army.mil/larc/larcsystem.stm.

Figure 3: The LARC vessel on its transport trailer.

Figure 4: The LARC surveying in swash zone.

Equipment

The LARC survey system uses HYPACK, Inc. hydrographic survey software to collect data. This software acts as an interface between the GPS data and the navigator in order to ensure that the LARC follows the pre-determined survey path. The LARC uses a Real-Time Kinematic Global Positioning System (RTK-GPS) to determine location and movement. The HYPACK software merges RTK-GPS data and echosounder data and creates raw data files that are post-processed and edited by staff at the USACE FRF using customized software. See http://www.frf.usace.army.mil/larc/larcsystem.stm for additional details of the USACE processing steps and equipment.

Knudsen 320BP Echosounder
Accurate depth information is needed as soon as the LARC enters the water and the tires are no longer in contact with the ground. To obtain this information, the 320BP Echosounder Transducer has a close proximity option that is capable of measuring depths as shallow as 10 centimeters (cm). The accuracy of the echosounder is determined to be 3 percent of the selectable depth range. The water depth readings from the echosounder are sampled at a rate of 10 hertz (Hz) (10 readings per second). The echosounder transducer is centered directly under the GPS antenna to ensure accurate readings.

Conductivity, Temperature, and Depth (CTD) Profiler
The Ocean Sensor's CTD gauge is capable of collecting information about salinity and speed of sound from the water surface down to the ocean floor. The echosounder transducer uses sound pulses to map the ocean bottom, and the accuracy of the echosounder depends on the speed of sound in water, which varies with water density. To monitor water density variations, CTD profiles are measured during the survey. For this survey, the CTD profile data were collected at the 8 m isobaths at the beginning and end of the survey day. The information was then incorporated into the custom USACE FRF post-data collection software analysis and correction process (see http://www.frf.usace.army.mil/larc/larcsystem.stm).

Real-Time Kinematic Global Positioning System (RTK-GPS)
The Trimble 4000 GPS System uses a base station setup in the operational area at a known reference point on land. The base station computes its location using signals from 5 to 9 satellites. By comparing this reading to the known reading, the base station calculates a correction which can then be sent to the LARC in real time. For this survey, the base station was set up on a benchmark that was established by the USGS. The USGS occupied the location for 3 consecutive days before submitting the coordinates to the Online Positioning User Service (OPUS) for precise quality control prior to the survey. The benchmark is located on a wooden walkway on the south side of a gated parking area at the National Park Service (NPS) Wilderness Visitor Center (VC) on eastern Fire Island (fig. 5). The benchmark location is approximately 2.7 kilometers (km) from the breach and the survey site (fig. 6).

The LARC acts as a rover station, so its GPS antenna is not given a differentially fixed position. Instead, the LARC computes its location from available transmitting GPS satellites. After the location is determined, the correction from the base station is received and applied to that computed position to produce the differentially corrected position. This base station correction results in an accuracy of ±2.0 cm in all three dimensions.

Figure 5: USGS-established GPS base station located at the National Park Service Wilderness Visitor Center on Fire Island. [larger version]

Figure 6: Map showing location of the USGS benchmark and the survey site around the breach. Orthophotograph is from the National Oceanic and Atmospheric Administration (http://storms.ngs.noaa.gov/storms/sandy/), taken on November 5, 2012. White box in bottom right is due to incomplete background data. [larger version]