Evaluation of a barrier-island breach created by Hurricane Sandy at Fire Island National Seashore, N.Y.—Continued.

Measurement of water velocities and depths on June 26, 2013 — methods and results

Objective:

On November 6 and November 20, 2012, and June 26 and November 13, 2013, U.S. Geological Survey (USGS) New York Water Science Center (NYWSC) personnel collected data to evaluate channel geometry, water velocity, and discharge of an open breach in the Federal Wilderness area of Fire Island National Seashore, N.Y. The breach resulted from major coastal flooding and overwash created by Hurricane Sandy on October 29, 2012. Data were collected using a Sontek¹ M9 Acoustic Doppler Current Profiler (ADCP) with Real-Time Kinematic (RTK) Global Positioning System (GPS). Data were processed into Geographic Information System (GIS) for interpolation and display. The following information generally summarizes the measurement of water velocities and depths within the Wilderness breach on June 26, 2013.

Approach:

An initial series of transects was collected starting about 2 hours before ocean high tide on June 26, 2013. Data were collected perpendicular to flow through the breach beginning at 0810 Eastern Standard Time (EST) and ending at 0830 EST. The first set of transects was taken near the bayward side of the breach, with subsequent sets collected progressively closer to the seaward side of the breach. Discharge ranged from 21,900 ft³/s (cubic feet per second) during the beginning set of transects to 19,200 ft³/s during the final set of transects.

A second series of transects was collected starting about 3 hours before ocean low tide on June 26, 2013. Data collection began at 1215 EST and ended at 1425 EST. The first set of transects was taken near the bayward side of the breach; subsequent sets were collected progressively closer to the seaward side of the breach. Discharge ranged from 13,500 ft³/s during the beginning set of transects to 18,200 ft³/s during the final set of transects.

Additional data were collected along the approximate centerline and adjacent to the shores of the channel, as well as perpendicular to the flood tidal channels bayward of the former Great South Bay (GSB) shoreline, to map the inundated area of the breach. The overall mapped area includes data collected during discharge measurements of both incoming and outgoing tide conditions. The Digital Elevation Model (DEM) was created by plotting the depths of all data points collected relative to North American Vertical Datum of 1988 (NAVD 88) and interpolating the elevation of the bathymetric surface between these points.

Collection of water levels within GSB adjacent to the breach is being be done with a modified USGS storm-tide sensor. This sensor, which normally collects 7 days of 30-second-interval data, has been reprogrammed to collect about 2 months of routine (6-minute-interval) tide-gage observations. Two sensors, an Onset¹ HOBO and an In-Situ¹ Aqua TROLL, were installed on June 19 and June 25, 2013, respectively, to help ensure that at least one would remain fully operational through the planned, 2-month deployment. The HOBO was installed at the East-sensor position on the Old Inlet dock near the former barrier island shoreline, and the Aqua TROLL was installed at the West-sensor position from the end of the dock nearest to Pelican Island. Both sensors were recovered in September 2013, however, data from the Aqua TROLL were considered unreliable due to instrument fouling and drift. Sensor data can be viewed here (sensor deployment notes, East-sensor record [plot, data]).

¹Use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Aerial Photographs of Wilderness breach—Before & After Hurricane Sandy

Oblique aerial photographs of Pelican Island and Fire Island, New York. The
view is looking northwest across Fire Island towards Great South Bay. This
location is within Fire Island National Seashore near Old Inlet—a very narrow
portion of the island that has experienced breaching in previous large storms.
The island breached during Hurricane Sandy, creating a new inlet. Despite the
breach, the fishing shack (yellow arrow) remained standing. Click on image to
enlarge.

Oblique aerial photos of the Wilderness breach

Aerial view of the Wilderness breach at Old Inlet, looking from the Atlantic Ocean
to Great South Bay, on Saturday, November 10, 2012. NPS Photo/Abell.