St. Petersburg Coastal and Marine Science Center
Focus: This study used Experimental Advanced Airborne Research Lidar (EAARL) data in order to derive volumetric analysis at the evaluated study areas. The U.S. Geological Survey (USGS), and the National Park Service (NPS) have collaborated to use EAARL to collect data for habitat mapping, ecological monitoring, storm-event assessment, and evaluating geomorphic change. In this study, 1-m digital elevation models (DEMs) were created from EAARL data to assess volumetric change along Fire Island National Seashore, (FIIS), a 41.8-km-long barrier island located on the south shore of Long Island, New York, and along the Gateway National Recreation Area (GATE), Sandy Hook, New Jersey, a 16-km-long barrier spit (fig. 1). The data collection periods for the two sites occurred in fall 2002, spring 2005, and spring 2007. Morphologic modification was calculated between each of the three acquisition periods by calculating net volumetric change per unit area. Both study areas exhibited short-term morphologic change along the beach face. Such changes are affected by the variability of sediment sources and sinks, episodic storms, and anthropogenic modification.
Study Areas: The south shore of Long Island consists of glaciated sand and gravel deposits which lie on the remnants of a submerged coastal plain (Panuzio, 1968). It has been proposed that the south shore barrier islands developed as westward growing spits and barrier bars from the predominant westerly long-shore drift (Panuzio, 1968). Fire Island is one of the central barrier islands along Long Island’s south shore; a majority of the island remains undeveloped with the exception of seventeen small residential communities primarily located on the western portion of Fire Island National Seashore (Psuty and others, 2005).
The Sandy Hook Unit is one of three sections of the Gateway National Recreation Area; it is separated from the mainland of New Jersey by Sandy Hook Bay (Pendleton and others, 2005). Sandy Hook formed as an elongated, recurved spit from the northward longshore drift (Allen, 1981). Despite growth along the northern end of the Sandy Hook spit, one section has been characterized as a critical zone by the NPS. It has become so narrow that past major storms breached the spit, turning the area into an island. To prevent erosion in the critical zone, the NPS has invested funds in replenishment projects and in the collection of Global Positioning System (GPS) and data to assist in morphological monitoring (Stevens and others, 2005).
Data Processing: The tool used for the post-flight processing of EAARL data is the custom-built Airborne Processing System (ALPS). ALPS has the ability to combine the laser return backscatter with aircraft positioning information. Processing algorithms are then used to convert raw waveform data acquired by the EAARL system into geo-referenced point (x, y, z) returns for first-surface and bare-Earth topography. Data acquired on the study sites was projected in Universal Transverse Mercator (UTM) and then converted to horizontal elevations referenced to North American Datum (NAD) 1983 and North American Vertical Datum (NAVD) 1988 vertical elevations using the GEOID03 model (Nayegandhi and others, 2006). Cut-fill analysis was performed on various Areas of Interest (AOI) within the study areas to determine topographic change; performed by use of the Environmental Systems Research Institute (ESRI) ArcGIS 9x software package. AOIs were selected based on past analysis with focus on erosion or accretion hot spots.
Example AOI Results: Davis Park is one of the seventeen small residential communities located within Fire Island National Seashore. Figures 2a-c showed the erosion that occurred from 2002-2007. The erosion dramatically increased after the April 2007 nor’easter. This erosion compelled Federal Emergency Management Agency (FEMA) and the State Emergency Management Office (SEMO) to restore snow fencing, import 18,000 cubic meters of sand by barge, and truck the sand to the most vulnerable areas of East to Second Walks in Davis Park. It was calculated that this replacement sand would restore half of what was lost in the April 2007 storm (CPE, 2007). Several other Fire Island National Seashore communities also plan to renourish their beaches with 1.5 million cubic meters of sand along 8,800 linear meters of shoreline (CPE, 2007).
Figures 3a-c showed the condition of the critical zone beach face in Sandy Hook, New Jersey from 2002-2007. There is much more sand on the beach face from 2002-2005 than from 2005-2007. This was in large part due to the critical zone receiving approximately 193,000 cubic meters of sand from the Sea Bright Borrow Area in November, 2002 (USACE, 2008).
Conclusion: The results of this study demonstrate the capacity of surveys for rapid collection of dense data over sizeable areas. This makes it a valuable tool in quantifying morphologic change especially when hurricanes and nor’easters play such a considerable role in shaping the coastline as seen in the two study sites. Because these barrier islands serve as important ecological and economic areas, it is recommended that the collection of data continues for these areas particularly after major storm events and on an annual or biennial basis.