Introduction

Quantifying the amount of shoreface sediment available to barrier islands is important for understanding how shorelines will respond to sea-level rise and storms (Kinsela and others, 2020). Further, understanding spatial variations in shoreface sediment availability may guide the development of sediment management priorities used by coastal managers. In October 2012, the Atlantic coastline from North Carolina to Connecticut was impacted by Hurricane Sandy (Blake and others, 2013). As a result of these impacts, many coastal communities pursued coastal restoration projects that sought to improve the condition of beaches and marshes (U.S. Army Corps of Engineers [USACE], 2016, 2019). To support a National Fish and Wildlife Foundation (NFWF)-led effort to evaluate the effectiveness of those restoration projects, the U.S. Geological Survey (USGS) received funding to conduct a remote sensing and in situ monitoring campaign of the shorelines, beaches, and marshes along the New York and New Jersey coastlines at three sites: Seven Mile Island, New York; Rockaway Beach, N.Y., alternatively called Rockaway Peninsula by Wei and others (2021), and Wei and Miselis (2022); and Fire Island, N.Y. Seven Mile Island, New Jersey (NFWF project no. 41991) and Rockaway Beach peninsula, N.Y. (NFWF project no. NPS-1A) sites (fig. 1B–C) were included within the NFWF project entitled, “Monitoring Hurricane Sandy Beach and Marsh Resilience in New York and New Jersey.” Fire Island, N.Y. (fig. 1D; Hurricane Sandy Supplemental Task USGS-GS2-2B) was added as a control, because it is less developed than the other two sites and the USGS had an existing monitoring project at this location. Remote sensing monitoring spanned the coastlines of New York and New Jersey, whereas in situ monitoring activities occurred at Seven Mile Island, N.J., and Rockaway Beach peninsula, N.Y.

One of the metrics that NFWF will use to evaluate restoration effectiveness is the active sand volume (ASV), defined here as the volume of sediment that could contribute to beach and shoreline behavior over short (annual to decadal) time scales. Barrier islands are composed of terrestrial (above-water) and submerged (below-water) environments (fig. 2). These environments contain sediment that is available for reworking and redistribution during fair weather and stormy conditions. However, the quantities and distribution of sediment volumes in submerged environments, particularly on the shoreface (fig. 2, inset), are poorly quantified relative to terrestrial environments. Over thousands of years, shoreface sediment availability is controlled by the geologic framework and often coincides with the Holocene-Pleistocene (H-P) boundary (Riggs and others, 1995; Belknap and Kraft, 1985; Schwab and others, 2014). However, other studies have identified boundaries above the H-P boundary that indicate not all of that shoreface sediment is available over annual to decadal time scales (Miselis and McNinch, 2006; Locker and others, 2017; Wei and Miselis, 2022; fig. 2). In contrast, ASV depends not only on the geologic framework, but also on the redistribution of sediment during and after storms (Harley and others, 2022) and on the wave conditions that define the offshore boundary of sediment transport, also known as the depth of closure (DoC) (Hallermeier, 1981).

To quantify the ASV at the three study sites, this report uses (1) previously interpreted geologic surfaces from chirp seismic surveys that define the geometry of shoreface sand units (Locker and others, 2017; Wei and others, 2021, 2025; Wei and Miselis, 2022; table 1); (2) repeat bathymetry surveys (multibeam echo sounder [MBES] or single-beam echo sounder [SBES]) (table 1) to assess interannual changes in seafloor elevation; and (3) estimates of the DoC for each survey year at each site (table 2).

Setting

The study sites discussed in this report are barrier environments along the mid-Atlantic shoreline and differ in wave and tidal influences, population density, coastal engineering activities, and island extent. The southernmost study site, Seven Mile Island, N.J. (fig. 1B), is a mixed-energy barrier island centered in a littoral cell with longshore transport from north to south (Wei and others, 2025). The area is heavily populated except for Stone Harbor Point Natural Conservation Area at its southern end, which is highly dynamic, uninhabited, and is the littoral endpoint of the barrier (Nordstrom and others, 2002; Barone, 2016). The island has undergone moderate coastal engineering, including repeat beach renourishment, dune building, and emplacement of a stabilizing hard structure at the northern end of the island, adjacent to Townsends Inlet (Nordstrom and others, 2002; Barone, 2016). Seven Mile Island has the shortest alongshore extent of the three sites, with a shoreline length of 11.39 kilometers (km).

Rockaway Beach peninsula, the westernmost barrier spit in New York (fig. 1C), is a sediment-starved, wave-dominated barrier island at the end of the New York Bight littoral cell (Wei and others, 2021; Wei and Miselis, 2022). The peninsula is heavily populated and has been heavily modified with beach renourishment, jetties, groins, and a large jetty at Rockaway Inlet, the western terminus and littoral endpoint of the peninsula (USACE, 2016, 2019). On its eastern end, the peninsula is separated from the Long Beach barrier island by East Rockaway Inlet, which is adjacent to the peninsula’s mainland attachment point. The peninsula has an alongshore extent of 15.79 km.

The northeasternmost study site is Fire Island, N.Y. (fig. 1D), a wave-dominated barrier island updrift from Rockaway Beach peninsula with longshore transport from east to west. The island is centrally located within the New York Bight littoral cell. Fire Island is lightly to moderately inhabited, with communities intermittently distributed along the length of the shoreline. Unlike the other two sites, large portions of Fire Island are uninhabited and part of the Fire Island National Seashore, including the Otis Pike High Dunes Wilderness area, which spans approximately 11 km of the island. Several beach nourishment projects have occurred within the communities, but there are few emplaced groins or jetties along the island to impede longshore transport. The study area at Fire Island does not include the littoral end of the island and does not cover the length of the entire island (~50 km) between Fire Island Inlet and Moriches Inlet, but instead spans 31 km of the shoreline from east of the Fire Island Lighthouse to west of the boundary of Smith Point County Park (fig. 1D). For more comprehensive information about the wave climate, geomorphic setting, and geologic framework of these islands, consult the following publications and references included therein: Wei and others (2025) (Seven Mile Island); Wei and others (2021) and Wei and Miselis (2022) (Rockaway Beach peninsula); Leatherman (1985), Schwab and others (2000, 2013, 2014), Locker and others (2017); and Ciarletta and others (2024) (Fire Island).

Methods

Previous studies at Seven Mile Island (Wei and others, 2025), Rockaway Beach peninsula (Wei and others, 2021; Wei and Miselis, 2022), and Fire Island (Locker and others, 2017; Schwab and others, 2013, 2014) interpreted the geologic framework and identified the distribution of sandy shoreface sediments, thereby providing the geologic context for ASV. From a hydrodynamic perspective, available sediment is defined as deposits that can be mobilized in the nearshore environment by waves and currents (Stive and de Vriend, 1995); however, measuring sediment transport using in situ instruments on timescales of years to decades is difficult (Anthony and Aagaard, 2020). To account for these shorter-term changes, this report refines the estimate of ASV beyond a geologic definition by using interannual bathymetry surveys and an estimate of DoC (Hallermeier, 1981) as the upper and offshore volume boundaries, respectively. To do this, the approach used was (1) identify interpreted geologic boundaries that represent the basal boundary of sandy shoreface sediments; (2) subtract the base surface from time-varying seafloor surfaces derived from repeat seafloor high-resolution MBES or SBES surveys; (3) identify the DoC, for each seafloor survey; and (4) calculate the volume of sediment between the base geologic boundary, the seafloor, and the DoC for each time period at each study site.

Interpreted Geologic Boundaries

Depending on the distinct geology at each of the sites, the lower geologic boundary of shoreface sediment availability as interpreted in previous publications represents different features (fig. 2). At Seven Mile Island, the H-P boundary was identified below several other geologic boundaries (Wei and others, 2025) (fig. 3). The H-P boundary can be both the lower boundary of sandy sediment availability and the base of the ASV package when no spatially consistent geologic boundaries can be identified above it, as is the case with the boundary at the Rockaway Beach peninsula shown in figure 4 (Wei and Miselis, 2022). Fire Island, similar to Seven Mile Island, includes other geologic boundaries identified above the H-P boundary (Locker and others, 2017) and represent both the lower boundary of sediment availability and the ASV (fig. 5).

At Seven Mile Island, the lower ASV boundary (fig. 3) coincides with the base of unit C, which is the uppermost Holocene unit composed predominantly of sand (Wei and others, 2025). At Rockaway Beach peninsula, the basal boundary used in this study (fig. 4) corresponds to the interpreted H-P boundary (Wei and others, 2021; Wei and Miselis, 2022). At Fire Island, the basal boundary used for the ASV (fig. 5) calculations is an interpreted composite boundary (R0 and R2) that separates reworked shoreface sediments from undisturbed sediments (Locker and others, 2017). The Rockaway Beach peninsula and Seven Mile Island boundaries interpreted from chirp profile data were exported as x,y,z data points from the seismic software Kingdom version 2021 (S&P Global, 2021), and the composite boundary at Fire Island x,y,z data points were exported from Chesapeake Technology’s SonarWiz version 10.7.4 software (Chesapeake Technologies, 2024). Each dataset was transformed from the acquisition datum to the North American Datum of 1983 (NAD 83) horizontal datum and North American Vertical Datum of 1988 (NAVD 88) GEIOD 12A (Fire Island) or 12B (Seven Mile Island and Rockaway Beach peninsula) using the National Oceanic and Atmospheric Administration’s (NOAA’s) VDatum software version 4.4.2 software (National Oceanic and Atmospheric Administration, 2025). In Global Mapper (GM) version 25.0 software (Blue Marble Geographics, 2024), each geologic basal boundary x,y,z dataset was gridded at a 50-m cell size following the same methodology used to create bathymetry digital elevation models (DEMs), discussed in the next section.

Bathymetry

Between 2014 and 2023, the shoreface elevations at each of the three study sites were surveyed three times. At Seven Mile Island, MBES bathymetry data were collected in 2018, 2021, and 2023. The 2018 MBES coverage at Seven Mile Island is incomplete in the southernmost part of the survey area closest to shore because of acquisition/weather difficulties encountered during the survey. At Rockaway Beach peninsula, MBES bathymetry were collected in 2019, 2021, and 2023. In 2019, SBES data were collected simultaneously with MBES and used to fill in shallower areas. At Fire Island, only SBES bathymetry data were collected in 2014, 2015, and 2018. The timing of each survey, data acquisition and processing details, and resulting geomorphologic interpretations can be found in the data releases, reports, and manuscripts cited in table 1.

Bathymetry x,y,z data were gridded to create DEMs in GM v.25.0 using the Create Elevation Grid tool with the Triangular Irregular Network algorithm at a 1-m cell size (MBES) and 50-m cell size (SBES) and a grid “no data” buffer distance set at 2 m for MBES data and at “full loose” 1 km for SBES data (figs. 6–8). Upon saving as a georeferenced tagged image file format (GeoTIFF) file from GM, all DEMs were aligned to either a 1- or 50-m blank “snap” raster using the “layer bounds” option to maintain consistency in cell center locations between surveys. Standard deviation raster maps of the three surveys at each site were created in Esri ArcGIS Pro v.3.2.1 using the StdDev tool to show the spatial and temporal variability in seafloor elevations (fig. 9).

The USGS collected SBES data concurrently with chirp subbottom profile data at Seven Mile Island and Rockaway Beach peninsula. However, because of issues with SBES collection at the Fire Island study site during the 2014 seismic survey, bathymetry data were not collected concurrently at that location. Because the USACE and USGS seismic surveys occurred over two legs in June and July (Forde and others, 2024), higher resolution USGS SBES data only overlapped with the first leg of the chirp survey. During this time, significant migration of nearshore bars occurred that would have affected chirp stratigraphy when the profiles were adjusted to the seafloor. As a result, the chirp profile data were not adjusted to the USGS SBES bathymetry, and instead, the chirp profile data were adjusted to a DEM created from tide-corrected, digitized chirp profile seafloor x,y,z data (Locker and others, 2017). The 2014 SBES data were used in volume calculations for consistency when using the SBES from the other two surveys.

Results

Seafloor elevations and associated DoC contours fluctuate spatially over time. While these variables and boundaries are constantly changing because of nearshore processes, the geologic interpreted base boundaries at each site are assumed to remain at a constant depth over years to decades, and this is supported by our finding that seafloor variability (fig. 9) does not exceed sediment thicknesses (figs. 3–5). Using a lower boundary from interpreted stratigraphy instead of extending the DoC depth boundary landward ensures that the ASVs account for geologic processes (Miselis and McNinch, 2006).

The volume change uncertainty exceedance (VCUE) indicates how effectively the methods capture true volume changes rather than noise associated with acquisition and gridding errors (table 5). For example, if the average volume uncertainty between years is less than measured volume change over that same time period (VCUE > 1), then volume changes can be resolved. On the other hand, if the average volume uncertainty between years is greater than measured volume change (VCUE < 1), then volume changes cannot be resolved. The higher the VCUE value is above one, the more likely volume changes are real and not artifacts associated with data acquisition and processing. Comparisons for the time periods at each site show that volume changes can confidently be resolved with our methodologies. One exception is at Rockaway Beach peninsula between 2021 and 2023, when changes in volume were below uncertainty thresholds. The alongshore extents and, therefore, the two-dimensional spatial areas among the three sites differ. The areas of the ASV polygons that were used to calculate volume change are shown in table 5. The difference in analysis area among the sites is substantial, especially when comparing Seven Mile Island and Rockaway Beach peninsula to the portion of Fire Island examined in this study. Normalizing the total volumes by the change areas shows that both Seven Mile Island and Rockaway Beach Peninsula have approximately 1.5 times the sediment per square meter compared to Fire Island (table 5).

The standard deviation in TASV reflects both changes in seafloor bathymetry and the cross-shore shift in the DoC contours through time (fig. 9 and table 5). Standard deviation in TASV at Fire Island exceeds those of Seven Mile Island and Rockaway Beach peninsula by approximately 4.5 × 10⁶ m³. This variability in total volume may indicate a more dynamic environment at Fire Island relative to the other two sites.

Summary

The active sand volumes (ASVs) in this report are defined as the amount of sediment within the shoreface that may be easily mobilized over time scales of years to decades. The calculated ASVs may represent only a portion of the geologically defined shoreface sediment described in previous publications. The ASV extents were determined from previously interpreted stratigraphic boundaries, seafloor elevations, and the depth of closure at each site. By incorporating the seaward depth of closure boundary, we arrive at a metric that accounts for shoreface geology and the hydrodynamics that mobilize shoreface sediment.

The volumetric uncertainty analysis as seen with exceedance values indicates that the volume changes observed at Seven Mile Island, Rockaway Beach peninsula, and Fire Island are real and not caused by acquisition, processing, or gridding errors. Although the total active sand volumes among the sites appear similar, by accounting for the differing extents of survey area, Seven Mile Island and Rockaway Beach peninsula have similar ASVs that are about 1.5 times greater than the ASV within the study extent at Fire Island. This is the first assessment of ASV for our study sites, and it is expected that this report will help to inform potential future work refining estimates of ASVs for different time scales using different boundaries.