*2.1. Study Area*

The barrier spit portion of the NPCA is a welded barrier [28], bounded by Watch Hill Point to the east and Napatree Point to the west (Figure 1). The entirety of the point/barrier complex extends 2.5 km across Little Narragansett Bay. The barrier spit itself is 2 km long. It has an average width of 100 m and an average volume of 300 m<sup>3</sup> m−1. Foredune heights range from 3.5 to 8 m MLLW (mean, 4.7 m MLLW), with a general increase in elevation from west to east. The headlands at each end of the barrier are composed of boulder diamict (glacial till) deposited as part of recessional end moraines during the Late Wisconsinan deglaciation [29,30]. Napatree Point headland ranges from 5 to 8 m in relief, and is unarmored, although the boulders eroded from the till bluffs at the far western

end form a natural 'revetment' in the manner of FitzGerald et al. [31]. The eastern bluff (Watch Hill Point) is 5 to 6 m in relief along the shoreline; however, shoreline protection structures (sea walls and revetments) have been in place along much of this shoreline since at least 1939. Offshore of the headlands, cobble-gravel to boulder pavement extends approximately 500 m to a water depth here of 7 to 8 m. These boulder-gravel areas are interpreted to represent erosion of the glacial headlands as the shoreline retreated during transgression [32]. The shoreface offshore of the Napatree barrier is largely sandy, and sorted bedforms are visible on aerial photographs, Lidar and unpublished side-scan sonar imagery. One linear ridge of cobble-gravel to boulder pavement is visible in both the aerial imagery and lidar of Figure 1 along the east end of the barrier.

NPCA, like the rest of the Rhode Island South Shore (RISS), is a microtidal, wavedominated shoreline in the classification of Hayes [33], and tidal range along the ocean shoreline (Block Island Sound) is 0.8 to 1.2 m. A U.S. Geological Survey gauge (Figure 1) installed in 2014 in the harbor behind the eastern end of the barrier does not report calculated tidal datums, but the range is similar to the open ocean conditions, suggesting tidal range is not significantly dampened within Little Narragansett Bay. Direct observations of wave conditions in Block Island Sound are limited, due to no long-term stations in the area. Approximately one year of observations between July 2010 and September 2011 reported that significant wave heights are typically <1 m with periods 5 to 11 s (see the location of the East Beach Buoy, Figure 1A) [34]. Fair weather waves approach predominantly from the southwest or south. Storm wave conditions during intense storms remain speculative; however, observations along the RISS in 10 m of water depth 18 km east of Napatree during Tropical Storm Irene (2011) measured significant wave heights of 4.1 m with a peak period of 10 s [34]. Modelled conditions during storms were calculated as part of the North Atlantic Coast Comprehensive Study (NACCS), and the station 200 m offshore of the Napatree barrier with approximately 6 m deep water (Figure 1) resulted in calculated the wave height for a storm with a 2-year return period of 4.7 m; the 10-year return period wave height was 5.4 m. The height was 5.7 m for a 100-year return period [35]. These values are lower for than the adjacent shoreline (east of Watch Hill Point, Figure 1), suggesting some reduction in wave energy relative to the RISS. Modelled wave heights (NACCS) are slightly lower at the east and west ends of the barrier; the calculated heights were 4.8 m for the east and west ends of the barrier during a 100-year return period storm. Two small groins were installed near the east end of the barrier, one on the southern (Block Island Sound) shoreline (100 m long, installed between 1948 and 1951) and one on the northern (Little Narragansett Bay) shoreline (160 m long, installed between 1945 and 1951) (Figure 1). Patterns of shoreline change suggest that longshore transport is towards the west on the southern shoreline and to the east on the northern shoreline. The longest sea-level record in southern New England is from the Newport, Rhode Island tide gauge (50 km northeast of Napatree (Figure 1)), with monthly mean sea-level values dating to 1930 (Figure 2). Linear regression through the data shows a reported rate of sea-level rise of 2.83 ± 0.16 mm yr−<sup>1</sup> [36]. Two stations closer to Napatree with shorter records (New London, CT, 20 km northwest; 2.72 ± 0.21 mm yr−<sup>1</sup> (1938–2020); Montauk, NY, 30 km southwest; 3.41 ± 0.25 mm yr−<sup>1</sup> (1947–2020)) [37,38] show good correlations with the Newport gauge (see supplemental Figure S1) suggesting a regional rate of sea-level rise of ~3 mm yr−<sup>1</sup> since 1930. Analysis of the record suggests the rate of sea-level rise at the Newport tide gauge has increased: between 1984 and 2014, it reached 4.1 mm yr−<sup>1</sup> [39]. The 30 year trend suggested by Carey et al. [39] is consistent with the measured global rate of sea-level rise since 1993 (3.3 ± 0.4 mm yr−1) [40] coupled with the vertical crustal motion of the region (−0.9 mm yr<sup>−</sup>1) [41].

**Figure 2.** Monthly average sea-level at the Newport, RI tide gauge (blue line). The red dashed line is a linear fit through the data [36].

Previous studies have documented the historical shoreline change of the Rhode Island shoreline, including NPCA, the results of which are summarized in Table 1. Shoreline change values are reported here as negative when the shoreline migrated landward and positive when the shoreline migrated seaward. Boothroyd and Hehre [24] reported the overall shoreline change rates as the net movement between 1939 to 2004, with a mean of −1 m yr−1. Hapke et al. [25] utilized T-sheet shorelines to extend the analysis to include the period between 1883 and 2004, and based on a linear regression of the shorelines, the average shoreline change was similar at −0.8 m yr−1. Hapke et al. [25] also reported a short-term shoreline change between 1975 and 2000 of +0.4 m yr−1. Boothroyd et al. [23] and Boothroyd and Hehre [24] reported the highest rates of shoreline change. Boothroyd et al. [23] calculated the rate using the most seaward shoreline in their study (1939) and the most landward shoreline (1975) based on the shoreline change envelope calculation in the Digital Shoreline Analysis System (DSAS), and the average rate for Napatree barrier was −1.4 m yr−1. Boothroyd and Hehre [24] reported a similar rate between 1939 and 1985 (Table 1).


**Table 1.** Shoreline change rates for the Napatree barrier (transects 20 to 50) from previously published shoreline change maps (Boothroyd and Hehre et al. [24]; Boothroyd et al. [23]; Hapke et al. [25]) and this study.
