**1. Introduction**

Coastal barrier spits and islands, which comprise approximately 10% of the world's coastline [1], are dynamic landforms that are impacted by storms and sea-level rise. Debate exists regarding the impacts of sea-level rise on shoreline change and barrier migration [2–6]. Over the millennia, sea-level rise and storms certainly drove the migration of shorelines across the continental shelf during the Holocene [5,7]. Barriers in the mid and northern Atlantic have largely been narrowing rather than migrating in the 20th century [8], and some authors argue the gradual recession of these areas is the result of sea-level rise, not storms [3,6]. Other authors have argued that roles of overwash and inlet dynamics in maintaining barrier systems drive shoreline changes at the decadal/century scale [5,9–11]. Difficulty remains in defining the direct cause and effect of sea-level rise and storms [12]. Gutierrez et al. [13] and Williams et al. [14] summarized likely responses of barriers to sealevel rise under various scenarios and concluded that it is virtually certain that barriers will experience morphological changes through erosion, overwash and washover fan deposition and the formation of inlets during storms. Subsequent research has focused on the various mechanisms related to barrier response to sea-level rise, including modelling studies focusing on the rate of sea-level rise, accommodation space and sediment availability [15,16] and the importance of tidal inlets and tidal deltas to barrier dynamics [17]. Understanding barrier dynamics in the face of potentially increased storminess (increased intensity) [18] and higher numbers of storms [19] is critical to understanding the response of barriers to forcings in the future.

**Citation:** Oakley, B.A. Storm Driven Migration of the Napatree Barrier, Rhode Island, USA. *Geosciences* **2021**, *11*, 330. https://doi.org/10.3390/ geosciences11080330

Academic Editors: Gianluigi Di Paola, Germán Rodríguez, Carmen M. Rosskopf and Jesus Martinez-Frias

Received: 7 May 2021 Accepted: 31 July 2021 Published: 5 August 2021

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**Copyright:** © 2021 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

This study documents the storm driven shoreline changes of the Napatree Point Conservation Area (NPCA), a 2.5 km headland bounded barrier spit located in southwest Rhode Island, extending across the mouth of the Little Narragansett Bay estuary (Figure 1). The barrier and adjacent headland are important for several key reasons: The barrier (1) Attenuates storm impacts (surge, waves) for the southern end of Little Narragansett Bay and adjacent shorelines and (2) provides a suitable habitat for several threatened or endangered species, including Piping Plovers (*Charadrius melodus*), American Oystercatchers (*Haematopus palliatus*) and Least Terns (*Sternula antillarum*). Napatree has been recognized by the Audubon Society as a Globally Important Bird Area [20]. (3) NPCA is managed as a conservation area and remains an important recreational destination with hundreds of visitors a day in the summer.

The objective of this study is to document historical changes in shoreline position and morphology of a barrier spit between 1883 and 2014 and examine the roles of storm frequency and magnitude on barrier migration. The erosion and subsequent recovery of foredunes is critical to understanding the style and rate of barrier retreat [21]. Increased storm frequency and sea-level rise are likely consequences of climate change [19], creating a feedback loop of increased frequency of overwash events and reduced periods of barrier recovery. This would lead to faster rates of shoreline migration as barriers potentially cross a geomorphic threshold [11,13,17]. Between 1938 and 1975 multiple storms (both tropical and extra-tropical) impacted Napatree Point, leading to overwash and 'roll-over' via washover fan deposition and migration of the barrier. The period before 1939 and after 1975 produced little to no net migration of the barrier, even with some moderate storm events. This suggests that the period of recovery was sufficient, such that the barrier was not overwashed in these events, even though similar storms overwashed the barrier in the past. The pattern of increased storm frequency with limited periods of recovery, even in the absence of rapid sea-level rise, provides at least a partial analog of future behavior of a barrier spit and furthers understanding of the importance of storm frequency in barrier migration.

**Figure 1.** (**A**). Location map of Napatree Point. (**B**). A 2014 Digital orthophotograph [22] with past shoreline positions superimposed in 1883 (white), 1939 (red) and 1975 (black) [23–25]. Note that Sandy Point was connected to Napatree Point in 1883 but has migrated north since the 1938 hurricane. White arrows indicate the 1938 hurricane [26]. The yellow star shows the location that the photograph in Figure 5D was captured at. Green circles indicate the locations of the NACCS buoys discussed in the text. The green star shows the location of the U.S. Geological Survey water level gauge. (**C**). A 2018 LiDAR Topobathymetric model of Napatree point and the adjacent Watch Hill, elevation in meters above NAVD88 [27].
