Storm Driven Migration of the Napatree Barrier, Rhode Island, USA

Bryan A Oakley

doi:10.3390/geosciences11080330

Abstract

Napatree Point, an isolated barrier in southern Rhode Island, provides a case study of barrier spit migration via storm driven overwash and washover fan migration. Documented shoreline changes using historical surveys and vertical aerial photographs show that the barrier had little in the way of net change in position between 1883 and 1939, including the impact of the 1938 hurricane. The barrier retreated rapidly between 1945 and 1975, driven by both tropical and extra-tropical storms. The shoreline position has been largely static since 1975. The removal of the foredune during the 1938 hurricane facilitated landward shoreline migration in subsequent lower intensity storms. Dune recovery following the 1962 Ash Wednesday storm has been allowed due to limited overwash and barrier migration over the last several decades. Shoreline change rates during the period from 1945–1975 were more than double the rate of shoreline change between 1939 and 2014 and triple the rate between 1883 and 2014, exceeding the positional uncertainty of these shoreline pairs. The long-term shoreline change rates used to calculate coastal setbacks in Rhode Island likely underestimate the potential for rapid shoreline retreat over shorter time periods, particularly in a cluster of storm activity. While sea-level rise has increased since 1975, the barrier has not migrated, highlighting the importance of storms in barrier migration.

Highlights

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
Other authors have argued that roles of overwash and inlet dynamics in maintaining barrier systems drive shoreline changes at the decadal/century scale [5,9,10,11]
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]

Summary

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 [2,3,4,5,6]. 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]. 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. 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

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