Journal of Marine Science and Engineering Article Assessing Natural and Mechanical Dune Performance in a Post-Hurricane Environment Jean T. Ellis * and Mayra A. Román-Rivera Department of Geography, University of South Carolina, Columbia, SC 29208, USA; [email protected] * Correspondence: [email protected] Received: 1 April 2019; Accepted: 29 April 2019; Published: 2 May 2019 Abstract: The purpose of this study is to document the geomorphic evolution of a mechanical dune over approximately one year following its installation and compare it to the recovery of a natural dune following the impact of Hurricane Matthew (2016). During the study period, the dunes’ integrity was tested by wave and wind events, including king tides, and a second hurricane (Irma, 2017), at the end of the study period. Prior to the impact of the second hurricane, the volumetric increase of the mechanical and natural dune was 32% and 75%, respectively, suggesting that scraping alone is not the optimal protection method. If scraping is employed, we advocate that the dune should be augmented by planting. Ideally, the storm-impacted dune should naturally recover. Post-storm vegetation regrowth was lower around the mechanical dune, which encouraged aeolian transport and dune deflation. Hurricane Irma, an extreme forcing event, substantially impacted the dunes. The natural dune was scarped and the mechanical dune was overtopped; the system was essentially left homogeneous following the hurricane. The results from this study question the current practice of sand scraping along the South Carolina coast, which occurs post-storm, emplacement along the former primary dune line, and does not include the planting of vegetation. Keywords: coastal stabilization; dune recovery; hurricane impact; aeolian geomorphology; beach scraping 1. Introduction Increasing storm frequency and growing populations along the coastal zone force coastal zone managers to intricately balance nature and society. When storms threaten to devastate this region, action must be taken to protect coastal infrastructure and life. Beach scraping is one such method employed after storms where nearshore sand is artificially transported to create a berm or a dune-like landform. This is a common practice along the U.S. East Coast [1]. Especially in South Carolina, it is believed that scraping protects coastal infrastructure from near-term high tide and king tide flooding events occurring shortly after substantial storms [2]. King tides are naturally-occurring elevated water levels that South Carolina has defined as meeting or exceeding 6.6 ft (2.0 m) at the Charleston Harbor tide gauge [3]. Following the hurricanes impacting the community of Isle of Palms, SC, scraped sand is placed along the former primary dune line. Here we describe the ~10 month evolution of these scraped (herein mechanical) dunes compared to the recovery of natural dunes following a hurricane and several mid-latitude cyclones. We also present the morphological changes after a hurricane impact (Irma) that impacted the system approximately 10 months after emplacement of the mechanical dune. The results from this study question the effectiveness of the current practice of post-storm sand scraping along the South Carolina coast, which is emplaced along the former primary dune line and does not include the planting of vegetation. Beach-dune systems provide a natural defense against coastal storms. The level of protection offered by these dunes is related to their size and their ability to withstand the threat of wind and wave attack [4]. Unfortunately, in many coastal locations, the size and extent of natural dune systems have J. Mar. Sci. Eng. 2019, 7, 126; doi:10.3390/jmse7050126 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2019, 7, x FOR PEER REVIEW 2 of 16 J. Mar. Sci. Eng. 2019, 7, 126 2 of 15 wave attack [4]. Unfortunately, in many coastal locations, the size and extent of natural dune systems have been reduced, as coastal infrastructure expands [5]. In developed areas, alternative dune buildingbeen reduced, strategies as coastal must be infrastructure employed to expands counteract [5]. the In developedeffects of diminishing areas, alternative natural dune coastal building land spacestrategies [4]. must be employed to counteract the effects of diminishing natural coastal land space [4]. 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