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Beach Erosion Under Rising Sea‐ Sedimentology (2016) doi: 10.1111/sed.12264 Beach erosion under rising sea-level modulated by coastal geomorphology and sediment availability on carbonate reef-fringed island coasts BRADLEY M. ROMINE*, CHARLES H. FLETCHER†, L. NEIL FRAZER† and TIFFANY R. ANDERSON† *University of Hawaii Sea Grant College Program, University of Hawaii at Manoa, Honolulu, HI, USA (E-mail: [email protected]) †Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu, HI, USA Associate Editor – Vern Manville ABSTRACT This study addresses gaps in understanding the relative roles of sea-level change, coastal geomorphology and sediment availability in driving beach erosion at the scale of individual beaches. Patterns of historical shoreline change are examined for spatial relationships to geomorphology and for tem- poral relationships to late-Holocene and modern sea-level change. The study area shoreline on the north-east coast of Oahu, Hawaii, is characterized by a series of kilometre-long beaches with repeated headland-embayed morpho- logy fronted by a carbonate fringing reef. The beaches are the seaward edge of a carbonate sand-rich coastal strand plain, a common morphological set- ting in tectonically stable tropical island coasts. Multiple lines of geological evidence indicate that the strand plain prograded atop a fringing reef plat- form during a period of late-Holocene sea-level fall. Analysis of historical shoreline changes indicates an overall trend of erosion (shoreline recession) along headland sections of beach and an overall trend of stable to accreting beaches along adjoining embayed sections. Eighty-eight per cent of headland beaches eroded over the past century at an average rate of À À0Á12 Æ 0Á03 m yr 1. In contrast, 56% of embayed beaches accreted at an À average rate of 0Á04 Æ 0Á03 m yr 1. Given over a century of global (and local) sea-level rise, the data indicate that embayed beaches are showing remark- able resiliency. The pattern of headland beach erosion and stable to accret- ing embayments suggests a shift from accretion to erosion particular to the headland beaches with the initiation of modern sea-level rise. These results emphasize the need to account for localized variations in beach erosion related to geomorphology and alongshore sediment transport in attempting to forecast future shoreline change under increasing sea-level rise. Keywords Beach, coastal, erosion, Hawaii, Oahu, sea-level, shoreline. INTRODUCTION 1987). Beach erosion and shoreline recession will increase on a regional to global scale with Coastal erosion is a problem in Hawaii (Romine increasing sea-level rise in coming decades & Fletcher, 2012), on continental shores of the (National Research Council, 2012; IPCC, 2014). United States (Morton et al., 2004; Morton & However, coastal scientists are limited in their Miller, 2005; Hapke et al., 2006, 2010; Hapke & ability to predict shoreline change, particularly Reid, 2007) and coasts around the world (Bird, on the scale of individual beaches and littoral © 2016 The Authors. Sedimentology © 2016 International Association of Sedimentologists 1 2 B. M. Romine et al. cells, because shoreline change can be domi- in beach erosion rates. Also, the implications of nated by localized sediment availability and these findings in projecting future shoreline transport, which can act partially or totally inde- change under increasing rates of sea-level rise pendently of sea-level change. Other factors are discussed. complicating the ability to forecast shoreline Historical shoreline positions and change change include alongshore and cross-shore vari- rates from Fletcher et al. (2012) are utilized ations in geology, lithology and coastal slope. (see Materials and methods section) to analyse Understanding how beaches will change in a spatial patterns of beach erosion and to evaluate future dominated by sea-level rise requires the importance of coastal geomorphology and insights into these localized processes governing alongshore sediment transport on observed and historical and modern shoreline changes. future coastal change. The primary focus is on Sea-level rise has been implicated as a driver of beaches of north-east Oahu due to: (i) the coastal modern (observed) shoreline change for island geology, which is characterized by a carbonate coasts (Singh, 1997; Ford, 2012; Romine et al., sand-rich strand plain (described in more detail 2013) and continental coasts (Leatherman et al., in the Physical setting section) – a common fea- 2000; Zhang et al., 2004; Brunel & Sabatier, 2009; ture on many tectonically stable carbonate reef- Guitierrez et al., 2011; Yates & Cozannet, 2012). fringed volcanic and atoll islands (Grossman However, the relative roles of coastal geomor- et al., 1998; Dickinson, 2001, 2004); (ii) the rela- phology, sediment availability, human influences tive abundance of historical shoreline positions and sea-level change in driving shoreline change from existing data sets (Historical shorelines sec- are not well-understood and add to uncertainty tion); (iii) the kilometres-long semi-continuous in these studies. These gaps in understanding of beaches with repeated headland-embayment the relative influence of individual processes on morphology allowing for investigation of charac- shoreline change highlight the need for further teristic longshore and cross-shore sediment studies that rely on observational data and mod- transport patterns; and (iv) the available maps of els and are applicable at the scale of individual nearshore sediment deposits and bathymetry beaches (Le Cozannet et al., 2014). (Conger et al., 2009). Cursory inspection of plots The reef-fringed coast of Oahu, Hawaii, is typi- of shoreline change along north-east Oahu in cal of other carbonate shoreline systems found on Fletcher et al. (2012) suggests a predominant tectonically stable islands in equatorial oceans trend of erosion along headland sections of worldwide. Throughout low-latitude areas of the beach and accretion within adjoining embayed Pacific and elsewhere, coasts have undergone a sections of beach. In addition to analysis of spa- unique history of relative sea-level fall during the tial patterns of shoreline change, potential rela- late Holocene due to post-glacial geoid subsi- tionships between the observed spatial patterns dence (Mitrovica & Peltier, 1991; Mitrovica & of shoreline change, existing coastal geomor- Milne, 2002) followed by recent eustatic sea-level phology and localized changes in late-Holocene rise due to global warming (IPCC, 2014). Millions sea-level are discussed. of people living on reef-fringed carbonate coasts around the world are exposed to increasing shore- line erosion with accelerating sea-level rise. It is PHYSICAL SETTING critically important that coastal managers are pro- vided with actionable scientific results that The island of Oahu, Hawaii, is located in the inform new policies to support decision making tropics of the central North Pacific (Fig. 1). The and planning for improved hazard resilience and coast is fringed by a carbonate reef platform resource management. comprised of a patchwork assemblage of fossil Using the Oahu coast as a representative set- Pleistocene reefs from interglacial high sea-level ting for carbonate, reef-fringed coasts in equato- stands of the past several hundred thousand rial waters, this study addresses gaps in current years (Fletcher et al., 2008). Hawaii lies in a understanding of the roles that coastal geomor- microtidal zone (tide range ≤1 m). The north- phology, sediment supply and sea-level rise play east Oahu study area is exposed to large waves in shoreline changes at the scale of individual in winter from the North Pacific (winter months) beaches. In particular, this study examines the and predominant trade winds year-round. As a role that coastal geomorphology and available result, wave-generated currents are the primary sediment, which are typically remnants of past drivers of sediment transport (Norcross et al., sea-level changes, play in alongshore variability 2003). © 2016 The Authors. Sedimentology © 2016 International Association of Sedimentologists, Sedimentology Beach erosion and sea-level rise on carbonate island coasts 3 158°15’ W 158°00’ 157°45’ 21° 45’ N Study area Pacific Ocean 21° 30’ Oahu 21° 0 5 Kilometres Fig. 1. Shoreline study area (black 15’ box) on the north-east coast of 0 5 Miles Oahu, Hawaii. Fig. 2. Low-lying sand-rich coastal plain and beach at Punaluu, east Oahu (view looking south-west). Note the embayed beach fronting the channel in the nearshore reef (left) and headland beach fronting the shallower nearshore reef (right) (photograph by Andrew D. Short, University of Sydney). Beaches along east Oahu, including the study within the study area, beaches are simply the area, are composed primarily of biogenic cal- eroded leading edge of a sand-rich coastal plain careous sand originally derived from nearshore (Fletcher et al., 2012) (Fig. 2). reefs (Harney & Fletcher, 2003). Volcanoclastic Historical shoreline studies indicate an overall sediment eroded from inland watersheds is typi- stable trend (region-wide average) on beaches cally a minor fraction of beach sand on Oahu. along east Oahu (Fletcher et al., 2012; Romine & Beaches are typically backed by low-lying Fletcher, 2012). Like other coastal regions in coastal plains comprised of a mixture of beach Hawaii, long-term shoreline trends along east deposits and dunes, lithified carbonate deposits Oahu are highly variable along the shore, when (including fossil
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