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A Sediment Budget Approach to Assessing and Managing the Impacts of Harbor Infrastructure on a Reef Island Shoreline: Masig, Torres Strait, Australia

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A Sediment Budget Approach to Assessing and Managing the Impacts of Harbor Infrastructure on a Reef Island Shoreline: Masig, Torres Strait, Australia
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  A Sediment Budget Approach to Assessing and Managing the Impacts of Harbor Infrastructure on a Reef Island Shoreline: Masig, Torres Strait, Australia Stephanie J. Duce a, *, Kevin E. Parnell a , Scott G. Smithers a   a   School of Earth and Environmental Sciences, James Cook University, Townsville, QLD 4811, Australia. * Corresponding Author. Tel.: +61-7-4781-4668 Fax: +61-7-4781-5581 Email: stephanie.duce@jcu.edu.au  Abstract: Reef islands are dynamic landforms composed almost entirely of unconsolidated sediments. They are low-lying and vulnerable to natural and human-induced environmental changes. Reef islands in Torres Strait experience a seasonal reversal of  prevailing winds, waves, and wave-generated currents (south-east for approximately 9 months of the year, to north-west during the summer monsoon). Sediment transport direction and island beach morphology respond to these seasonal changes. Construction of harbor infrastructure including a shore perpendicular rock barge ramp has disrupted natural beach processes on the north-western shore of Masig Island, a  populated sand cay located in central Torres Strait. Severe erosion has affected the  beach east of the harbor whereas the beach to the west has significantly accreted over the period since harbor construction. We have quantified the impact of the harbor on beach and shoreline dynamics by accurately measuring and analyzing seasonal shoreline and beach volume changes. The eroding shoreline east of the harbor has an annual deficit of ~ 120 m³ while the accreting area west is in surplus by more than 700 m³. Although the south-easterly wind season is longer, more sediment is moved during the shorter north-westerly monsoon due to greater wave penetration to shore over a narrower and deeper reef flat. If mitigation measures are not implemented, current shoreline change trends will continue. Based on the sediment budget calculated, annual renourishment of ~ 500 m³ on the eastern side of the jetty, or assisted sediment bypassing would correct the sediment deficit and stabilize the eroding shoreline. Keywords:  Reef islands, Sediment budget, Coastal erosion, Harbor infrastructure, Sediment transport, Torres Strait, Coastal management 1.   Introduction: Most of the world’s sandy coastlines are showing long-term erosional trends (Hennecke, et al. 2004). Coastal erosion, defined as a landward translation of the shoreline (Komar 2000), presents serious problems when coastal infrastructure and important coastal land is threatened. Reef islands, like Masig, are particularly vulnerable to coastal erosion given their limited land area and low elevation (Nicholls, et al.  2007). Coastal erosion, shoreline position and beach morphology are a function of the coastal sediment budget and physical sediment transport processes at a range of temporal and spatial scales (Leont'yev 2003, Reeve 2006). The sediment budget of a coastal system considers the volume of sediment in, and moving through, a coastal compartment (ASCE. 1994, Short 1999). It equates to the sediment gains minus the sediment losses over a given time period within a defined area. Deficits in a sediment  budget usually result in erosion while surpluses are usually associated with accretion (Rosati 2005). Coastal erosion can be a natural phenomenon or can be caused or exacerbated by anthropogenic activities (Cooper & Pethick 2005). All coastal engineering works alter natural geomorphic processes and have environmental impacts, some favorable and some adverse (Finkl 2002). Examples of inappropriately designed engineered structures causing or exacerbating coastal erosion are common on reef islands (e.g.  Asher 1994, Gillie 1997, Nunn 1994, Ragoonaden 1997). Though examples are  plentiful, few studies take pragmatic steps towards the quantification of such  problems or the mechanisms which drive them. Kench et al . (2003)   examined the use and environmental consequences of engineered structures in the Maldives. They note an almost 180 degree oscillation in beach  position in response to the seasonal reversal of the wind regime and discuss the implications of these processes to the introduction of engineered structures in such environments. While their study elucidated the likely impacts of engineered structures which interrupt transport pathways around a reef island, and synthesized valuable ‘lessons’ for the use of engineering structures in these environments, it did not quantify the impacts of any existing structures. The effects of the Masig Harbor on the island beach and shoreline had not been quantitatively measured and the sediment transport regime and sediment budget were unknown prior to this study. To address the lack of quantitative understanding and mitigate the erosion problem in a sustainable and effective manner this study quantified the effects of harbor infrastructure on the Masig Island shoreline and beach. The seasonal changes in beach volume near the harbor were measured and a simple sediment budget was calculated. This paper demonstrates how this improved understanding of sediment transport processes and the sediment budget can be used to inform practical erosion management measures. It is anticipated that the sediment  budget approach outlined in this study could be replicated on other reef islands to  better understand, predict and manage coastal erosion problems resulting from coastal infrastructure.    2. Study Site Masig (also known as Yorke Island) (9º 45´ S, 143º 43´ E) is a relatively small (~154 ha), inhabited, reef island, located in central Torres Strait, between the northern tip of Australia and Papua New Guinea (Long, et al. 1995) (Fig. 2.1). The tidal regime in Torres Strait is complex, semi-diurnal, and meso–tidal, with a mean tidal range of ~ 3 m. Wind direction reverses seasonally in Torres Strait, where south-easterly winds  prevail from April to November (winter) but are replaced by north-westerly monsoonal winds from December to March (summer) (Brander, et al. 2004). The south-easterly winds are typically stronger, averaging 7 – 9 m/s, whereas the north-westerlies average less than 4 m/s. Maximum wind speeds reach 15 m/s during the south-easterlies and up to 11 m/s during the north-westerly season. This seasonal reversal is of great geomorphic significance to the low reef islands of Torres Strait, driving sediment movement around their shorelines (Kench & Brander 2006). Masig Island is the larger of two reef islands on the boomerang-shaped Masig-Kodall reef flat. Masig Harbor is located on the north-western shoreline of Masig Island, directly east of the airstrip (Fig. 2.2). Harbor infrastructure includes a 90 m long pre-cast concrete and rubble barge ramp with an adjoining 30 m long timber jetty. A dredged channel approximately 4.5 to 6 m deep (below Mean Sea Level) and 140 m in length connects the harbor to open water (Yorke Island Council 2000) (Fig. 2.3). Since construction of the harbor in 1992 the beach to the west has accreted significantly whereas the beach to the east has severely eroded, threatening a water
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