Elsevier

Marine Geology

Volume 344, 1 October 2013, Pages 132-143
Marine Geology

The influence of coastal reefs on spatial variability in seasonal sand fluxes

https://doi.org/10.1016/j.margeo.2013.07.016Get rights and content

Highlights

  • Investigated how coastal reefs cause spatial variability in seasonal sand fluxes

  • Revealed alongshore ‘zones’ with alternating spatial and seasonal modes of flux

  • Zone boundaries determined by reefs, headlands, infilled lagoons, and longshore current jets

  • Seasonal variability in sand fluxes driven by littoral drift, wave power and sea level

Abstract

The effect of coastal reefs on seasonal erosion and accretion was investigated on 2 km of sandy coast. The focus was on how reef topography drives alongshore variation in the mode and magnitude of seasonal beach erosion and accretion; and the effect of intra- and inter-annual variability in metocean conditions on seasonal sediment fluxes. This involved using monthly and 6-monthly surveys of the beach and coastal zone, and comparison with a range of metocean conditions including mean sea level, storm surges, wind, and wave power. Alongshore ‘zones’ were revealed with alternating modes of sediment transport in spring and summer compared to autumn and winter. Zone boundaries were determined by rock headlands and reefs interrupting littoral drift; the seasonal build up of sand over the reef in the south zone; and current jets generated by wave set-up over reefs. In spring and summer, constant sand resuspension and northerly littoral drift due to sea breezes allowed a sand ramp to form in the South Zone so that sand overtopped the reef to infill the lagoon. This blocked the main pathway for sand supply to downdrift zones which subsequently eroded. In autumn and winter, with the dominance of northwesterly storms and reversal in the direction of littoral drift, the South Zone eroded and sand travelled through the lagoon in the current jet to nourish the northern beaches. Inter-annual and seasonal variation in sea level, storm frequency and intensity, together with pulsational effects of local sand fluxes at Yanchep due to inter-seasonal switching in the direction of littoral drift determined marked differences in the volumes of seasonal sand transport. These seasonal ‘sediment zones’ highlighted interesting and unexplored parallels between coasts fronted seaward by coral reefs and rock formations.

Introduction

Perched beaches are underlain or fronted seaward by hard structures that are shallowly buried or outcrop above the sediment (Gallop et al., 2012b). 75% of the world's coast is rocky (Davis, 1996) and global estimates of the area of coral reef coverage range between 112,000 to 393,0000 km2 (Spalding and Grenfell, 1997). Beaches can be perched on all types of rock, such as beachrock that occurs in tropical as well as temperate regions such as Australia, Greece, Brazil, Israel and the Seychelles (e.g. Vousdoukas et al., 2007); limestone such as in southwestern Australia (Playford et al., 1975, Gallop et al., 2011a, Gallop et al., 2011b); and sand stone such as in Recife, Brasil (Branner, 1904). Many beaches that are perched on structures in the cross-shore direction also have lateral structural boundaries that form embayed and pocket beaches. These beaches are associated with interrupted longshore sediment transport due to their indentation; trapping-effects and hence limitations in sediment exchange to adjacent beaches (Storlazzi and Field, 2000, Ranasinghe et al., 2004, Bowman et al., 2009). The presence of reefs and platforms, and the lithological mix is a key determinant of coastal complexity (Porter-Smith and McKinlay, 2012) and there is a large gap in knowledge of how reefs and platforms affect sediment transport (Larson and Kraus, 2000, Stephenson and Thornton, 2005, Naylor et al., 2010). McNinch (2004) stated that while many coastal geologists have suggested that framework geology plays a key role in determining large-scale and long-term shoreline behaviour (e.g. Demarest and Leatherman, 1985, Riggs et al., 1995, Kraus and Galgano, 2001); it is unknown how it influences shorter-term coastal morphodynamics, although the extent, and mechanisms are still unclear. For example, in Hawaii, many beaches are eroding creating a societal hazard while others are accreting despite an increase in mean sea level (MSL) (Norcross et al., 2002). Moreover, often eroding and accreting beaches are adjacent to each other presumably because of variability in sediment supply due to spatially-variable topography (Fletcher et al., 1997). Therefore, to investigate how coastal reefs influence coastal morphodynamics over months and seasons, the current research focused on Yanchep Lagoon in southwestern Australia, where sandy beaches are perched on calcarenite limestone reefs.

The coast of southwestern Australia consists of low bluffs and extensive limestone reefs and platforms alternating with sandy beaches (Bird, 1985, Semeniuk and Searle, 1987, Collins, 1988). Spring and summer are dominated by strong, southerly sea breezes that blow parallel to the coast and result in northerly littoral drift. Winter is dominated by storms, often from the northwest which can reverse the direction of littoral drift (Kempin, 1953, Masselink and Pattiaratchi, 2001). There are limited records of temporal variability in beach morphology in Western Australia, but it has been suggested that a key driver of beach growth and recession is cumulative longshore winds (Clarke and Eliot, 1983, Eliot and Travers, 2011). The rate of northerly littoral drift in southwestern Australia due to the longshore energy flux driven by wind waves was estimated as 150,000 m3 yr 1 by Pattiaratchi et al. (1997), and between 138,000 and 200,000 m3 yr 1 by Tonk (2004). On a beach perched on a limestone platform in Cottesloe (Fig. 1a), plentiful sediment supply in summer in combination with smaller and less-steep waves and less storm surges causes the limestone platform to be covered in sand. This sand is eroded in autumn and winter due to more frequent storm-surges, steeper waves, and northwesterly winds (Doucette, 2009). On a finer temporal scale, the influence of sea breezes and storm events on sand transport was investigated hourly and daily at Yanchep Lagoon in 2010 (Gallop et al., 2011a, Gallop et al., 2011b, Gallop et al., 2011b). Longshore variation in reef topography had a dominant influence on the mode and magnitude of cross-shore and longshore sand transport due to wave refraction, current jets and scour-steps. Research reported in the current paper examines the cumulative effect of sea breezes in spring and summer; and of storm events during autumn and winter on the coastal morphology at Yanchep Lagoon.

The aim was to investigate the effect of reefs on seasonal erosion and accretion. Objectives were to: (1) examine how alongshore variation in reef topography drives alongshore variation in the mode and magnitude of seasonal beach erosion and accretion; and (2) understand the effect of intra- and inter-annual variability in metocean conditions on seasonal sediment fluxes. To achieve this, sub-aerial beach surveys were undertaken monthly for two years, and hydrographic surveys were measured five times during late summer and late winter, out to 10 m above MSL. Numerical models using XBeach (Roelvink et al., 2009, Roelvink et al., 2010) were used to determine how the coastal reefs influence currents at Yanchep.

Section snippets

Geological setting

The coastal bathymetry of southwestern Australia is dominated by a series of shore-parallel reefs of calcarenite limestone. These reefs are present from 20 km offshore (Fig. 1a; Playford et al., 1975, Semeniuk and Johnson, 1982), to the coast such as at Yanchep, located 60 km north of the City of Perth (Fig. 1a, b). The beaches of Yanchep Lagoon face southwest and consist of well-sorted medium sand with d50 of 0.4 mm (Murphy, 2011) made of mostly quartz and skeletal material (Semeniuk and Johnson,

Sub-aerial beach surveys

Nine sub-aerial beach transects (T1 to T9) were surveyed monthly on a 1 km-long stretch of coast from February 2010 to March 2012 (Fig. 1b). Profiles were surveyed from temporary benchmarks at the top of the frontal dune ridge to instantaneous sea level. All elevations are relative to Australian Height Datum (AHD) where zero AHD corresponds to MSL. Due to the low tidal range (mean of 0.45 m during the study period), the level of the tide at the time of survey was negligible. Transect 4 (T4) was

Sub-aerial beach volume

Alongshore variability in reef elevation, placement and longshore continuity was expected to lead to modifications in beach morphodynamics. This expectation was confirmed by the marked degree of spatial variation in the mode and magnitude of seasonal sand transport along the kilometre long-stretch of coast where sub-aerial beach profiles were surveyed monthly. The sub-aerial beachface landward of the reef (between the vegetation line at the toe of the dune ridge and sea level) was consistently

Sediment zones formed by the reefs

Four seasonal ‘sediment zones’ were identified, with boundaries formed by rock headlands and reefs interrupting littoral drift; the seasonal build up of sand over the reef in the south zone; and current jets generated by wave set-up over reefs. The division between the South and Middle Zones at the headland was also identified by Stul (2005), who defined sediment cells of Perth beaches based on alongshore variation in topography, bathymetry, geology and beach type. However, the boundary due to

Conclusions

This research focused on the effect of coastal reefs on seasonal erosion and accretion, in particular how alongshore variation in reef topography drives the mode and magnitude of seasonal beach erosion and accretion; and the effect of intra- and inter-annual variability in metocean conditions on seasonal sediment fluxes. Inter-annual variability in sea level, storm frequency and intensity, together with seasonal switching in the direction and magnitude of wave power and littoral drift in

Acknowledgements

This work was led by S.L.G as part of a PhD at The University of Western Australia and was funded by the Samaha Research Scholarship and the Western Australian Marine Science Institution (WAMSI, Project 6.1). For the hydrographic surveys and survey benchmarks we are grateful to the Department of Transport, in particular Charlie Bicknell, Karl Ilich, Lucya Roncevich, Fangjun Li and Rebecca Popperwell. For the aerial images thanks to Nearmap and Landgate and for wind, sea level and wave data to

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