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

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.

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