Shoreline variability of an urban beach fronted by a beachrock reef from video imagery

Abstract

This contribution presents the results of a study on the shoreline variability of a natural perched urban beach (Ammoudara, N. Crete, Greece). Shoreline variability was monitored in high spatio-temporal resolution using time series of coastal video images and a novel, fully automated 2-D shoreline detection algorithm. Ten-month video monitoring showed that cross-shore shoreline change was, in some areas, up to 8 m with adjacent sections of the shoreline showing contrasting patterns of beach loss or gain. Variability increased in spring/early summer and stabilized until the end of the summer when partial beach recovery commenced. Correlation of the patterns of beach change with wave forcing (as recorded at an offshore wave buoy) is not straightforward; the only discernible association was that particularly energetic waves from the northern sector can trigger changes in the patterns of shoreline variability and that increased variability might be sustained by increases in offshore wave steepness. It was also found that the fronting beachrock reef exerts significant geological control on beach hydrodynamics. Hydrodynamic modelling and observations during an energetic event showed that the reef can filter wave energy in a highly differential manner, depending on its local architecture. In some areas, the reef allows only low-energy waves to impinge on the shoreline, whereas elsewhere penetration of higher waves is facilitated by the low elevation and limited width of the reef or by the presence of an inlet. Wave/reef interaction can also generate complex circulation patterns, including rip currents that appeared to be also constrained by the reef architecture.

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Acknowledgments

This work was supported by the projects BEACHTOUR (11SYN-8-1466) of the Operational Programme ‘Cooperation 2011, Competitiveness and Entrepreneurship’ and AKTAIA (09SYN-31-711) of the Operational Programme ‘Operation 2009—Partnerships of Production and Research’ co-funded by the European Regional Development Fund and the Greek State. NCMR and HNMS are thanked for the provision of wave and wind data. M.I. Vousdoukas acknowledges funding from the European Union Seventh Framework Programme FP7/2007–2013 under Grant Agreement No. 603864 (HELIX: ‘High-End cLimate Impacts and eXtremes’; www.helixclimate.eu), as well as by the JRC institutional project Coastalrisk. The authors would like to thank G. Alexandrakis, S. Poulos and S. Petrakis for their help during fieldwork as well K. Chatzikonstantinou for his assistance in the generation of Fig. 1. The two anonymous reviewers, whose comments improved substantially the quality of the manuscript, are gratefully acknowledged.

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Velegrakis, A.F., Trygonis, V., Chatzipavlis, A.E. et al. Shoreline variability of an urban beach fronted by a beachrock reef from video imagery. Nat Hazards 83, 201–222 (2016). https://doi.org/10.1007/s11069-016-2415-9

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Keywords

  • Beach morphodynamics
  • Coastal video imagery
  • Perched beaches
  • Beachrock reef