The morphodynamic effects of incident wave groupiness and tide range on an energetic beach

Abstract

The roles played by daily tide range and incident wave groupiness in affecting the short-term morphodynamic responses of an energetic beach are examined. Demodulation of original tide and wave time series from the moderate energy Narrabeen Beach, Australia, resulted in new time series of daily tide range and a daily grouping factor which expresses the relative amplitude of the alternation between groups of high and low waves. Using these new time series together with time series of daily beach state and changing breaker conditions, statistical analyses were performed aimed at determining the contributions made by tide and groupiness in explaining time-varying beach state additional to those made by time-varying wave conditions. Eigenvector analyses of monthly surveyed beach and surf zone profiles provided a set of time series on modes of profile configuration change which were also analyzed to determine the relative effects of waves and tide range on profile shape. Both tide range and groupiness are significant determinants of beach state. The six beach states are reflective, low-tide terrace, transverse bar and rip, rhythmic bar and beach, longshore bar trough, and dissipative (Wright and Short, 1984). Provided that wave conditions are within the appropriate range to favor beach states at the reflective end of the sequence, spring tides will tend to favor the low-tide terrace over the transverse bar and rip state. Higher tide ranges are also associated with more subdued bar-trough topography; the more accentuated bar-trough profiles are developed when tide range is minimal. Overall, higher relative incident wave groupiness favors the more dissipative states. Incident wave groupiness is the dominant factor determining whether the low-tide terrace state or transverse bar and rip state prevails, provided incident wave steepnesses permit one or the other of these states. Groupiness is also important in discriminating between the transverse bar and rip and rhythmic bar and beach states.