Coastal Restoration Trust of New Zealand

Coastal Dune Ecosystem Reference Database

Beach cut in relation to net offshore bar migration Journal Paper

Shand, R.D.; Hesp, P.A.; Shepard, M.J.
Journal / Source
Journal of Coastal Research
Plan-view morphological configurations were identified for the inner sub-tidal bar at Wanganui, New Zealand, and configuration behaviour was related to wave height, longshore current (in terms of longshore wind components), tidal range (in terms of neap tide-spring tide variation), and antecedent morphology to derive a conceptual model for this type of coast. The Wanganui coast is characterised by fine sand, multiple sand bars, frequent storm waves, longshore current, a neap tidal range of 0.8 m and a spring tidal range of 2.4 m. Transverse configurations occur more frequently and were more persistent than the other 3 types of configuration used in the study: linear, undulating and subdued. The predominant class transitions consist of transverse configurations changing to subdued and undulating; undulating configurations changing to transverse; linear configurations changing to undulating, and subdued configurations changing to transverse. Configuration change to transverse, undulating and linear occur under lower (neap) tidal conditions, with linearity increasing with increasing wave height and strength of the longshore current. Change to subdued configurations corresponds with higher (spring) tides, lower wave height and weaker longshore current. The direction of longshore currents also influences configuration change. In particular, the formation of transverse configurations are facilitated by longshore currents aligned with the oblique rip currents which characterise the Wanganui coast. By contrast, the formation of subdued and undulating configurations appear to be facilitated by longhore currents opposing obliquely orientated rip currents. Morphological feedback enables transverse configurations to resist greater wave height and stronger longshore current than occur during their formation. While undulating and linear configurations remain coherent during higher tidal range, bar-crests broaden in the cross-shore direction with the opposite occurring during times of lower tidal range. Transverse configurations may also persist during higher tidal range; however, in this situation bar-crest broadening and channel infill can result in a class change to subdued configurations.