Longshore realignment of shore-parallel sand-bars at Wanganui, New Zealand

Abstract

The disconnection and realignment of shore-parallel nearshore sand-bars in the longshore direction is a recently identified morphological behaviour which is referred to as bar switching. This phenomenon has been observed in data from multi-bar coasts in The Netherlands, in North Carolina on the east coast of the USA, and on the west coast of the New Zealand North Island. This paper identifies the characteristics of bar switching along a 6 km stretch of coast at Wanganui, New Zealand. Analysis of a 6.3 yr record of image-based morphological data identified nine periods or episodes of bar switching. Switching occurred within transition zones which had longshore lengths between 500 and 1000 m. Episodes occurred throughout the study period at intervals ranging from 0 to 64 weeks ( mean=25 weeks) . Episode duration ranged from 8 to 27 weeks ( mean=14 weeks) . Episodes tended to occur sequentially at centres located 2000–3000 and 4400–5200 m from the nearby Wanganui River mouth which marks the southeastern boundary of the study area. Two types of switching episodes were identified. Shoreward propagating episodes originate in the outer surf zone and the location of switching then moves landward. By contrast, stationary episodes begin and remain within the mid-surf zone. Episodes of bar switching are characterised by strong longshore currents, peak significant wave height values that are usually greater than the 1% exceedence level (3.2 m), and significant wave heights above the 5% exceedence level (2.6 m) for at least 4.5% of the switching period. While high-energy conditions are necessary for bar switching to occur, such energy levels do not always result in this type of morphological behaviour. Antecedent morphology and other hydrodynamic factors may also play important roles in the morphodynamics of bar switching.