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

The disconnection and realignment of shore-parallel nearshore sand-bars in the longshore direction is a recently identified morphological behaviour wh ich is referred to as bar switching. This phenomenon has been observed in d ata 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 str etch of coast at Wanganui, New Zealand. Analysis of a 6.3 yr record of imag e-based morphological data identified nine periods or episodes of bar switc hing. Switching occurred within transition zones which had longshore length s between 500 and 1000 m. Episodes occurred throughout the study period at intervals ranging from 0 to 64 weeks (mean = 25 weeks). Episode duration ra nged from 8 to 27 weeks (mean = 14 weeks). Episodes tended to occur sequent ially at centres located 2000-3000 and 4400-5200 in from the nearby Wanganu i 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 energ y levels do not always result in this type of morphological behaviour. Ante cedent morphology and other hydrodynamic factors may also play important ro les in the morphodynamics of bar switching. (C) 2001 Elsevier Science B.V. All rights reserved.