An Inter-Site Comparison of Net Offshore Bar Migration Characteristics and Environmental Conditions

Abstract

In this paper we quantitatively identify behavioural characteristics of net offshore bar migration (NOM) and present the results of an inter-site comparison. The net offshore migration of sandbars on multi-bar coasts has been reported at sites on the Dutch coast, the eastern USA seaboard, and the New Zealand west coast. The NOM phenomenon is repetitive, with the life-cycle of each bar consisting of three stages: bar generation near the shore-line (stage 1), systematic offshore migration of the bar across the surf zone (stage 2), and finally bar disappearance in the outer surf zone (stage 3). The NOM sites are on multi-bar coasts with relatively short period waves and a narrow range of storm strength wind and wave conditions. They encompass a wide range of geometrical dimensions, physical boundary conditions and wind and wave approach angles relative to the shoreline. Parameters measuring migration width, duration, return period and rate of NOM are used to identify bar migrational characteristics for each stage. There is wide variation in the parameter values, both between the zones representing the NOM stages at each site, and between corresponding zones at different sites. NOM duration for stage 2 is identified as the system index parameter. An inter-site correlation analysis between NOM duration and key environmental parameters shows NOM activity to increase, i.e. NOM duration decreases, with increasing nearshore slope and decreasing wave height. NOM duration also decreases when the predominant wind direction tends towards a maximum of 40 to 45 degrees from the coastline. It is suggested that bar size and longshore currents influence NOM and possible mechanisms are discussed.