Rd. Shand et al., An inter-site comparison of net offshore bar migration characteristics andenvironmental conditions, J COAST RES, 15(3), 1999, pp. 750-765
In this paper we quantitatively identify behavioural characteristics of net
offshore bar migration (NOM) and present the results of an inter-site comp
arison. 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 Ne
w 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). T
he NOM sites are on multi-bar coasts with relatively short period waves and
a narrow range of storm strength mind and wave conditions. They encompass
a wide range of geometrical dimensions, physical boundary conditions and wi
nd and wave approach angles relative to the shoreline. Parameters measuring
migration width, duration, return period and rate of NOM are used to ident
ify bar migrational characteristics for each stage. There is wide variation
in the parameter values, both between the zones representing the NOM stage
s at each site, and between corresponding zones at different sites. NOM dur
ation for stage 2 is identified as the system index parameter. An inter-sit
e correlation analysis between NOM duration and key environmental parameter
s shows NOM activity to increase, i.e. NOM duration decreases, with increas
ing nearshore slope and decreasing wave height. NOM duration also decreases
when the predominant wind direction tends towards a maximum of 40 to 45 de
grees from the coastline. It is suggested that bar size and longshore curre
nts influence NOM and possible mechanisms are discussed.