Mesoscale Behaviour of Longshore Bars — Net Onshore or Net Offshore Migration

Abstract

The aim of this study is to compare long-term (30 yr) bar migration characteristics at two field sites on the Danish North Sea coast. One site displays systematically onshore migrating bars (Skallingen) while the bar migration at the other site (Vejers) is systematically offshore. Using cross-shore profile data and a sediment transport model, the paper discusses the mechanisms that cause divergent longshore bar behaviour at the two sites. The main reason for the difference appears to be larger undertow speeds at Vejers, caused by a steeper outer shoreface and a larger bar relief, leading to breaking wave reformation between bars. INTRODUCTION With the emergence of data sets on long-term profile change in e.g. the Netherlands and on the East coast of the US, it has been realized that multiple bar systems often display a tendency towards a systematic offshore migration of individual bars. Observations of such systematic seaward bar displacements were reported by Kroon (1990) and Wijnberg and Terwindt (1995) off the Dutch coast. Similar offshore migration tendencies were observed at Duck, N.C. (Plant et al., 1999) and in New Zealand (Shand and Bailey, 1999). The characteristics of the offshore bar migration cycle are similar at most sites. Longshore bars are formed close to the shoreline, they oscillate back and forth in the inner surf zone for a few years and then they begin to migrate consistently offshore. In water depths of 4-7 m, the migration cycle slows down and stops, subsequent to which the bars decay and eventually disappear. The return period for the bar migration cycle ranges between 1-13 years for the different study sites (Ruessink and Terwindt, 2000; Ruessink et al., 2003). Coastal Sediments '07 © 2007 ASCE