Video observations of nearshore bar behaviour. Part 2: alongshore non-uniform variability

Abstract

A considerable portion of the variability in nearshore sandbars is related to changes in the plan shape of quasi-rhythmic alongshore non-uniform features, such as rip channels and crescentic shapes. These changes may include changes in their alongshore length, cross-shore amplitude and alongshore position. Here, we use complex empirical orthogonal eigenfunction analysis to quantify these changes from a 3.4-year data set of almost daily time-exposure images of the double-barred coast at Noordwijk (Netherlands). The observed alongshore non-uniform features had alongshore lengths between 380 and approximately 3000 m and lifetimes in the order of months, considerably longer than the characteristic time scale of individual wave events. Transitions from one feature to another were mostly gradual, resulting from an alongshore differential growth in amplitude. Abrupt transitions, that is, the existing features disappeared entirely and were subsequently replaced by different features, were barely observed and did not always take place during high-energy wave events. The amplitude of the non-uniform features varied between 0 and 30 m on a weekly to monthly scale, unrelated to variations in the wave height. In addition, the features migrated back and forth along the shore with typical rates of O(10 m/ day) on weekly scales with the rates increasing with an increase in the alongshore component of the wave power. On the whole our observations suggest that alongshore non-uniform sandbar variability is governed by free behaviour rather than by the direct forcing of the prevailing wave conditions.