Video observations of nearshore bar behaviour. Part 1: alongshore uniform variability

Changes in nearshore sandbar morphology comprise of an alongshore uniform and non-uniform component. The former reflects the overall on/offshore migration, while the latter expresses changes in quasi-rhythmic non-uniformities, such as crescentic plan shapes. Here we focus on the alongshore-uniform component, quantified from a 3.4-year data set of daily time-exposure video images of the double barred nearshore at Noordwijk, Netherlands. The high temporal resolution and the long duration of the data set allowed us to quantify the cross-shore bar migration at weekly, seasonal and inter-annual time scales and, accordingly, to compare the contribution of all three components to the total variability in cross-shore bar position. The maximum observed offshore-directed weekly, seasonal and inter-annual bar migration rates were 10, 0.5 and 0.2 m/day. Maximum onshore rates at weekly and seasonal scales were 8 and 0.5 m/day, while onshore migration at the inter-annual scale was not observed. The inter-annual bar migration dominated the bar crest variability over time spans longer than 10–13 months, whereas changes on the weekly scale were the dominant source of variability on time spans shorter than 7–10 months. Seasonal bar migration only dominated the bar crest variability at the outer bar on time spans between 7 and 13 months. In general, Noordwijk appears to be a site with a strong inter-annual signal, with limited seasonal variability, and with fluctuations at weekly scales that are long compared to the characteristic time scale of individual events, suggesting a response to sequences of events rather than to individual events.