Analysis of observed two- and three-dimensional nearshore bar behaviour

Shore-parallel sandbar systems often exhibit considerable quasi-regular alo ngshore variations, such as crescentic plan shapes. Therefore, morphologica l change in such systems commonly consists of two- and three-dimensional va riability. The former is related to overall on/offshore bar migration, wher eas the latter is induced by the horizontal amplitude growth, migration, or length scale change of the quasi-regular topography. In this paper, we use d a six-week data set of bathymetric surveys and video images of breaking-i nduced foam to quantify short-term (days-weeks) two- and three-dimensional variability in the bar-crest: position of the double barred beach at Egmond aan Zee (Netherlands). The alongshore-uniform response was computed as the mean of each bar-crest line, whereas the alongshore non-uniform behaviour was characterised by the amplitude around each mean. A more sophisticated d escription of the inner-bar data set was provided by complex empirical orth ogonal function (CEOF) analysis. The first complex mode, containing about 8 5% of the variance in the data set, corresponded to the amplitude growth an d longshore migration of an approximately 600 m long alongshore non-uniform ity. The longshore migration rate varied between 0 and 150 m/day and was fo und to be well related to the longshore component of the offshore: wave ene rgy flux. The second complex mode explained about 10% of the variance and l argely described the alongshore-averaged cross-short: bar migration. The CE OF results suggest that short-term variability in bar-crest position is lar gely due to changes in the quasi-regular topography and not to alongshore-u niform on/offshore oriented behaviour. (C) 2000 Elsevier Science B.V. All r ights reserved.