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

• Published in: Marine geology Vol. 169; no. 1; pp. 161 - 183
• Main Authors: Ruessink, B.G, van Enckevort, I.M.J, Kingston, K.S, Davidson, M.A
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.09.2000
• Subjects: Crescentic bars; Marine; Nearshore bars; Nearshore environment; Netherlands; Rhythmic topography; ANE, Netherlands, Noord-Holland, Egmond aan Zee; Crescentic bars; Nearshore environment; Nearshore bars; Rhythmic topography
• ISSN: 0025-3227; 1872-6151
• DOI: 10.1016/S0025-3227(00)00060-8

Shore-parallel sandbar systems often exhibit considerable quasi-regular alongshore variations, such as crescentic plan shapes. Therefore, morphological change in such systems commonly consists of two- and three-dimensional variability. The former is related to overall on/offshore bar migration, whereas the latter is induced by the horizontal amplitude growth, migration, or length scale change of the quasi-regular topography. In this paper, we used a six-week data set of bathymetric surveys and video images of breaking-induced 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 description of the inner-bar data set was provided by complex empirical orthogonal function (CEOF) analysis. The first complex mode, containing about 85% of the variance in the data set, corresponded to the amplitude growth and longshore migration of an approximately 600 m long alongshore non-uniformity. The longshore migration rate varied between 0 and 150 m/day and was found to be well related to the longshore component of the offshore wave energy flux. The second complex mode explained about 10% of the variance and largely described the alongshore-averaged cross-shore bar migration. The CEOF results suggest that short-term variability in bar-crest position is largely due to changes in the quasi-regular topography and not to alongshore-uniform on/offshore oriented behaviour.