The behaviour of a multiple bar system in the nearshore zone of Terschelling, the Netherlands: 1965–1993

Abstract The behaviour of a multiple bar system in the nearshore zone of the island of Terschelling, the Netherlands, is investigated on the time scale of years using a data set of soundings. The data set covers the period from 1965 to 1993. The behaviour is analysed in terms of bar crest position and in terms of morphometric parameters, such as bar crest depth, height, width, and volume. Two or three breaker bars are permanently present in the study area. Each individual bar passes through three stages during its existence: (1) generation close to the shore, (2) seaward migration and (3) degeneration at the outer margin of the nearshore bar zone. The key factor controlling the behaviour of the inner bars is the crest depth of the outer bar. Three different couplings between the behaviour of the individual bars have been observed: (1) bar stage changes due to the degeneration of the outer bar, (2) prevention of the transition of an inner bar from the first to the second stage, caused by the appearance from alongshore of a new outer bar having a small crest depth and (3) difference in mean annual seaward migration rate caused by the possible presence of a more seaward positioned bar. It is hypothesized that the crest depth of the outer bar governs the relative importance of processes inducing shoreward and seaward migration in the inner nearshore bar zone and therefore controls the behaviour of the nearshore bar zone on the time scale of years.