Barred beaches

Seven different bar types are distinguished to provide a framework for comparing morphodynamic studies conducted in different areas. Five types occur in semiprotected or open coast settings, of which two are intertidal and three are subtidal. Two types occur in highly protected settings. The occurrence of a certain bar type is generally determined by the wave energy and tidal range, although the nearshore slope may also be a differentiating boundary condition. The theory behind the generation, evolution and decay of bars has evolved most for the subtidal bars in the semiprotected and open coast settings, for which three types of competing mechanisms have been formulated (breakpoint, infragravity waves, self-organisational). Most research has focused on these processes on the time scale of storm events and post-storm recovery. However, to understand the longer-term behavior of bar systems, knowledge of the role of relaxation time and morphologic feedback is needed as well. At present, such knowledge is very limited. We think it can best be obtained from the analysis of long time series of morphology and forcing conditions, rather than from intensive field experiments. In case of a feedback-dominated response (self-organisational), we expect to find no correlation between the time series of external forcing and the morphologic response. In case of a relaxation time-dominated response, we do expect to find such a correlation, albeit filtered. This discussion is illustrated by a case study of the Dutch coast. (C) 2002 Elsevier Science B.V. All rights reserved.