Shoreline evolution of the Holland coast on a decadal scale

The decadal evolution of the dunefoot position along 120 km of the Holland coast is analysed. Firstly, a new definition of dunefoot is introduced base d on hydrodynamic and morphologic considerations, which is not affected by local and instantaneous processes occurring immediately before the bathymet ric survey. The dunefoot evolution over decadal scales indicates the existe nce of spatial and temporal oscillations in the shoreline position with mag nitudes of 2-3 km length and a periodicity of 4-15 years. Two main controll ing factors of this behaviour are identified: (1) influence of sub-aqueous bar systems, and (2) changes in the storm-wave conditions reaching the coas t. Although the precise controlling processes of the relation between the d unefoot and the subaqueous profile still remain unclarified, we introduce t he concept that the development of a morphological bar cycle requires a fix ed amount of time-integrated forcing that is proportional to the cumulative effect of storm waves. Beach mobility along the Holland coast on decadal scales (10-20 m) is simil ar or lower than mobility introduced by storms or by seasonal cycles. Howev er, it is important to consider these changes for the possible implications on the local vulnerability of the coast to extreme events. Copyright (C) 1 999 John Wiley & Sons, Ltd.