Decadal morphodynamic behaviour of the Holland shoreface
The shoreface, the 'buffer zone' between the land and sea, is one of the most important coastal regions directly influencing the coastal sediment budget through its role as a sediment sink or source. It therefore has a potentially significant impact upon large-scale (10 km; decades) shoreline movement. However, knowledge of shoreface activity over the medium- (I km; years) and, more particularly, the large-scale remains scarce, primarily as a consequence of data limitations. This empirical thesis extends existing knowledge to the large-scale through the observation of the temporal and spatial characteristics of shoreface morphodynamic behaviour over a 32 year period. It takes a data-orientated approach using the unique JARKUS data set which is composed of cross-shore bathymetric profiles covering the entire Holland coast to a maximum offshore distance of 3 km (approximately 16 m depth). The observations made are also used to i) evaluate the ability of existing models to predict shoreface morphodynamic behaviour; and ii) examine the evolution of shoreface activity beyond the data limits. It is shown that not only is there a cross-shore limit to significant depth change on the upper shoreface (as previously observed in short-scale studies), but over the longer temporal periods (2: 10 years), the middle and lower shoreface typically undergoes significant erosion. These observations are forthwith named the 'shoreward depth of closure' and 're-opening zone', respectively. The observed shoreface activity has also been classified as a function of the cross-shore extent of the activity as either 'non-', 'partially-' or 'fully-active'. Shoreface activity is strongly spatially- and temporally dependant, such that i) the Holland coast can be divided into two longshore provinces of similar morphodynamic characteristics; and ii) after 100 years, the north Holland shoreface is predicted to become fully active i. e. there is no cross-shore limit to the activity. Shoreface processes are under the control of internal dynamics e. g. the nearshore bar system, and external forcing e.g. hydrodynamics. The relative significance of these forcings is temporally dependent; for example the nearshore bar system has a greater relevance on the upper shoreface activity over the shorter time periods. Although most readily applicable to wave-dominated coastlines with similar characteristics to the Holland coast e.g. a near shore bar system, the fundamental ideas arising from this work could also be applied to coasts with different environmental conditions e.g. tidally dominated. Essentially this study shows that shoreface activity is more widespread in the cross-shore than previously appreciated. One consequence is that there will be a greater sediment volume in transport than formerly acknowledged and accounted for in, for example, sediment budgets.