MODELING SHOREFACE PROFILE EVOLUTION

Current knowledge of hydro-, sediment and morpho-dynamics in the shore face environment is insufficient to undertake shoreface-profile evolut ion modelling on the basis of first physical principles. We propose a simple, panel-type model to map observed behaviour. The internal dynam ics are determined by slope-dependent, wave-induced cross-shoreface tr ansports, while the external driving factors are lateral sediment supp ly and sea-level rise. This model concept is tested with reasonable su ccess against the observed behaviour of the Central Holland Coast, con sidering two hindcast periods, one covering the evolution over the las t century, the other the Subboreal/Subatlantic evolution. A limitation of this model is that the cross-shoreface dynamics are solely steered by the variations of shoaling, short waves. Since a variety of other wave and current dynamics may be expected to be present in the coastal boundary layer, it may well be that the effects of the mechanisms and conditions which are not represented are hidden in the coefficients o f the sediment-transport formula. This limits the accuracy of the coef ficients as used, and our findings should be considered as an-order-of -magnitude estimate only. Indeed, behaviour-oriented modelling implies that generalization of results to arbitrary situations and conditions is not straightforward. Yet, we expect that some of the conclusions a re more generally applicable. This concerns the substantiation of the assumption that the upper shoreface responds on a much smaller time sc ale than the lower shoreface, and the idea that the shoreface profile is not always and everywhere in equilibrium with its forcing. A worthw hile observation from the Holland Coast application is, that the botto m slope effect on the transport is only important at geological time s cales. The profile evolution at the engineering time scales (say 10 to 100 years) is effectively quasi-static, in that there is no feedback between the long-term averaged transport and the state of the profile. This implies that at these smaller scales the profile changes can be predicted on the basis of a static sediment balance. This does not mea n that the gravitational downslope transport is unimportant as a physi cal phenomenon in coastal profile evolution: It is only unimportant if a highly aggregated model like this is applied at relatively short ti me scales.