SEA-LEVEL RISE AND SHORE NOURISHMENT - A DISCUSSION

The role of sea-level rise in the long-term evolution of the world's coastlines is generally acknowledged in a qualitative sense. Hence it is logical that it is taken into account in the design of shore nourishment schemes. However, because of the large-scale character (in both time and space), it is difficult to quantify its impacts. The state-of-the-art in deductive modelling does not allow for an approach based purely on mathematical physics. Therefore, an alternative approach is proposed, utilizing conceptual models which are made quantitative and validated with the aid of inductive ideas, such as those inferred from observed or through analogy expected behaviour. An example is a conceptual LSCE model in which sea-level rise is one of the "sinks" acting on the active zone of a coastal cell. This model describes the coastal evolution at the time scale of the sea-level rise. At a somewhat smaller time scale, say the lifetime of an individual nourishment, a profile behaviour model, deductive or inductive and defined for a specific site and a specific class of situations and phenomena, can be the appropriate answer. With the aid of these design tools one does not have to fall back in the too simple and usually false assumption that shoreline retreat before and after a nourishment is the same. This allows us to include the increased effects of an accelerated sea-level rise to predict the longevity of shore nourishment. Thus, the conceptual model approach appears to be promising. Designing shore nourishment specifically to compensate for the effects of sea-level rise requires a good understanding of the coastal evolution on the longer time and space scales. Because of the uncertainties associated with these large scales this requires flexibility which, fortunately, can be achieved by combining shore nourishment with appropriate monitoring. Thus, shore nourishment is an effective mechanism to prevent shore retreat due to long-term sea-level rise.