Méthodologie pour une simulation des transformations granulométriques de sables de la zone d'avant-côte

Abstract

The grain size of mature coastal sands changes rapidly in response to the variation of the energy regime which provokes either erosion or deposition of fractions of the textural assemblage. Whatever the dynamic conditions, questions of paramount importance such as the size of grain populations transported simultaneously, the most common size of the grains in movement and the grain-size range of the sediment fraction involved are still poorly understood. The present paper deals with an attempt to describe this fraction without any consideration of the processes responsible for the observed changes. A major problem results from the fact that the distribution of the mobilized fraction cannot be known and described by direct comparison between the initial and the final grain-size curves. The only way to characterize the mobile fraction between two subsequent grain-size states is to simulate various kinds of removal or deposition. The first question is to express the shape of the actual distributions of the sediments mathematically. The simple Gaussian approximation being irrelevant, distributions can be expressed using advanced computer programmes which can provide mathematical expressions for all distributions. The cubic-spline approximation was chosen. The mobile fraction was assumed to have a Gaussian distribution. Various tests were made in order to simulate qualitative phenomena observed in nature (bimodality, grain-size parameter changes…) and to assess the three parameters which define the characteristics of the fraction deposited or removed: modal value, dispersion of the mobile population and relative amplitude of the change. An application was carried out on the Senegalese coast offshore from the Senegal delta and on the Mediteranean coast in the Gulf of Lions. It suggests that the dynamic agents have a great selectivity. Further developments are considered.