CONCEPT FOR INFERRING THE LITTORAL DRIFT TREND

Abstract

To infer the rate and prevailing direction of littoral transport along a given coast is of prime importance in the functional and rational designing of coastal structures as well as in the environmental consideration of coastal zones. Many efforts have been made to develop the inferring method from field evidences such as 1) coastal landforms and shore configurations in the vicinity of existing structures1'~°', 2) alongshore variations in beach and bed sediment properties V)~12)j 3) statistical analysis of incident wave properties 13)~16)/ 4) movements of natural or artificial tracers 17)-20)^ an(j so on_ Among these evidences, the alongshore variations of indigeneous beach materials have served as a powerfull, practical and economical tool in the inference of general trend of littoral transport. Properties of beach sediment, such as grain size of sand and volume and shape of gravels, indicate some progressive trends with alongshore distance from supply sources2D. Thus the littoral transport direction along a given coast is inferable from variations in beach sediment properties. However, inferring criteria of prevailing direction from variations in various beach sediment properties have not yet been fully established. For example, the prevailing direction of littoral transport should be defined to be alongshore component of intergrated vector of littoral transport during several years or decades along a given coast. But this direction is often confused with the alongshore dislocating or traveling direction of beach sediment, which is indicated by the variations. It has also been concluded simply that the variation series obtained from backshore samples show a long-term trend of littoral transport, whereas those from foreshore indicate a short-term trend such as seasonal changes. In this inferring procedure any considerations have never been paid on differences in grain size of beach materials (sand and gravels) and in wave conditions during the period of field study. Rate or intensity of littoral transport has scarcely been inferred from the variation series of beach sediment properties even quantitatively. To solve these problems, the present study proposed a new concept on the basis of field evidences obtained on the Enshu Coast, Central Japan.