EROSIVE WAVES IN SHORELINE CHANGE DUE TO THE REDUCTION OF A RIVER DELTA

Yoshito Tsuchiya

Abstract


Using observations of long-term data of shoreline change during the reduction process of a river-mouth delta, space-time shoreline changes and their variations are clarified by the moving-average method. There clearly exist two modes of shoreline changes due to distinct types of erosive wave. The first mode is defined to be an erosive wave propagating down-coast as a diffusion phenomenon. The second was estimated from the variation from the erosive wave and defined to be an erosive wave propagating down-coast as a wave phenomenon. We suggest new terminology for these two kinds of erosive wave. The erosive waves of the first mode are subject to change as positive and negative erosive waves due to the initial and boundary conditions at the river mouth in relation to the sediment input from the river and local change in the submerged river delta. The erosive wave of the second mode propagates down coast faster than the first mode erosive wave. We therefore conclude that shoreline change due to beach erosion can be described as space-time change propagating down-coast as a diffusion phenomenon, upon which shoreline variation propagates as a wave phenomenon faster than the shoreline change. Additionally, a theory for the diffusionwave phenomenon of shoreline change is attempted using a set of equations of longshore sediment transport rate and continuity of shoreline change to find the dispersion relation of erosive waves.

Keywords


erosion; shoreline change; river delta; delta reduction

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