Longshore propagation erosion of beach in the vicinity of tsunami-induced concave shoreline

Abstract

The 2011 Great East Japan Earthquake and Tsunami caused significant changes in the morphology of the Sendai Coast, Miyagi Prefecture, Japan. These tsunami waves and the return flows resulted in the formation of concave shorelines in areas like lagoon locations adjacent to river mouths or where sandy beaches located on pre-existent (closed) river mouths were breached. This study investigates the erosion of sandy beaches adjacent to the concave shoreline at the Nanakita River mouth and the Akaiko breaching areas after the tsunami through an analysis of aerial photographs and analytical solutions of the one-line model. Soon after the tsunami, sandy beaches on both sides of the concave shoreline experienced erosion that was propagating along the coast. Analysis of the analytical solution of the one-line model indicates that the erosion propagation distance is proportional to square root of elapsed time. Measured data of erosion propagation distance from aerial photograph is found to match well with theoretical results. An approach to estimate diffusion coefficient, which is an important parameter in numerical simulation of shoreline change, is introduced.