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.

2011年的日本东部大地震和海啸导致日本宫城县仙台海岸的形态发生了重大变化。这些海啸波和回流导致在邻近河口的泻湖地区或先前存在（闭合）河口的沙滩被破坏的地区形成凹形海岸线。本研究通过航拍照片分析和单线模型的解析解，研究了南秋田河入海口凹海岸线和赤子违规区域附近沙滩的侵蚀情况。海啸过后不久，凹形海岸线两边的沙滩经历了沿海岸蔓延的侵蚀。单线模型解析解的分析表明，侵蚀传播距离与经过时间的平方根成正比。发现从航空照片到侵蚀传播距离的测量数据与理论结果非常吻合。介绍了一种估算扩散系数的方法，该方法是海岸线变化数值模拟中的重要参数。