THEORY FOR BACKFILLING OF TSUNAMI-INDUCED BEACH EROSION

The 2011 tsunami formed concave shoreline around the Nanakita River mouth and Akaiko breaching areas, Sendai Coast, Miyagi Prefecture, Japan. The recovery process of morphology in these areas is much depended on the longshore sediment which is transported from adjacent sandy coasts. Coastal structures on both sides of the concave portion are considered as rigid boundaries. Analytical solutions of one-line model, which describe the evolution of shoreline around the concave portion in cases without and with rigid boundaries, have been discussed. When the dimensionless of concave width to the length of bounded area is getting smaller, the dimensionless recovery time obtained from solution for the case with rigid boundaries is getting larger and asymptotic the one for case without rigid boundaries. An analytical solution for estimating the proportion of backfilling of sediment deposition in the concave portion for case with rigid boundaries is introduced. It is asymptotic to the one without rigid boundaries when the dimensionless of concave width to the length of bounded area is getting smaller. The comparison between theoretical results and measured data of cases of the Nanakita River mouth and the Akaiko breaching areas are presented.