Serious erosion of the southern Sendai Coast due to the 2011 tohoku earthquake tsunami and its recovery process

Abstract

We investigate morphology change of the southern Sendai Coast due to the 2011 Tsunami by analyzing topography and aerial images before and after the tsunami. The results show the characteristics such as erosion in the longshore direction behind seawalls, landward sediment transport during tsunami runup, seaward sediment transport from shore during backwash especially through crevasses of the seawalls, and coastal stabilization by coastal structures such as seawalls, breakwaters and headlands. At the seriously eroded Yamamoto Coast, more than half of the total amount of eroded shore sand above sea level was estimated to be transported seaward due to backwash. After 1 year from the tsunami, the eroded coasts were recovered to form pocket beaches. After another year, the coastal morphology had not changed apparently but seawalls started to be reconstructed. At present, after 3 years from the tsunami, the seawalls with a height of 7.2 m have been reconstructed along the coast. With the reconstruction, the foundation ground of the seawalls has been recovered, but the eroded beaches still remain disappeared. The coast act in Japan was established in 1956 to protect the coast from disasters, and amended in 1999 to also preserve both the coastal environment and its utilization. From the perspective of long-term coastal management, it is strongly required to consider the vision of the future coast.