Serious Erosion of the Southern Sendai Coast Due to the 2011 Tohoku Earthquake Tsunami and Its Recovery Process - Post-Tsunami Hazard: Reconstruction and Restoration - page 221

Fig. 15.3 Aerial photographs of coastal areas in September 2010, March 2011, July 2011,
September 2012, and November 2013. The locations of ( a ) and ( b ) correspond to those of
Fig. 15.2c, d (Source: Tohoku Regional Bureau, Ministry of Land, Infrastructure, Transport and
Tourism of Japan)
15 m, whereas deposition expands in the narrower and shallower area in Area A. In
Area B, the river mouth terrace was eroded and slight deposition was observed on
the seaward side. Also, deposition occurred around the breakwaters. In Area D,
deposition occurred near the headlands (Fig. 15.1d ).
Characteristics of the sediment transport during the tsunami are discussed using
the aerial video. In Area A, erosion occurred behind the seawall and water channels
were formed. The video image (Fig. 15.5a ) of the Fukanuma Beach, located 10 km
northeast of the Sendai Airport, where the topography conditions such as beach
profi le and beach width were the same as those of Area A, shows sediment suspen-
sion behind the seawall and succeeding landward sediment transport during the
tsunami runup; this must be a major mechanism of erosion behind seawalls (Kato
et al. 2013 ). In the vicinity of the breakwater in Area B where deposition was
occurring, eddies were formed and sediment concentration increased (Fig. 15.5b ).
In Area C and D where beach erosions were apparent and depositions occurred
extensively over the sea area (Fig. 15.1c, d ), sediment concentration in the sea area
increased during backwash, and most signifi cant deposition occurred at the seaward
border of the sediment transport zone (Fig. 15.5c, d ). This indicates that the deposi-
tion was caused by the seaward sediment transport during backwash through the
seawall crevasse. These facts demonstrate that coastal morphology change due to
tsunami strongly depends on the coastal structures.