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 222

Fig. 15.4 Seabed elevation change from November 2009 to January 2013 (see Fig. 15.1 ). Land
elevation change is also shown only in the fi gures from September 2010 to April 2011 (Source:
Tohoku regional Bureau, Ministry of Land, Infrastructure, Transport and Tourism of Japan)
Especially in Area C and D where inundation depths were large, deposition area
was widely expanded at the sea side of signifi cantly eroded beach (Fig. 15.1c, d ),
and enormous sediment was transported from land to sea. Sediment deposition vol-
umes over 0.2 m in the sea area were calculated to be 1.6 × 10 5 m 3 and 7.4 × 10 5 m 3
in Area C and D, respectively, which corresponds to more than half of the total
amount of shore sand above mean sea level.
Considering the characteristics of both the elevation changes and the sediment
transport comprehensively, the beach erosion mechanism due to the tsunami is
estimated as follows: Surging tsunamis transported sediment landward while
causing erosion in the shallow sea area. When the tsunami overtopped the seawall,
scouring behind the seawall caused even greater volumes of sediment to be trans-
ported landward. Also, return fl ows concentrated at the points where the seawall
was destroyed and at water channels newly formed, eroding the beach and trans-
porting sediment seaward. Erosion was relatively suppressed where seawalls
remained existed.