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 218

Morphology changes of the 2010 Chile Earthquake Tsunami (2010 tsunami) and
the 2011 tsunami were compared by Tanaka et al. ( 2011 ) and Haraguchi et al.
( 2012 ). Tanaka et al. ( 2011 ) indicated that, in Sendai Bay, the 2010 tsunami ran up
rivers without causing any apparent morphology change at the river mouths, whereas
the 2011 tsunami caused notable morphology changes at the river mouths and ran
up the rivers for extensively long distances. Haraguchi et al. ( 2012 ) reported that, in
Kesennuma Bay, the 2010 tsunami caused the morphology change by about 50 cm
at maximum, whereas the 2011 tsunami induced the elevation change up to 7 m,
proving massive impact of the 2011 tsunami.
This study aims to clarify the characteristics of coastal morphology change due
to the 2011 tsunami and its recovery process on the southern Sendai Coast by
analyzing topography data before and after the 2011 tsunami and aerial images.
Also, reconstructions of coastal structures and recovery of ecosystems after the
tsunami are shown.

15.2 Study Area and 2011 Tsunami

Figure 15.1 shows the shore morphology of the southern Sendai Coast in 2006 obtained
from airborne laser scanning data, and the nearshore morphology in 2009 obtained
from bathymetry data provided by Tohoku Regional Bureau, Ministry of Land,
Infrastructure, Transport and Tourism in Japan (hereinafter referred to as Tohoku
Regional Bureau). Along the coast from the shoreline toward land, areas below 1 m in
altitude lie broadly. The Sendai Plain had been formed due to sediment supply from the
Nanakita, Natori, and Abukuma Rivers at a rate of 0.4-1.2 m year −1 for approximately
5,000 years since the period when sea level stabilized (Matsumoto 2001 ). Seabed slope
was 0.01-0.02 in the areas less than 5 m in depth, 0.005-0.01 in the areas between 5
and 15 m in depth, and below 0.002 expanding widely in the areas deeper than 15 m.
On the southern Sendai Coast, the dominant direction of longshore sediment
transport by waves is north, and sandbanks at the river mouths extend from south to
north. Notable coastal erosion began from the 1970s, particularly on the south side,
resulting from measures to prevent the erosion of cliff coast in Fukushima Prefecture
which coast was a source of sediment supply, and from the improvement of Soma
Port. This led to extensive measures for preventing coastal erosion in the late 1990s
(Tohoku Regional Bureau 2009 ). Total 19 headlands were planned to be constructed,
and 7 groins and 2 headlands had been constructed until the tsunami.
In Fig. 15.1 , the blue dashed line indicates the boundary of the inundation area
(Earth Environmental Engineering Group, Institute of Industrial Science, the
University of Tokyo 2011 ) and a line a few kilometers inland from the coastline
with an altitude of approximate 5-10 m shows the Sendai Tobu Road, indicating
that inundation at the land side of the road mainly occurred around low altitude
areas. The maximum inundation height and runup height in the study area were
13.5 m and 9.6 m respectively at the Yamamoto Coast (The 2011 Tohoku Earthquake
Tsunami Joint Survey Group 2012 ).