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 224

than 5 m from April 2011 to February 2012; however, after which period changes
were insignifi cant. By comparing morphology changes before and after the tsunami,
we found that the change in Area D was likely to decrease after the tsunami even if
the effect of nourishment in 2010 is taken into account.
The erosion of shallow sea area in Area C and D just after the tsunami is consid-
ered because of landward sediment transport and this contributed to the recovery of
shore morphology after the tsunami, based on the facts that shore morphology
recovered rapidly just after the tsunami and that the amount of erosion in the
shallow sea area was enormous. The reason of the afterward decrease of morphology
changes in Area D is unclear and further study is required.

15.4.3 Reconstruction Process of Coastal Structures

and Recovery of Ecosystems

Due to the tsunami, seawalls of total 190 km out of 300 km were damaged along the
coasts in the three most damaged prefectures (Iwate, Miyagi, and Fukushima; Kato
et al. 2013 ). In the study area, Tohoku Regional Bureau had constructed 17 km
temporary seawalls at a height of 3.8-6.2 m relative to Tokyo Peil (T.P.) datum until
August 2011 (6 months after the tsunami) in order to prepare for storms in the
period from late summer to autumn. Until March 2015, 21 km seawalls at a height
of 7.2 m relative to T.P. are planned to be reconstructed. The seawall height was
determined by the sum of the mean spring high tide level, the maximum storm surge
deviation, 30-year return period wave height, and allowance height, because storm
surges are the greatest threat in the study area. Tsunami debris of total 110 thousand
m 3 is used as banking material. As of December 2013, 60 % of the seawalls had
been constructed and 30 % are being constructed (Fig. 15.3 ; Tohoku Regional
Bureau 2013 ). With the reconstruction, the foundation ground of the seawalls has
been recovered, but the eroded beaches still remain disappeared. On the other hand,
the construction plan of headlands and nourishment in progress before the tsunami
are revised to continue until the 2070s (Tohoku Regional Bureau 2012a ).
Before the seawall constructions, effects of the seawalls on landscape and envi-
ronment were discussed in committees composed of people of learning and experi-
ence, and comments from the committee members on the landscape were considered
in seawall design (Tohoku Regional Bureau 2012b ). On the environment, monitor-
ing surveys of fl ora and fauna were conducted in autumn and winter in 2002, autumn
and winter in 2011, and summer in 2012, which continue until 2015 in order to
investigate the recovery ecosystems (Table 15.1 ; Tohoku Regional Bureau 2012b ).
According to that, the number of plant, bird, and land insect species at beaches
increased after the tsunami from 67 in 2002 to 248 in 2012, from 37 in 2002 to 55 in
2011 and then 45 in 2012, from 147 in 2002 to 229 in 2012, respectively; however,
it remains unclear whether they were caused by the tsunami because the data before
the tsunami was obtained in 2002, that is, 9 years before the tsunami. At the