Blogs from the Coastal Structures Team - Tohoku, Japan Earthquake and Tsunami of 2011 - page 73

11:30 am where we had a good view from the port's interior shoreline of the Hotaro
breakwater. This breakwater has a crest elevation of 5 meters above mean sea level
and was constructed of concrete caissons 7 to 8 meters in height placed on top of a
rock-lined earthen-filled mound. The mid-section of the breakwater had been
severely damaged and the outer end section had a change in alignment and was
completely missing above the water level. The caissons failed inward (landward),
with about 100 caissons coming off the breakwater into the port's inner basin.
The tide range in this area of Japan is about 1.5 meters. Even though the tsunami
occurred during low tide, when the water level was approximately 0.4 meters below
the mean sea level, the first tsunami wave measured approximately 8 meters at the
harbor entrance and 5 meters at the port facilities. The entrance channel,
approximately 10 meters deep prior to the tsunami, was scoured down to 17 meters at
its deepest part. There was also significant damage to the port's ferry terminal
building. We spent some time with the ferry terminal supervisor, Gonohe-san,
discussing the sequence of events after the tsunami warning was issued. Because the
ferry operators had about an hour of warning before the first tsunami waves, the boats
had time to leave the harbor and there was relatively little vessel damage.
Continuing south, our next stop was Tanesashi Beach. This area has both an open
space area with scenic views of the rocky shoreline and, to the south, a small fishing
harbor protected by a breakwater. The park area and bathroom facilities were
inundated but the harbor was not impacted by the tsunami. We then stopped at Oja
(which means Big Snake), visited a school located approximately 18 meters above
sea level and above the inundation level for the recent March 11, 2011 tsunami. The
school grounds had been inundated during both the 1896 tsunami and 1933 tsunami,
and while at the site, we visited a monument for the 1896 and 1933 tsunamis.
Because of the water levels from these past events, the school site is not considered
suitable as an evacuation site.
After stopping for a quick lunch we drove south to Noda. Here we had our first
realization of the enormous destructive power of the tsunami generated by Tohuku
Pacific earthquake when we observed the Noda tsunami seawall and town directly
behind the seawall. The town was protected by an extensive seawall that had been
constructed in precast segments. The existing older tsunami wall was constructed 25
to 30 years ago and was about 8 meters above sea level. Over time, a large amount of
armor units had been placed on the seaward face of the wall, adding to the protection.
A newer, higher concrete tsunami seawall, about 10 meters above sea level, was well
under construction to replace this older wall. At the time of the tsunami,
approximately 300 meters of the new seawall was already complete. The new wall
was also constructed with a concrete walkway and a landward concrete protected
slope behind the wall. The tsunami destroyed most of the older wall and overtopped
the newly completed sections of the 10-meter high seawall. There was massive
flooding and damage (complete destruction) to everything well inland of this tsunami
seawall. The waves even stripped the branches from the pine trees located landward
of the seawall and the 10-meter high branchless trunks were a good indicator of the