Topics

Report on a survey of damage to riverlevees caused by tsunami triggered by theGreat East Japan Earthquake

HATTORI Atsushi (Phd), Head
FUKUSHIMA Masaki (Phd.), Senior Researcher
River Department, River Division(Key words) Tsunami runup, levee erosion, overtopping
1. Introduction
In response to this tsunami damage, theEmergency Proposal of Countermeasures for River
Runup of a Tsunami (River Tsunami
Countermeasure Study Committee, August 2011)
was compiled, organizing the handling of tsunami
external force in river management and concept oflevee height in facility planning. At the end ofOctober 2011, the Law For the Creation of Tsunami
Disaster Prevention Regions was approved by theCabinet, and plans for the protection of regions
from tsunami were enacted based on hypotheticalinundation regions. In response to these events, it isnecessary for river levees to also fulfill theirstipulated functions as levee systems linked tocoastline levees, but to achieve this, it is necessary
to consider the fact that river levees are, structurally,
extremely susceptible to damage by overtopping.
For example, when exposed to action of a tsunami alittle larger than the tsunami hypothesized forfacility planning, it is predicted that temporary
overtopping will occur even in river levee sections,
so it is important to understand levee behavior inthis event. Based on this point, the division lookedback at the results of a survey of the state of damage
along the Kitakami River, Abukuma River, etc.,
performed immediately after the earthquake toreorganize the characteristics of river levee damage
caused by a tsunami.
2. Characteristics of damage to river levees bytsunami
In addition to the results of the on-site survey,traces of the tsunami runup, aerial photographs, andvideo and other types of data obtained by theTohoku Regional Development Bureau and theGeospatial Information Authority were used toorganize the water level and direction of flow oftsunami runup, and at the same time, link thesedifferent types of information to organize the formof and degree of damage to levees etc. The findings
are described below.
[1] The second most severe form of damage after
the breaching of levees is erosion of slopes on
the protected side of levees caused byovertopping. Erosion of the slopes on the riverside by the flow of the tsunami runup in theriver course generally did not cause damage tothe levee sections which appears very serious.
[2] One reason why levees were not breached byovertopping estimated to have exceeded about
1m is presumed to be the effects of a watercushion that restricted erosion of the toe and theslope of the levee.
[3] The flow on the front slope surface which
occurred during tsunami runoff was of shortduration, even though its flow rate was high, soin some cases, slope erosion is restricted byvegetation.
Judging from the above, erosion of the levee
body caused by the flow of the tsunami as it ran up
and flowed back downstream inside the river course,
was generally smaller than that caused byovertopping, reconfirming that levees aresusceptible to damage by overtopping ． It wasassumed that for levees laid out parallel to thedirection of the runup of the tsunami, it is vital toconsider not only the overtopping depth, but thewater level difference between the river side andprotected side of the levee and the duration of thisdifference, which are related to the scale of theeffects of the water cushion.
3. Conclusions
The estimation was made by combining theresults of the on-site survey with information which
could be used until now. To increase the certainty of
the results of the estimation and clarify the responseof the river levees during overtopping based on
differences between their structures etc., aerial
measurement data obtained before and after theearthquake will be used to clarify the actual state of
deformation of river levees in greater detail.。
[Reference]
Early report on the survey of damage to civilengineering structures by the Great East Japan
Earthquake of 2011, Technical Note of the NILIM,
No.645, pp. 243-341, July 2011.