A Case of Utilizing Results

Technical support in relation to resilient structures ofcoastal dikes
SUWA Yoshio,HeadKATO Fuminori, Senior Researcher
HATOGAI Satoshi, Researcher
Coast Division, River Department(keywords) resilient structures, tsunami, overflow, scour
1.

Introduction
In a ‘Basic concept in relation to restoration of coastaldikes, etc. suffered from the 2011 off the Pacific coast ofTohoku Earthquake and tsunami,(2) which was compiled
based on a view of ‘Structures which can produce
resilient effect of facilities even in case of exceeding thedesigned height of tsunami’ presented in a report ofExpert Examination Committee of Central Disaster
Prevention Council1), prevention of scour by back bottom
slope protection works and prevention of washout by
securing member thickness of back slope covering works,etc. were pointed out as structural devices of coastal dikes,etc. In order to reflect such structural devices in
restoration after disaster, etc., River Department ofNILIM conducted a technological study in cooperation
with Coast Division of Water and Disaster Management
Bureau and Tohoku Regional Bureau.
2. Scope of the study
Focusing on the scour of back bottom slope of coastal
dikes with a general form of trapezoid cross section andthe stability of back slope protection works, devices for
improvements of scour resistance and stability wereexamined by hydraulic model experiments, etc.
3. An example of resilient structures
Main points to note among the devices confirmed by
the hydraulic model experiments are shown in the table.
Table: Main points to note about structural devices
It is important to completely change the flow directionof overflow flowing down the back slope not to run into
the foundation (securing the flat ground of back bottom
slope) in order to keep the scour of back bottom slope
away from the main body of the dike. →To lay down
the soil stabilization as well as the foundation work andsecure the flat ground of back bottom slope as shownin the Table.
It is important to avoid unevenness because theunevenness increases the active surface ofhydrodynamic force and the degree of destabilization if
unevenness occurs in back slope covering works. →To form a structure difficult to produce unevenness
against water flow by fitting together cutout blocks.
It is important to form a structure in consideration ofnegative pressure produced at back slopes. → To
integrate slope blocks and armoring work on crown sothat one unit of block does not cover the negative
pressure range.

5.0m

法肩ブロックと天端被覆工の一体化
基礎工
2.25m

2.25m
地盤改良

切り欠きブロック
1.0m
1.0m

5.0m

Figure: An example of resilient structures(dimensions are local equivalent value)
4. Conclusion
The results of this study were published in NILIM
Technical Flash3) in May and August, 2012 and they werereflected in the disaster restoration works in South coastof Sendai Bay (partial completion scheduled at the end ofMarch, 2013)

Photo: Laying works of cutout blocks
References(1) Expert Examination Committee of Central Disaster
Prevention Council on countermeasures against
earthquakes and tsunami learning lessons from The 2011
off the Pacific coast of Tohoku Earthquake: Expert
Examination Committee report on countermeasuresagainst earthquakes and tsunami learning lessons fromThe 2011 off the Pacific coast of Tohoku Earthquake,
http://www.bousai.go.jp/jishin/chubou/higashinihon/hou
koku.pdf(2) Committee on tsunami protection measures at coastal
areas: Basic concept about restoration of coastal dikes,
etc. suffered from the 2011 Tohoku-Pacific Ocean
Earthquake and tsunami,
http://www.mlit.go.jp/river/shinngikai_blog/kaigantsun
amitaisaku/kangaekata/kangaekata111116.pdf(3) Ministry of Land, Infrastructure, Transport and Tourism,
Research Center for Land and Construction
Management: Study on the structure of coastal dikes
which can produce resilient effect, NILIM TechnicalFlash, 2012,
http://www.nilim.go.jp/lab/bcg/sokuhou/index.html