IJERD – International Journal of Environmental and Rural Development (2013) 4-2

Research article
Evaluation of Tide Embankment and Protection Forest
Width on Tsunami Disaster Using Tsunami Simulator
YOUKI MAEDA
Graduate School of Agriculture, Tokyo University of Agriculture, Tokyo, Japan

HIROMU OKAZAWA*
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Email: h1okazaw@nodai.ac.jp

YASUSHI TAKEUCHI
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, JapanTOMONORI FUJIKAWA
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Received 14 December 2012

Accepted 10 June 2013(*Corresponding Author)

Abstract The tsunami that occurred after the Tohoku Earthquake of March 11, 2011,devastated the infrastructure in Tohoku, including roads and water channels. This study
reports the results of a tsunami simulation experiment using an open channel forevaluation of tide embankments and protection forest in controlling the force of tsunami
waves and flow of sand carried by tsunami. Coastal areas in Minami Soma City,
Fukushima Prefecture, that were devastated by the Tohoku Earthquake were assumed as
the experimental location. A 0.3 m-wide, 12 m-long open channel was used for thetsunami experiment. To simulate a tsunami, a removable barrier was set near the upstream
end of the channel to retain water. The barrier was lifted to generate a bore. The scale forthe model was 1/100. A model protection forest and a tide embankment made of acrylic
were placed at the longitudinal midpoint of the channel. To examine the sand control
effect of the tide embankment and protection forest at the time of tsunami, Toyoura silica
sand was laid in the channel bottom upstream of the tide embankment and protection
forest. The sand left behind after the tsunami simulation was measured for dry weight. The
speed of the wave beyond the protection forest tended to decrease with increases in forest
width. The amount of sand carried by the tsunami was found to decrease with increases inforest width. The results, however, are from a simplified model channel experiment. It isnecessary to perform experiments that more closely reproduce the original sites in terms oftopography and vegetation.
Keywords tsunami, protection forest, tide embankment, sand, earthquake

INTRODUCTION
Japan is among the most earthquake-prone countries in the world. The tsunamis that occurred after
the Tohoku Earthquake of March 11, 2011, devastated the infrastructure in Tohoku, including
roads and water channels. A total area of 443 km2 was inundated, and 238 km2 (54 %) of this wasalmost paddy field (The Geospatial Information Authority of Japan, 2011). Coastal sand carried by
the tsunamis deposited in the affected paddy fields in wide areas. Considerable time and labor havebeen spent in removing the deposited sand.
Restoration and reconstruction of the infrastructure and agricultural lands are underway in thedisaster-hit areas. At the same time, the value of establishing tide embankments and tidewater
control forests has come under review as a part of efforts to make tsunami-resistant communities.
Two tsunami mitigation effects were suggested by Shuto (1985): 1) the forests prevent objects suchas boats that drift from sea to land from moving into areas with houses and other buildings, and 2)© ISERD
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the forests reduce the speed of tsunami waves by exerting drag, which limits the flooding. However,
the effects of tidewater control forests in mitigating damage have not been fully examined. Nostudies have addressed the scale of tidewater control forest required to mitigate damage against
large-scale tsunamis (wave heights of 10 m or greater) caused by mega-earthquakes (magnitudes
around M9.0).
OBJECTIVE
This paper reports the results of tsunami simulation experiments using an open water channel forevaluation of tide embankments and tidewater control forests of several widths in controlling theforce of tsunami waves and the amount of sand carried by the tsunamis. The disaster prevention
measures of Minami Soma City in Fukushima Prefecture (Fig.1), which was devastated by theTohoku Earthquake tsunamis, were used as a reference case.

Seismic intensity
the seismiccenter
SendaiMinamisoma

Tokyo

Fig. 1 Seismic intensity’s map of the Tohoku Earthquake (March 11, 2011)
Source: Japan Meteorological Agency

METHODOLOGY
A 0.3 m-wide, 12 m-long open channel was used for the tsunami experiment (Fig. 2). To simulate atsunami, a removable barrier was set near the upstream end of the channel to retain water. Thebarrier was rapidly lifted to generate a bore. The scale for the model was 1/100. The height of themodel waves was set as 12 cm, based on the 12 m height of the actual waves that reached thecoastal areas (Takahashi et al., 2011). Tidewater control forest and tide embankment models wereplaced at the longitudinal midpoint of the channel. The dimensions and alignment of the modeltrees of the tidewater control forest were determined based on the study by Shuto (1985). Acrylicsticks 2 mm in diameter and 10.5 cm in length were arranged in a staggered pattern. The tideembankment had a trapezoidal cross section, a top width of 12 cm, a bottom width of 16 cm and aheight of 45 cm. To examine the sand control effect of the tide embankment and tidewater control
forest at the time of tsunami, Toyoura silica sand (Japanese standard sand), which was chosen toreproduce the sea bottom sand at the site, was laid in the channel bottom upstream of the tideembankment and tidewater control forest. The 4.0 kg of sand was laid 30 cm wide, 80 cm long, and0.9 cm thick. The sand left behind after the tsunami simulation was measured for dry weight.
The experiment conditions are given in Table 1. There were 12 experiment treatments:© ISERD
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without a tide control forest (indicated as 0cm) and with a tide control forest of 20 cm, 30 cm, 40cm, 50 cm or 60 cm in width, in each case with and without a tide embankment. The tsunami
simulation was videotaped, and the speed of the waves after they passed the tidewater control forest
was calculated from the video data.
Step 1← Tide control forest ↑↑
Sand(4 kg)
Tide embankment 
12mm

0.3

Step 2

Rapidly lifted Tsunami

Fig. 2 Outline of the Tsunami Simulator using experimental open channel(0.3 m-wide, 12 m-long)
Table 1 The experiment conditions using Tsunami Simulator
With a tide embankment
Without a tide embankment

Width of tide control forest
0 cm
10 cm
20 cm
0 cm
10 cm
20 cm

30 cm
30 cm

40 cm
40 cm

50 cm
50 cm

60 cm
60 cm

As the flow velocity of waves changed in a short time in this experiment, it was impossible toapply the Froude’s law of similitude, which is used in normal channel experiments. Referring toKimura et al. (1968) and based on the condition that the density and gravitational acceleration ofwater were the same for the original site and for the model, the following two rules of similitude
were determined (Eqs.(1)- (3)).
1T2 ⎛ L2 ⎞ 21=⎜ ⎟ = 1T1 ⎝ L1 ⎠n 2(1)−1
V2 L2 ⋅T21
1
1== 1 =
1 =−1
V1 L1 ⋅T1n 2 100 2 10(2)∴V1 = 10 V2(3)where L is length (m), n is reduction scale, V is flow velocity (m/s)and T is time (s). The
subscripts 1 and 2 indicate the original and the model, respectively.
After conversion by using Eq.(1) to (3), the speed of tsunami waves measured in the modelexperiments was found to be 1/10 that of the flow velocity observed at the original site.

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RESULTS AND DISCUSSION

2.0

100

1.680

Flow velosity* (%Flow velosity of Tsunami (m/sFig. 3 shows the relationship between forest width and flow velocity of tsunami waves. It wasclarified that the flow velocities for the case with a 0.2 m-wide forest were reduced to 77% (casewithout a tide embankment) and to 84% (case with a tide embankment) that of the case without atide control forest (forest width 0m). However, the differences in flow velocities were small under
the condition with forest width of 0.3 cm or wider. Therefore, it can be assumed that flow velocities
decreased for the forests with widths of up to 0.2 cm, but did not decrease appreciably at widthsgreater than that. For cases with a forest of a given width, it was found that, in the case with a tideembankment, the maximum flow velocity decreased to 80% that of the case without a tideembankment.

1.2
0.8
0.4

60
40
2000.0
000.2
0.4
0.6
Protection forest width (m)with tide embankment without tide embankment

0.2
0.40.6
Protection forest width (m)with tide embankment without tide embankment

Fig. 3 Relationship between flow velocity of tsunami and protection forest width*Ratio of flow velocity (m/s) in the case of 20-60 m forest width on that of 0 m forest width
100
Amount of sand carriedby tsunami* (%)Amount of sand carriedby tsunami (kg)

2.0
1.5
1.00.50.080
60
4020000.2

0.40.6

Protection forest width (m)with tide embankment

00.2

0.40.6

Protection forest width (m)without tide embankment

with tide embankment

without tide embankment

Fig. 4 Relationship between flow velocity of tsunami and amount of sand carried by tsunami*Ratio of sand (kg) in the case of 20-60 m forest width on that of 0 m forest width

The relationship between forest width and amount of sand carried by tsunami (sediment) isshown in Fig.4. At the forest width of 0m, i.e., without a tidewater control forest, sediment was
1.040 kg in the case with a tide embankment and 1.550 kg in the case without a tide embankment.
It was clarified that even without a tide control forest; sediment reduction to 67% was achieved byusing a tide embankment. For the cases with a forest of a given width, the sediment for the casewith a tide embankment was found to be constantly about 60% that of the case without a tideembankment. Based on the above findings, it is thought installation of tidewater control forests
combined with tide embankments can reduce sediment to about 60% that of tidewater control
forests alone.
When the relationship between forest width and changes in sediment deposition was examined,
it was found that sediment deposition tended to decrease for the forest widths of 0.2 m and 0.3 m,irrespective of the presence of a tide embankment. Sediment in the cases with a forest width of 0.3m was 56% with the tide embankment and 60% without the tide embankment of those in the cases© ISERD
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without a forest (forest width of 0m). However, the changes in sediment were small in the caseswith forests of 0.3 m or greater in width. The above findings clarified that sediment reduction to
60% is possible by installing a forest of 30 m in width (equivalent to the 0.3 m width in the model)
and that increases in forest width beyond 30 m would not afford greater reductions in sedimentdeposition.
CONCLUSION
These experiments examined the effects of a tidewater control forest and a tide embankment on thespeed of tsunami waves and the amount of sand carried by a tsunami under the assumption of atsunami with the scale of those that occurred after the Tohoku Earthquake in Japan on March 11,
2011. Laboratory experiments demonstrated that the minimum width for a tide control forest
effectively reduces the speed of tsunami waves and the amount of sand carried by a tsunami is 20
to 30 m. It was also clarified that use of a tide embankment in combination with a tidewater control
forest is effective in mitigating the tsunami damage. The results, however, are from a simplified
model channel experiment. It is necessary to perform experiments that closely resemble the originalsites in terms of detailed topography and vegetation.
ACKNOWLEDGEMENTS
This research was supported by Eastern Japan Support Project from Tokyo University ofAgriculture, Japan. We would like to express our gratitude to Prof. Dr. Shigeyuki MIYABAYASHI(Tokyo University of Agriculture). The authors are also grateful for the research assistanceprovided by the students of the laboratory of Hydro-structure Engineering, Tokyo University ofAgriculture.
REFERENCES
Kimura, H., Kudo, K. and Shibayama, A., 1968. A Model Experiment on the Tsunami Breakwaters at theport Kochi. Report of the Port and Airport Research Institute. 7-2. 3-64. (in Japanese).
Shuto, N., 1985. Effect of salty wind protection forests on protection against tsunami and its limit. Coastal
Engineering Journal. 32. 465-469. (in Japanese).
Takahasi, S., et al. 2011. Urgent survey for 2011 great east japan earthquake and tsunami disaster in ports
and coasts. Technical Note of the Port and Airport Research Institute. 1231. 31-44.
The Geospatial Information Authority of Japan. 2011. (http://gisstar.gsi.go.jp/2011TaiheiyouOki/LandUse/
LandUseArea.pdf (2012.12.13 Perusal))© ISERD108