IJERD – International Journal of Environmental and Rural Development (2014) 5-1

Research article
erd

Effects of Coastal Road Spacing on Deposition ofTsunami-Borne Sand
YOUKI MAEDA
Aoi Engineering Inc., Aichi, Japan

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

YASUSHI TAKEUCHI
Faculty of Regional Environmental Science, Tokyo University of Agriculture, Tokyo, JapanTOMONORI FUJIKAWA
Faculty of Regional Environmental Science, Tokyo University of Agriculture, Tokyo, JapanSHIGEYUKI MIYABAYASHI
Faculty of Regional Environmental Science, Tokyo University of Agriculture, Tokyo, Japan
Received 1 November 2013

Accepted 1 March 2014(*Corresponding Author)

Abstract Large-scale infrastructure for tsunami damage mitigation includes tidewater
control forests and tide embankments. It is said that installing a tidewater control forest of
200 m in width along the coast is effective in mitigating tsunami damage. Since the East
Japan Great Earthquake, tide embankments of about 7 m high have been underconstruction on coasts of tsunami-hit areas. Restoration and installation of tidewater
control forests require many years, and the construction of large-scale tide embankments
requires enormous cost. This study focuses on existing roads in the coastal areas. We
investigated the effects of road, depending on their conditions of spacing, on tsunami sanddeposition. A 0.3 m-wide, 12 m-long open channel was used for tsunami experiments. To
simulate a tsunami, a removable barrier was set near the upstream end of the channel toretain water. The barrier was lifted suddenly to generate a bore. The scale of the model
was 1/100. A certain amount of sand and models of a tidewater control forest, a tideembankment and roads made of acrylic were placed at the longitudinal midpoint of thechannel. The movement of sand carried by the reproduced tsunami was observed. Several
combinations of the tidewater control forest, tide embankment and three roads were usedin the tsunami reproduction experiment, with the spacing changed for each experiment. As
the result of experiments, it was clarified that when the spaces between the roads werewide, the amount of deposited sand was controlled better. It was also clarified that theamount of sand deposition at the time of a tsunami in a coastal area was able to becontrolled effectively by combining roads with wide spacing between them with atidewater control forest or a tide embankment.
Keywords tsunami, tidewater control forests, tide embankments, road, sand

INTRODUCTION
Tsunamis generated by the Sumatra-Andaman Earthquake of 2004 (Mw 9.1) and the East Japan
Great Earthquake of 2011 (Mw 9.0) caused catastrophic damage in coastal areas (i.e. Kandasamy etal., 2005). In the East Japan Great Earthquake in particular, large amounts of sand were carried bytsunami waves from the sea bottom to coastal areas. The sand deposition was up to 30 cm thick,and the deposition was widespread. Because of the extreme amount and wide area of sanddeposition, removal work has not progressed yet at 2013.
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Large-scale infrastructure for tsunami damage mitigation includes tidewater control forests
and tide embankments. It is said that installing a tidewater control forest of 200 m in width along
the coast is effective in mitigating tsunami damage (Maeda, 2013). Since the East Japan Great
Earthquake, tide embankments of about 7 m high have been under construction on coasts oftsunami-hit areas. Restoration and installation of tidewater control forests require many years, andthe construction of large-scale tide embankments requires enormous cost.
OBJECTIVE
This study focuses on existing roads in the coastal areas. Roads take less time and money toconstruct than tide embankments and tidewater control forests take. We investigated the effects ofroad, depending of their conditions of installation, on tsunami sand deposition.
METHODOLOGY
A 0.3 m-wide, 12m-long open channel was used for tsunami experiments. To simulate a tsunami, aremovable barrier was set near the upstream end of the channel to retain water. The barrier waslifted suddenly to generate a bore. The scale of the model was 1/100. A certain amount of sand andmodels of a tidewater control forest, a tide embankment and roads made of acrylic were placed at
the longitudinal midpoint of the channel. The movement of sand carried by the reproduced tsunami
was observed. Several combinations of tidewater control forest, tide embankment and three roads
were used in the tsunami reproduction experiment, with the spacing changed for each experiment.
The dimensions and alignment of the model trees in the tidewater control forest weredetermined based on the study by Shuto et al. (1985). Acrylic sticks of 2 mm in diameter and 10.5
cm in length were arranged in a staggered pattern. The width of the tidewater control forest was set
as 200 cm (200 m in field scale), which was determined based on the Disaster Prevention Plan of
Minami Soma City, Fukushima Prefecture. The dimensions of the trapezoidal cross-section of thetide embankment were determined, based on the same plan, as 5 cm wide at the top, 13 cm wide at
the bottom and 7.5 cm high. The road model, simulating an agricultural road, was created as arectangle of 4.0 cm in width and 0.5 cm in height. The road models were placed immediately
downstream of the tidewater control forest model or the tide embankment model in three patternsof 15 cm, 20 cm or 25 cm intervals in each experiment. To evaluate the degree of sand control of
the road, the tide embankment and the tidewater control forest at the time of tsunami, Toyoura
silica sand, which was chosen to reproduce the sea bottom sand at the site, was laid in the channelbottom upstream of the tide embankment and tidewater control forest. 4.0 kg of sand was laid at 30
cm in width, 80 cm in length and 0.9 cm in thickness. The sand left in the assumed tsunami control
zone, consisting of the tide embankment, the tidewater control forest and roads, after the tsunami
simulation was measured for dry weight. Five experimental channel conditions were used: (1) nocontrol structures (no models), (2) only the road models, (3) the roads and tide embankment, (4) theroads and the tidewater control forest, and (5) the roads, the tide embankment and the tidewater
control forest. The interval of road was set as 15 cm, 20 cm or 25 cm in each experiment that used
the road models. To reproduce tsunamis in the experiment flume, a law of similitude was used.
Froude’s law of similitude is generally used for studies involving open flume experiments;
however, it was impossible to apply the Froude’s law of similitude, because there were locations
where water level differences were rapidly created by the presence of the tidewater control forest
and the embankment models. In this experiment, use of a law of similitude, in which the water
density and gravitational acceleration are the same for the original site as for the model, wasdetermined based on the conditions described in the Port and Airport Research Institute Report(Kimura et al., 1968). The geometric similarity in this experiment was set as 1/100, and the heightof tsunami that reached the coastal areas was assumed as 12 m, based on the Port and Airport
Research Institute Report (Takahashi et al. 2011). The tsunami velocity measured in the experimentusing the models was 1/10 that observed in the disaster area, which is the same as the result
obtained using Froude’s law of similitude (Maeda et al. 2013).
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IJERD – International Journal of Environmental and Rural Development (2014) 5-1

Structures for mitigating Tsunami damage
Three Roads

Model scale is 1:100

Tidewater Control Forest 

Tide Embankment Acrylic sticks

4 cm

10 cm

30 cm
5 cm height

7.5 cm200 cm width

Tsunami Experiment Section
Several combinations of three infrastructuresets on this section

Removal barrier

Water4 kg sand for evaluatingsand deposition

Lower(Ground)

Tsunami Experiment Section
Several combinations of three infrastructuresets on this section

Upper(Sea)The barrier waslifted suddenly
to generate
TSUNAMI.

TSUNAMI

After Tsunami attacked,
Sands washed to downstream

Fig. 1 Outline of Tsunami experiment
RESULTS AND DISCUSSION
Fig. 2 shows the relationship between the road interval and the amount of sand deposited by thewater flow in the experiment. The values are the average and the standard deviation of threeexperimental flows. When road models were not installed, the sand deposition was 3.3 kg out of
the 4 kg of sand in the flow. When the road models were installed perpendicular to the flow atintervals of 15 cm, 20 cm and 25 cm, the respective sand depositions were 3.1 kg (78%), 2.7 kg(68%) and 2.5 kg (63%) and it’s standard deviation were 0.36, 0.16 and 0.11. It was clarified thatthe amount of sand deposited by the tsunami waves decreases with increases in the interval of roads.
It was demonstrated that the tsunami drag force decreased when roads were installed.
Fig. 3 shows the result of experiments with four different conditions. (1) Without any
structures, 3.2 kg of sand was deposited by tsunami waves. It was clarified that the amount of sanddeposited by the tsunami waves decreased in stages by combining the roads with a tideembankment and a tidewater control forest, in the following order of least to most increase: (3)with roads and a tide embankment, (4) with roads and a tidewater control forest, and (5) with roads,
a tide embankment and a tidewater control forest. It was clarified that combining the roads with atidewater control forest was effective in mitigating sand deposition caused by tsunami, because theamount of sand controlled by the tidewater control forest was particularly large.

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Amount of sand depositedby the tsunami waves (kg)

IJERD – International Journal of Environmental and Rural Development (2014) 5-1

4.0

Before experiment

After experiment

3.0
2.0
1.0
0.0
Initial laid sand(4kg)

Nothing is roads

15 cm intervalroads

20 cm intervalroads

25 cm intervalroads

Fig. 2 Relationship between the road interval and the amount of sand depositedby the water flow in the experiment

Amount of sand deposited
By the tsunami waves (kg)

4.0
4.0

0.5
0.4

3.0

0.3
2.0
2.0
0.2
1.0
1.0

0.1

0.0

0.0
Nothing

Only Road

Road and TE

Road and TWPF Road, TE andTWPF

TE and TWPF means tide embankment and tidewater embankment.
Red indicates the result of tsunami experiment without any structures. Blue, Yellow and Orange also shows the results of15 cm, 20 cm and 25 cm interval three roads, respectively.

Fig. 3 Result of experiments with five different conditions
As the result of experiments, it was clarified that when the spaces between the roads werewide, the amount of deposited sand was controlled in better way. It was also clarified that theamount of sand deposition at the time of a tsunami in a coastal area could be controlled effectively
by combining roads with wide spacing between them with a tidewater control forest or a tideembankment.
CONCLUSION
In this study, the effect of roads and each additional measure in controlling sand deposition causedby tsunami was examined and the effective interval of roads installed parallel to the coastline in
mitigating tsunami damages was explored.
It was clarified that the amount of sand controlled by the roads increases with increases in theroad interval and that a reduction of 98% or more was achieved in the condition with control
measures compared to without control measures, assuming that a tidewater control forest of 200 cm
in width (200 m in field-scale) was installed in each case.
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However, these results are from laboratory flume experiments that did not consider thetopography of the sea bottom or of coastal areas. The similarity of sand was also ignored in thisstudy. To further study measures against sand deposition damage by tsunamis, it is necessary toconduct experiments that consider the similarity of sand and the topography of the sea bottom andthe coastal areas in question.
ACKNOWLEDGEMENTS
This research was supported by Eastern Japan Support Project from Tokyo University ofAgriculture, Japan. The authors are grateful for the research assistance provided by the students ofthe laboratory of Hydro-structure Engineering, Tokyo University of Agriculture.
REFERENCES
Kandasamy, K. and Narayanasamy R., 2005. Coastal mangrove forests mitigated tsunami, Estuarine, Coastaland Shelf Science, 65, 601-606.
Kimura, H., Kudo, K. and Shibayama, A., 1968. A model experiment on the tsunami breakwaters at the port
Kochi. Report of the Port and Airport Research Institute. 7(2), 3-64. 1968, (in Japanese).
Maeda, Y., Okazawa, H., Takeuchi, Y. and Fujikawa, T., 2013. Evaluation of tide embankment andprotection forest width on tsunami disaster using tsunami simulator. International Journal ofEnvironmental and Rural Development, 4(2), (in press).
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 portsand coasts. Technical Note of the Port and Airport Research Institute, 1231, 31-44.

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