Journal of Natural Disaster Science, Volume 34, Number 1, 2012, pp79-90

Performance evaluation of pedestrian bridge as verticalevacuation site during the 2011 tsunami in Japan
Abdul MUHARI*
Shunichi KOSHIMURA*
Fumihiko IMAMURA**Graduate School of Engineering, TOHOKU University(Received March 31, 2012 Accepted July 20, 2012)
ABSTRACTWe evaluated the performance of pedestrian bridges as vertical evacuation sites during
the 2011 Tohoku tsunami in six prefectures in Japan. Evaluation was performed by considering the vulnerability and tsunami hazard characteristics as parameters that influence thedamage probability. Vulnerability is represented by the exposure, indicated by the bridge’s
position from the shoreline relative to the maximum inundation distance. In contrast, tsunami hazard was identified from the observed flow depths relative to the bridge’s height.
We found that pedestrian bridges that were positioned inside an area less than 0.2 times thatof the maximum inundation distance (which is 6 km), or where the tsunami flow depth exceeded that of the bridge’s height by more than 1.5 times, have a high probability of being
damaged by a tsunami. It is necessary to consider such limitations when determining theplacement criteria of pedestrian bridges as evacuation shelters for the above areas. Starting with the concept of sudden expansion phenomenon, a series of numerical exercises was
conducted to parameterize hydraulic conditions that affect the sudden drop in water level
due to the building configuration at an intersection. We searched the areas where pedestrian
bridges could still be used for evacuation with the existing height. Preliminary results indicate that if the sudden expansion ratio (D) > 1.5, and the ratio between the widths of road to
the width of residential blocks (β) < 0.1, sudden expansion occurs with hydraulic gradientdependent on the Froude number (Fr). In this condition, the bridges can be used for evacuation as long as they are installed on the side of the road parallel to the coastline, or on the
landward side of the road at an intersection. These results provide the opportunity to apply
more distributed evacuation shelters to relatively flat and populated areas.
Keywords: tsunami evacuation, pedestrian bridge, fragility curve, sudden expansion

1. Introduction

coast in Iwate Prefecture, the villages are surroundedby hills. Hence, they are easily reached once a tsunami evacuation order has been issued. However, inthe southern part of the Sanriku coast, and especiallyon the Sendai plain, Miyagi Prefecture, high groundis situated far from the coast and the designated verti-

Geographical aspects and level of awareness in
responding to a tsunami warning are influential parameters in determining the successful rate of evacuation. In the central to northern part of the Sanriku
79

A. MUHARI, S. KOSHIMURA, F. IMAMURA

cal evacuation sites are not well distributed. During
the 2011 tsunami, therefore, people in the plain areas
tended to use cars to evacuate. While cars are very
important for evacuating elderly people who cannot
walk long distances, they can cause problems such as
traffic jams that will delay the evacuation to safety.
A joint survey conducted by the Cabinet Office, Internal Affairs and Communication Ministry, Fire andDisaster Management Agency and Meteorological
Agency in July 2011 revealed that among the 870 respondents, 55.7% of them evacuated by car and 34%
were stuck in traffic jams (Cabinet Office, 2011).
Most people who used cars said that they returned
home to look for family members and collect them. A
similar tendency of people using cars to evacuate wasalso revealed in Kesennuma City; a survey conductedby the municipal government and Ministry of Land,
Infrastructures and Transport indicated that 45% of
6,730 survivors who evacuated by car –apart from the
6% who abandoned their cars and evacuated on foot–stated that cars were necessary to evacuate elderly
people and children because evacuation sites were
far away (MLIT, 2011). The above experiences could
be interpreted in two linked ways: first, it is impossible to avoid the use of cars during evacuation due tophysical and psychological reasons. Second, the morecars involved during evacuation, the greater the possibility of traffic jams. Therefore, in order to ensure
the safety of evacuees in plain areas, it is necessary todevelop more distributed temporary evacuation sites.
Along the national and prefectural roads inJapan, pedestrian bridges have been installed at an
average height of 4.5 to 5 m. Depending on the local height of tsunami inundation; pedestrian bridges

can be utilized as temporary vertical evacuation sites.
Figure 1 shows a snapshot of an amateur video takenfrom a pedestrian bridge in Yuriage, Miyagi Prefecture, during the 2011 Tohoku tsunami in Japan.
During the evacuation, most traffic jams occurred at intersections (Southworth, 1991). Where
pedestrian bridges are available at intersections inside tsunami affected areas, the evacuees can simply
leave their cars and climb-up onto the bridge if they
become trapped at an intersection due to a traffic jam.
We assumed that pedestrian bridges were an appropriate solution for rapid evacuation in plain areas, especially when it is difficult to prohibit the use of cars.
To verify our hypothesis, we conducted a study toevaluate the performance of pedestrian bridges during
the 2011 tsunami in six prefectures in Japan, namely
Ibaraki, Fukushima, Miyagi, Iwate, Aomori and Hokkaido. Fifty-two bridges including those in train stations were examined by considering the vulnerability
indicated by the exposure (position from the shoreline), and the hazard in terms of tsunami flow depths
around the bridge as parameters that influence thedamage probability.
Previous studies have assessed the structural
fragility in particular buildings subjected to tsunami
flow (e.g., Shuto, 1993, Koshimura et al. 2009). In thecontext of bridge structures, their fragility to seismic
ground motion and Peak Ground Acceleration (PGA)
parameters were also well assessed (e.g., Shinozuka.
et al. 2000, Karim and Yamazaki. 2003). In the context of bridge fragility subjected to tsunami force,
Shoji and Moriyama (2007) analyzed bridge performance during the 2004 Indian Ocean tsunami in Banda Aceh and Sri Lanka. In their study, tsunami force isFigure1. Tsunami wave front observed from survivor in pedestrian bridge at Yuriage town, Miyagi.
(source. http://www.youtube.com/watch?v=DXbeoKgQ9x0)80

PERFORMANCE EVALUATION OF PEDESTRIAN BRIDGE AS VERTICAL EVACUATION SITE DURING THE 2011 TSUNAMI IN JAPANrepresented by the estimated flow depth obtained byadjusting the field survey data to the still water levelof the river where bridges were installed. In anotherstudy, based on field observation of damage to bridges
in Banda Aceh after the 2004 Indian Ocean tsunami,
Iemura and Pradono (2005) reported that preventionof the seismic unseating problem might be applicable
to tsunami force as well. Under the Development of
Performance Based Tsunami Engineering (PBTE,
NEES. 2010), a set of hydraulic experiments in addition to numerical models were conducted to obtain indepth analysis of fluid-structural interaction with thefocus on the tsunami effect on coastal infrastructures(dry bed). Since tsunami generated impact on structures may differ in wet- and dry-beds, the estimationof tsunami impact on structures in dry beds is still
under development.
In our study, the performance of a bridge on a
dry bed subjected to a tsunami was evaluated quantitatively. We limited our interest to the performanceof a pedestrian bridge struck by a tsunami based onfield observation data supported by remote sensing
analyses, and determined conditions that influence
the damage probability of pedestrian bridges. Then,
we conducted a set of numerical exercises to optimize
their function by studying the tsunami flow characteristics at intersections where the bridges are situated.
The findings were then used as a basis to determine
placement criteria should such structures be selected
for use as temporary evacuation shelters.

facility available on Google maps (Google, 2012).
The database included 21 pedestrian bridges at train
stations from 6 prefectures as listed in Table 1. Next,
we conducted a cross-check analysis to ensure thebridges were placed inside the inundation zone bysuperimposing them with the results of a post-tsunami
survey conducted by the Japan Society of Civil Engineers (JSCE, 2011). If placement was affirmed, wethen confirmed whether there were field observation
points available around the bridge.
As a result, 16 bridges were excluded from further analysis since there was insufficient run-up or
flow depth data on them. For those that were selected
for the subsequent steps, we interpolated the closest
run-up or flow depth observation points near the pedestrian bridges using a simple interpolation scheme
shown by Eq.1,nh = [Σi ri]/n(1)Here, h is the actual flow depth around the bridge, n
is the number of observed run-up or flow depth data
and r is the observed values of run-up height and flow
depths. The results of cross-check analysis and calculated observed flow depths are given in Table 2.
In terms of exposure, we measured the distanceof bridges from the shoreline by generalizing thebeach strike to the north without considering specificfeatures such as small rivers, ports, ponds and soforth.
2.2. Methodology
2.2.1. Developing fragility curves for pedestrian
bridges
After compiling the field observation data in addition to the satellite image analysis and oblique photographs provided by the Geographic Survey Instituteof Japan (GSI, 2011), damage levels of pedestrian

2. Data and methodology
2.1. Data
We first compiled 68 pedestrian bridges assumed
to be constructed inside tsunami affected areas basedon satellite image analysis and field survey data. The
searching process was supported by the street view

Table 1 Number of collected bridges along the east coast of Japan81

A. MUHARI, S. KOSHIMURA, F. IMAMURA

Table2. Observed bridges and flow depth during 2011 tsunami

Table3. Damage criteria of pedestrian bridge

82

PERFORMANCE EVALUATION OF PEDESTRIAN BRIDGE AS VERTICAL EVACUATION SITE DURING THE 2011 TSUNAMI IN JAPANBy applying these equations, fragility curves for pedestrian bridges are developed and discussed in thefollowing chapters.

bridges were classified into three types (see Table3):(A) is heavy damage, which describes the condition
where bridges are swept away or their decks are damaged, (B) is partial damage indicated by damage tothe stairs or barriers so repair is required before reuse, and (C) is the condition where bridges are inundated only and can be directly used after a tsunami.
We first analyzed the fragility of pedestrian
bridges in terms of tsunami hazard. In general, a higher tsunami flow depth will increase the damage probability of a pedestrian bridge. In other words, damage
probability should have a linear correlation with theincrement of tsunami flow depth. Even though thedamage itself might be caused by the impact of debris
collision, in this study we used tsunami flow depth as
the only observable parameter from the field survey.
Some of the debris trapped on the bridge’s stairs was
visible from satellite imagery, but incomplete figures
for all of them might yield inconsistent interpretation. Therefore, debris impact is excluded from theanalysis. The cumulative damage probability is given
through the widely used fragility equation as follows,[]

Pi (x) = Φ Inx ˗ u’
σ’

2.2.2. Searching criteria of pedestrian bridge fortsunami evacuation
In order to ensure the safety of evacuees whenusing pedestrian bridges as evacuation sites, thedeck’s height should be higher than the maximum surrounding tsunami flow depth. However, in Japan theheight of pedestrian bridges is almost the same in therange of 4.5 – 5 m. In this case, there are two options
available if pedestrian bridges are to serve as evacuation shelters. First is adjusting the height according to the maximum estimated tsunami flow depth.
This option implies that the bridge height might beincreased and become unsuitable for the elderly. The
second option is searching for an appropriate location
where the maximum tsunami flow depth is lower thanthe bridge’s height. However, this requires further
placement criteria to utilize the bridge at the existingheight. This section focuses on the second option,
which will be further elaborated.
In populated areas where streets between two
residential blocks are fenced by rows of houses, tsunamis might have specific features when they passthrough narrow roads. For instance, flow depth alonga road prior to an intersection might be higher than
the flow depth after the intersection. Here, a sudden expansion might occur because of the spread ofwater mass in different directions and the building
configuration around the intersection (e.g., Muhari
et al. 2011). Therefore, we assumed that once the influential factors affecting the flow characteristics at
an intersection can be parameterized, the placement
criteria of pedestrian bridges can be established.
Many engineering applications have shown thatsudden expansion generally depends on the expansion
ratio, which is described as the ratio between channel
width before and after expansion. However, most of
the previous researches conducted experiments using a closed tube to keep the flux constant before andafter the expansion (e.g., Oliviera and Pinho, 1997).
Thus, the head loss was only determined by the expansion ratio. In the tsunami case, however, more
complex aspects need to be considered i.e., housing
geometry at an intersection, effect of the width ofresidential blocks, and so forth. Goto and Shuto (1983)(2)

Here, P is the cumulative probability of specific damage classification (i), Φ is the standardized lognormal
distribution function, x is the median of the observed
tsunami flow depth, u’ and σ’ are the median and standard deviation of random variable of x, respectively.
In order to obtain the last two statistical parameters,
we first calculated the inclination (uy) and intercept (σy)
through the least-square fitting plot of ln x and the inverse of lognormal distribution function Φ-1. Next themedian and standard deviation were determined by(-uy/σy) and (1/σy), respectively (e.g., Shoji and Moriyama, 2007).
For pedestrian bridge fragility in terms of exposure, the previously mentioned approaches should be
modified since the damage probability will be reduced
together with the increment of the distance from theshoreline. In other words, the safety probability will
increase if bridges are placed farther from the coast.
Equation 2 therefore is modified as follows,[]

1-[Pi (x)] = 1- Φ Inx ˗ u’
σ’

(3)

83

A. MUHARI, S. KOSHIMURA, F. IMAMURA

conducted both a hydraulic experiment and numericalanalysis to parameterize factors that influence the expansion coefficient when a tsunami flows along lined
obstacles. However, their works were limited to twoparallel obstacles where there was no actual representation of an intersection after the rearmost region.
In this study, we performed numerical exercises
to preliminarily parameterize the influential factors
on tsunami characteristics at an intersection. A hypothetical town was set up as shown in Fig 2. Residential blocks prior to the intersection occupied allareas, while the one after the intersection is not. This
design allows the flux at an intersection to spread out
in different directions, and minimize the unnecessaryreflecting flows from the side walls. In reality, thisdesign is only able to represent areas where the widthof the residential block is much larger than the widthof the road between them.
A set of non-linear shallow water equations asgiven by Imamura (1996) was used to model the
propagation and inundation of the tsunami. The tsunami is initialized by a sine wave as shown in Fig. 2.

The reliability of 2D Shallow Water Equation (SWE)
to represent flow passing through obstacles had been
demonstrated by Goto and Shuto (1983), and Tsudaka
et al. (2011). They validated the result of numericalcalculation using SWE with experimental data andobtained good agreement. However, his approach has
limitations on reproducing flood water behind buildings (Tsudaka et al. 2011). Also, the absence of the
momentum diffusion term in the SWE might yield aless accurate modeled water level due to the lack oflateral diffusion at intersections.
The numerical simulations were conducted inseveral scenarios, and the initial tsunami wave waspropagated at various amplitudes as given in Table 4.
The influence of the expansion ratio (D) was assessed by increasing the road width after the intersection (d2); while the road width prior to the intersection(d1) remained unchanged. The effect of roads parallel to the coast (l2) was analyzed by increasing theirwidth while the length of residential blocks prior to
the intersection (l1) was constant. We also investigatedthe effect of the reduction of the length of a residen-

Figure2. Design of hypothetical town for numerical simulation of tsunami at intersection

Table4. Scenarios for numerical simulation of tsunami at intersection84

PERFORMANCE EVALUATION OF PEDESTRIAN BRIDGE AS VERTICAL EVACUATION SITE DURING THE 2011 TSUNAMI IN JAPANRegression analysis gives the parameters shownin Table 7 which were used to determine the best fitof the fragility curve with the obtained coefficientcorrelations (R2) for the exposure component as 0.74
and 0.61 for damage classification (A) and cumulative (A+B). In terms of susceptibility, the coefficientcorrelations were obtained as 0.64 for damage classification (A) and 0.81 for cumulative damage classification (A+B).
The fragility curves were then developed byplotting the damage probability to the non-dimensional number for each fragility parameter. The nondimensional number for exposure is defined as theratio between the locations of the respective bridgefrom the shore (l) to the maximum tsunami inundation
distance (L), which is assumed to be 6 km. As for thefragility curve subjected to tsunami hazard, the nondimensional parameter is given by the ratio between
the observed tsunami run-ups (h) to the height of thepedestrian bridge (H). The results are shown in Fig. 3
as follows,
The above results reveal that most pedestrian
bridges have a more than 50% probability of being
damaged by a tsunami if they were placed closed tothe shore. This is a common understanding on vulnerability analysis. However, an important point to note
is up to what distance the risk of using a pedestrian
bridge for evacuation is relatively high. Here, ourtial block on the head-loss at an intersection if partsof them were damaged by a tsunami. Lastly, we estimated the effect of the ratio between road widths (d1)to the width of residential blocks (d3) symbolized by(β) on influencing the hydraulic gradient at a specificexpansion ratio (D). From the results of the modeledwater surface of each scenario, a cross section at the
middle of the road perpendicular to the shoreline was
extracted for analysis. The numerical scenarios wereapplied to flat and sloping (1/100 and 1/50) topography with a cell size of 0.5 m. Grid size was selectedaccording to an empirical relation of dx/(g hmax)1/2 T <3×10-3 to obtain 0.5 < predicted value/measured value< 2 as proposed by Fujima (2012). The wave periodof 0.5 hours was selected for the sin wave initially.
This period is similar to the average observed periodof the first wave during the 2011 Tohoku tsunami at
Sendai buoys.
3. Results and discussions
3.1. Fragility curves
We classified the damage rank, cumulative frequency and cumulative damage probability in a 1
meter range as shown in Table 5. In terms of theirlocation, damage rank, cumulative frequency and cumulative damage probability were analyzed at intervals of 250 m (Table 6).

Table5. Number of analyzed bridges and their cumulative damage probability subjected in term of tsunami hazard characteristic85

A. MUHARI, S. KOSHIMURA, F. IMAMURA

Table6. Number of analyzed bridges and their cumulative damage probability in term of exposure

Table7. Statistical parameters for fragility function obtained from regression analysis

Figure3. Fragility curves for exposure (left) and susceptibility (right) parameters. Solid line indicates the fragility for criteria (A) and dashed line for cumulative (A) + (B)86

PERFORMANCE EVALUATION OF PEDESTRIAN BRIDGE AS VERTICAL EVACUATION SITE DURING THE 2011 TSUNAMI IN JAPAN

result shows that up to 1.2 km from the shore (or0.2 times that of the assumed maximum inundation
extent); the probability of a pedestrian bridge being
damaged by a tsunami remains high. The damage
probability is more than 50% for those within an areaof 0.1 times that of the maximum inundation extent.
Even though the coefficient correlations (R2) for
both fragility parameters are relatively good, somedata are scattered especially when it represents the
cumulative damage (A and B) in the exposure component. The cause for this is attributed to two possibilities: first, the bridges located far from the shore
still incurred some damage. Second, bridges that weresituated near the coast were not damaged by the tsunami. For the first possibility, we visualized examplesof bridges in Ishinomaki City that incurred damage(B) even though they were placed 1 to 1.5 km fromthe shore. Using satellite images after the disaster, it
can be seen that a bridge was hit by a house that was
swept along by the tsunami (Fig. 4). For the second
possibility, on the contrary, some bridges in Miyako
City were much closer to the shore (< 100 m), but nodamage was observed. Even though several available
amateur videos showed that much floating debris suchas fishing boats was brought inland by the tsunami,
fortunately none of it directly hit the bridges. Basedon the above facts, we should note that the above
described quantitative analysis only shows the correlation between some observable parameters fromthe field observation data or remote sensing analysis
to the damage degree of pedestrian bridges. Thismight not be sufficient to describe the exact factorsthat destroy or damage bridges such as debris collision. However, it is still assumed that the selectedfragility components can represent such unobservableparameters quantitatively due to the limitation of scientific evidence. This is the reason why we acquired
scattered data on the fragility curves. Further detailed
analysis especially of the debris source and its impact
is required to analyze the mechanism of damage topedestrian bridges.
The result from the exposure perspective can also
be described to some extent by the developed fragility
curves for the tsunami hazard component. Here, welearned that for criteria (A), pedestrian bridges have amore than 50% probability of being swept away by atsunami if the surrounding flow depths are 1.5 times
that of the height of the bridge’s deck or around 6 to
7.5 m. This height of the tsunami was experienced byalmost all areas on the Sanriku coast within a distanceof 0.5 to 1 km from the shore (e.g. Mori et al. 2011).
If the cumulative damage is taken into account, it canbe seen that the bridge will start to have a high probability of being damaged by a tsunami if the observed
run-up height is close to the height of the bridge.
The above results have various implications.
First, the use of pedestrian bridges in an area close to
the shore (up to 1.2 km from the shore), or bridges
placed in areas with predicted flow depth close to
their height may still be dangerous as vertical evacuation sites. In reality, however, areas within 1 km of
the shore –where the 3 to 5 m tsunami run-up is more
likely to occur– might be critical for rapid evacuation.
Particularly in plain areas, this region might have lim-

Figure4. Debris collision observed from satellite image (left) and oblique photographs (right, GSI) after tsunami
in Ishinomaki.
87

A. MUHARI, S. KOSHIMURA, F. IMAMURA

long as the expansion ratio at the intersection meetsthe requirements discussed previously.
Considering the influence of Froude number (Fr),we conducted numerical experiments in the rangeof Fr higher than its critical condition (> 1). This isbecause the bore is usually observed under supercritical flow. The correlation between tg α and expansion
ratio (D) in the range of Froude number 1.13 to 1.34
is given in Fig. 6 (A). The higher Fr yields a larger
hydraulic gradient at an intersection.
The influence of (β) as seen in Fig. 5 panel [4]affects the determination of the initial point of the hydraulic gradient at an intersection. Higher β produces
larger ΔL (see the description of ΔL in Fig. 5 panel[1]). In this context, we drew a correlation between
β with the non-dimensional parameter ΔL/l2. The lastterm indicates the ratio between initial points of thehydraulic gradient from the rearmost cell of a roadprior to the intersection (ΔL) with the l2. The result
is presented in Fig. 6 (B), where ‘A’ denotes the amplitude of modeled waves. The result indicates that
ΔL will overlap l2 if β > 0.25 because the flow depth
starts to decline when the tsunami front has alreadypassed the intersection. This means there will be no
hydraulic gradient observed at the intersection even
if the expansion ratio meets the criteria as explained
previously. Therefore, in this condition, the maximum
tsunami flow depth used for the design height will be
the same for all sides of the intersection.
The above discussed phenomena were only
observed in a flat topography scenario. We did not
obtain similar results from sloping topography (1/100
and 1/50). Thus, dynamic features of tsunami flow at
an intersection can only be used for consideration on
determining the design height if the surrounding topography is relatively flat.

ited evacuation time and potential problems such as
traffic jams during evacuation. Therefore, to support
the need for pedestrian bridges for evacuation in thisarea, we determined the placement criteria to apply
pedestrian bridges as evacuation sites in the area described above.
3.2 Placement criteria for pedestrian bridge
We first analyzed the influence of the expansion
ratio (D) as given in Fig. 5, panel [1]. If d1 (symbols
refer to Fig. 2) is relatively similar to d2, then thehydraulic gradient (tg α) will not be observed (see description of tg α in Fig. 5, panel [2]). It will start to bevisible if the expansion ratio (D) is 1.4, and saturated
when D equals 2. Therefore, in a similar range of
Froude number (Fr) examined in this study, a higher
expansion ratio (D > 2) will not give an even higher
hydraulic gradient. These indicate that the influenceof the expansion ratio is crucial; if a pedestrian bridge
is to be positioned at an intersection with sudden expansion phenomena, then the most appropriate place
for the pedestrian bridge is on the landward side andthe side parallel to the coastline of an intersection.
On these sides, the maximum predicted tsunami flow
depths will be lower than that on the sea-side of theintersection.
The subsequent analyses took into account theeffect of road width parallel to the coastline (l2), andthe effect of road length prior to the intersection (l1)on influencing the hydraulic gradient. Our results
show that there was no significant effect from thesefactors. This means first, the water mass that flows
along roads parallel to the coastline will not significantly influence the observed hydraulic gradient at
the intersection; it simply creates a slight differencein the initial position of the hydraulic jump, which is
not considered in this study (Fig. 5, panel [2]). Second, the road length prior to the intersection (l1) does
not influence the hydraulic gradient. Their effect islimited only to a slight reduction of the flow depth
depending on the road length (Fig. 5, panel [3]). Thiswas explained by Goto and Shuto (1983). They statedthat when a tsunami flows between lined obstacles,
the only energy loss is caused by friction due toroughened walls, which is considered to be negligible.
Nevertheless, even if some houses along the roadprior to the intersection are damaged by a tsunami,
the sudden expansion phenomena will still occur as

4. Conclusions
We evaluated the performance of pedestrian
bridges by developing their fragility curves subjected
to a tsunami from two perspectives. The first is the
exposure in terms of their location from the shore;
the second is the hazard characteristics in terms of thetsunami flow depths surrounding them. The results
indicate that pedestrian bridges positioned in an area
up to 1.2 km from the shore, or in an area where theestimated tsunami flow depths are 1.5 times that of88

PERFORMANCE EVALUATION OF PEDESTRIAN BRIDGE AS VERTICAL EVACUATION SITE DURING THE 2011 TSUNAMI IN JAPAN

Figure5. The influence of the following parameters to the modeled water surface at intersection (refer to panel [1]to [4]): [1] expansion ratio (D), [2] road width parallel to the coastline (l2), [3] the length of residential
block (l1) and [4] β (ratio between d1 and d2)

Figure6. Plotting correlation between: [A] expansion ratio (D) and hydraulic gradient (tg α), [B] correlation between the initial position of hydraulic gradient with the β89

A. MUHARI, S. KOSHIMURA, F. IMAMURA

the deck’s height have a high probability of being
damaged by a tsunami.
We discussed the opportunity of utilizing pedestrian bridges for evacuation in the areas describedabove by explaining the placement criteria based ontsunami flow characteristics at an intersection. The
criteria were revealed by considering the dynamic
features of tsunami flows at an intersection in a flat
topographic condition. Depending on the estimatedFroude number, intersections with geometric configuration that have an expansion ratio D > 1.5 and β< 0.15 may be appropriate for pedestrian bridges asvertical evacuation sites if they are installed on the
landward side or on sides parallel to the coastline of
an intersection.

ference on Earthquake Engineering, Taipei, Taiwan.
Japan Society of Civil Engineers (JSCE), 2011, Tohoku
earthquake-tsunami information, available at http://
www.coastal.jp/tsunami2011/, last access March 2012.
Karim, R. K. and Yamazaki, F., 2003, A simplified methodof constructing fragility curves for highway bridges,
Earthquake Engineering and Structural Dynamic, 32,
1603-1626.
Koshimura, S., Oie, T., Yanagisawa, H., Imamura, F., 2009,
Developing fragility function for tsunami damage
estimation using numerical model and post-tsunami
data from Banda Aceh, Indonesia, Coastal Engineering
Journal, 51 (3), 243-273.
Ministry of Land, Infrastructure and Transport (MLIT),
2011, Post-tsunami survey, available at http://www.
yomiuri.co.jp/dy/national/T110807002350.htm, last

Acknowledgement

access March 2012.
Mori, N., Takahashi, T., Yasuda, T., Yanagisawa, H., 2011,

We are very grateful to the JST-JICA project(Multidisciplinary hazard reduction from earthquakes
and volcanoes in Indonesia) group 3, and the Ministryof Education, Culture, Sports, Science and Technology (MEXT) Japan for financial support throughout
study No.(22241042).

Survey of 2011 Tohoku earthquake tsunami inundation
and run-up, Geophysical Research Letter, 38, L00G14,
doi:10.1029/2011GL049210.
Muhari, A., Imamura, F., Koshimura, S., Post, J., 2011, Examination of three practical run-up models for assessing tsunami impact on highly populated areas, Natural
Hazard and Earth System Science, 11, 3107-3123.

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