Proceedings of Coastal Engineering, JSCE, Vol.3, 2012, November

TSUNAMI-DECK: A NEW CONCEPT OFTSUNAMI VERTICAL EVACUATION SYSTEM
Abdul Muhari1†, Fumihiko Imamura1* and Shunichi Koshimura1‡
1†International Research Center for Disaster Science (IRIDeS), Tohoku University, aam@tsunami2.civil.tohoku.ac.jp
1*International Research Center for Disaster Science (IRIDeS), Tohoku University, imamura@irides.tohoku.ac.jp
1‡International Research Center for Disaster Science (IRIDeS), Tohoku University, koshimura@irides.tohoku.ac.jp

A new type of tsunami vertical evacuation structure is proposed to be applied in densely populated
area. The proposal is a combination utility of pedestrian bridge and tsunami tower, which is then installedat intersection. This simple structure does not require a specific place and land occupancy for its
development. Also, the easiness on the construction and maintenance makes it applicable in developing
countries. However, limitation of this idea in practical is found in the design of the height, especially inarea where the predicted tsunami height is more or less similar to the height of pedestrian bridge. In orderto keep the existing height of pedestrian bridge to be used in tsunami-deck, placement criteria are needed
to ensure the safety of evacuee. Starts from the concept of sudden expansion phenomena, a set ofnumerical exercise is conducted to parameterized hydraulic condition that will allow the sudden drop ofwater level due to the building configuration at intersection. Preliminary results indicate that if the suddenexpansion ratio (D) > 1.5, and the ratio between the widths of road to the residential blocks (β) < 0.1, thesudden expansion will occur at intersection with hydraulic gradient depends on the Froude number (Fr).
In this condition, a tsunami-deck can be installed by adjusting the deck’s design according to thecharacteristic of surface elevation at intersection.
Key Words: Tsunami-deck, pedestrian bridge, sudden expansion, hydraulic gradient

1. INTRODUCTION
In pre-disaster condition, difficulty to find spacefor vertical evacuation structure is a common
problem in densely populated areas. Utilizing theexisting tall buildings especially in developing
countries is sometimes risky because the structuralquality is not adequate against the earthquake. As animplication, repeatedly seen congestion that occur
along the roads used for evacuation due to thedependence on horizontal evacuation system and thein-avoided tendency to use cars for evacuation.
To overcome the above situation, a new type ofvertical evacuation structure is proposed.
Tsunami-deck is a combination of pedestrian bridge
and tsunami tower to put at an intersection (Fig.1).
It has several advantages on the applicability indensely populated area ,as it has a multipurpose on
daily basis i.e. as pedestrian bridge, easy toconstruct and easy to maintain, and the mostimportant thing is that the structure solves thecongestion during the evacuation period directly

where it occurs.
Since the idea is based on the extended functionof pedestrian bridge, the limitation of the function
may come from the design height. The existingheight of pedestrian bridge in Japan is around
4.5–5m. Open structure higher than this height forevacuation shelter might not be appropriate for theelderly. Therefore, placement criteria according tothe tsunami flow characteristic at intersection arenecessary.

- 71 -Fig.1 Conception of tsunami-deck

The objective of this paper is to introduce theconcept, and numerically models the placement
criteria for for constructing tsunami-deck. Started
from the concept of sudden expansion phenomena(e.g. Goto and Shuto, 1983), a set of numerical
exercise is performed to set parameters of hydrauliccondition that will allow the sudden drop of tsunami
flow depth due to the building configuration atintersection.

2. METHODOLOGY
A hypothetical town is set as shown in Fig 2.
Residential block prior the intersection occupied allareas, while the second is not. This design allows
the flux at the intersection to spread out in different
direction, and minimizing the unnecessary reflecting
flows from the side walls. In reality, this design can
only represent areas where the width of residential
blocks is much bigger than the width of roads
among them. The numerical simulations wereconducted in several scenarios as shown in Table1.
Initial of tsunami wave is represented by sin wave
with various amplitudes.

Imamura (1996) is used to model the tsunami
propagation and inundation. Reliability of the 2D
Shallow Water Equation (SWE) approach to modeltsunami flow passes through obstacles had beenvalidated by Goto and Shuto (1983), and Tsudaka etal. (2011). They obtained result of fairly good
agreement with experimental data. However, one
should note that this approach has limitation on
reproducing flood water behind single building(Tsudaka et al. 2011). Also the absence of diffusion
term in the momentum equation might yield
inaccuracy of the modeled water level atintersection.
Referring to Fig.2, the influence of expansion
ratio (D) was assessed by increasing d2 while d1keeps remain. The effect of road parallel to the coast
is analyzed by increasing l2, while l1 keeps constant.
The effect the potential building damage prior theintersection and their effect on the hydraulicgradient at intersection is analyzed by reducing thel1 while the l2 remains. Lastly, we estimate the effectof the ratio between road widths to the width ofresidential blocks –symbolized as (β)– on
influencing the hydraulic gradient. The above
15 m

T: 1800 sec
0.5 m

4–5 – 6 m

1/100 – 1/505m
1/100
250 m

25 m

250 md1dx = dy = 0.5 m

d2d3

β=d 1 /d 3

l1
l2

Fig.2 Design of hypothetical town for numerical simulation of tsunami at intersection

A set of non-linear shallow water equation inmentioned numerical scenarios were applied into

Table1. Scenarios for numerical simulation of tsunami at intersection- 72 -

flat and sloping (1/100 and 1/50) topography with
0.5 m cell size. Selection of the grid size isaccording to an empirical relation of dx/(ghmax)1/2 T< 3×10-3 to obtain 0.5 < predicted value/measuredvalue < 2 as proposed by Fujima (2012). The wave
period of 0.5 hours is selected for the sin wave asinitial. This period is similar to the average observed
period of first wave during the 2011 Tohoku
tsunami at Sendai buoys.
From the results of each scenario, a cross sectionin the middle of the road perpendicular to theshoreline is extracted to be analyzed as given thefollowing section.

slightly reduce the flow depth depending on the road
length (Fig. 5). The latter was explained by Gotoand Shuto (1983) as the energy loss when flow
passing through lined obstacle only caused by
friction due to roughened walls which areconsidered as neglect able. Nevertheless, theimportant thing to note is that even if some ofhouses along the road prior to the intersection aredamaged by tsunami, the sudden expansion
phenomena will still occur as long as the expansion
ratio at the intersection meets the requirement as
discussed previously.

3. RESULTS AND DISCUSSIONS
We first analyzed the influence of expansion ratio(D) as given in Fig. 3. If d1 ≈ d2, hydraulic gradient
will not be observed at intersection. It will start tobe visible if the expansion ratio (D) 1.4, andsaturated when D ≥ 2. Thus, it can be said that in thesimilar range of Froude number examined in thisstudy, higher expansion ratio (> 2) will not givehigher hydraulic gradient (tg α) at intersections.

Fig.4 Pattern of the tsunami flow depth by the change of roadwidth l2

Fig.3 Pattern of the tsunami flow depth depending on theexpansion ratio at intersection

The next analyses is taking into account thechange of road width parallel to the coastline, andthe effect of road length prior the intersection on
influencing the hydraulic gradient. The numericalresults show that no significant effect from thesefactors. It means: first, the water mass that goes toroads parallel to the coastline will not significantly
influence the hydraulic gradient at intersection. It
just creates a small different on the initial positionof hydraulic jump after the expansion, which is notconsider in this study (Fig. 4). The second, thelength road prior to intersection is not influencing
the hydraulic gradient. Their effect is limited only toFig.5 Pattern of the tsunami flow depth by the change of roadwidth l1

Considering the influence of Froude number (Fr),the numerical experiments were conducted using
Froude number above its critical condition. This isbecause bore usually observed under supercriticalflow (Fr > 1). The results confirmed that higher
Froude number yields to larger hydraulic gradient at
the intersection as shown in the correlation between- 73 -

hydraulic gradient and expansion ratio with Fr
ranges from 1.13 to 1.34 (Fig. 6).

already passed the intersection. In the other words,
there will be no hydraulic gradient at intersection on
this condition even if the condition of expansion
ratio allows the sudden expansion to occur.

Fig.6 Correlation between expansion ration (D) with thehydraulic gradient (tg α)

The influence of (β) affects to the initial point of
the decline of water level after the rearmost part of
the residential block (ΔL, see definition in Fig. 4). If
d3 is not significantly larger than the d1, thehydraulic gradients were not observed at theintersection. On that condition, ΔL is larger than l2(Fig. 7). To simplify the correlation, a graphbetween β and the non-dimensional number of ΔL/l2
is drawn in Fig.8.

Fig. 8 Correlation between β with the non-dimensionalparameter ΔL/l2

The above discussed phenomena only observedin scenario of flat topography. We did not obtained
similar results from sloping topography (1/100 and1/50). Therefore, this becomes another limitation of
the developed criteria that it is only applicable if thesurrounding topography is relatively flat.
The practical implications of the above
discussion; first, the tsunami-deck is applicable only
if the predicted tsunami flow depth is lower thantheir height. Second, if the predicted flow depth iscomparable to the deck’s height, than (1) it shouldbe placed at intersection with geometricconfiguration of buildings allows the sudden
expansion to occur. (2) An adjustment should bemade on the deck’s design to ensure the flow
passing through beneath the structure according theΔL (Fig.9).

Fig.7 The influence of β on determining the head loss atintersection

The (ΔL/l2) indicates the ratio between initialpoints of hydraulic gradient relative from therearmost cell of road prior to intersection, with thewidth of road parallel to the shoreline (l2). Here, ‘A’denotes the amplitude of modeled waves. It can beseen that ΔL will overlap l2 if β > 0.25 because theflow depth starts to decline when tsunami front

Fig.9 Adjustment on the deck shape to accommodate ΔL

4. CONCLUSIONS- 74 -

A new type of tsunami vertical evacuation
structure is proposed to be applied in densely
populated areas, particularly in developingcountries. The easiness/feasibility of theirconstruction as well as the maintenance, and theflexibility on their placement (at intersection) are
combined with the large space available on the deck
creating an ‘easy to find’ evacuation shelter. Thus,
this structure can be a solution for the congestion
problem during evacuation.
Limitation due to the existing height is solved bydetermining criteria for their placement. Utilizing
the sudden expansion phenomenon at theintersection, the tsunami-deck can be installed in thearea with predicted flow depth less than the deck’sheight. Moreover, adjustment on the deck’s shape isnecessary according to the ΔL at intersection withgeometric configuration of (D) > 1.5 and (β) < 0.15.
The present study described only thehydrodynamic part for the introduction oftsunami-deck concept. It is highly acknowledgedthat the structural analysis and the potential of solid
debris impact should be taking into account for thefurther study which is now currently on going.
ACKNOWLEDGMENT: We express our deep

appreciation to JST-JICA project (Multidisciplinary
hazard reduction from earthquakes and volcanoes inIndonesia) group 3, and the Ministry of Education,Culture, Sports, Science and Technology (MEXT)
Japan for financial support through the study
No.(22241042).
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Goto, C and Shuto, N.: Effect of large obstacle on tsunami
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Tsudaka, R., Shigihara, Y., Fujima, K. (2011). Accuracy oftsunami numerical simulation with high-resolution
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- 75 -(Received June 15, 2011)