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Proceedings of

the Sixth International Workshop

on Coastal Disaster Prevention

December 1 - 2, 2009

Holiday Inn Bangkok, Bangkok, Thailand

Port and Airport Research Institute, Japan

Development Institute of Technology, Japan

Ministry of Land, Infrastructure, Transport and Tourism, Japan

Department of Civil Engineering, Chulalongkorn University, Thailand

Climate and Disaster Science and Technology Center, Ministry of Science and Technology, Thailand


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CONTENTS

Tsunami Research - Its past, present and near future - ..................................................1

Nobuo Shuto

Tsunami Risk Assessment and the Planning and Implementation of Strategic Mitigation

Measures- Case Study City of Galle..............................................................................25

Samantha Hettiarachchi, Saman Samarawickrama and Nimal Wijeratne

Real Time Monitoring for Mega Thrust Earthquakes and Tsunamis around the Nankai

Trough Southwestern Japan - Towards to understanding mega thrust earthquakes anddisaster mitigation - .......................................................................................................37

Yoshiyuki Kaneda, Katsuyoshi Kawaguchi, Eiichiro Araki, Hiroyuki Matsumoto,

Takeshi Nakamura, Shinichiro Kamiya, Keisuke Ariyoshi, Takane Hori, Hide Sakaguchi,

Maddegedara Lalith and Toshitaka Baba

Tsunami Warning System in Japan................................................................................47

Kenji Nakata

Experimental Study on Tsunami Power ........................................................................51

Taro Arikawa

Necessity of Advanced Tsunami Damage Index ...........................................................61

Koji Fujima

Detailed and Real-time Estimation Methods of Tsunami..............................................71

Takashi Tomita, Daisuke Tatsumi and Kazuhiko Honda

Tsunami Evacuation Simulation in an Urban Area of Japan .........................................89

Kentaro Kumagai

Tsunami Disaster Reduction in Japanese Ports and Harbors.........................................93

Naokazu Takata

Recent Progress on Tsunami Disaster Mitigation in Indonesia ...................................107

Subandono Diposaptono and Enggar Sadtopo


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TSUNAMI RESEARCH ITS PAST, PRESENT AND NEAR FUTURE

Nobuo SHUTO

ARISH, Nihon University, Tokyo, Japan, [email protected]

Abstract

Each time when a tsunami occurred, research seeds were found and tsunami research made

progress. After the 1933 tsunami, a small scale experiment began to understand the

generation mechanism of tsunamis. Before 1960, the mainstream of tsunami research was

theoretical works. Generation of tsunamis and T-waves was predicted. Need of the

dispersion term was theoretically recommended in the analysis of far-field tsunamis. In

1960, large-scale experiments and numerical simulation started. After 1970s, numerical

simulations with the shallow-water theory have been growing with remarkable speed. The

Mansinha-Smylie method provided a way to determine the initial profile of homogeneous

fault model, which is being replaced by heterogeneous fault model. In 1983, a video

clearly showed soliton fission in the sea that requires the non-linear dispersive equation.

In 1993, a tsunami trace in a narrow valley urged the introduction of the 2D/3D hybrid

simulation. In addition to the analysis and simulation of tsunamis, there are many needs

from a viewpoint of disaster mitigation.

Keywords: Numerical simulation, initial profile, propagation, run-ups, damage assessment

1. IntroductionFrom the birth to the coastal effects of tsunamis, each research topics are described in order of time.

In Section 2, causes of tsunami are summarized. Although sub-marine earthquakes are the major

subject in the present paper, another cause such as volcanic activity should be paid attention, if we

consider the Krakatau eruption that claimed over 36,000 lives.

In Section 3, generation of tsunamis and related problems is discussed. A sub-marine earthquake

generates not only tsunami but also T-wave. The tsunami itself generates infrasound. Static and

dynamic generation mechanisms are explained.

In Section 4, the method to estimate tsunami initial profile is described. Before 1971, the inverse

propagation diagram was used. With the Mansinha-Smylie method, the fault parameters determined

from seismic data are used to estimate tsunami initial profile. After several experiences, assumption of

Proceedings of the 6th International

Workshop on Coastal Disaster Prevention,

Bangkok, Thailand, December 1-2, 2009


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homogeneous fault model is being replaced by heterogeneous fault model.

In Section 5, tsunamis in the deep sea are discussed. For a near-field tsunami, the liner long wave

theory is applicable while for a far-field tsunami the linear Boussinesq equation is necessary if the Kajiura

number pais smaller than 4.Section 6 is for tsunamis in the shallow sea. The fundamental equations are the shallow-water

theory with bottom friction included. If a tsunami front shows cnoidal bore, dispersion effect should be

taken into consideration. There is no theory applicable to edge bores, running along the shoreline with

breaking front.

Section 7 is for tsunamis on land. Run-up front needs q special assumption of the moving boundary.

For a special topography, three dimensional equations are required. To deal with this condition, a 2D/3D

hybrid simulation is introduced. For a judgment whether or not a numerical simulation is carried out

satisfactorily for practical application, Aida measures Kand are used.

In Section 8, four topics important from a practical point of view are selected; damage to houses and

buildings, impact of waves and floating materials, erosion due to tsunami-induced currents, and tsunami

control forests.

Section 9 is for research topics to be solved in the near future. Section 10 is concluding remarks.

2. Causes of Tsunamis

2.1 The oldest document

Thucydides, a Greek historian, was the first person who recorded a tsunami and thought that the

tsunami was generated by an earthquake. In summer of 426 B.C. during the Peloponnesian war, the sea

receded after an earthquake, and then a huge wave hit the city of Orobiae at the northwestern coast of

Euboea Island. A part of the city subsided and became the sea. At the island of Atlanta at the other side

of the strait was also hit by the tsunami. A part of Athenian fortifications was swept away. Thucydides

considered that the full force of the earthquake drew the seawater from the shore and then the sea

suddenly swept back again even more violently.

2.2 Submarine Earthquake

Most of the causes of tsunamis are submarine earthquakes. Not the ground shaking but the vertical

displacement of sea-bottom generates a tsunami. The greater an earthquake is, the larger is the verticaldisplacement; hence the larger tsunami is generated. This is a rule for a tsunamigenic earthquake. An

exception is a tsunami earthquake. Much larger tsunamis than expected from earthquake seismic waves

can be generated.

2.3 Landslide

Landslides can also generate tsunamis. Lituya Bay in Alaska repeatedly experienced huge local

tsunamis in 1958, 1936, 1899, 1853-1854, and probably in 1900. This bay is about 11 km long, 1 km

wide and 169 m deep. On July 10, 1958, an earthquake caused 30 million cubic meters of rocks

weighing 90 million tons to slide from the northern shore from an average height of 600 m with the

dimensions of 700 m to 900 m and the average thickness of 90 m. The slide forced the water surge up to


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the height of 520 m on the opposite shore. Then, the water ran down into the bay to form a huge tsunami

higher than 30 m in the bay (Lander, 1996).

2.4 Volcanic ActivityAnother mechanism is volcanic actions. In May 1883, the volcanic eruption of Krakatau in the

Sunda Strait between Java and Sumatra Islands began. On August 27, a giant tsunami was generated.

Because of thickly falling ashes, no one could see the tsunami offshore. When coastal residents noticed

the white cap of the tsunami, the tsunami was just in front of them. There was no time for evacuation.

It claimed more than 36,000 human lives. Its maximum runup was higher than 30 m Simkin and

Fiske).

3. Generation of Tsunamis, T-waves and Infrasound

3.1 Tsunami Earthquake

The Meiji Sanriku Great Tsunami on June 15, 1896, opened the gate to the tsunami science. It was a

typical tsunami earthquake. Many coastal residents felt no ground shaking and did not try to evacuate.

A giant tsunami with the maximum runup height of 38 m hit and claimed 22,000 lives. Since the ground

shaking was too weak, scholars considered that the cause might be a submarine landslide or an action of a

submarine volcano. Imamura (1899) checked the wave length on the tide records that reflected the size

of the tsunami source area, and found that the source area should be wider than several ten kilometers, too

wide for the possible size of a landslide or an eruption of volcano. He concluded the submarine

earthquake was the cause.

To understand the mechanism of a tsunami earthquake, we have to wait until 1977 when Tanioka,

Ruff and Satake (1997) proposed an assumption of the horst-graven topography.

3.2 Tsunamigenic Earthquake

(1) Static Generation

On March 3, 1933, the Showa Great Sanriku Tsunami hit after a strong ground shaking. The modern

tsunami science and technology began with this tsunami. Takahashi (1934) began hydraulic

experiments of tsunami generation efficiency with apparatus

shown in Fig.1. Speed and stroke of the piston were adjusted.He found that a large stroke with the high speed of the piston

movement yielded high waves.

Kajiura (1970) discussed theoretically the generation

efficiency based upon the long wave approximation. The

energy transfer from the sea bottom to the water was examined

in relation to the duration of the bottom movement. If the

duration is less than several minutes, the deformation may be

considered to be abrupt as far as the tsunami is concerned.

This idea leads to the static theory of tsunami generation; that

is, the final vertical displacement of sea bottom surface is

Figure 1: Takahashis experimental

setup


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equal to the tsunami initial profile.

(2) Dynamic Generation

Omachi et al. (1999) carried out a dynamic analysis of tsunami generation and showed that the

tsunami height could be larger than the static displacement. If the period of the Rayleigh waveapproaches the water wave period that is determined by the water depth, the tsunami is amplified. For a

tsunami generated in the shallow sea, this mechanism may become non-negligible.

3.3 T-Wave Generation

Kajiura (1970) also predicted that if the movement is completed in a few second, the energy transferred

to the compressional water waves (T-wave) might be larger than the tsunami energy. Iwasaki (1992)

discussed the relation between the movement of sea bottom surface and characteristics of T-wave. The

generation of the compressional water waves was proved by a record of the 2004 Indian Ocean Tsunami

(Okal et al., 2007).

3.4 Infrasound Generation

When a tsunami appears on the sea surface, this vertical displacement compresses the air and

generates the compressional air waves (Iwasaki, 1992; Izumiya and Nagaoka, 1994). This infrasound, not

perceived by human, propagates with the speed of 340 m/s faster than tsunamis. When the 2004 Indian

Ocean Tsunami hit Sri Lanka and Thailand, elephants detected this low-frequency sound and evacuated

from the sea shore. This sound also recorded on a CTBT microphone (Garces, M. et al., 2005).

4. Tsunami Initial Profile

4.1 Inverse Propagation Method

Before 1971, the inverse propagation method was used to estimate the initial profile. Starting from a

tide gauge station, wave fronts (inverse wave fronts) are drawn, on assuming the propagation velocity of

linear long waves. The inverse wave front that corresponds to the time between the earthquake and the

arrival of the tsunami, gives the position from any place of which the tsunami can starts and arrives at the

tide station. An envelope of the inverse wave fronts from available tide gauge stations is the landward

boundary of the tsunami source area. From the profile of the first wave, wave length is calculated to

determine the width of the tsunami source, i.e., seaward boundary of the tsunami source. Horizontaldistribution of rise and fall of the water surface is also estimated, using tide records.

4.2 Use of Fault Parameters

Mansinha & Smylie (1971) introduced a way to calculate the sea-bottom displacement based on fault

parameters. From analysis of the seismic wave record, a fault movement is described by geometrical

characteristics (location, depth, strike-, dip- and slip-angles of the fault plane), physical characteristics

(length, width and dislocation of the fault plane) and dynamic characteristics (rupture direction, rupture

velocity and rise time of the fault movement). With fault parameters (except for dynamic

characteristics), the static displacement of sea bottom surface can be computed, assuming that a fault

movement occurred in a semi-infinite elastic, homogeneous body. This assumes a homogeneous


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movement in a fault plane that leads us to a simple tsunami profile: one crest and one trough in the area of

the generation. More complete study was carried out by Okada (1985) with correction of misprints in

Mansinha & Smylie paper.

If the fault movement is not large, this estimation works well. An example is shown in Figs. 2 and 3.

Figure 2: The source of the 2004 Kii Hanto-oki Figure 3: Comparison between the measured

tsunami. and measured.

The main shock of MJ=7.4 occurred at 23:57 off the Kii Peninsula generated a tsunami (Fig.2). The

record of a GPS wave gauge, after 150 second moving average, shows the tsunami as in Fig.3. The upper

line in Fig. 3 is the computed by Koike et al. (2005).

4.3 Actual Examples

However, for a big fault motion, the Mansinha-Smylie method does not always give satisfactory

results. Fig. 4 shows the initial profiles of the 1993 Hokkaido Nansei-Oki tsunami (MW=7.8) proposed

by different researchers. The left was computed based upon the Harvard CMT solution assuming one

fault plane. The middle was by Kikuchi who used the seismic records near the epicenter and assumed

two fault planes. Both used only seismic data. If started with these initial profiles, the tsunami was not

explained.

The DCRC of Tohoku University repeated trials 24 times to find DCRC-17a that satisfies earthquake,

tsunami and topographic change (Takahashi et al. 1995).

Figure 4: Initial profile proposed by Harvard (left), Figure 5: Measured profile of the

Kikuchi (middle) and DCRC (right). 1964 tsunami.


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Only one example of the measured initial tsunami profile is found in the case of the 1964 Great

Alaska earthquake. Figure 5 is obtained by Plafker (1965). A section along the line A-A is shown in

the lower figure. There is a gentle wavy deformation 450 km long with one crest and one trough. Its

wave height is about 6 m. Near the crest, there is a sharp rise about 6 m high and about 30 km wide atits base. We can compute this long deformation with the Mansinha-Smylie method but not the sharp rise,

a result of a sub-fault developed in the accretionary prism, although this rise makes the tsunami height

almost double.

4.4 Heterogeneous Fault Model

There have been several efforts to obtain more realistic initial

profile using seismic records (for example, Fukuyama and Irikura,

1986) or tide records (Satake, 1989). Inversion introduced by

Satake (1989) is as follows. The fault plane is first divided into

several subfaults and the deformation on the ocean bottom is

computed for each subfaults with a unit amount of slip. Using

this as an initial condition, tsunami waveforms are numerically

computed on actual bathymetry. The observed tsunami

waveforms are expressed as a superposition of the computed

waveforms as follows,

Aij(t)xj= bi(t) (1)

Where Aijis the computed waveform, or Greens function, at the

ith station from the jth subfault; xjis the amount of slip on the jth

subfault; and biis the observed tsunami waveform at the ith station.

The slip xjon each subfault can be estimated by a least-squares inversion of the above set of equation.

Figure 6 is for the 1968 Tokachi-oki tsunami by Satake.

Recent development of asperity accelerates the need of the heterogeneous fault estimation.

5. Propagation in the Deep Ocean

5.1 Assumption of Long Waves

When generated, a tsunami has a wavelength several tens kilometer long that is much longer than thewater depth. For example, the average depth of the Pacific Ocean is about 4.2 km. This water wave is

categorized as long waves, for which the hydrostatic water pressure is a good, first-order approximation.

The initial height of the tsunami is several meters, quite small compared to the water depth and the wave

length. This water wave belongs to the small amplitude waves and is analyzed with the linear long wave

theory.

In the sea shallower than 200 m, a tsunami height becomes of finite magnitude, not infinitesimally

small compared with the water depth. The shallow-water theory, the first-order non-linear long wave

equation, is used. If its water surface develops to have a non-negligibly wavy shape, the effect of

surface curvature that is called as the dispersion effect should be taken into consideration.

Figure 6: Satakes heterogeneous

fault model.


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5.2 Phase Velocity Under the Long Wave Approximation

Corresponding to this process of development, the phase velocity is expressed as follows.

(2)

The first term is for a linear long wave, determined only by the gravitational acceleration gand the water

depth h. The second term expresses the contribution of the finite amplitude . The higher point

propagates faster, and the wave front becomes steeper approaching the shore. The third term is the

effect of the local curvature of the water surface. A wave crest for which the curvature takes a negative

value propagates slower. Once the second and third terms cancel out each other, a long wave of finite

amplitude can propagate keeping its profile. Typical example is a solitary wave.

5.3 Dispersion Effect for Far-field Tsunamis

For a tsunami in the ocean deeper than 200 m, the effect of the finite amplitude is always negligible

but the dispersion effect is, sometimes, not negligible. The dispersion effect means that high frequency

component propagates with lower phase velocity. This difference, although very small, results in

non-negligible deformation in wave profile, if the travel distance is long as in case of a far-field tsunami.

A parameter pa proposed by Kajiura (1970) is used to judge whether the dispersion effect should be

included or not:

pa= (6h/R)1/3

(a/h) (3)

where his the water depth, athe horizontal dimension of the tsunami source and Rthe distance from the

source. If pais smaller than 4, the dispersion effect should not be neglected and the linear Boussinesq

equation that includes the first-order effect of the phased dispersion should be used. The Coriolis effects

are also required. In placed of the Cartesian coordinates, the latitude-longitude coordinate system is

used.

5.4 Use of the Imamura Term

The difference equation of the linear Boussinesq equation becomes complicated because of the

dispersion term and needs more CPU time for numerical simulation. Imamura et al. (1990) introduced a

clever way to reduce the computation time but keeping the same accuracy. The linear Boussinesqequation is as follows and the third term is the dispersion term.

(4)

When the linear long wave equation is expressed by a difference equation as follows, the first term of

truncation error takes the similar form to the physical dispersion term.

(5)

where Kis the Courant number, the ratio of the physical wave propagation velocity C0to the artificial one


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(x/t). Select the spatial grids and time steps so as to make the coefficients of the third terms in both

equations the same, and then the linear long wave difference equation gives the same result as the

physically correct linear Boussinesq equation.

5.5 Dispersion due to the Coriolis Force

The Coriolis force has a dispersion effect (Imamura et al, 1990). The phase velocity of the linear

long waves is modified to

C=(gh)1/2[1-(f/)2]1/2 (6)

where is the wave number and f is the Coriolis coefficient. For a huge tsunami such as the 1960

Chilean tsunami, the Pacific Ocean behaves like a small pond. Average water depth of the Pacific

Ocean 4.2 km, gives the tsunami a propagation velocity faster than 730 km/hr. The tsunami traveled

17,000 km from the source off Chilean coast to Japan within 23 hours. It started toward Japan with the

crest at its front but arrived at Japan with a big ebb. The first crest became unrecognizably small and the

following trough began to grow near the Hawaiian Islands. This change was the result of the dispersion

due to the Coriolis Effect.

5.6 Topography Effects

(1) Scattering by Sea Mounts

Traveling over the ocean, a far-field tsunami is affected much by the bottom topography. When a

tsunami passes a sea mount, scattered waves are induced according to the scale of the sea mount, and

propagate as the concentric circles from the mount. Tsuji (1977) discussed the wave scattering over a

sea mount or rise. In case of the 1960 Chilean tsunami, 40% of the incident tsunami was lost due to

wave scattering during its travel from the coast of Chile to Japan. Mofjeld et al. (2001) analyzed the

scattering capability of the topography in the Pacific Ocean, and found a narrow band of strong scattering

running across the ocean from the northwest (Emperor Seamount Chain) to the southeast (Easter Island

Fracture Zone). Based upon their results, they recommend that numerical models of trans-Pacific

tsunamis must resolve the effects of the small-scale topography in order to accurately simulate their wave

patterns and amplitudes.

(2) Transmission by Submarine RidgeA submarine ridge acts as a good wave guide. The shallower the water depth is, the slower is the

wave propagation. Waves change their propagation direction toward the shallower ridge crest. This

refraction causes concentration and effective transportation of tsunami energy along an ocean ridge. The

energy of the 1996 Irian Jaya tsunami was transported to Japan although Japan is not located on the major

direction of initial tsunami energy radiation. A numerical simulation clarified that the South-Honshu

ridge acted as an effective wave guide (Koshimura et al., 1999).

(3) Edge Waves on the Continental Shelf

Tsunamis behave as edge waves on the continental shelf. When a far-field tsunami is incident

almost parallel to the shoreline, multiple reflected waves are excited and are trapped on the slope. The


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interference between these reflected waves and incoming incident waves could be a cause of unexpected

tsunami amplification on the shelf (Koshimura, 2002). In case of a near-field tsunami, edge waves

through different propagation path determine the tsunami. Radiated from the source of two-dimensional

extension, the tsunami front enters the slope with locally different incident angle. The first wave at asite is the component that propagates relatively deep area, and then other components of high energy

arrive later after a slow propagation on the slope along the shore.

6. Tsunamis in the Shallow Sea

6.1 Amplification Mechanism

In the shallow sea where a tsunami becomes of non-negligible wave height compared to the water

depth, the effect of finite amplitude is taken into consideration. The tsunami heightHvaries according

to the Green Law that assumes the constant energy transmission rate as the water depthhand the width of

a channelBchange. The former is the shoaling effect and the latter is the focusing effect.

(gh)1/2H2B=constant, H~h-1/4B-1/2 (7)

Another important amplification phenomenon that occurs in relation to topography is resonance in

bays and harbors. If a bay satisfies the following relation with an incident tsunami, the oscillation at the

head of bay becomes quite high.

4l= (gh)1/2T (8)

where l is the length of the bay and Tis the wave period of the tsunami.

6.2 Tsunami Front

The shallow-water equations applicable to this long wave include the first-order non-linear,

convection terms. The water pressure is still assumed to be hydrostatic. The non-linear effect in the

phase velocity makes the higher rear part to go forward faster, then tsunami profile becomes step-wise,

and the tsunami front steepens. This is a bore.

When the steep bore front continues breaking, it is the breaking bore and is solved with the

shallow-water theory.

When the effect of the local water surface curvature prevents the breaking, a train of short-period

waves appears and develops at the front. This is the soliton fission that was clearly recorded on videos

(NHK, 1983). Tsunamis in river often take this type of bore. This bore is called as a wavy bore or acnoidal bore, according to cn(x, k) of the Jacobian elliptic function that is a periodic solution of the

non-linear dispersive long wave equation. As the modulus k of the cn-function tends to 0, a cnoidal

wave becomes a solitary wave. The water pressure is modified by the centrifugal acceleration from the

hydrostatic distribution. For a tsunami initial profile that has high frequency components, the dispersion

terms should be taken into consideration, even if the source area is close to land.

6.3 Edge Bores

A breaking bore propagating along the shore is an edge bore. This was also recorded on video

published by NHK (1983). There is no theory applicable to the edge bore. Uda et al. (1988) carried

out large scale hydraulic experiments and found several interesting phenomena. An edge bore


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propagates sometimes following the ordinary refraction law and sometimes neglecting the topography.

A small difference in the side boundary condition introduces a big difference in wave profile, thus

suggesting also a big difference in wave force.

6.4 Non-linear Dispersive Equations

There are many kinds of non-linear dispersive equations. Different expressions are resulted from the

different basic parameters for perturbation. The Peregrine (1967) used the section-averaged velocity as

the fundamental variable. Madsen & Sorensen (1992) selected the depth-integrated variable, i.e. the

discharge flux. Equations by Iwase et al. (1998) are composed of the same terms as those of the

Peregrine equations but expressed in terms of the discharge flux. Other perturbations are possible, using

the bottom velocity (Mei & LeMehaute, 1966) or the surface velocity (Dingemains, 1973). Iwase et al.

(2002) carried out numerical computation using different 8 equations and compared the results with the

theoretical prediction and hydraulic experiments. The best agreement was obtained with the Iwase et al.

equations and the Madsen & Sorensen equation. The former, in other word, the Peregrine equation in

terms of the discharge flux is more convenient because it is simpler than the latter.

(9)

The use of the Iwase et als modified Peregrine equation is recommended for numerical simulation of a

near-field tsunami from its source in deep sea to the shallow water region.

7. Tsunami Run-up on Land

7.1 Theoretical Approach

Although our most concern is tsunamis in the near-shore zone and on land where human activity is

intense, it is not easy to simulate the tsunamis and their effects. Before the numerical computation with

electronic computer was introduced, such simple but basic problem as one-dimensional run-up on a

sloping beach needed a skilful method of the Carrier-Greenspan transform (1958) for waves of finite

amplitude. The same problem was solved with the linear long wave theory expressed in the Lagrangian

description (Shuto, 1967). These kinds of theoretical consideration can not be applied to solve any

practical problems but only supply data necessary for examination of the numerical scheme.

7.2 Numerical Simulation with the Shallow-Water Theory

The shallow-water equations are successfully applied in the numerical simulation of tsunami run-up

even if tsunamis have a breaking bore front. A careful design of numerical scheme is, of course,

necessary. In order to ensure a stable computation, the grid lengthx and the time stept should

satisfy the following CFL condition.

x/t(2gh)1/2 (10)

It is not easy to solve the tsunami front that runs up and down the land with the shallow-water

theory expressed in the Eulerian description. We need a moving boundary assumption. In the

leap-frog numerical scheme, grid points are alternatively located for velocity and water level. Assume


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that the water level is already computed at a computation cell. Then, compare the water level with the

bottom height of the next landward cell. If the water level is higher than the latter, the water may flow

into the landward cell. Iwasaki and Mano (1979) assume that the line connecting the water level and the

bottom height gives the surface slope to the first-order approximation. Hibberd and Peregrine (1979)proposed more accurate method that required repeated computation. Aida (1977) and Houston - Butler

(1979) used weir formula to determine the discharge into the landward dry cell. For the practical

application, the Iwasaki and Mano assumption is widely used, because of its simplicity.

When a difference equation is solved, we have to control numerical errors that depend upon the

scheme used in the computation. Under the condition that the shallow-water equations are discretized

with the leap-frog scheme, one local tsunami wavelength should be covered by more than 20 grid points.

Otherwise, the computed tsunami height experiences numerical decay (Shuto et al., 1986). Imamura and

Goto (1988) discussed more thoroughly the possible numerical errors in relation to local wave length.

If the Iwasaki-Mano boundary condition is used, the following condition should be satisfied at the

front.

x/gT2


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essentially important. The third is the selection of basic differential equations. If the original equations

are used without any approximation, the CPU time is too long for practical application. An

approximation, in which some terms are neglected or simplified, does not express the real phenomena.

In the difference equations, we have to expect inevitable truncation errors. Accuracy of measured datathat are used to verify the numerical results is not always reliable. Tsunami trace heights sometimes

reflect the splash that cannot be simulated. Tide records give smaller amplitudes than the actual tsunami

(Satake et al., 1988).

Aidas K and are used to judge whether or not a simulation gives satisfactory run-up heights

(Aida, 1978).

(12)

(13)

The measure K is a geometric mean of Ki, the ratio of the

measured run-up heightRito the computedHi,. The corresponding standard deviation is. Based upon

rich experience of simulation in Japan, if Kfalls between 1.2 and 0.8 andis less than 1.4, the simulation

is judged satisfactorily carried out for practical use.

8. Mechanism and Damage Assessment

8.1Damage to Houses and Buildings

(1) Damage to a Village Inundated

Hatori (1984) defined the damage percentage of housesRHDin a given village as follows.

RHD= (a+ 0.5b)/(a+b+c) (14)

where a is the number of houses washed away and completely destroyed, b is the number of houses

partially damaged, and c is the number of houses only flooded without structural damage. Degree of

damage is judged by on-site inspection. Number of data thus obtained is of the order of several tens at

most. He tried to find a relation betweenRHD and tsunami height. If numerical results are available,

RHD can be explained in terms of drag forces that is

proportional to current velocity (for example; Shuto, 1993).

Koshimura et al. (2009) developed Hatoris damagepercentage to fragility functions that treat tremendous

number of houses. From a comparison of the pre-and

post-tsunami IKONOS satellite imageries for Banda Ache

hit by the 2004 Indian Ocean Tsunami, they counted the

16,474 destroyed houses, the roofs of which disappeared,

and the 32,436 survived houses of remaining roofs.

Combining the damage probability and inundation depth

(or, current velocity or hydrodynamic force), they obtained Figure 7: Example of fragility function.

the fragility functions as shown in Fig. 7


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(2) Damage to Individual Building

Degree of damage depends upon the structure of houses and the tsunami force. Data in the past are

given in terms of the inundation depth (Shuto, 1993). Roughly speaking, a Japanese wooden house can

withstands if the inundation depth is smaller than 1 m and is completely washed away if the inundationdepth is larger than 2 m. All the reinforced concrete buildings in the past example could withstand

tsunami forces even if the inundation depth was 5 m, except one example, the Scotch Cap Lighthouse that

was completely washed away by the 1946 Aleutian tsunami of 20 m inundation depth.

Iizuka and Matsutomi (2000) expressed quantitatively damage conditions in terms of inundation depth,

current velocity and/or hydrodynamic force. A wooden house will be destroyed if the inundation depth

is over 2 m, or if current velocity is over 4.9 m/s, or if hydrodynamic force is over 27 kN/m.

Koshimura et al. (2000) obtained that the structures were significantly vulnerable when the local

inundation depth exceeds 2 or 3 m, the current velocity exceeds 2.5 m/s or hydrodynamic load on a

structure exceeds 5 kN/m.

8.2 Impact of Waves and Floating Materials

(1) Wave pressure to vertical wall on land

Asakura et al. (2000) carried out hydraulic

experiments of tsunami force on buildings due to

tsunami run-up front without breaking. Two types of

front, with and without formation of solitons, were

examined. A run-up front without solitons gives

hydrostatic wave pressure distribution pH(z) as

follows, where hc is the maximum inundation depth

measured on the seaward wall of building. Figure 8: Pressure distribution without and

with solitons.

PH(z) /g = (3hcz) (15)

For a run-up front with solitons, the lower part of the pressure distribution is modified as shown in

Fig.8 and the total pressure increases by 20 %. Including the two conditions, pressure distribution is

expressed as follows.

PH(z)/g= max(5.4hcz, 3hcz) (16)

Arikawa et al. (2006) carried out large-scale hydraulic experiments and showed that the equation

above was applicable for Froude number smaller than unity. For Froude number that exceeds unity, a

tsunami can break just in front of wall to generate larger impact force. The pressure distribution is not

hydrostatic but has a peak value at the inundation height.

The total force FDis defined by Iizuka and Matsutomi (2000) as follows,

FD= 0.61gCDhc2B (17)

where CD (=1.1~2.0) is the drag coefficient andBis the breadth of building. Large range of variation in

the value of CDmeans that the shape of tsunami hitting the wall is important factor to generate whether

impact force or hydrostatic pressure.


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(2) Force caused by breaking bore

The first hydraulic experiment of breaking bore in a large scale was carried out by Fukui et al.

(1962a).

The propagation velocity of the breaking front, c, was theoretically derived as follows.c = [g(H + h)(H + 2h)/2(H+h H)]

1/2 (18)

whereHis the height of breaking bore, his the water depth, is the total water depth and is the resistance

coefficient.

The impact total force, Pi, due to breaking bore was expressed in terms of this propagation velocity.

Pi= Kgc4/g

2H (19)

where Kis non-dimensional coefficient that takes 0.51 for vertical wall and tends to 0.33 as the wall slope

becomes gentle (Fukui et al, 1962b).

Run-up and force by breaking bores were experimentally studied by Mizutani and Imamura (2000).

They found three peaks in wave pressure on structures. The first is the dynamic wave pressure (D.W.P)

caused by impact of an incident bore. The second, the sustained wave pressure (S.W.P.), appears during

the high rise of water level because of continuous incidence of the bore. The third, the impact standing

wave pressure (I.S.W.P), is a result of impulsive collision between the incident and reflected bores.

Takahashi, Fujima and Asakura (2001) succeeded the numerical simulation of this phenomenon with a

method of numerical wave flume, CADMAS-SURF

(SUper Roller Flume for Computer Aided Design of

MAritime Structure) (CDIT, 2001; Fujima, 2002). Figure 9 compares the time series of pressure along

the bottom surface and Fig. 10 compares the vertical distribution of wave pressure.

Figure 9: Computed and measured time series Figure 10. Computed and measured wave pressure

of wave pressure. distribution.

(3) Impact of floating materials

Figure 11: Pressure distribution with impact of lumber. Figure 12: Matsutomi diagram for lumber impact.


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Hydraulic experiments (for example Matsutomi, 1993) showed the increase of force due to floating

materials.

Fig. 11 is an example of the vertical distribution of pressure when a lumber hits with the breaking

bore front. In addition to bore-induced water pressure, impact of a lumber acts locally but strongly.Matsutomi (1999) provided a diagram (Fig.12) in terms of dimensionless impact and dimensionless

collision velocity to evaluate the impulsive force due to lumbers floated by breaking bores and surging

fronts. In the figure, Fmis the impact, , D and L are the unit weight, diameter and length of lumber,

and VA0is the approaching velocity just on collision.

After the 2004 Indian Ocean tsunami, similar studies but for different materials are being carried out,

for example, on ships and boats (Ikeya et al, 2006; Fujii et al, 2007), on cars (Anno et al., 2007), and on

containers (Kumagai et al., 2007) .

Transport of lumbers was first solved by Goto (1983). Lumbers stored in timber yard on land begin

to be floated if the tsunami inundation depth exceeds their diameter and those in a timber basin when

tsunami force on timbers exceeds the strength of mooring wire. Then, lumbers are transported by the

tsunami-induced current, scattering by diffusion effect.

8.3 Erosion due to Tsunami-Induced Current

(1) Damage to Coastal Road Embankment

Massive structures such as coastal road embankments made of soil are not destroyed by the strong

impact of tsunami front but are eroded and damaged by the water current induced by tsunamis. The first

type occurs when the tsunami height is lower than the crest of embankment. Stopped by a long coastal

embankment, the water concentrates to the openings such as underpass or bridge with increasing velocity.

Then, the neighborhood of the openings is soured. The second is the case when a tsunami overflows the

structures and hit the rear slope and rear toe that are usually not protected with solid covers. The

overflowing tsunami is an unsteady flow that is sub-critical on the crest, super-critical on the rear slope

and returns to sub-critical after a hydraulic jump. Erosion process under this complicated flow is solved

by Fujii et al. (2009). They used CADMAS-SURFfor flow computation combining with an erosion law

they established through hydraulic experiments.

(2) Toe of Quay Eroded by Backwash Water FallsWhen a tsunami recedes after landing, the water falling from the top of quay wall directly hit the sea

bottom nearly exposed. The toe is scoured to lead to destruction of quay walls (Shuto, 2009).

Gotoh et al. (2002) used the moving-particle semi-implicit method (MPS) to solve the nap

formation of the falling water from the quay and the erosion of sand bottom at the toe.

Their result qualitatively explained the results of scouring obtained in large-scale hydraulic

experiments of Noguchi et al. (1997). However, there is no trial to explain some of damage

examples in the past.

(3) Tsunami-Induced Current in Narrow Waterways (Shuto, 2009)

The second case is the tsunami-induced current at narrow waterways in harbors or bays. Strong


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currents scour the sea bottom just at the toe of structures and destroy them. A good example is found in

case of the gravity-type quay wall of the Konakano Fish market, Hachinohe Harbor, in Iwate Prefecture,

Japan.

The fish market was completed in August, 1959, one year before the 1960 Chilean Tsunami, near theentrance of Hachinohe Industrial Harbor which was built by using the mouth of the Niida River. Near

the entrance of the water area 200 m wide and 2 km long, the Fish Market was built. The whole

harbor was very much influenced by the 1960 Chilean Tsunami. At the entrance of the harbor, the

maximum ebb flow velocity was estimated to be 13 m/s and the maximum flood flow velocity 8 m/s.

By this current, the Fish Market was affected and damaged. Figure 13 shows a section of the market

before and after. The maximum amplitude of the 1960 Chilean Tsunami was about 6 m. The toe of the

quay wall -3m deep was scored to -9 m. In addition, the soil- and residual water- pressures from behind

during abnormally low water destroyed the basis of caissons and pushed them forward.

Five of 8 caissons, each of which was 10 m long, were overturned or subsided, as shown in Fig.13.

A person, who witnessed from the opposite side of harbor 200 m far, told that the quay wall collapsed

during an ebb tide from 06:31 a.m. to 07:03 a.m.

Figure 13: A section of the fish market damaged in 1960.

(4) Current Measurement

Different from the tsunami height, there are almost no measured data of tsunami-induced current.

Takahashi et al. (1991) used aero photographs taken for the Kesen-numa Bay in case of the 1960Chilean Tsunami. From the movement of floating materials and the Cameron effect, they determined

the current velocity distribution and compared with the results of their numerical simulation as in Fig. 15.

White circles were obtained from the numerical simulation for the sea bottom bathymetry before the

tsunami and black triangles are for that after the tsunami. There are big differences.

The computed current velocity are less than half the measured, although their computation simulated very

well a tide record in the bay.

Nagai et al. (2004) concluded the generation of edge waves along an arch-shaped coast of Hokkaido

in case of the 2003 Tokachi-Off Earthquake Tsunami, using the records obtained with super-sonic current

meters installed by NOWPHAS (Nationwide Ocean Wave Information network for Ports and HAbourS) .

Use of videos was begun with the 2004 Indian Ocean Tsunami. Fritz et al. (2006) analyzed an


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eyewitness video record with PIV (planar particle image velocimetry) and obtained time series of flow

velocity at two locations which were within the range of 2 to 5 m/s.

Figure 14: Plan of Kesen-numa Bay Figure 15: Computed and measured velocity

8.4 Tsunami Control Forest

There are discordant opinions about the effectiveness of a forest along a shoreline on the reduction of

tsunami energy.

Affirmative views assert that a forest is effective because; 1) it stops driftwood and other floating

materials, 2) it reduces water flow velocity and inundation water depth, 3) it provides a life-saving means

by catching persons carried off by tsunamis, and 4) it collects wind-blown sands and raises dunes, which

act as a natural barrier against tsunamis. A representative negative opinion is that a forest may be

ineffective against a huge tsunami, and at worst, trees themselves could become destructive forces to

houses if cut down by the tsunami. Shuto (1987) collected forty-five examples for five huge tsunamis in

Japan and gave Fig. 12 in case of pine trees.

The ordinate is the summed tree

diameter nd, where nis the average number

of trees along the direction of water flow in

a rectangle with a frontage of unit length of

shoreline and a depth equal to the width offorest, and dis the diameter breast high.

In region A, no trees are damaged. The

number of trees is not enough to reduce

tsunami energy but is sufficient only to stop

boats and drift-wood. In Region B, trees

may be damaged. In Sub-region C-1, if there

is dense undergrowth in the forest, the

reduction of tsunami energy as well as the

stoppage of floating materials is expected.

In Sub-region D-1, the forest is thick enoughFig.16 Shuto diagram for the effect of coastal forest.


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and a similar effect to Sub-region C-1 is expected, even without undergrowth.

Harada and Imamura (2003) opened the way for quantitative evaluation of tsunami control forest,

using the shallow-water equations in which the effect of trees was expressed by Morison formula with

drag and inertia forces. Experiences in 2004 are attracting many researchers to this topic, for example,Tanaka et al. (2006) and Tanimoto et al. (2007).

9. Research Topics in the Near Future

Our knowledge of tsunami, tsunami damage and countermeasures is still limited. Among many

research topics, the present author would like to put emphasis on the following three subjects.

The first is related with tsunami-induced currents. Current velocity measured for real tsunamis is too

poor, in number and in quality. This is a big hindrance for further development of numerical simulation

and design of countermeasure structures. In addition to increase number of instruments in the field,

large scale hydraulic experiments are necessary. With these data, such defect as discussed in 8.3.(3) will

be improved. The simulation technique thus improved will become a powerful tool in the tsunami

archeology too, with which tsunamis not recorded on documents are found from tsunami deposit.

The second is the further development of numerical technique such as CADMAS-SURF. We need

large-scale hydraulic experiments to supply verification data for numerical simulation. An extension of

CADMAS-SURF to the three-dimensional space may become a powerful means to solve practical

problems.

The third is the use of CG animation in public education. The last way to save lives is an early

evacuation. Human beings, however, are optimistic. Even in an emergency, we are likely to consider,

Im OK, and do not take necessary action. Due to this normalcy bias, many people have lost their

lives. In order to break this preconception, CG animation works well. Katada et al (2004) developed a

kind of dynamic hazard map, and used in public education. Visit his home page

http://dsel.ce.gunma-u.ac.jp/, you can down load some of his results. Another use of CG technique is

the virtual realty, in which visitors can have a pseudo-experience of tsunami risk.

10. Concluding Remarks

Tsunami research started in Japan, in 1933 when the Showa Great Sanriku Tsunami hit.

Until 1960, it progress was slow but the theoretical approach was carried out by few pioneers, as thetsunami science. After the 1960 Chilean tsunami, the tsunami engineering appeared to deal with the

actual risks. At that time, there were two countries where tsunami research was earnestly carried out,

Japan and USA.

In 1970s, tsunami researchers in USA, except for forecasting people, moved to other field such as

ocean engineering. In 1968, a local tsunami hit the Pacific coast of Japan and was nearly perfectly

prevented by coastal structures built after the Chilean tsunami. Then, it was not easy to continue

tsunami research in Japan, too. Under these conditions, supported by the electronic high-speed

computer and the Mansinha-Smylie method to determine the tsunami initial profile, both appeared in

1970s, tsunami science began a big step of progress with quite a few tsunami researchers.

In 1983 when a huge tsunami hit the Japan Sea coast and gave tremendous damages, USA people


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were awoken and number of tsunami researcher in Japan began to increase, in the field of science and

engineering.

In September, 1992, the first international tsunami survey team (ITST) was formed to study the

Nicaraguan tsunami. Then, in December of the same year, another tsunami hit the Flores Island,Indonesia, ITST was also organized. In July, 1993, a huge tsunami occurred in the Japan Sea. This

frequent occurrence of tsunami disaster worked to increase number of tsunami researchers in the world.

Knowing a fact that the 1993 tsunami easily overflowed the man-made structures, it was confirmed that

human action was vitally important in an emergency. This accelerated the participation of the social

scientists, with an emphasis on public education.

Each time when a tsunami occurs, tsunami research shows a progress and residents renew their

awareness. However, with the elapse of time, generation changes and precious experience will be lost.

Then, a new tsunami may ask new victims again. We, not only coastal residents but also tsunami

researchers, should resist this tendency, the decay of memory and prevention technique.

References

Aida, I. (1977): Numerical Experiments for Inundation of Tsunamis, Susaki and Usa, in the Kochi

Prefecture, Bulletin of Earthquake Research Institute, University of Tokyo, Vol.52, pp.441-460 (in

Japanese).

Aida, I. (1978): Rreliability of a Tsunami Source Model Derived from Fault Parameter, Journal of Physics

of Earth, Vol.26, pp.57-73.

Anno, K., T. Nishihata and Y. Moriya (2007): Numerical and Experimental Study on Drift Due to

Tsunami for Several Drifting Bodies, Annual Journal of Coastal Engineering, JSCE, Vol.54,

pp.866-870. (in Japanese)

Arikawa, T. et al. (2006): Large Model Test on Surge front Tsunami Force, Annual Journal of Coastal

Engineering, JSCE, Vol.53, pp.797-800.

Asakura, R. et al. (2000): An Experimental Study on Wave Force Due to Tsunamis That Overflow the

Upright Seawall, Proc. Coastal Engineering, JSCE, 47, pp.911-955 (in Japanese).

Carrier, G. F. and H. P. Greenspan (1958): Water Waves of Finite Amplitude on a Sloping Beach, Journal

of Fluid Mechanics, Vol.4, Part 1, pp.97-107.

CDIT (Coastal Development Institute of Technology) (2001): CADMAS-SURF, Research andDevelopment of Numerical Wave Channel, 296p (in Japanese) .

Dingemans, M. W. (1973): Water Waves Over an Uneven Bottom A Discussion of Long-Wave

Equations, Report R 729-II, Delft Hydraulic Laboratory, Delft, The Netherlands.

Fritz, H. M., J. Borrero, C. E. Synolakis and J. Yoo (2006): 2004 Indian Ocean Tsunami Flow Velocity

Measurements from Survivor Videos, Geophysical Research Letters, Vol.33, L24605,

doi:10.1029/2006GL026784, 5p.

Fujii, H., S. Hotta and N. Shuto (2000): Numerical Simulation of Damage to Soil Embankment Caused

by Overflowing Tsunamis, Journal of Disaster Research, Vol.4, No.6 (in press).

Fujii, N., M. Ohmori, M. Takao, S. Kanayama and H. Ohtani (1998): On the Deformation of the Sea

Bottom Topography Due to Tsunami, Proc. Coastal Engineering, JSCE, 45, pp.376-380 (in


24/123

- 20 -

Japanese).

Fujii, N., et al. (2007): Experimental Study on Variability in Drifting Behavior Due to Tusnmi and Its

Evaluation Method, Annual Journal of Coastal Engineering, JSCE, Vol.54, pp.241-245 (in

Japanese).Fujima, K. and T. Shigemura (2000): Determination Grid Size for Leap-frog Finite Difference Model to

Simulate Tsunamis around a Conical Island, Coastal Engineering Journal, Vol.42, No.2,

pp.197-210.

Fujima, K., K. Masamura and C. Goto (2002): Development of the 2D/3D Hybrid Model for Tsunami

Numerical Simulation, Coastal Engineering Journal, Vol.44, No.2, pp.373-397.

Fujima, K. (2002): Review: Development of Numerical Wave Flume CADMAS-SURF ((SUper Roller

Flume for Computer Aided Design of MAritime Structure), Proceedings of Coastal and Ocean

Engineering in Korea, Vol.13, pp.1-14.

Fukui, Y., H. Siraishi, M. Nakamura and Y. Sasaki (1962a): A Study on Tsunami; (1) Propagation Velocity

of Bore, Proceedings of Coastal Engineering in Japan, JSCE, Vol.9, pp.44-49 (in Japanese).

Fukui, Y., H. Siraishi, M. Nakamura and Y. Sasaki (1962b): A Study on Tsunami; (2) Effect of Bore on

Coastal Dikes, Proceedings of Coastal Engineering in Japan, JSCE, Vol.9, pp.50-54. (in Japanese)

Fukuyama, E. and K. Irikura (1986): Rupture Process of the 1983 Japan Sea (Akita-Oki) Earthquake

Using a Waveform Inversion Method, Bulletin of the Seismological Society of America, Vol.76,

No.6, pp.1623-1640.

Garces, M. et al. (2005): Deep Infrasound Radiated by the Sumatra Earthquake and Tsunami, EOS,

Transactions, American Geophysical Union, Vol.86, No.36, pp.317-320.

Goto, C (1983): Numerical Simulation of Timbers Transported by Tsunamis, Proceedings of Coastal

Engineering in Japan, JSCE, Vol.30, pp.594-597 (in Japanese).

Goto, C. and N. Shuto (1983): Numerical Simulation of Tsunami Propagations and Run-up, in K. Iida and

T. Iwasaki (eds), Tsunamis: Their Science and Engineering, Terra Science Publication Co.,

Tokyo/Reidel, Dordrecht, pp.439-451.

Gotoh, H., T. Sakai, M. Hayashi, K. Oda and H. Ikari (2002): Gridless Analysis of Toe Scouring of

Seawall Due to Tsunami Return Flow, Proc. Coastal Engineering, JSCE, 49, pp.46-50 (in Japanese).

Harada, K. and F. Imamura (2003): Utilization of Forest Control to Mitigate the Tsunami Damage and Its

Evaluation, Proc. Coastal Engineering, JSCE, Vol.50, pp.341-345 (in Japanese).Hatori, T. (1984): On the Damage to Houses Due to Tsunamis, Bulletin of Earthquake Research Institute,

Vol.59, pp.433-439 (in Japanese).

Hibberd, S. and D.H. Peregrine (1979): Surf and Run-up on a Beach: A Uniform Bore, Journal of Fluid

Mechanics, Vol95, pp.323-345.

Houston, J. R. and H. L. Butler (1979): A numerical model for tsunami inundation, WES Technical

Report HL-79-2.

Ikeya, T. et al. (2006): Experimental and Analytical Study of Impulsive Forces by a Drifter Due to

Tsunami, Annual Journal of Coastal Engineering, JSCE, Vol.53, pp.276-280 (in Japanese).

Iizuka, H. and H. Matsutimi (2000): Estimation of Damage Caused by Tsunami Flow, Proceedings of

Coastal Engineering, JSCE, Vol.47, pp.381-385 (in Japanese).


25/123

- 21 -

Imamura, F. and C. Goto (1988): Truncation Error in Numerical Tsunami Simulation by the Finite

Difference Method, Coastal Engineering Journal, Vol.31, No.2, pp.245-263.

Imamura, F., N. Shuto and C. Goto (1990): Study on Numerical Simulation of The Transoceanic

Propagation of Tsunami, Part 2: Characteristics of Tsunami Propagating Over the Pacific Ocean,Zisin, Journal of the Seismological Society of Japan, Vol.43, pp.389-402 (in Japanese).

Iwasaki, S. (1992): Tsunami Forecasting Technique Using Underwater Sounds and Air pressure waves,

Tsunami Engineering Technical Report, DCRD Tohoku University, No.9, pp.65-83. (in Japanese)

Iwasaki, T. and A. Mano (1979): Two-dimensional Numerical Simulation of Tsunami Run-ups in the

Eulerian Description, Proceedings of 26thConference on Coastal Engineering, JSCE, pp.70-74 (in

Japanese).

Iwase, H., T. Mikami and C. Goto (1998): Practical Tsunami Numerical Simulation Model by Use of

Non-linear Dispersive Long Wave Theory, Journal of Hydraulic, Coastal and Environmental

Engineering, JSCE, No.600/II-44, pp.119-124 (in Japanese).

Iwase, H., T. Mikami, C. Goto and K. Fujima (2002): A Comparative Study of Nonlinear Dispersive Long

Wave Equations for Numerical Simulation of Tsunami, Journal of Hydraulic, Coastal and

Environmental Engineering, JSCE, No.705/II-59, pp. 101-114 (in Japanese).

Izumiya, T. and H. Nagaoka (1994): Generation of Acoustic Gravity Waves by Tsunamis and Possibility

of Tsunami Forecasting, Proceedings of Coastal Engineering, JSCE, Vol.41, pp. 241-245 (in

Japanese).

Kajiura, K. (1970): Tsunami Source, Energy and the Directivity of Wave Radiation, Bulletin ERI, Vol.48,

pp. 835-869.

Katada, K., N. Kuwasawa and H. Yeh (2004): Development of Tsunami Comprehensive Scenario

Simulator for Disaster Education and Risk Management, National Geophysical Research Institute,

Hazards2004, 75p.

Koike, N. et al. (2005): Tsunami Height Survey of the 2004 Off the Kii Peninsula Earthquake, Annual

Journal of Coastal Engineering, JSCE, Vol.52, pp. 1336-1340 (in Japanese).

Koshimura, S. (2002): Multiple Reflection of Tsunamis Incident on a Continental Slope, Journal of

Hydraulic, Coastal and Environmental Engineering, JSCE, No.705/II-59, pp. 151-160 (in

Japanese).

Koshimura, S., F. Imamura and N. Shuto (1999): Propagation of Obliquely Incident Tsunamis on a Slope,Part II Characteristics of On-ridge Tsunamis, Coastal Engineering Journal, Vol.41, No.2, pp.

165-182.

Koshimura, S., T. Oie, H. Yanagisawa and F. Imamura (2009): Developing Fragility Functions for

Tsunami Damage Estimation Using Numerical Model and Post-tsunami Data from Banda Ache,

Indonesia, Coastal Engineering Journal, Vol.51, No. 243-273.

Kumagai, K., K. Oda and N. Fujii (2007): Simulation Model for Drift Behavior of Container Due to

Tsunami and Collision Force, Annual Journal of Coastal Engineering, JSCE, Vol.54, pp. 236-240

(in Japanese).

Lander, J. F. (1996): Tsunamis Affecting Alaska 1737-1996, NGDC Key to Geophysical Research,

Documentation No.31, 195 p.


26/123

- 22 -

Madsen, P. A. and O. R. Sorensen (1992): A New Form of the Boussinesq Equations with Improved

Linear Dispersion Characteristics, Part 2; A Slowly-Varying Bathymetry, Coastal Engineering,

Vol.18, pp.183-204.

Manshinha, L. and D. E. Smylie (1971): The Displacement Fields of Inclined Faults, Bulletin ofSeismological Society of America, Vol.61, pp.1433-1440.

Matsutomi, H. (1993): Laboratory Study on Maximum Impulsive Force of Timbers Drifted by Bore-like

Tsunamis, Journal of Hydraulic, Coastal and Environmental Engineering, JSCE, No.467/II-23,

pp.19-28 (in Japanese).

Matsutomi, H. (1999): A Practical Formula for Estimating Impulsive Force Due to Driftwoods and

Variation Features of the Impulsive Force, Journal of Hydraulic, Coastal and Environmental

Engineering, JSCE, No.621/II-47, pp. 111-127 (in Japanese).

Mei, C. C. and B. LeMehaute (1966): Note on Equations of Long Waves Over an Uneven Bottom,

Journal of Geophysical Research, Vol. 71, pp. 393-400.

Mizutani, M. and F. Imamura (2000): Experiments on Bore Wave Pressure on Structures, Proceeding of

Coastal Engineering, Vol. 47, pp. 946-950 (in Japanese).

Mofjeld, H. O., V. V. Titov, F. I. Gonzalez, and J. C. Newman (2001): Tsunami Scattering Provinces in the

Pacific Ocean, Geophysical Research Letters, Vol. 28, No. 2, pp. 335-337.

Nagai, N., H. Ogawa, K. Nukada and M. Kudaka (2004): Characteristics of the Observed 2003

Tokachi-Off Earthquake Tsunami Profile, Annual Journal of Coastal Engineering, JSCE, Vol. 51, pp.

276-280 (in Japanese).

NHK (1983): A Huge Tsunami Witnessed Records of the Nihon-Kai Chubu Earthquake-, Video or DVD.

Noguchi, K., S. Sato and S. Tanaka (1997): Large-scale Experiments on Tsunami Overtopping and Bed

Scour around Coastal Revetment, Proc. Coastal Engineering, JSCE, Vol. 44, pp. 296-300 (in

Japanese).

Okada, Y. (1985): Surface Deformation Due to Shear and Tensile Faults in a Half-space, Bulletin of the

Seismological Society of America, Vol. 75, pp. 1135-1154.

Okal, E. A., J. Talandier and D. Reymond (2007): Quantification of Hydrophone Records of the 2004

Sumatra Tsunami, Pure and applied Geophysics, Vol. 164, pp. 309-323.

Omachi, T., H. Tukiyama and H. Matsumoto (1999): Evaluation of Tsunami Simulation Considering

Dynamic Crustal Displacement Accompanying a Fault Movement, Proceedings of CoastalEngineering, JSCE, Vol. 46, pp. 321-325 (in Japanese).

Peregrine, D. H. (1967): Long Waves on a Beach, Journal of Fluid Mechanics, Vol. 71, Part 4, pp.

815-827.

Plafker, G. (1965): Tectonic Deformation Associated with the 1964 Alaska Earthquake, Science, Vol. 148,

pp. 1675-1687.

Satake, K., M. Okada and Ku. Abe (1988): Tide Gauge Response to Tsunamis: Measurements at 40 Tide

Gauge Stations in Japan, Journal of Marine Research, Vol. 46, pp. 557-571.

Satake, K. (1989): Inversion of Tsunami Waveforms for the Estimation of Heterogeneous Fault Motion of

Large Submarine Earthquakes: 1968 Tokachi-oki and 1983 Japan Sea earthquakes, Journal of

Geophysical Research, Vol. 98, pp. 4553-4565.


27/123

- 23 -

Shuto, N. (1967): Run-up of Long Waves on a Sloping Beach, Coastal Engineering in Japan, JSCE, Vol.

10, pp. 23-38.

Shuto, N. (1987): The Effectiveness and Limit of Tsunami Control Forests, Coastal Engineering in Japan,

Vol.30, No. 1, pp. 143-153.Shuto, N. (1993): Tsunami Intensity and Disasters, Advances in Natural and Technological Hazards

Research, Vol.1, pp197-216.

Shuto, N. (2009): Damage to Coastal Structures by Tsunami-Induced Currents in the Past, Journal of

Disaster Research, Vol. 4, No. 6 (in press).

Simkin, T. and R, S, Fiske, (1983): Krakatau 1883, Eruption and its Effects, Smithsonian Institution Press,

464p.

Takahashi, R. (1934): A Model Experiment on the Mechanism of Seismic Sea Wave Generation, Part 1,

Bulletin ERI, Supplementary Vol.1, Tokyo University, pp. 152-181.

Takahashi, T., F. Imamura and N. Shuto (1991): A study on Tsunami-induced Current and Sea Bottom

Deformation, Proc. Coastal Engineering, JSCE, Vol. 38, pp. 161-165 (in Japanese).

Takahashi, T., F. Imamura and N. Shuto (1992): Tsunami Simulation with Movable Sea Bottom, Proc.

Coastal Engineering, JSCE, Vol. 39, pp. 231-235 (in Japanese).

Takahashi, T., F. Imamura, and N. Shuto (1993): Examination of Applicability of Movable Bed Model by

Tsunamis, Proc. Coastal Engineering, JSCE, Vol. 40, pp. 171-175 (in Japanese).

Takahashi, T., T. Takahashi, N. Shuto, F. Imamura and M. Ortiz (1995): Source Models for the 1993

Hokkaido Nansei-Oki Earthquake Tsunami, Pure and Applied Geophysics, Vol. 144, No. 3/4, pp.

747-767.

Takahashi, T., K. Fujima and R. Asakura (2001): An Application of Numerical Wave Flume to

Transformation of Bore, Proceedings of Civil Engineering in the Ocean, JSCE, Vol. 17, pp.

281-286 (in Japanese).

Tanimoto, K. (2007): Numerical Simulation of Tsunami Prevention by Coastal Forest with Several

Species of Tropical Tree, Annual Journal of Coastal Engineering, JSCE, Vol. 54, pp. 1381-1385 (in

Japanese).

Tomita, T., T. Kakinuma and A. Shimada (2004): Numerical Simulation of Tsunamis around and Behind

Breakwater using a 3D Model, Annual Journal of Coastal Engineering, JSCE, Vol. 51, pp. 296-300

(in Japanese).Tanaka, N., T. Takemura, Y. Sasaki and M.I.M. Mowjood (2006): Breaking Condition of the

Representative Coastal Tree Species in Sri Lanka and the Effect of Vegetation on the Delay in

Tsunami Arrival Time, Annual Journal of Coastal Engineering, JSCE, Vol. 53, pp. 281-285 (in

Japanese).

Thucydides, (for example): History of The Peloponnesian War, Penguin Classics, p.247.

Tsuji, Y. (1977): A Study on the Scattering Wave Induced by Tsunami Passing Over a Sea Mount or Rise,

Ocean Science, Vol. 9, pp. 117-125 (in Japanese).

Uda, T. et al. (1988): Two-dimensional Deformation of Nonlinear Long Waves on a Beach, Report No.

2627, Public Works Research Institute, Ministry of Construction (in Japanese).

Yoneyama, N., M. Matsuyama and H. Tanaka (2002): Numerical Analysis for Locally High Runup of


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1993 Hokkaido Nansei-Oki Seismic Tsunami, Journal of Hydraulic, Coastal and Environmental

Engineering, JSCE, No. 705/II-59, pp. 139-150 (in Japanese).


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Tsunami Risk Assessment and the Planning and Implementation of

Strategic Mitigation Measures- Case Study City of Galle

Samantha HETTIARACHCHI

Department of Civil Engineering, University of Moratuwa, Moratuwa, Sri Lanka, [email protected]

Saman SAMARAWICKRAMA

Department of Civil Engineering, University of Moratuwa, Moratuwa, Sri Lanka, [email protected]

Nimal WIJERATNE

Faculty of Engineering, University of Ruhuna, Galle, Sri Lanka, [email protected]

1. Risk - Components of Risk and Assessment

Planning post tsunami rehabilitation and conservation of the Sri Lankan coastline should ideally be

undertaken within a multi hazard coastal risk assessment framework giving due consideration to all thecoastal hazards. Even when risk assessments are undertaken only for the tsunami hazard it is important to

conduct such studies on a platform which can accommodate other coastal hazards. For risk assessment against

the tsunami hazard it is important to assess scientifically and establish the basis and criteria on which such an

exercise is undertaken. Planning based on observations arising from a single extreme event without

scientifically analyzing the true character of potential events, their impacts and future threats and risks should

be avoided.

Coastal communities all over the world are under severe pressure resulting from population growth in

coastal areas, human induced vulnerability, increases in frequency and magnitude of coastal hazards and

impacts of global climate change. These unprecedented changes are placing communities at increasing risk

from coastal hazards such as severe storms, tsunamis leading to coastal erosion, flooding and environmental

degradation. In this respect coastal community resilience is identified as the capacity to absorb and withstand

such impacts of hazards, emerge from disaster events and adapt efficiently to changing conditions.

The Indian Ocean Tsunami focused attention on globally on the severe impacts of tsunamis. It was also

recognized that coastal communities are increasingly at risk from a number of hazards which can be broadly

classified as Episodic and Chronic hazards. These hazards which may arise from natural phenomena or

human induces events have severe impact on coastal communities and eco-systems.

Proceedings of the 6th International

Workshop on Coastal Disaster Prevention,

Bangkok, Thailand, December 1-2, 2009


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Episodic hazards include severe storms, earthquakes, tsunamis and oil spills all of which have limited

predictability and may result in major disasters. The communities should be made aware of these hazards,their vulnerability and risks and should be educated on the importance of preparedness in responding to

potential disasters which usually require long term post event recovery efforts. Chronic conditions include

shoreline erosion, flooding, sedimentation, sea level rise and coastal environmental and resource degradation.

These condition which may result or increase from disasters arising from episodic hazards, relate to processes

which could be measured and monitored. They require long term planning measures and restoration efforts to

reduce risks.

Risk is usually expressed by the notation Risk = Hazard x Vulnerability. In this expression hazard includes

exposure. Risk represents the probability of harmful consequences or expected losses (in terms of deaths,

injuries, property, livelihoods, economic activity disrupted or environment affected) arising from interactions

between natural or human hazards and vulnerable conditions. Vulnerability can be broadly classified into

several components including, physical, human, socio- economic, functional and environmental vulnerability.

Hence vulnerability is dependent on several factors belonging to the said components, including population

density, building density and status, distance from the shoreline, elevation and evacuation time. Prior to the

Indian Ocean Tsunami (IOT), Sri Lanka had not adopted a planned approach towards preparedness in relation

to disasters, an aspect which is considered vital in saving lives. Hence the notation, Risk = Hazard x

Vulnerability x Deficiencies in Preparedness seems more appropriate. The additional term represents certain

measures and tasks the absence of which could reduce the loss of human lives and property in the specific

interval of time during which the event is taking place. This term is also commonly identified as the inverse of

Capacity. Hence there are many literature in which the notation, Risk = (Hazard x Vulnerability)/ Capacity isused.

For detailed assessment of risk it is necessary to quantify the three main components of risk. However

quantifying all three terms is a challenging task in view of the wide range of diverse parameters associated

with the respective components of risk. In particular, there are no standard techniques for such assessment and

a number of methods have been used by researchers. These include qualitative methods, quantification based

on qualitative description and quantification based on detailed analysis of respective parameters. It is

therefore difficult to develop comprehensive risk assessment studies which capture the significance off all the

three components. However it is important that risk assessment studies are conducted within the framework

defined by the above formulae. This aspect has to be kept in mind when reviewing the outputs from studies on

risk assessment. The assessment of risk is an important element of coastal community resilience.

Communities must be made aware of the hazards, their exposure, vulnerability and be encouraged to address

issues on awareness, early warning, emergency planning, response and recovery and hazard mitigation.

Enhanced coastal community resilience enables populations at risk to live with risk with a greater degree of

confidence.

2. Increased Exposure of the City of Galle

Many coastal cities of Sri Lanka were severely affected by the Indian Ocean Tsunami due to the exposure

to the hazard. One of the principal coastal cities devastated was the historic port city of Galle (Figs. 1 and 2).


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Incidentally the first recorded Tsunami to have affected Sri Lanka was on 27 thAugust 1883, arising from the

eruption of the volcanic island of Krakatoa. On this occasion too, unusually high water levels followed by thereceding beach were observed in Galle around 1.30 pm. The water level fluctuations were not severe and

there was no inundation. The said time corresponds well with the tsunami travel time for tsunami waves

which would have been generated by the largest eruption of the volcano earlier in the morning. However, on

26thDecember 2004, Galle received the severe impact of Tsunami waves, their magnitude having increased

due to near-shore transformations. Galle is one of the many coastal cities around the world, which remains

heavily exposed to the tsunami hazard. Poorly constructed buildings and inadequate drainage contributes to

the vulnerability.

The tsunami waves, which reached the offshore waters of Galle were primarily diffracted waves,

diffraction taking place around the southern coast of Sri Lanka. In the context of Tsunamis the location of

Galle is extremely vulnerable. It lies besides a wide bay and a natural headland on which is located the

historic Galle Fort with very reflective vertical non-porous walls on all sides. Furthermore, there exists the

Dutch canal west of the headland, conveying water through the city centre. The waves in the vicinity of Galle,

which were increasing in height due to reduced water depths were further subjected to a series of near-shore

processes which increased their heights even further. The canal was a facilitator in conveying the massive

wave and associated flow towards the city centre.

In the vicinity of the headland on which the Galle Fort is located, the wave energy concentrates due to

refraction. These waves then reflected from the vertical solid walls of the Fort and moved around the

headland. Such walls reflect almost all the incident wave energy with very high wave heights at the wall itself.

There is hardly any dissipation. On the west of the headland the waves moved ferociously into the DutchCanal. On the east it moved along the bay. The wide bay in Galle further contributed to the increase in wave

height by modifying the shoaling process via reduced wave crest width to accommodate the bay shape. The

combined effect of this phenomenon and the wave coming around the eastern side of the Fort caused a

massive wave of destruction along the Marine Drive (see Fig. 3). It is certainly not surprising that many

survivors referred to a moving large black wall similar to that of the Galle Fort.

The city of Galle is therefore not only exposed to tsunami waves which will diffract around the southern

part of Sri Lanka it is even more exposed in the context of near-shore coastal processes which will further

increase wave heights. This aspect is identified as increased exposure within the risk assessment framework.

Figs. 1 and 2 give the testified tsunami wave heights and arrival times around Sri Lanka. Project location

Galle is indicated in Fig. 1.

3. Investigations for Risk Assessment Case Study

In order to safeguard lives and protect infrastructure a Risk Assessment Case Study was undertaken for the

City of Galle. One of the main objectives was to develop a Tsunami Hazard Map and an Evacuation Plan for

the City of Galle coastal area. Field surveys were carried out to collect data on:

Inundation height

Direction of first wave of tsunami

Possible evacuation paths & locations


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Figure 1: Testified Tsunami wave heights Figure 2: Testified Tsunamiarrival times

in meters. (highest wave).

Figure 3: Galle Bay and Headland.

79

515

345

5

58

610

712

11

79

511

49

410

45

23

12


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Area under the study was divided into 250m x250m grids and people living within the respective areas were

interviewed for all grids. The collected data were used to identify the Inundated area

Inundation contours with wave direction and

Risk level of the area

Results of the study were also useful in identifying the

Safe areas and safe buildings

Evacuation routes and refuge areas

Proposed locations for fixing sign boards on evacuation routes

Fig. 4 gives the data collection points.

Figure 4: Data collection locations.

4. Numerical Modeling of Tsunamis

Numerical modeling of tsunami phenomena was carried out to obtain information on the coastal region of

Sri Lanka that could be affected by potential tsunamis. General coarse grid modeling was carried out for the

coastal region in the southern parts of the island and detailed fine grid modelling, including tsunami run-up

and inundation was carried out for the City of Galle. The results of the modelling was used for the preparation

of Hazard Map for Vulnerability for the City of Galle.

Generation and deepwater propagation of the tsunami waves were modeled using the AVI-NAMI model.

The module for co-seismic tsunami generation of AVI-NAMI uses the method developed by Okada (1985)

and the module for tsunami propagation solves Nonlinear Shallow Water Equations. ANUGA fluid dynamics

model based on a finite-volume method for solving Shallow Water Wave Equations was used for the


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inundation modeling. In the ANUGA model the study area is represented by a mesh of triangular cells having

the flexibility to change the resolution of the mesh according to the area of importance. A major capability ofthe model is that it can simulate the process of wetting and drying as water enters and leaves an area and

therefore suitable for simulating water flow onto a beach or dry land and around structures such as buildings.

High resolution near shore bathymetric data obtained for new Galle Port Development (2007) and high

resolution Topographic Data obtained after the 2004 Tsunami were used for study. (LIDA Surveys, 2005).

Broad scale deep water propagation Modeling was carried out for a number of source scenarios selected

from the Sunda/Java Trench (Table 1). Fault length of 500 km, a width of 150 km, Dip angle of 80, a Slip

angle of 1100and a displacement of 40 m was used for the study.

Table 1: Source details and the maximum and minimum wave amplitudes from the propagation modeling

Longitude Latitude Strike Angle Max. Amplitude

(m)

Min.

Amplitude (m)

Scenario 1 92.00' E 8.52' N 350' 2.015 -1.501

Scenario 2 94.26' E 3.09' N 329' 3.477 -2.391

Scenario 3 97.01' E 2.07' N 329' 1.419 -1.33

Scenario 4 97.60' E -0.60' N 329' 2.608 -2.081

Based on the results of the deep water model, inundation modeling was carried out using the ANUGA

model. Modelling results give valuable information on the coastline of Galle that could be affected by

potential tsunamis. The model results are very useful for the preparation of Hazard maps. Fig. 5 gives theinundation modeling results for 4 scenarios.

5. Countermeasures against the Tsunami Hazard - Classification and Planning

5.1 Classification of countermeasures

There are many countermeasures that could be adopted in coastal zone management when planning for a

tsunami and other coastal hazards that accompany high waves and high inundation. These include early

warning systems, regulatory interventions in the form of extending existing setback defense line and physical

interventions such as protection structures and utilizing the full potential of coastal ecosystems. These have to

be supplemented with public awareness on disaster preparedness, efficient evacuation procedures,

incorporating planned evacuation routes and structures that effectively integrate with the overall planning

process.

Countermeasures can therefore be broadly classified into two categories, namely, those which promote

successful evacuation from tsunamis and those which mitigate the impact of tsunami.


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(a) Scenario 1 (b) Scenario 2

(c) Scenario 3 (d) Scenario 4

All inundation depths are in meters

Figure 5: Inundation Modelling for Galle- four selected scenarios.

5.1.1 Countermeasures that promote successful evacuation from tsunami

Countermeasures that promote successful evacuation from tsunami are listed below

1. Early Warning Systems

2. Public Warning Systems

3. Hazard, Vulnerability and Risk Maps

4. Set Back defense line

5. Evacuation Routes and Structures


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5.1.2 Countermeasures that mitigate the impact of tsunami

Countermeasures that mitigate the impact of tsunami1. The implementation of artificial measures for protection including tsunami breakwaters, dikes

and revetments

2. The effective use of natural coastal ecosystems including Coral Reefs, Sand Dunes and Coastal

Vegetation (Mangrove Forests)

3. Tsunami Resistant Buildings and Infrastructure

5.2 Planning Countermeasures via Policy and Management Options

It is important that post disaster planning should be undertaken in the context of overall coastal hazards one

of which remains Tsunamis, however remote the chances of an extreme event such as that of the 26 th

December taking place. It is recognized that a Coastal Hazard Protection plan for a city which is an integral

part of an overall Coastal Zone Management Plan has to be based upon Policy and Management Options.

These options reflect the strategic approach for achieving long term stability in particular for sustaining

multiple uses of the coastal zone giving due consideration to the threats and risks of hazards.

Policy and Management options must be formulated on a sound scientific basis preferably to function within

the prevailing legal and institutional frameworks. However, if the need arises institutional improvements

should be affected and new laws should be imposed. In this process high priority should be given to

stakeholder participation. Extreme care has to be exercised when obtaining the active participation of

stakeholders who have witnessed and suffered heavily in terms of life, property and economic avenues from

one of the most sever natural disasters to have affected mankind. Most of them are yet to recover completelyfrom their traumatic experiences.

Policy Options identify possible courses of action on shoreline, as,

(1) Maintain existing defence line

(2) Setback defence line

(3) Retreat

(4) Advance

In order to implement the Policy Options various Management Options are considered provided they are

appropriate for the coastal classification. They are summarized as,

(1) Do nothing

(2)

Reinstate to previous state

(3) Modify the existing design

(4) Develop new design

Once the Risk Assessment study is completed mitigation options will be developed within the framework

of Policy and Management Options with due consideration given to stakeholder consultations.

5.3 Classification of Physical Interventions (Artificial and Natural)

In the light of the discussion in Sections 5.1.2 and 5.2, mitigation by physical interventions is classified into

three types, depending on their location and function in protecting the coast. These interventions may be


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achieved not only by artificial methods via Coastal Engineering Design but also by harnessing the full

potential of natural coastal ecosystems. The types of interventions and typical examples for each category arelisted below.

(i) Reduce the impacts of tsunami waves prior to reaching the shoreline.

(eg. Tsunami Breakwaters, Coral Reefs)

(ii) Protect the coastal zone by preventing the inland movement of tsunami waves.

(eg. Tsunami Dike, Sand Dunes)

(iii) Mitigate the severe impacts of tsunami waves on entry to the shoreline.

(eg. Tsunami Dikes, Revetment, Mangrove Forests)

On many occasions both methods can be adopted in parallel to develop well-integrated hybrid solutions

satisfying environmental concerns.

5.4 Development of Guidelines for tsunami resistant buildings

The coast is an area of hi

proceedings of coastal disaster prevention.pdf

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