Abstract: Exceedance probability for various magnitudes of tsunami

Abstract: Globally,
all official disaster management systems rarely focus in possible destructive
events of tsunami risks because of the lack of information in historical data.
Frequent hazards like floods and cyclones are considered in assessing natural
hazards. Return period of huge tsunami’s are not predictable, it may be
thousands of years but in the recent past there were two disastrous tsunamis
occurred in 2004 and 2011 in Indian ocean and west pacific ocean respectively. Totally
both caused more than 250,000 fatalities and about LKR 40 trillion direct monetary
losses. Under water earth quakes are the main cause for worst tsunami
occurrences in the past. Landslides and volcanic eruption also can cause worst
scenarios. So those sources also need to be considered for tsunami hazard
assessment. For that numerical modelling of tsunami are needs to be done with
fine inputs. By inputting the details of possible tsunami events without
uncertainty and accurate bathymetry details will result the propagation of
tsunami wave and inundation in the dry lands. From these results a hazard
innundation graph will represent the Exceedance probability for various magnitudes
of tsunami thresholds. In this paper, the actual activities done by various
parties in past to find these results are described. Especially this focuses on
analyses made for 2004 Indian Ocean tsunami in eastern coast of Sri Lanka and
about the sources needed to set up numerical computer based models with high–resolution
inputs to get the outputs of tsunami generation, propagation and inundation
details, with which to research which sources among earth quake, landslides and
volcanic eruption may govern the inundation in the dry land area in coastal
areas of Batticaloa, Ampara and Trincomalee districts.

Keywords: Disaster; Earth
quake; Tsunami; Landslides; Volcanic eruption; Numerical modelling; Inundation graph

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1.   
Introduction

In the recent past possible giant tsunami
waves in eastern coast of Sri Lanka is mostly caused due to the Techtronic
plate movement in sunda trench near Sumatra. Sunda trench is around 3200 kilometres
in length and the maximum depth is around 7.7 kilometres in a particular
location. As this is a much larger trench with various possible locations for
earth quakes and landslides it was much harder to give a fine database which
consist all the possible tsunami occurrences by different sources. The
intention of the research is to provide a hazard curve which clearly represents
the inundation height in the dry lands of eastern coast. Previously in even it
lots of improved precautions (Eg. Sea walls) were made before 2011 Japan
tsunami a large amount of monetary losses occurred because of this poor estimation
of inundation height and the distance in the dry land. It tends for various
researched on hazard map to explicitly indicate the uncertainty of tsunami hazard
assessment. These errors were caused due to lack of good knowledge in the
source mechanisms (Non accuracy of the parameters of earthquakes and submerged
landslides), difficulties in finding the return period of these critical events,
limitations of the data input and the human errors while approximating the data
on the model. While these past procedures were done using worst scenarios, a need
for database with hazard analysis parameters which includes all possible tsunami
sources and the exceedance rate is becoming essential in this world.

2.     
Hazard Assessment

Probabilistic tsunami hazard assessment
(PTHA) should assess by inputting bathymetry and topography data of the eastern
coast and the actual sources of them. Numerical modelling can be done using
commit model interface which is developed by NOAA – National Oceanic atmospheric
administration. Data can be collected from various parties as it is belongs to
either manual or satellite surveys. Gridded bathymetry data of Indian ocean can
be collected from Sri Lanka survey department or by paying for third party
sources like GEBCO – General Bathymetry chart of the ocean or any other
geological survey institutions.

 

Description

TABLE 1: Details of Input data for numerical model

Input data

 

Where to find

Details

Bathymetry &
Topography

Survey Department, GEBCO

Eastern
coastal area

Sources

Geological
Institutions

Along
Sunda Trench

Past worst case
scenarios

Meterological
department, Marine, EIA institutions and geophysical associations

At
least after 1900s

Effect of previous
tsunmais

Local Government
orgganizations, Disaster management ministry and non profittable organizations
worked during tsunami relief activities

For
2004 Tsunami

Numerical Sample Models

NOAA, Engineering
Community

For
2004 Tsunami

If data of sources are given, it can be
integrated into the model or from the PTHA observed results, the database can
be checked.

FIGURE 1:

 

The above sketch shows the main
scenarios when a worst case tsunami occurs due to the earth quake. The fault
slip’s edge location is known as sunda trench. By inputting the topography and
bathymetry details the model out put will show the propagation od wave and the inundation
in the dry land.

3. Computer Modelling

3.1. Brief Description

To model the possible tsunami occurrences
and from that to get the PTHA results a software of ComMIT will be used. It was
developed after the 2004 tsunami by national oceanic atmospheric administration
(NOAA) in pacific marine environmental laboratory. Commonly community model is
freely available in the internet. This will provide the tools to develop models
for different cases in finding inundation maps in dry land area in real-time
tsunami forecast applications.

The access for the ComMIT is given freely
to Indian Ocean countries. So from local databases any government organizations
or others in this region can access easily. This is giving great opportunities
in keeping geo-spatial data locally for the secure input purposes and it allows
every people to analyse and get the results so those results can be integrated
and a reliable database can be created.  Most importantly it  is creating a global community of modellers
who are using same approaches  so that
they can share the information within them.

3.2. ComMIT Procedure

Models with inundation and propagation details
should be created using the input mentioned in TABLE 1.

To access the ComMIT interface it is essential
to go through open-source software. It will lead to present the results easily.
And ComMIT was written in JAVA programming language so NetCDF format will be
the input for ComMIT interface. For this commonly used NetCDF format provides several
third party models. For this research MOST model will be used. Following
procedure will show the way to develop and analyse a model using numerical
methods.

1.      
Deep water (Indian Ocean) and
Eastern coastal topography

2.      
Boundary conditions in both
ends – Trench and Coastal area

3.      
Scale of model, Time-step,
required resolution and the period of model run will be inputted in ComMIT.

The ComMIT model interface allows the
users to share the bathymetry data and the analysed PTHA databases with other
ComMIT users. ComMIT also has a platform to display the PTHA output results in
animated format.

4. Conclusion

Providing the numerically analysed
results and a database with all PTHA results will incense the government to carry
out the fastest way to alert the people and to develop good early warning
systems without any wrong confusion. In future it will lead the researcher to
invent real-time monitoring systems of tsunami generation and propagation. This
will be a great revolution. And if geotechnical side can able to predict the time
and failure criteria for a trench with magnitude then it will be easy to
predict the worst case scenarios of tsunamis. So in my words, we can shift the
people very earlier before disaster period.

5. Acknowledgements

Many data of previous researches and
some topographical details was given by Miss. Udayanga Edirisooriya (Instructor
at Civil Engineering Department) I would like to thank her for the sources.
Lecturer Mr.Harsha Ratnasooriya sir guided me in collecting the references and
the details of previous literatures. I would like to thank him too.