HAZARD MAP AS A MEANS OF TSUNAMI DISASTER MITIGATION IN ...

HAZARD MAP AS A MEANS OF TSUNAMI DISASTER
MITIGATION IN GALLE, SRI LANKA

NIMAL WIJAYARATNA, D.P.L.RANASINGHE, A.JAYANTHIRAN,
C.M.WEERAKKODI, W.M.N.WIJESENA

Department of Civil and Environmental Engineering, Faculty of Engineering,
University of Ruhuna, Galle, Sri Lanka

Galle city has been identified as a highly vulnerable area for tsunamis. As a disaster
mitigatory measure, a tsunami hazard map is proposed for Galle city. Hazards and
vulnerabilities were assessed using the flow directions, inundation levels, run-up
distances, number of casualties and extent of damages during the Indian Ocean tsunami
on December 26th 2004. Evacuation routes and places were decided based upon the most
preferred options of the residents.

1. Introduction

Indian Ocean tsunami on December 26th 2004 provoked many fatalities and
major losses in many cities of Sri Lanka, including the port city of Galle in the
south coast. It demonstrated the high vulnerability of coastal communities in case
of tsunamis, lack of awareness regarding tsunamis and weaknesses in disaster
preparedness. Lethal combination of two headlands and a shallow bay with
inadequately protected shoreline have helped the tsunami to accelerate into the
city. Overwhelmed by the amount of devastation, many disaster mitigatory
measures were proposed for Galle city. Structural measures such as seawalls,
offshore breakwaters, elevated evacuation structures, relocating city facilities to
safe areas and non-structural measures such as set back lines or buffer zones
were extensively discussed. However due to many reasons some effective
mitigatory measures are yet to see the day light.

Hazard maps and evacuation plans are often described as effective measures
to reduce the human casualties. Especially when funding, legal and other issues
delay conventional miitigatory measures, hazard maps and evacuation plans
become effective but quickly implementable mitigatory measures. It requires
only a small fraction of funds which might spend over the conventional methods.
One of the major problems encountered in implementation of other non-
structural measures such as buffer zones is that the opposition from the coastal
communities. Tsunamis been extremely rare events in Sri Lanka, setting back is
argued as a meaningless effort. In contrast, hazard maps can be implemented
with least amount of disputes. However, there should be an accurate early
warning system in order that a hazard map will effectively save human lives.
Indian Ocean tsunami early warning system which will be in place in the near
future will fulfill this requirement.

2. Experiances on Hazard Maps in Sri Lanka

There had been no hazard maps or evacuation plans in Sri Lanka at the time of
2004 Indian Ocean tsunami. Immediately after the initial recovery stage, coastal
communities themselves have identified the escape routes that they should
follow in case of a similar event in the future. In some places they have gone to

the extent that vulnerable areas were identified and evacuation routes were
demarcated. However these efforts were not coordinated by the government or
followed any recognized standards or guidelines. First ever evacuation plan
prepared using a scientific methodology was done by a research group headed by
the United Nations university. They prepared evacuation plans for Galle city
and nearby Unawatuna tourist village. However they have not utilized all
possible evacuation locations or routes. Especially the existing buildings were
not identified as places for vertical evacuation. A hazard map for Matara city in
the southern coast of Sri Lanka was prepared by a group of consultants
(PADECO, NIPPON KOEI et al. 2006) funded by the Japan International Co-
operation Agency. Amount of vulnerability in terms of inundation heights were
shown but the evacuation plans are not included in the map. All these maps have
been submitted to the relevant authorities of the central government who has the
authority to disseminate the information to the local communities.

3. Guidelines on Preparation of Hazard Maps

In parallel with the above efforts the preparation of the hazard map presented in
this paper was started soon after the immediate recovery stage. There were no
guidelines or standards for the preparation of hazard maps in Sri Lanka at that
time. Therefore the guidelines which were being used in Japan (Coastal
Development Institute of Technology, 2004) were followed, however with
several limitations. One of the main issues was that the amount of vulnerability
was decided by the amount of inundation recorded during 26th December
tsunami. However, according to the manual, vulnerabilities due to all possible
disaster mechanisms should be considered. There were no past records of
tsunamis in Galle or no inundation simulations have been done for Galle city
area at that time. However this tsunami was the largest ever to record in the
region and it was expected that a tsunami with a higher devastating power was
extremely unlikely to occur. Further it was a far field tsunami and Sri Lanka
cannot expect near field tsunamis. There will be no major change in the
inundation pattern due to diffraction and refraction at the continental shelf when
far field tsunamis with different origins are concerned.

4. Data Collection

It is a well known fact that the local community should actively involve in the
preparation of hazard maps. Especially the evacuation plans should be prepared
by themselves. However there had been no legal provision for such a community
based hazard map at that time and as such the community participation was kept
at a very informal level. Only a selected group of local residents were requested
to share their experiences on December 26th Tsunami and their preferred
evacuation paths and locations. Information necessary to evaluate the amount of
hazard and vulnerability were collected along with the data for evacuation plans.

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In order to get a spatially, evenly distributed set of information, city area of
approximately 8.5 km2 was divided into a set of blocks, each 250 m long and
250 m wide squares. There were 138 squares in total and one to three households
were selected in each to represent the community in the respective block. Data
collection was carried out in a form of an interview survey and the discussions
were concentrated on collecting following information.

 Inundation depth, flow direction, run-up distance etc.
 Extent of damage, failure mechanisms
 Number of residents in the block
 Residents’ preferred evacuation routes and safe areas
 Additional information to propose alternative evacuation routes and

safe areas

Data collected were rechecked for consistency at site by comparing with
figures at neighboring blocks, data available at other sources and water level
marks remaining in the buildings. Inconsistent data were disregarded and the
opinion of a second or a third person was taken. Figure 1 shows the locations in
each block where the data were collected. There were several blocks where the
destruction was so severe that residents have not yet come back or they all were
killed on the December 26th 2004. Inundation height and the flow directions
were estimated using whatever the traces in the buildings and trees left behind by
the tsunami. Figure 2 shows the flow directions and inundation heights at each
location for the central part of the city.

Figure 1: Locations of data collection

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1.5 – Inundation Depth
– Flow Direction

Figure 2: Tsunami flow directions and inundation heights (central part of Galle city)

5. Inundation Map and Amount of Vulnerability
Inundation height data were reduced to mean sea level datum and an inundation
map was produced (Figure 3). Due to the unavailability of any other historical or
possible future inundation patterns, inundation contours in Figure 3 was used to
demarcate vulnerable areas.

Figure 3 Inundation Contours (m MSL)

Areas where the ground elevation was at least two meters higher than the
elevation at the nearest run up location were designated as safe areas for a future
tsunami. Several buildings were also identified as places for vertical evacuation
where there is no close by safe high grounds. In addition to the safe height, easy

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access throughout 24 hours, enough space and strength for some estimated
number of people, willingness of the owners to offer the building to use as an
evacuation center are considered in designating a building as a safe building.
Figure 4 shows how the safe areas and safe buildings are located in Galle city.

Figure 4: Safe Areas and Safe Buildings

Maximum distance to a safe area or a safe building from a vulnerable place
is approximately 400 m. An early warning reaching the residents at least 10
minutes before the arrival of tsunami would be enough for safe evacuation.

6. Evacuation Plan

Evacuation plan of the hazard map was prepared by merging the most preferred
evacuation routes of the residents in each block. Figure 5 shows the evacuation
plan in the central part of Galle city. All the routes directed towards highland or
inland were not designated as evacuation routes. Especially the roads laid along
the banks of the canals were omitted from the list of escape routes. Shore
perpendicular canals had acted as resistance free passages for tsunami flow and
the tsunami flow has come inland very quickly without dissipating much energy
on the way. Therefore the residents were on the opinion that they would not use
those roads in the race against a tsunami.

7. Hazard Map
A significant number of people who have interacted with the group who
collected data for this hazard map have found that it was difficult to understand
the topographic drawing on which the evacuation plan was printed. Therefore a
high resolution satellite image was used as a base map which has helped them to
identify the evacuation routes and locations with respect to familiar
surroundings. Figure 6 is a part of the hazard map for Galle city showing
inundation levels and evacuation plan. Arrival time of tsunami is not marked in

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the map as the whole area will flood within two to three minutes after the
tsunami reaches the shoreline. Therefore practically there is no time difference in
flooding at different parts of the city. Forecasted arrival time of tsunami at Galle
in the early warning message should be treated as the flooding time at every parts
of the city for all practical purposes.

Figure 5 Evacuation plan (Central part of Galle city)

Figure 6. Hazard Map for Galle City (a part of)

In contrast to many disaster prepared countries, residents of Galle city prefer
to move to a relative’s place or to a friends place in a safer place rather than

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going to a designated evacuation center. Main reason they sited was there is no
guarantee that at least the basic needs of the refugees will be satisfied within a
reasonable time frame. Providing identified evacuation centers with some basic
facilities is an urgent requirement, which should be attended immediately.
References
Coastal Development Institute of Technology, Japan. 2004, Outline of Manual
for Tsunami and Storm Surge Hazard Maps
PADECO Co. Ltd., NIPPON KOEI Co. Ltd. and OVERSEAS AGRO-
FISHERIES CONSULTANTS Co. Ltd. 2006 Recovery, Rehabilitation and
Development Project for Tsunami Affected Area of Southern Region in the
Democratic Socialist Republic of Sri Lanka.

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