Tsunami preparedness and coastal buffer zone: Have we lost the plot?

by Somaweera Sirisinghe, New Zealand
(Courtesy The Island 2/04/2005):

The proposed one-hundred meter buffer zone (or set-back) as an integral part of tsunami preparedness is a serious topic that merits careful consideration. Rather than addressing it as a tsunami hazard mitigation measure, the issue has been turned into a political football. Conservationists and commercial interest groups have waded into the politically charged waters and one engineer cum leisure management entrepreneur showed no hesitation to call it the hangman's noose. It is unfortunate that a responsible political party in the opposition took an early stand to oppose it tooth and nail. It is equally tragic that the government laid a far-fetched claim that the buffer zone concept worked wonders on Monday's tsunami alert. The victim seems to be the common sense.

Tsunami is a hazard that brings in its wake serious harm to human life and destroys national wealth. It can not be prevented or eliminated. Hence, the hazard has to be assessed and the potential harms need to be mitigated. Adverse impacts such as loss of life and property can be reduced with proper planning. Therefore, tsunami hazard mitigation or tsunami preparedness is common sense.
It is claimed that more people have been killed since 1945 as a result of tsunamis than as a result of earthquakes. Killer tsunamis have wreaked havoc in the recent past. These are caused by massive earthquakes that occur in the subduction faults, where one of the earth's plates is diving or subducting beneath another plate. This type of faults has produced the largest earthquakes ever recorded that include the December 26 earthquake off Aceh. On 22 May 1960, a massive earthquake of 9.5 Richter scale occurred in the subduction zone off the coast of Chile and generated a tsunami that affected the entire Pacific Rim. According to Geoscience experts other type of earthquake faults, such as strike-silp faults where earth's plates slide past each other generally do not produce the surface displacement necessary to cause a tsunami, unless the quake causes subsequent under sea landslides that can trigger the displacement. The tsunami that destroyed a large number of villages in Papua New Guinea in 1998 is an example.

Whether the next tsunami happens tomorrow or in hundred year's time, Sri Lanka is vulnerable to the tsunami hazard because of the existence of the subduction fault in the Sunda Sea off Sumatra. Hence, tsunami preparedness must be treated as a national issue sans political bickering.

Two countries that have developed very high levels of tsunami preparedness are Japan and the United States. It is said that more than 25% of reported Pacific tsunamis since 1895 have originated near Japan that sits precariously near the colliding margins of four tectonic plates. (The word tsunami is from Japanese language "tsu" = Harbour and "nami" = Wave). Hence over the years Japan has made heavy investments in tsunami mitigation that has included tsunami research, comprehensive education and public awareness, effective warning systems, shoreline barrier forestation, sea walls and other coastal fortifications.

Vast progress made by the United States in tsunami preparedness is especially due to its perceived vulnerability to tsunamis originating from earthquakes occurring in the Cascadia subduction zone off the coast of Washington, Oregon and North California, The last tsunami generated by a 9 to 9.5 Richter scale quake in that subduction zone occurred on 26 January 1700; the actual date confirmed by Japanese historical records of tsunamis. Scientists expect a big quake to occur in this zone at anytime. As a result the US National Oceanic & Atmospheric Administration (NOAA) and Federal Emergency Management Agency (FEMA) have extensively planned tsunami hazard mitigation.
Japan and the US deploy the state-of-the art computer technology for tsunami preparedness, which includes Tsunami Inundation Mapping Efforts (TIME), Inundation Modelling, Inundation Animation and Risk Models to map out the impact etc that are used at Regional or State level and community level to plan responses and evacuation. The strategy used is that "more informed the people are, better the chances for survival". Hence "Tsunami Ready Community Programme" has been successfully launched in the US.

A striking feature of Japanese and US Tsunami Preparedness is that it is founded on the results of scientific research on inundation mapping and modelling rather than basing it on a "one-size-fits-all" policy of declaring a buffer zone for the entire coastal belt.

If willing, Sri Lanka can learn and benefit from the experience of these two countries. The major part of tsunami hazard mitigation in those countries lies in early warning systems, increasing community awareness of coastal communities at risk, ability to disseminate emergency information, Emergency Operation Centres and their co-ordinating ability and tsunami hazard plans that include evacuation plans. Sri Lanka's distance to the known subduction zone in the Sunda seas that allows at least three hours for emergency evacuation and proposed early warning systems for the Indian Ocean region makes it imperative that we should act steadfastly towards these hazard mitigation goals.
The present debate on the buffer zone concept should be re-focused towards an un-biased evaluation of its real contribution to tsunami hazard mitigation and tsunami preparedness. Otherwise the buffer zone or the set backs currently in operation under the Coastal Zone Management Plan (CZMP) prepared under the Coast Conservation Act (CCA) should be allowed to take control of conservation issues.

The universal application of a coastal buffer zone of a prescribed distance from the sea does not sit comfortably with scientifically analysed tsunami inundation patterns.

According to Geoscientists tsunami waves are barely noticeable in the deep ocean but when it approaches the shallower water above a continental shelf, the friction with the shelf slow the front of the waves. As it nears the shore the trailing waves pile on to the waves in front causing them to grow in height up to10 to 15 meters before hitting the shore. The turbulent wall of water (called a bore) crashes on the shore with great destructive power. Behind it comes a deep and fast moving flood that can pick up and sweep away anything in its path. Minutes later the water will draw away as the trough of the tsunami wave arrives some times exposing great patches of sea floor. Then the water rises again as before. The destructive cycle can repeat many times.

Experts agree that the size and the impact of tsunami waves are determined by the off shore and coastal morphology. Reefs, bays, entrances to rivers, under sea features and the slope of the beach etc modify the tsunami as it attacks the coastline. Refraction by bumps and grooves on the sea floor can shifts the wave direction as it travels into shallow water. A particular significance is that the wave fronts tend to align parallel to the shoreline. That makes the waves to wrap around a protruding headland before smashing into it with much increased fury. What happened in the public bus terminal opposite Galle Fort that lies on a protruded headland is a case in point. Hence it is the morphology of the land that determines a tsunami's inundation distance and the height of the wave that is called the run-up elevation. Therefore it is possible that one coastal community not suffering any damaging wave activity while a nearby community suffering great destruction. Some times flooding can spread inland by 300 meters or more covering large expanses of land with water and debris.

Under this scenario the effectiveness of a universal application of a buffer zone as a tsunami hazard mitigation measure seems to be of questionable value compared to more viable alternatives of using already developed techniques of inundation mapping and modelling.

They have proved to be more effective for tsunami hazard planning, disaster response and evacuation. Any shore line barrier forestation, facilitation of sand dune formation or the construction of sea walls or other fortifications should be determined after inundation mapping/modelling to prevent any identified hazards of potential inundation.

The Sri Lanka International Tsunami Survey jointly conducted by US Geological Survey and Geological Survey and Mines Bureau of Sri Lanka with ten other American, New Zealand and local associates that included Upali Group of Companies could well be followed on with an exercise of inundation mapping/modelling funded by international aid.

In the buffer zone debate two professional contributors have commented on the feasibility of erecting buildings within the buffer zone to withstand tsunami waves. The engineer cum leisure management entrepreneur endorsed "vertical evacuation" in a tsunami disaster by allowing well-designed multi storied buildings with proper foundations in the coastal buffer zone that could provide safe places to evacuate. A cadastral surveying professional commented that it is not the "distance from the sea" that maters but the "elevation" and went on to recommend houses on concrete pillars within the buffer zone.

Both professionals have presumed that it is an easy to build structures capable of resisting the extreme forces of a tsunami. Some expert comments on building science issues relating to tsunami preparedness in the US quoted below show that it is not that simple. "For average structures would not be economically feasible to construct to withstand extreme loads of a tsunami." The conclusion of the authors of FEMA Coastal Construction Manual that provides design and construction guidance for structures built in coastal areas throughout the US is that " tsunami loads are far too great and that, in general, it is not feasible or practical to design normal structures to withstand these loads".

According to tsunami mitigation information available from the US, vertical evacuation becomes the only feasible alternative if evacuation has to be effected within minutes of an earthquake that could generate a tsunami, provided that structures could be constructed to resist tsunami loads. This applies to populated areas closer to Cascadia Subduction Zone. It would only be possible with specially designed structures built to withstand specific tsunami loads without collapse to provide community shelters for vertical evacuation. Same special design criteria could be possibly used if the buildings were to house large occupancy loads such as larger seaside resorts. In the lines of this argument Sri Lanka's physical location substantially away from the nearest subduction zone rules out the necessity for vertical evacuation

Let's hope that an objective an impartial analysis is completed with an open mind before "one-size-fits-all" solutions are forced-fed in the guise of Tsunami Preparedness.

(The writer is a Geography graduate of Peradeniya University and is saddened by the passing away of one of his teachers, the devoted Geographer Prof. Kusuma Gunawardene. This article is dedicated to her.)



Copyright 1997-2004 www.lankaweb.Com Newspapers Ltd. All rights reserved.
Reproduction In Whole Or In Part Without Express Permission is Prohibited.