The tsunami of 26 December 2004 was a major natural disaster, killing some 229 866 men, women and children and causing billions of dollars in damage (United Nations, 2007). With a moment magnitude, Mw, between 9.1 and 9.3, the earthquake that caused the tsunami was the largest in the last forty years and second largest in instrumental history (Bilham, 2005). Yet, the devastation caused by the 2004 tsunami (like most other tsunamis) could have been greatly reduced in many of the thirteen countries that were hit, particularly in those countries farther from the earthquake epicentre and subject to less massive tsunamis.
While it is well documented that the lack of an adequate early-warning system for the Indian Ocean was largely to blame for the high casualty rate, the tragedy occurred for another reason, as well. Much of the coastline in many parts of Asia and the Pacific is heavily populated – an increasingly growing phenomenon seen around the world. As a consequence of this development, coastal vegetation – and the associated setback – that would have provided natural protection from hazards such as storms, cyclones or even tsunamis has been degraded, severely altered or completely removed.
In many countries the requirement for setbacks is written into land use legislation and regulations. So far, these have not been uniformly enforced and, moreover, most settlements and other developments are not planned by taking into consideration the potentially massive destruction associated with coastal hazards. Although huge, massively destructive tsunamis may have a 100-year return period, smaller, but potentially devastating tsunamis, are much more frequent in some regions. It should be recalled that Sri Lanka had exceedingly high casualties and property damage, despite being far from the epicentre and struck by waves less than a quarter of the size those striking Aceh in Indonesia. In Sri Lanka about 68 percent of wave height measurements fell between 3.0 and 7.5 meters, with a median height of only 5.0 meters. It is the lack of preparedness in many coastal areas that increases vulnerability to disaster. There will always be some degree of vulnerability in developed coastal areas, but such risks can be minimized with proper planning.
Coastal area development entails changes to the natural landscape. However, many types of development do not necessarily have to come at the expense of vegetation cover. In heavily developed urban areas the establishment of coastal forests for protection may not be easy, but it is not inconceivable. In rural coastal areas, the integration of protective forests with rural development should be the norm. In fact, the impact of the 2004 tsunami was not limited to populous cities, but included a multitude of rural communities strung along the coastline. Where mangroves and beach forests no longer existed, the damage caused by the tsunami was generally more severe. Where forests were present they mitigated the impact of the tsunami in many cases. Early warnings systems could have saved many lives. Coastal forests could have saved property, as well as lives, where the tsunami was not extremely large.
Though coastal forests are only partially effective against flooding, particularly when caused by successive, non-breaking waves of a long-period tsunami,1 they greatly reduce impact forces and flow depths and velocities, which in turn limits the extent of flooding. Nevertheless, almost complete protection from impact damage of 6-7 meter waves can be achieved. It is even possible that a large, well-designed coastal forest could substantially mitigate the damage of a tsunami up to 8, or even 10 meters. This, of course, would also depend on the suitability of the site for tree growth, ground elevation, and the near-shore run-up slope that determine wave form and force of the waves of similar height.2 Appropriate set-back distances, large enough to incorporate the coastal forest, would also be necessary. Yet, in rural hamlets and villages, coastal forests generally integrate well with people’s livelihoods and economies.
1 Non-breaking waves represent about 75 percent of tsunamis.
2 It is important to note that inundation depth (flow depth), rather than wave height, is critical variable determining if a forest is able to withstand a tsunami. Inundation depth or flow depth is wave height adjusted for tide level and ground elevation (see Fig. 2 and Table 1). Consequently, depth may anywhere from 0.5 to 3.0 or more meters less than estimated or measured wave height at any location. Forests need to be designed for the expected flow depth and velocity of a tsunami.
