There is considerable evidence that coastal forests can reduce the force, depth and velocity of a tsunami, lessening damage to property and reducing loss of life. Numerous anecdotes, field surveys and scientific studies in India, Indonesia, Japan, Malaysia, Maldives, Myanmar, Sri Lanka, and Thailand of the 2004 tsunami and other tsunamis show a connection between areas with the highest levels of damage and the absence of coastal forests.3
The destructive force of a tsunami is subject to local factors which are often unavailable for analysis (e.g. local bathymetry and coastline configuration) and therefore the protection offered by trees and forests may not be fully quantifiable. On a case by case basis, however, studies often show reductions in the degree of damage to trees with distance from the leading edge of a coastal forest, implying that the force of the tsunami is reduced by the forest and areas to the rear are afforded protection. An additional source of information is provided by studies in which adjacent areas of coastline, with and without trees, are compared. Such studies provide core evidence of the mitigation potential of forests. Empirical findings are also supported by experiments using models and mathematical analogues of tsunami-forest interfaces. Such methods add further weight to claims of protection by forests against tsunamis.
Data from field studies across Asia shown in Figure 1 and Table 1 (below), show that where coastal forests failed, waves were very large or forest width was limited. In other cases, although waves were less substantial and widths were adequate, forests could still fail to provide mitigation where trees were widely spaced,of small diameter, or without branches near ground level as denoted by the symbols w, s, and b, respectively.4 Conversely, some cases of successful mitigation may possibly be partially attributed to other contributing factors such as higher ground elevation or less exposure to the sea. Data allowing, Table 1 accounts for elevation in the estimates of tsunami flow depth – the most important variable determining success or failure.5
Figure 1: Evidence from 2004 Indian Ocean and 2006 West Java tsunamis of coastal forest’s protective role relative to wave height and forest width. Solid shapes indicate substantial mitigation and damage reduction. Source: compiled by Keith Forbes
In the case of mangroves, for any particular elevation or distance from the sea front, tsunami hazard is consistently lower for areas behind mangroves. Furthermore, plantations of pine in Japan have proved effective against various tsunamis. Many casuarina shelterbelts in India, Sri Lanka and Thailand, established to protect coasts from cyclones, tsunami and other coastal hazards were effective against the 2004 Indian Ocean tsunami as well. Natural beach forests and plantations of tree crops, such as cashew nut with their low, widely-branching canopies or pandanus with mangrove-like stilt roots and dense foliage, have also protected coasts in many instances.6
There are also a significant number of cases where coastal forests failed to protect coastlines from a tsunami. Rather than an indictment of coastal forests in general, however, these failures can be attributed to a rare, massively large tsunami or insufficiency of one or more forest attributes such as forest width, density, age or some other parameter important in providing protection. This was frequently the case with degraded or altered beach forests with widely spaced trees, replacement tree species susceptible to breaking, or sparse undergrowth.
Casuarina shelterbelts were also ineffective in situations where they were too narrow or had become too old and were therefore without flow-resisting branches lower down on the trunk. As casuarina and similar species like pine mature, the branches and foliage at lower heights die off and the drag they provide is lost. Similarly, coconuts provide very little resistance as their trunks have no branches.
Coastal forests have also been reported to have a role protecting lives and property beyond wave energy mitigation. In India and Malaysia, there are stories of how the presence of large mangroves saved the lives of people who climbed or were able to cling to trees and escape from being dragged out to sea. Some moderately tall tree species with wide canopies growing on beaches in altered forest and plantations also provided important refuge. Coastal forests have also obstructed boats, timber and similar ship cargo and other debris from washing inland where they would cause many casualties and great damage.
A narrow shelterbelt of pine trees near Shizugawa (Miyagi Prefecture), Japan appears to have protected the houses within its shadow during the 1960 Chilean tsunami. Waves came from the Pacific (top of photo) and river mouth (left side of photo). Destruction in foreground also includes debris left by river inundation.
3 See, for example, Aksornkoae and Hawanon 2005, Chang et al 2006, Dahdouh-Guebas 2005, Danielsen et al 2005, Hiraishi 2006, IUCN 2005, Izumi et al 1961, Kathiresan and Rajendran 2005, Latief and Hadi 2006, MSSRF 2005, Padma 2006, Parish 2005, Ramanamurthy 2005, Ranasinghe 2006, Shuto 1987, Siripong 2006, Tanaka et al 2007, UNEP 2005, and Yasuda et al, 2006.
4 Note, only maximum forest width and minimum wave height, where there was range in the data, are plotted (see Table 1) to give a greater safety margin in interpretation.
6 Though cashew nut plantations may have widely-spaced trees, mitigation capacity comes from the high density of the branches and foliage brought close to the ground � a growth form common to the species. Wide spacing, thus, has less influence on limiting mitigation.
