There are two important caveats concerning the protective role of coastal forests. In some situations, the presence of a coastal forest can be detrimental. The most important caveat is the risk of complete destruction of a coastal forest and the hazard from the debris flow that results. The second caveat is the tendency for gaps in the coastal forest at river mouth openings and elsewhere to accelerate the tsunami flow rate and channel more energy towards a smaller area.
Given a large enough tsunami, all coastal forests can be a liability.17 Moreover, even in the case where a forest could conceivably mitigate a tsunami 6, 7, or perhaps up to 8 or 10 meters, it could become a liability if it is “under-designed”. If forest width, density, tree diameter, or soil substrate strength are insufficient, a tsunami can uproot trees or break tree trunks and branches, and level the forest. The broken material becomes debris that can be carried inland by the tsunami. This was particularly evident in near-field zones such as the coastal areas of Aceh, Indonesia in the 2004 Indian Ocean tsunami where mangrove debris was found 2-3 km inland. The damage caused by debris laden water flows can exceed the damage caused by water alone because of the greater mass and inertial forces of the objects carried along.
Considering the mechanics involved in breaking or uprooting a tree, five factors are relevant:
While the last two factors cannot be controlled, the environmental conditions can be managed to minimize the chances of breaking or uprooting. The ability of tree stems, branches and roots to withstand tsunami forces depends on their diameter and on the density and structure of the wood, which can be manipulated through management.
Stem and branch diameter (or more precisely cross-sectional area) is a major factor determining horizontal breaking strength. Even a small increase in stem diameter will dramatically increase the breaking strength and mechanical stability of a tree.
Wood density and structure directly affect strength, and depend on tree species and growing conditions. The wood of species that are resistant to breakage are either rigid and dense (with sufficient diameter), or elastic and forgiving. Growing conditions that encourage rapid growth in conifers can lower wood density, shorten fibre length and increase proportion of juvenile wood (Evans and Turnbull, 2004). For ring-porous species fast growth enhances strength, while for diffuse-porous species growth rate has little effect on strength (Jagels, 2006).
The characteristics of the root-soil interface are also critical in determining the resistance of a tree to a tsunami. Failure of the anchorage to hold the tree firmly upright will result in uprooting and loss of mitigation effect, as well as additional tsunami debris, potentially endangering people and infrastructures. Resistance to uprooting depends on the soil properties and the nature of the root system (i.e., rooting depth and root mass), which are determined by tree species and growing conditions (Gardiner et al, 2000).
Generally, a gap in the coastal forest will increase risks and potential damage. Gaps are found at the mouth of rivers and mangrove channels opening onto the sea. Homesteads, beach access, and roads also create openings in the forest barrier. A tsunami encountering a gap in a barrier will be funnelled into the gap with the flow accelerating as it moves into the constriction. Yet, for very large gaps the acceleration field does not develop and the presence of a coastal forest would not increase hazard. Moreover, the acceleration hazard is localized and limited to the area within and immediately behind the gap. Areas behind the coastal forest can still be protected from the tsunami. In the channel or gap, increased flow velocity and likely increased force, can be expected although flow depth will actually drop in most cases (Struve et al., 2003).
Over a length of coastline the net mitigation effect of a coastal forest would be positive in most cases. In other words, tsunami hazard will be much greater in the absence of a coastal forest, as opposed to one in which there are some gaps. If, however, habitation is concentrated in the gaps, then total cost of damage may exceed the value of protection. Also, if there are many gaps in the forest barrier relative to forest length, then the increased hazard at the gaps may exceed the mitigation potential of the forest beyond the gaps. Mangroves seem to be a special case, because of the numerous channels that weave through the forest and allow for rapid lateral dissipation of water volume and energy. Beach forests, altered forests and plantations cannot laterally disperse the water as fast. The rapid dispersion in mangroves would reduce effect of gaps. Nevertheless, gaps constitute a deficiency that increases tsunami hazard at specific locations, and because it is not realistic to consider a coastal forest without frequent breaks in the barrier, careful planning of settlement location in conjunction with forest establishment will always be necessary.
School saved by coastal forest? The impact of the 1998 PNG tsunami on the coastal forest near Sissano, north coast Papua New Guinea is shown here. The tsunami cleared 550 m of forest after overtopping a palm fringe. The inundation was only 70 cm when it reached the school. The school and students survived.
17However, in the case of a very large tsunami (20-30 meters in height) the extra force in the debris would be just ‘over-kill’, as the wave itself would be sufficient to level all buildings not built to tsunami code. This would imply that building standards would need to consider debris forces in their code as well.
