The function of a barrier – whether coastal forest, breakwater, seawall, or cliff – is to absorb the impact forces and to retard the flow of large storm waves and tsunamis. A seawall, if tall enough, reflects the wave back out to sea. On the other hand, permeable structures, like breakwaters and coastal forests, partly reflect and partly transmit the water. In the case of a coastal forest, energy is progressively absorbed as it passes through the forest. Without the forest barrier, the tsunami will run-up to a maximum height determined by the magnitude and nature of the seismic event that created the tsunami and local factors such as the coastal profile, offshore bathymetry and beach slope that modify the wave’s force.7 Once the tsunami comes on shore, the amount of reduction in water depth, velocity, and force depends on how much water is reflected and energy adsorbed by the coastal forest (see figure 2).
Figure 2: Tsunami wave run-up with and without coastal forest barrier. Source: Keith Forbes
7Bathymetry refers to the underwater topographic relief found offshore.
