Narrow Results

Extensive studies have been carried out to study the performance of mangrove forests in wave height reduction. In this study, the reduction of the inundation and run-up of leading tsunami waves by mangrove forests was investigated through a series of laboratory experiments conducted in a long wave tank. The inundation and run-up were measured using a high speed CCD camera. Solitary waves were used to model the leading tsunami waves. Five vegetation models representing three forest densities and two tree distributions were examined on an impermeable sloping beach, and they were compared with the non-vegetated slope in view of wave reflection, transmission, and run-up. Results show that both incident wave height and run-up could be reduced by up to 50% when the vegetation was present on the slope. Dense vegetation reduced the wave transmission because of the increased wave reflection and energy dissipation into turbulence in vegetation. Normalized wave run-up on the beach decreased with the increase of both normalized incident wave height and forest density. Effect of forest density on the wave run-up on the sloping beach was further examined, and an empirical formula with the density incorporated was proposed. The study also highlighted the importance of tree distribution to wave interaction with vegetation on the slope when the forest density was unaltered, and run-up reduction difference between tandem and staggered arrangements of the trees could reach up to 20%.

Debris produced by coastal forest destruction due to the tsunami after the Great East Japan Earthquake caused secondary damage to buildings by collision. To limit such destruction, the trapping action of a finite-length forest was examined in a flume considering the effects of ‘forest density’, ‘debris length to forest width ratio’, and ‘forest width-length ratio (aspect ratio)’ because the trapping height greatly affects the rate of damage to the forest itself. Higher forest density and a higher aspect ratio decrease the velocity in front of the forest. Debris having a specific gravity up to 0.80 floated after collision but oscillated vertically as forest density was increased. With debris of a higher specific gravity (0.90–1.05), increased forest density resulted in debris attachment closer to the ground, which reached a plateau beyond a forest density of 0.48 cylinders/cm². In sparse forest, when debris was longer than the forest width; most debris fell at the foot of trees, while it was caught in the upper half of water depth in dense forest. The flow structure in front of and around a forest greatly affected the debris trapping capacity. It was deducted that the inland forest with a density of 0.48 cylinders/cm² and an aspect ratio of 1.7 trapped most of the debris of all lengths at the foot of trees.

In this paper, the effects of tsunami condition, forest density and forest width on tsunami flow velocity behind the coastal forest with an open gap were investigated by numerical simulations. A numerical model based on two-dimensional nonlinear long-wave equations was developed to account for the effects of drag and eddy viscosity forces due to the presence of vegetation. The numerical model was validated with good agreement by the experimental results. The numerical model was then applied to the coastal forest of Pandanus odoratissimus with a straight open gap (gap width=15m) perpendicular to the shoreline. It is found that the normalized maximum velocity behind the gap and vegetation¹ patch are greatly different. When tsunami period becomes large, the increase of velocity at the gap exit becomes large. Both forest width and forest density primarily influence on the velocity; as those increase, the normalized maximum velocity at the gap exit increases, while it decreases at the behind the vegetation patch. The enhancement of velocity at the gap exit is strongly dependent on the forest density but it is weakly influenced by the tsunami height.