Author(s): M. Rashedunnabi A. H; Norio Tanaka
Linked Author(s): Norio Tanaka
Keywords: No Keywords
Abstract: High level tsunami propagates inland with high inundation depth and with disastrous energy. Vegetation has been recognized as an effective measure to reduce the inundation depth, as well as the fluid force in the land area. This study, conducted flume experiments to investigate the flow structure and energy reduction mechanism of a two layer rigid vegetation against a high level tsunami. Short vegetation in single and a combination of short and tall vegetation with different porosities were selected for this investigation. Vertically two layered vegetation was selected for increasing the mitigation effect. Firstly, a selected type of tall vegetation was set on a flat plate and a secondary short vegetation was incorporated within the tall vegetation to construct the double layer. Assuming a high level tsunami, an inundation depth above 5 m in real scale was chosen as the flow condition in the laboratory experiment with a physical scale of 1/100, where the short vegetation remained submerged and the tall vegetation remained in an emergent condition. The combined (submerged& emergent) and single (submerged) layer vegetation models were set against the selected flow condition in a flume. The water surface height was measured throughout the center of the flume before and after the placement of the vegetation model, velocity profile was measured vertically inside the vegetation, and video analysis was carried on to understand the flow structure, percentage of energy reduction, as well as the mechanisms causing a reduction in energy. The study found that the energy reduction is highly dependent upon the porosity (density) of the short layer. The combined vegetation reduced a significant amount of energy by offering a large water rise in front of the vegetation, very mild water surface slope inside the vegetation, air entrainment within the water surface and creating a low-velocity zone at the back side of the forest. The experiment showed a maximum energy reduction of 52% for the combination of a dense short layer and a sparse tall layer vegetation model.