Author(s): Yuta Akasaki, Yoshiya Igarashi, Norio Tanaka
Linked Author(s): Yoshiya Igarashi
Keywords: Ecosystem-based disaster reduction, energy dissipation, hydraulic jump, fluid force, fragility curve
Abstract: Eco-system based Disaster Risk Reduction concept recommends taking advantage of the natural elements including sand dune, coastal forest, and tidal flats area like a lagoon for mitigating disasters. This study aims to clarify the tsunami mitigation effect of a lagoon on the surrounding area and the study site is selected around the Torinoumi Lagoon, which is the damaged area by the 2011 Japanese tsunami located on the southern side of the Abukumagawa River. In order to clarify the mitigation effects of the lagoon, the inundation depths around the lagoon were reproduced by numerical simulation using a two-dimensional (2D) depth averaged long-wave model, where the reduction of velocity and fluid force were analyzed. To clarify the lagoon effect on mitigating tsunami fluid force, the lagoon volume which can store the tsunami is considered an important parameter. The lagoon volume is changed by changing the land elevation height assuming with/without the lagoon. Furthermore, using a fragility curve for the house damaged by the fluid force index u2h (u: velocity, h: water depth), the reduction of house damage probability is analyzed. The reduction of the percentage of washed out houses was more than 4 % in cross-stream direction due to the existence of lagoon in the 2011 tsunami. The tsunami fluid force index decreases around 20-40 (m3/s2) at maximum due to the existence of lagoon especially in the cross-shore direction, where the tsunami current from lagoon is not strong because of it getting mixed with the inundated current from seaside. Fluid force index in this area shows larger value for the same house damage probability compared to in another district. This means that the energy reduction by hydraulic jump that actually occurred behind the sand dune near the lagoon mouth is not reproduced well in the traditional 2D depth averaged equations
Year: 2017