Author(s): Yoshiya Igarashi, Norio Tanaka, Takehito Zaha
Linked Author(s): Yoshiya Igarashi
Keywords: Energy reduction, hydraulic jump, multiple defense, overflow from embankment
Abstract: The 2011 Great East Japan tsunami caused catastrophic damage to people and buildings in the Tohoku and Kanto districts of Japan and revealed the limit of using merely sea embankment as defense. After the tsunami, the importance of multiple defense systems for mitigating tsunamis has been recognized. This study was conducted to clarify the energy reduction of a tsunami due to a compound defense system by combination of a sea embankment and lined vertical piles by which tsunami energy of overflowing water is reduced through hydraulic jump and/or resistance of piles. This study investigates the relationship between flow structure and energy reduction with changing pile height and spacing. Even in the case of single embankment, energy reduction rates were approximately 38-81% because the overflowing water from embankment collided with the bed. In compound system, the flow structure after overtopping the embankment can be classified into ten types. The change of flow structure patterns was classified into two by pile spacing. The energy reduction rate was large with lined pile when the pile spacing was small. In some cases, due to resistance offered by lined piles, the standing wave was formed at immediately downstream of piles which resulted in decrease of energy loss. The energy reduction rate gradually decreased when the pile sank in water. However, the energy can be expected to reduce if the pile height is large and/or the pile spacing is small because of the large resistance of piles. Eddies are generated just behind the piles when the pile spacing is small and the water depth at the pile is larger than the pile height. This is due to the large difference of flow velocity inside the piles and over the piles. Therefore, the energy loss can also be expected in this case
Year: 2017