Abstract
As a countermeasure against tsunami inundation, the present study conducted a series of laboratory experiments using a compound mitigation system in which a seaward embankment (E) followed by landward coastal vegetation (V) over a mound (M) (EMV) was investigated in supercritical flow conditions. The changes of flow around the mitigation system and energy reduction were clarified under varying conditions of mound height and vegetation density. Cases of an embankment followed by only a mound (EMNV) were also considered for comparison. Experimental results showed that three basic types of flow structures were observed within the mitigation system in EMV cases. A water cushion was created within the mitigation system mainly due to the combined effects of the mound and vegetation. It significantly reduced the maximum total energy in EMV cases by approximately 41–66%, whereas in EMNV cases, the maximum energy reduction was found to be 23–65%. Increments in both mound height and vegetation density increased the intensity of the water cushion within the mitigation system by offering more drag and reflecting the flow, and hence, significantly reduced the energy of the flow.
Highlights
The 2011 Great East Japan Tsunami (GEJT) significantly exceeded the disaster prevention capability of the present single coastal embankment system and resulted in loss of human lives, and extensive damage to the buildings and coastal forests in the Tohoku and Kanto regions of Japan [1,2]
When a hydraulic jump was formed within the mitigation system, it was classified as a type-A jump [32,33,34]
When a submerged hydraulic jump was formed on the downward slope of the embankment, it was classified as a type-B hydraulic jump [32,33,34]
Summary
The 2011 Great East Japan Tsunami (GEJT) significantly exceeded the disaster prevention capability of the present single coastal embankment system and resulted in loss of human lives, and extensive damage to the buildings and coastal forests in the Tohoku and Kanto regions of Japan [1,2]. After this disastrous event, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) of Japan divided tsunamis into two groups, with level 1 (level of disaster prevention) having a recurrence interval less than hundred years, and level 2 (level of disaster mitigation) having a recurrence interval several hundred to a thousand years [3]. The literature reveals the necessity of development of a system with a different mitigation capability
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