Author(s): Md. Abedur Rahman; Norio Tanaka; A. H. M. Rashedunnabi; Yoshiya Igarashi
Linked Author(s): Norio Tanaka
Keywords: Hybrid defense system; Coastal forest; Overtopping flow; Hydraulic jump; Tsunami mitigation
Abstract: The countermeasure against tsunami inundation is drifting from only sea embankment to a hybrid system comprising embankment and coastal forest after the 2011 Great East Japan tsunami. Thus, the present experimental study investigated the defense system comprising an embankment (E) followed by a coastal vegetation (V) on a mound (M). The loss of energy head of the tsunami flow was examined with varying condition of mound length and forest density against different overtopping depths while keeping the mound height fixed. Different types of hydraulic jump were formed between E and V which also contributes to the energy loss significantly. The loss of energy head against the maximum ho’ = 0.48, (where, ho’ (non- dimensional overtopping depth) = overflowing water depth/ height of embankment) for the embankment followed by vegetation on a mound (EVM) was found to be 43%, whereas it was found to be 23% for the embankment followed by mound (EM) only. Moreover, the energy of the flow against the considered range of h0’ was significantly reduced by 43%-65% in case of EVM, whereas it was reduced by 23%-64% in case of EM. An exponentially decreasing trend in energy dissipation was observed with increase in h0’ for both cases because the effect of energy reduction by hitting the ground became relatively low with the increase in overflowing water depth.