Tsunami mitigation by combination of coastal vegetation and a backward-facing step

Abstract

Since the 2011 Great East Japan tsunami, many improvements have been made in both hard and soft solutions for tsunami mitigation. After the 2011 Great East Japan tsunami, a post-tsunami survey was conducted along the coast in Miyagi Prefecture, which was one of the most tsunami-affected sites because rapid acceleration of the tsunami currents broke and washed away the trees, resulting in extensive damage to inland houses. In contrast, some of the houses located inland and away from vegetation with a dropping step survived. This shows a possibility that a step combined with the vegetation offers greater tsunami energy reduction by providing additional resistance. Laboratory experiments were conducted to investigate the energy reduction through a compound defense system (vegetation and a backward-facing step). Vegetation with a step (VS) showed greater energy reduction compared to that of only vegetation without a step (OV) due to additional loss by collision with the bed surface. However, the relative energy reduction in OV remained almost constant with the increase in the initial Froude number (F0, where the Froude number is obtained from a model without vegetation in a flume), whereas the relative energy reduction in VS showed a decreasing trend with increasing F0 because the energy reduction due to collision decreases with increase in water depth or F0.