Countermeasure against local scouring and tsunami damage by landward forests behind a coastal embankment

Abstract

In the Great East Japan Tsunami, many coastal embankments collapsed due to excessive local scouring at the embankment toe. Thus, as a countermeasure against local scouring and tsunami damage, the present study performed a series of flume experiments with landward forests models (the combination of a short frontal forest (SF) and a landward tall forest (TF)) behind a coastal embankment model (EM) on a gravel bed. The damming of washed-out trees as debris occurs due to the excessive local scouring around the embankment; if they are properly managed, they could also contribute to reducing the tsunami energy. Thus, the effect of damming by tsunami-borne driftwood on scouring was also investigated. The maximum reductions of the scour depth at the EM toe, maximum scour depth, and scour length were found to be approximately 44.1–65.2%, 30.2–58.6%, and 21.9–41.9% compared with the single embankment system when the SF was placed between the EM and TF (i.e., Case 3). Moreover, damming (i.e., Case 5) also effectively reduced the local scouring around the EM toe. However, at the highest overflow depth, significant scouring was observed beneath the formed driftwood dam, which implies the washout of TF. Both Case 3 and the damming case increased the energy reduction due to the greater shift of the position of a submerged hydraulic jump toe toward the EM crest and an increased backwater rise in front of TF. However, considering the vulnerability of TF, Case 3 is recommended to mitigate a large tsunami.