Role of hybrid structures on the control of tsunami induced Large Driftwood

Abstract

Tsunami induced Large Driftwood (TLD) can cause significant damage to a coastal environment. In a tsunami, TLD can become an additional impact and a drag force against a coastal structure. Modern research is now showing that hybrid-structures provide a more optimal solution against mitigating tsunami-related damages. Hybrid-structures involve structures like moats, embankments, and forests. This study considered the use of a dry moat and investigated the benefits of a hybrid-structure in controlling TLD's damage potential. Through a laboratory-scale experiment, this paper elucidated the reduction potential of impact velocity and impact moment of a TLD. The experiment was conducted in a channel that generated a quasi-steady-flow, imitating a long period wave flow. As a result of the strong reflection of flow, generated within the moat structure of an embankment-moat design, this study observed the retention of TLD. Moreover, due to the turbulence and energy loss, the TLD after it had overtopped the hybrid-structure, had a lower impact velocity and impact moment. Changes to the upstream vegetation characteristic, for example, the ratio of trunk height to tree height, showed variation in the probability of retention. A trunk height to tree height ratio of 0.3 had a 71% probability of retention, while a TLD that had a trunk height to tree height ratio of 0.8 had a 42% probability of retention. Indicating the variation in the upstream tree's characteristics influences the retention capability of an embankment-moat design. This same hybrid design, vegetation upstream of an embankment-moat design, also showed a rotation capability of TLD. The rotation of TLD against the streamlines can increase the role of trapping by a second vegetation patch, potentially further reducing the damage that can happen because of TLD. Therefore, the application of an embankment-moat design can be used to control the damage potential of TLD.