Author(s): Rowan Sachika De Costa; Norio Tanaka
Linked Author(s): Norio Tanaka
Keywords: Disaster prevention engineering; Driftwood management; Inland forest design; Tsunami mitigation; Trapping of driftwood
Abstract: An inland forest can be used to not only dampen the impact by a tsunami but also prevent impact by tsunami borne driftwood, by trapping them within a forest. This paper hopes to develop the understanding of an inland forest model (IFM) by elucidating the trapping function of an IFM and the effect of trapped driftwood on the incoming flow. Variables of an IFM include spacings between the tree trunks (G/D; gap between cylinders/ diameter of the cylinder; cylinder representing the trunks of the tree), number of rows and the initial Froude number (Fr) (Initial being the flow condition when no IFM exist). Two different laboratory experiments were conducted using a small coniferous plant. The first experiment elucidated the effect of tsunami borne large driftwood (TLD) angle against the streamlines and the trapping function of an IFM with G/D ratios of 10.6, 6.8 and 3.0. It showed that TLD that attacks an IFM in a cross-stream position increased the trapping function of an IFM than the TLD being in a streamwise position, an additional increase of 20% in G/D of 6.8. The second experiment showed that the G/D ratio of 4.53 had a different trapping pattern than a G/D ratio of 0.88. Therefore, the effect of trapped TLD on the incoming flow was different. The G/D ratio of 4.53 showed an approximately 5% decrease to the downstream flow.