Effect of Nearshore Islands on Tsunami Inundation in Shadow Zones
Abstract
Field surveys performed in the wake of the 2010 Mentawai tsunami event have described the belief of local residents that offshore islands serve as possible tsunami sheltering mechanisms, reducing the corresponding inundation on beaches behind the islands, despite the fact that deduced inundation from debris lines show this to be in fact untrue (Hill et al. 2012). Recent numerical model studies (Stefanakis et al. 2014) have shown that inundation levels on beaches behind conical islands are indeed higher than they are on open coastlines. While work has been done on tsunami amplification on the lee side of islands (Briggs et al. 1995), no work has been done concerning tsunami inundation on beach areas behind the islands. A series of experiments to address this were conducted in the Directional Wave Basin (DWB) at the O.H. Hinsdale Wave Research Laboratory at Oregon State University in summer 2016. A series of four sheet metal islands (two with a full conical section, two truncated at the water line) were placed at varying distances from the toe of a 1/10 sloping beach. Incident wave conditions consisting of solitary waves and full-stroke "dam break" waves were run over the islands. Free surface elevations, velocities, and beach runup were measured, with the intent of determining relationships between the wave condition, the island geometry and distance from the beach, and the tsunami characteristics. A series of runup measurements from a particular set of experiments can be seen in Figure 1. Based on these preliminary analyses, it was determined that: A) inundation was always amplified behind the island relative to areas outside this shadow zone; and B) inundation was generally highest with the island closest to the beach, except in the case where the tsunami wave broke prior to reaching the island. In this latter scenario, the inundation behind the island increased with island distance from the beach. The development of relationships between the inundation levels, island geometry and distance, and wave conditions will be described. References: Briggs, M.J., et al. (1995), Pure Appl. Geophys., 144, 569-593. Hill, E.M., et al. (2012), J. Geophys. Res. - Sol. Ea., 117, B06402. Stefanakis, T.,et al. (2014), Proc. R. Soc. A, DOI: 10.1098/rspa.2014.0575.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMNH12A..07G
- Keywords:
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- 3225 Numerical approximations and analysis;
- MATHEMATICAL GEOPHYSICS;
- 4332 Disaster resilience;
- NATURAL HAZARDS;
- 4341 Early warning systems;
- NATURAL HAZARDS;
- 4564 Tsunamis and storm surges;
- OCEANOGRAPHY: PHYSICAL