The Role of Near-Shore Bathymetry During Tsunami Inundation in a Reef Island Setting: A Case Study of Tutuila Island
On September 29, 2009 at 17:48 UTC, an Mw = 8.1 earthquake in the Tonga Trench generated a tsunami that caused heavy damage across Samoa, American Samoa, and Tonga. One of the worst hits was the volcanic island of Tutuila in American Samoa. Tutuila has a typical tropical island bathymetry setting influenced by coral reefs, and so the event provided an opportunity to evaluate the relationship between tsunami dynamics and the bathymetry in that typical island environment. Previous work has come to differing conclusions regarding how coral reefs affect tsunami dynamics through their influence on bathymetry and dissipation. This study presents numerical simulations of this event with a focus on two main issues: first, how roughness variations affect tsunami run-up and whether different values of Manning’s roughness parameter, n, improve the simulated run-up compared to observations; and second, how depth variations in the shelf bathymetry with coral reefs control run-up and inundation on the island coastlines they shield. We find that no single value of n provides a uniformly good match to all observations; and we find substantial bay-to-bay variations in the impact of varying n. The results suggest that there are aspects of tsunami wave dissipation which are not captured by a simplified drag formulation used in shallow-water waves model. The study also suggests that the primary impact of removing the near-shore bathymetry in coral reef environment is to reduce run-up, from which we conclude that, at least in this setting, the impact of the near-shore bathymetry is to increase run-up and inundation.
KeywordsTsunami numerical modeling manning roughness island reef environment American Samoa
This publication makes use of data products provided by National Oceanic and Atmospheric Administration (NOAA), Pacific Marine Environmental Laboratory (Contribution number 4618). This publication is partially funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement NA15OAR4320063, Contribution No. 2018-0131. Hermann Fritz generously provided the post-tsunami run-up survey datasets. We are grateful to Randall Leveque, Joanne Bourgeois, Frank Gonzales, Hongqiang Zhou, Christopher Moore, Marie Eble, Diego Arcas, Lijuan Tang, Jose Borrero and the anonymous reviewer for their endless advice and help.
- Borrero, J.C., Greer, S.D., Lebreton, L., & Fritz, H.M. (2011). Field surveys and numerical modelling of the 2009 South Pacific and 2010 Mentawai Islands tsunamis. In Proceedings of Coasts and Ports Conference, Perth, Australia, September 28–30, 2011.Google Scholar
- Chow, V. T. (1959). Open-channel hydraulics. New York: McGraw-Hill Book Company.Google Scholar
- Jaffe, B. E., Gelfenbaum, G., Buckley, M. L., Steve, W., Apotsos, A., Stevens, A. W., & Richmond, B. M. (2010). The limit of inundation of the September 29, 2009, Tsunami on Tutuila, American Samoa: U.S. Geological Survey Open File Report, 2010–1018.Google Scholar
- Leschka, S., Kongko, W., & Larsen, O. (2009a). On the influence of nearshore bathymetry data quality on tsunami runup modeling, part I: Bathymetry. In Proc. of the 5th International Conference on Asian and Pacific Coasts (APAC 2009) (Vol. 1, pp. 151–156). Singapore: Soon Keat Tan, Zenhua Huang.Google Scholar
- Levin, B. W., & Nosov, M. A. (2016). Propagation of a tsunami in the ocean and its interaction with the coast. In Levin, B. W., & Nosov M. A. (Ed.), Physics of tsunamis (pp. 311–358). Springer. https://doi.org/10.1007/978-3-319-24037-4.
- Lynett, P. J. (2007). Effect of a shallow water obstruction on long wave runup and overland flow velocity. Journal of Waterway, Port, Coastal, and Ocean Engineering, 133(6), 455–462. https://doi.org/10.1061/(ASCE)0733-950X(2007)133:6(455).CrossRefGoogle Scholar
- NTHMP, National Tsunami Hazard Mitigation Program. (2012). Proceedings and Results of the 2011 NTHMP Model Benchmarking Workshop. Boulder: U.S. Department of Commerce/NOAA/NTHMP, (NOAA Special Report), 436.Google Scholar
- Titov, V. V. (1997). Numerical modeling of long wave runup, PhD Thesis, University of Southern California, p. 141.Google Scholar
- Titov, V. V. (2009). Tsunami forecasting. In A. N. Bernard & A. R. Robinson (Eds.), The Sea. Tsunamis, Chap. 12 (Vol. 15, pp. 371–400). Cambridge, MA: Harvard University Press.Google Scholar
- Titov, V., & González, F. I. (1997). Implementation and testing of the Method of Splitting Tsunami (MOST) model. NOAA Tech. Memo. ERL PMEL-112 (PB98-122773), NOAA/Pacific Marine Environmental Laboratory, Seattle, WA, p. 11.Google Scholar
- Titov, V.V., Gonzalez, F. I., Mofjeld, H. O., & Venturato, A. J. (2003b). NOAA TIME Seattle tsunami mapping project: Procedures, data sources, and products. NOAA Tech. Memo. OAR PMEL-124, NTIS: PB2004-101635, p. 21.Google Scholar
- Titov, V. V., Kânoğlu, U., & Synolakis, C. (2016). Development of MOST for real-time tsunami forecasting. Journal of Waterway, Port, Coastal, and Ocean Engineering, 142(6), 03116004. https://doi.org/10.1061/(asce)ww.1943-5460.0000357.CrossRefGoogle Scholar