The objective of this grant for rapid response research (RAPID) is to assess the type of debris generated by tsunami events and quantify the resulting impact damage to structures. To achieve this objective, a field investigation of the northeast coast of Honshu, Japan, inundated by the 11 March 2011 Tohoku tsunami, will be conducted by a team of researchers from Lehigh University, Oregon State University, University of Hawaii, and Japan's Nagoya University. The research team will acquire field data on tsunami generated debris, document cases of impact on structures in Japan, and use this field data to validate results of an ongoing experimental and analytical NSF-supported research project on tsunami generated debris impact. The research will assess the type and size of debris demands typical for coastal communities, assess structural damage patterns generated as a result of impact events, and validate models developed as part of the ongoing NSF-supported study. This research team will also coordinate with the UNESCO-led International Tsunami Survey Team.
The knowledge gained from the proposed research will be used for enhancement of U.S. infrastructure code recommendations and design practice for coastal communities where the threat of a tsunami event exists. Identification of debris type is critical for determination of potential design loads. Video footage and post-event images indicate that the Tohoku event generated a spectrum of debris ranging from wood, vehicles, and shipping containers, to entire houses and ships. Assessment of what size and type of debris is likely in a given region is critical for the development of design recommendations. In addition, the occurrence of impact events may not be associated with all types of debris, and some structural systems may be more sensitive to a type of impact demand. These issues will be quantified with the field studies conducted in Japan.
On March 11, 2011 a magnitude 9.0 earthquake occurred off the northeast coast of Honshu, Japan. The Tohoku Earthquake, as it is now known, was focused in the subduction zone plate boundary resulting in the formation of a tsunami event radiating across the Pacific. The tsunami inundation across the northeast coast of Japan was severe, in one case resulting in a run-up height of 23.6 m above sea level. The tsunami event resulted in widespread destruction to coastal ports and communities. Estimates by the National Police Agency put the number of dead and missing at over 27,000 with approximately 92% of those resulting from drowning. The tragedy reinforces the need to improve our understanding of the effects of tsunami inundation on modern coastal communities. In particular, field observations of structural systems that remained standing, as well as those that did not, provide an opportunity to enhance design recommendations for tsunami susceptible regions. The research program revolved around a field investigation of the tsunami inundated zones in Japan. A reconnaissance of Sendai, Ofunato, Minami-sanriku, Kesennuma, Rikuzentakata, Onagawa, and Ishinomaki were conducted in June 2011 by Professors Naito and Cox, and Dr. Yu of Degenkolb Engineers. The field investigations were supported by Japanese researchers Prof. Norimi Mizutani, Nobuhito Mori, Tomohiro Yasuda, and Mr. Daiki Tsujio. During the reconnaissance visual records of damage cases, debris types and impact scenarios were recorded. Further processing of this information has resulted in a number of recommendations for improving design against tsunami effects. An assessment of the potential of debris strikes on structures during tsunamis is not typically carried out. However, the field investigations have shown that structural damage from such events does occur, and at least some structures should be designed to resist the strikes. Based on the observations a framework was established that can be used to classify potential debris types, quantify the debris potential for a given site, and then translate the debris and debris potential to impact loads on structures. Debris in coastal regions can be categorized into small, moderate, and large debris based on four quantitative characteristics: mass, stiffness, size, and buoyancy. Categorization is framed within the context of the magnitude of forces that could be generated by impact with a structure. The findings follow intuition in that the inundation depth at the point of debris origin, the surrounding building height, building construction, and topography control dispersion of debris. For debris sources surrounded by steel or concrete industrial buildings and a low relative inundation depth the dispersal will be minimized, while for high inundation depths, planar topography, and frangible construction the dispersal may be high. A method to calculate delivery potential given a debris source and inundation zone has been developed based on this work and is being integrated into the next version of ASCE 7 Minimum Design Loads for Buildings and Other Structures. The method provides a reasonable approach to identify regions with high likelihood of debris impact. A summary of this facet of the research is available in Naito et al. [2013]. Another key impact of the research effort was a study on the impact to above ground fuel storage facilities in the inundated region. As a result of this tsunami event fuel storage containers located on the coast were subject to crushing, tie down failures and lateral movement due to the hydrodynamic demands. As a result containers were moved from their containment yards to regions over 1 km away. In some cases this resulted in fuel contamination to the region. Observations of the modes of failure in Onagawa and Ishinomaki were made and detailed in Naito et al. [2012]. The case study indicates that prevention of tie down failure of containers is difficult if the container happens to have a low fuel level during the event. To minimize the damage a new cable tie system is proposed. References Naito, C., Cercone, C., Riggs, R., Cox, D., "A Procedure for Site Assessment of the Potential for Tsunami Debris Impact," ASCE Journal of Waterway, Port, Coastal, and Ocean Engineering, ASCE, Manuscript WWENG-400R2, Accepted: June 2013. Naito, C., Cox, D., Yu, Q., Brooker, H., "Fuel Storage Container Performance During the 2011 Tohoku Japan Tsunami," ASCE Journal of Performance of Constructed Facilities, Accepted February 2012. DOI: 10.1061/(ASCE)CF.1943-5509.0000339
