Education and Outreach
The proposed research will develop the methodology and tools for implementation of site specific Performance Based Tsunami Engineering (PBTE) for use in the analysis, evaluation, design and retrofit of coastal structures and facilities. We have patterned PBTE after Performance Based Earthquake Engineering (PBEE), which has been under development in the last several years. FEMA recently initiated project ATC-58 to develop PBEE for building code implementation, which will serve as a model for development of PBTE. Site-specific PBTE has 3 primary components: 1) probabilistic tsunami hazard analysis (PTHA); 2) site-specific performance simulation; and 3) societal impact assessment. Component 2 is the least developed aspect of tsunami research, and is therefore the major focus of the proposed study. The interdisciplinary collaboration required in PBTE is indicated in the figure below.
The majority of tsunami research to date has focused on two major areas. The first is Tsunami Modeling which involves investigation of the tsunami source mechanism, either earthquake or landslide, and modeling of the open ocean propagation of the resulting tsunami. This research has led to the development of Probabilistic Tsunami Hazard Analysis which has been applied to numerous coastal communities, particularly around the Pacific Ocean. The one area of research in tsunami modeling that we feel needs more attention is the coastal inundation that results when a tsunami makes landfall. The effects of coastal bathymetry, fringing reefs, coastal profile, topography and the presence of obstructions in the form of vegetation and coastal structures have not yet been addressed adequately. At the other extreme, in the area of social sciences and public policy considerable effort has been applied to development of warning systems and evacuation plans, especially subsequent to the Indian Ocean tsunami of December 2004. There are also renewed efforts to estimate consequences of tsunamis in the form of life and economic losses. These research efforts could be referred to as a Societal Impact Assessment. One area of public policy that requires further research is the development of Performance Levels that society requires of various coastal infrastructure. These performance levels will differ for light-framed residential construction compared with engineered multistory buildings or port facilities, and may also differ from community to community, but they need to be established to guide the design of future coastal infrastructure.
Between these two research fields, there is a major gap of information. The focus of this project is to provide the link between coastal inundation and structural performance. This will involve an understanding of fluid-structure interaction, scour and sediment transport, loading on structural elements, and the design of those elements and the overall structure so that the response satisfies the required performance level. To perform this work, we have assembled a team of ocean, hydraulic and structural engineers, and our objective is to develop Performance Based Tsunami Engineering, or PBTE.
PBTE begins with a probabilistic tsunami hazard assessment (PTHA). A PTHA study requires an analysis of potential source mechanisms and the probability of tsunami generation for each of these events. Major submarine earthquakes are the primary source of tsunami due to vertical movement of the sea floor, or initiation of sub-sea landslides. Whether or not a tsunami is generated depends on numerous factors, and if a tsunami is generated, its effect may be limited only to local coastal regions, or it may result in an ocean-wide tsunami threat. Open ocean tsunami propagation models are then used to predict the anticipated wave height and frequency at the specific site of interest. Modeling of the propagation of tsunami in open ocean conditions and the resulting coastal run-up has been a primary focus of the tsunami research community for many years, and numerous computer simulations are currently available (e.g., Shuto et al 1986; Liu et al., 1995; and Titov and Synolakis, 1998). The result of a PTHA is a series of return periods or probabilities for various tsunami wave heights impacting the specific shoreline. A number of studies have been performed to develop PTHAs for various sites around the world (Natural Disaster Research, 2001, Downes & Stirling, 2001). PTHA has been applied successfully to a number of tsunami prone locations in the Pacific Rim, most notably a recent study to develop anticipated tsunami height and return periods for Hilo Bay and other locations in Hawaii (Gica & Teng, 2005). PTHA is not a focus of the proposed NEESR study, but it does provide the input information for the coastal inundation modeling.
Given the results of a PTHA, a societal impact assessment of the site and facility being evaluated or constructed will provide a measure of the consequences of a particular size tsunami in terms of life and economic losses. A large amount of past research on tsunamis has considered public policy issues, such as development of warning systems, evacuation procedures, and public education and awareness. These have been addressed to varying degrees by communities in tsunami hazard areas. Considerable work is still required in this area, but successful models are already in place in Japan and Hawaii for others to emulate. The recent Indian Ocean tsunami has resulted in renewed attention to development of early warning systems, and public education and awareness programs in tsunami prone areas.
As a result of the societal impact assessment, performance levels will be established based on the community and owner requirements for functionality during and after a tsunami. Given the coastal inundation and performance levels from components 1 and 3, the engineering community is required to provide the necessary design for new or retrofit construction that meets the specific performance levels. Unfortunately, relatively little research has been performed in the engineering aspects of tsunami effects on the built environment. A recent pilot study by co-PI Ian Robertson and Harry Yeh of Oregon State University highlighted the lack of research into tsunami loading effects on coastal infrastructure (Yeh & Robertson, 2005).
The focus of the proposed research will be the engineering aspects of PBTE highlighted in the center of the above figure. This component involves the simulation of fluid-structure interaction, sediment transport and scour modeling, development of structural loading time histories, and application of these to the structure in a non-linear structural analysis to determine the expected performance of the constructed facility.
The deliverables from this project will consist of simulation tools and procedures, and design guidelines for PBTE. The simulation tools will include models for tsunami coastal run-up, 3-D RANS models for tsunami flow around structural elements, tsunami scour models, and non-linear analysis models for structural response to tsunami loading. Design guidelines will be developed as code adoptable provisions for PBTE, and as experimentally validated hydrodynamic loading expressions for tsunamis.