Spatial Statistics of Tsunami Overland Flow Properties

Numerical models are a key component of methodologies used to estimate tsunami risk, and model predictions are essential for the development of tsunami hazard assessments (THAs). By better understanding model bias and variability and, if possible, minimizing them, more reliable THAs will result. In this study, the authors compare run-up height, inundation lines, and flow-velocity field measurements between an open-source tsunami model and the method of splitting tsunami (MOST) model predictions in the Sendai Plain in Japan. Run-up elevation and average inundation distance are, in general, overpredicted by the models. However, both models agree relatively well with each other when predicting maximum sea surface elevations and maximum flow velocities. Furthermore, to explore the variability in numerical models, MOST is used to compare predictions from six different grid resolutions (90, 60, 30, 20, 15, and 10 m). Results of this work show that predictions of statistically stable products (run-up, inundation lines, and flow velocities) do not require the use of high-resolution (<30-m) digital elevation maps at this particular location. When predicting run-up heights, inundation lines, and flow velocities, numerical convergence was achieved by using the 30-m resolution grid. In addition, the Froude number variation in overland flow and aMOST sensitivity analysis are presented. Also, run-up height measurements and elevations from the digital elevation map were used to estimate model bias. The results provided in this paper will help provide an understanding of the bias and variability in model predictions and locate possible sources of errors within a model.DOI: 10.1061/(ASCE)WW.1943-5460.0000363.© 2016 American Society of Civil Engineers. Author keywords: Tsunami; Hazard; Numerical modeling; Run-up; Flow velocity. Introduction and Background On March 11, 2011, a MW 9.0 earthquake generated a tsunami 130 km off the Japanese coast near Sendai (Mori et al. 2011). This event was one of the worst in Japanese history, killing more than 15,000 people and causing more than $200 billion in damage. Available data show that in some areas, run-up elevations reached 40 m and flow velocities reached more than 14 m/s (Mori et al. 2011; Koshimura and Hayashi 2012). This event raised safety concerns for many coastal communities. Along the Sendai Plain, the tsunami traveled more than 5 km inland with a maximum measured run-up of approximately 9.4 m and at an average of 2.5 m above mean sea level (Mori et al. 2011). The tsunami velocities measured by Koshimura and Hayashi (2012) at different locations on the Sendai Plain ranged from 2 to 8 m/s. Because of the measurements collected during and after the Tohoku event, researchers have a great opportunity to model, study, and understand the nearshore and onshore hydrodynamics of tsunamis. Numerical models are a key component in the development of tsunami hazard assessments (THAs), regardless of whether these assessments are probabilistic or deterministic in nature. When field measurements andmodel predictions are available, there is an opportunity to better understand model bias and sensitivities, resulting in more accurate and reliable THAs, which will lead to improvement in risk assessment and hazard mitigation in coastal areas susceptible to tsunamis. In past decades, numerical models that can accurately predict tsunami run-up, inundation, and flow velocity were developed. Because of the surge in state-of-the-art numerical models and their widespread use in this field, there is a need to better understand model predictions and variability for better evacuation and construction planning. In this study, the method of splitting tsunami (MOST) (Titov and Synolakis 1995, 1998) and GeoClaw (LeVeque et al. 2011; Berger et al. 2011) tsunami models are used to compare run-up and flow-velocity results to measured field data. Available field-survey data and video-footage analysis measurements are used to compare model run-up and flow-velocity predictions. Possible sources of error are analyzed and discussed. This study includes detailed comparisons between observations and numerical simulations in Sendai and focuses on the Sendai Plain area. Field Measurements andObservations The field-survey data published by Mori et al. (2011) and the flowvelocity measurements from Koshimura and Hayashi (2012) are used to compare the accuracy and reliability of numerical model predictions. More than 5,300 measurements were recorded by a large group of scientists and researchers. A total of 63 universities and 297 people were involved in this project, which covered 2,000 km of the Japanese coast. In Sendai, the maximum measured run-up elevation was 9.4 m (2011 Tohoku Earthquake Tsunami Joint Survey Group 2011). Only 10% of the run-up measurements were greater than 5 m. For this study, the authors focus on the Sendai Plain (particularly from 38.10°N to 38.28°N). The wave Ph.D. Student, Research Assistant, Tsunami Research Center, Univ. of Southern California, Los Angeles, CA 90089 (corresponding author). E-mail: [email protected] Professor, Tsunami Research Center, Univ. of Southern California, Los Angeles, CA 90089. E-mail: [email protected] Principal Seismologist, AECOM, 915 Wilshire Blvd., Los Angeles, CA 90017. E-mail: [email protected] Tsunami Scientist, AECOM, 915 Wilshire Blvd., Los Angeles, CA 90017. E-mail: [email protected] Note. This manuscript was submitted on November 24, 2015; approved on June 8, 2016; published online on August 17, 2016. Discussion period open until January 17, 2017; separate discussions must be submitted for individual papers. This paper is part of the Journal of Waterway, Port, Coastal, and Ocean Engineering, © ASCE, ISSN 0733950X. © ASCE 04016017-1 J. Waterway, Port, Coastal, Ocean Eng. J. Waterway, Port, Coastal, Ocean Eng., 2017, 143(2): -1--1 D ow nl oa de d fr om a sc el ib ra ry .o rg b y So ut he rn C al if or ni a U ni ve rs ity o n 05 /1 9/ 17 . C op yr ig ht A SC E . F or p er so na l u se o nl y; a ll ri gh ts r es er ve d.