A Numerical Model for Tsunami-Induced Morphology Change
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Pure and Applied Geophysics
A Numerical Model for Tsunami-Induced Morphology Change BABAK TEHRANIRAD,1,2
JAMES T. KIRBY,1 and FENGYAN SHI1
Abstract—We describe the development of a time-resolved model for tsunami-induced morphology change, based on an existing Boussinesq model for weakly-dispersive free-surface waves coupled to a depth-integrated model for sediment concentration and an equation for bed level change. The model allows for spatial variability in bottom friction coefficients and accounts for the presence of non-erodible beds or finite depths of available sediment. The model is verified using one laboratory data set and against field observations of morphology change in the Crescent City, CA harbor during the 2011 Tohoku tsunami event. The model is then applied to a hypothetical example illustrating the impact of tsunami inundation on an undeveloped barrier island. Keywords: Tsunami, sediment transport modeling, morphology change.
1. Introduction Tsunamis can cause significant changes to coastal morphology. Recent events have provided numerous examples of erosion of barrier islands, sand dunes and coastal protection systems (Choowong et al. 2008; Fritz 2011; Gelfenbaum and Jaffe 2003; Goto et al. 2011, 2012a, b; Gusman et al. 2018; Haraguchi et al. 2012; Naruse and Abe 2017; Ramalho et al. 2018; Sugawara et al. 2014a, b; Szczucin´ski et al. 2012; Takashimizu et al. 2012; Tanaka et al. 2012; Tanaka and Sato 2015; Udo et al. 2012, 2016; Wilson et al. 2012). Post-tsunami measurements suggest that during tsunami inundation significant amounts of sand are eroded from sandy coasts and deposited further onshore (Goto et al. 2011, 2012a; Udo et al. 2016). In many cases, dune protection systems were
1 Department of Civil and Environmental Engineering, Center for Applied Coastal Research, University of Delaware, Newark, DE 19716, USA. E-mail: [email protected] 2 Present Address: USGS Pacific Coastal and Marine Science Center, 2885 Mission St, Santa Cruz, CA 95060, USA.
completely eroded, where eroded sediment deposited onshore behind the dunes (Haraguchi et al. 2012). After the 2004 Indian Ocean Tsunami on the western coast of Thailand, field surveys showed that coastal dunes close to the shoreline were the source for sand deposits extending 1 km onshore, covering the lowlying coastal areas (Choowong et al. 2008; Hori et al. 2007). The 2011 Tohoku-oki tsunami, which devastated the most tsunami-prepared coastline in the world, further revealed the potential for tsunamis to change coastal morphology in sandy regions (Goto et al. 2012a, b; Takashimizu et al. 2012; Udo et al. 2012, 2016). Significant morphological changes were observed on coasts with intermittent coastal protection, where severe damage and erosion in less protected areas were observed (Goto et al. 2012a; Tanaka et al. 2012). In addition to sandy coasts, tsunami-induced morphological impacts has been significant in other coastal areas, especially in regions neighboring coastal structures. Considerable erosion and deposition has been observed inside
