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This experimental project investigated the debris accumulation in front of structures during tsunamis (debris damming), which leads to an increase in the forces imposed by tsunami flow on structures. The study was conducted through the construction of a 1:20 geometric scale physical model. Tsunami-like waves were generated over an idealized slope and transported different shapes of multi-debris, representing shipping containers, over the flat test section to measure debris loadings on elevated column structures. The experiment optically measured the debris impact and damming process, along with the corresponding loads on the entire column structure using a Force Balance Plate and separately on an individual column in the front row using a load cell. This unique data set will help to understand the impact of various factors on debris-driven damming loads, including wave characteristics, specimen configurations, and debris shapes. This data will also help to develop and validate numerical models that predict the motion and dynamics of floating debris during extreme coastal events. This project is the outcome of “Collaborative Research: Experimental Quantification of Tsunami-driven Debris Damming on Structures” with collaborators from the University of Hawaii at Manoa, Louisiana State University, and Oregon State University. 

Tsunami overland flow induces hydrodynamic loads on coastal structures and may transport various objects located within the inundation zone, which could become debris and exacerbate hydrodynamic loading. In the process of adopting “the first national, consensus-based standard for tsunami resilience” (Chock, 2016) in the form of ASCE 7-16 Chapter 6: Tsunami Loads and Effects, emphasis was placed on evaluating debris transport and impact forces. This is evidenced by the robust body of literature regarding physical model experiments of these processes and thorough design procedures for both load considerations in current structural engineering standards (ASCE, 2022). Debris damming forces, resultant of debris being transported and accumulating against structures, are less thoroughly studied, having only recently begun to transition from steady flow experiments to transient flow conditions representative of coastal inundation events. A recent pair of experiments bridges this gap, comparing debris damming via steady-state, subcritical flow conditions to that caused by a dam-break style hydraulic bore (Stolle et al., 2018). That paper aimed to study debris dam formation, stability, and loading as well as runup of the flow onto idealized structural columns. Another study varied debris quantity, orientation, and arrangement to determine the effect had on damming and impact loads (Shekhar et al., 2020), however neither compared findings to current standards. The experimental work presented herein represents initial findings of a multi-year experimental campaign to better understand the mechanisms that lead to debris damming and increased structural loading. This work builds upon previous studies by using larger scale debris elements, more numerous debris fields, and more trials to better model such a stochastic process as debris damming. Three different incident wave conditions also led to varied hydrodynamics at the column specimen. In later phases, this campaign will also investigate the debris damming consequences of heterogeneous debris, which more accurately represent highly variable debris fields observed in post-event site surveys (Nistor et al., 2017). This paper aims to compare experimental debris dam loading parameters to those in the current ASCE 7-22 standard (ASCE, 2022). Namely, evaluating conservatism of ASCE 7-22 design values for: overall drag force on buildings, minimum closure ratios used in load determination, and empirical rectilinear structure drag coefficients during both debris accumulation and quasi-steady debris damming phases.