Flood-Entrained Debris Impacts on Structures: A Structural-Hydraulic Numerical Analysis

Abstract

Tsunamis are extreme hydrodynamic events that have devastating economic, social, and environmental impacts on coastal communities. Recent devastation caused by extreme hydrodynamic events, such as the 2010 Tsunami in Chile, 2011 Tohoku Tsunami in Japan, and the 2018 Indonesian Tsunami, have increased awareness about the effectiveness of design codes used to estimate loads generated during such extreme events. The American Society of Civil Engineers (ASCE) released Chapter 6 of ASCE 7-16, which was the world's first standard that addressed tsunami resilient design prescriptions in mandatory language. Chapter 6 of ASCE 7-16 classifies tsunami loads into three categories: (1) Hydrostatic loads; (2) Hydrodynamic loads; and (3) Debris impact loads. Previous literature has mainly focused on classifying the hydraulic loading conditions and up until recently there has been limited research that addressed debris impact loads generated from objects entrained within the tsunami flow. Hence, the focus of this thesis will be on debris impact loads generated from extreme hydrodynamic events. There are two main objectives of this thesis: (1) To investigate how Chapter 6 of ASCE 7-16 defines debris impact loads and how the assumptions made about debris impact loads in ASCE 7-16 influence structural response parameters; and (2) investigate the validity and practicality of numerically simulating dam-break wave generated debris impact events. The first objective of this thesis is achieved by investigating how Chapter 6 of ASCE 7-16 defines debris impact loads and comparing these loads to alternative methods for estimating debris impact loads. Furthermore, these debris impact loads are applied to a numerical model of a physical structure and the response of the model is compared to response of the physical structure. It was shown that when debris impact loads are generated by following the guidelines outlined in Chapter 6 of ASCE 7-16 the resulting force response of the structural model overestimated both the magnitude and frequency of the experimental force response. Additionally, how the debris impact force history shape influences the response of the structural model was also investigated to understand the significance of the assumptions made about debris impact loads in Chapter 6 of ASCE 7-16, which assumes a rectangular impact shape. It was shown that the most conservative force response was generated when the debris impact force history shape was assumed to be a rectangular pulse. Finally, a study of how the structural response changed as the stiffness of the numerical model increased was also completed. From this study, it was shown that within the elastic range of a column, the forces developed in the column increased and the deformations decreased as the structural stiffness increased. The second objective of this thesis is achieved by numerically simulating a dam-break wave generated debris impact event and comparing the numerical model to an experimental program of dam-break wave generated debris impact events. The numerical model was shown to be capable of estimating the water surface elevations, hydrodynamic forces, debris impact loads, and debris transport when compared to experimental data. Furthermore, it was also shown that the numerical model is capable of simulating multi-debris transport and impact events, while not requiring an unreasonably large amount of computational time and cores.