Debris Hazard Assessment in Extreme Flooding Events

Title: Debris Hazard Assessment in Extreme Flooding Events
Authors: Stolle, Jacob
Date: 2019-09-13
Abstract: Coastal areas are often important to economic, social, and environmental processes throughout the world. With changing climate and growing populations in these areas, coastal communities have become increasingly vulnerable to extreme flooding events, such as tsunami, storm surges, and flash floods. Within this new paradigm, there has been an effort to improve upon current methods of hazard assessment, particularly for tsunami. Recently, the American Society of Civil Engineers (ASCE) released the ASCE 7 Chapter 6 which was the world’s first standard, written in mandatory language, that addressed tsunami resilient design in a probabilistic manner for several of its prescriptions. While often the focus tends to be on mapping the hazards related to hydraulic loading conditions, post-tsunami field surveys from disaster-stricken coastal communities have also shown the importance of also considering the loads exerted by solid objects entrained within the inundating flows, commonly referred to as debris loading. Limited research has addressed debris hazard assessment in a comprehensive manner. Debris loading can be generally divided into two categories: impact and damming. Debris impact loads are caused by the rapid strike of solid objects against a structure. Debris damming loads are the result of the accumulation of debris at the face of or around a structure, causing thus an obstruction to the flow. The primary difference between these loads is the time period over which they act. The rapid loading due to debris impacts requires structural properties be considered in assessing the associated loads whereas debris damming loads are generally considered in a quasi-static manner. In assessing the hazard associated with both impact and damming loading conditions, methodologies must be developed to consider the likelihood of the load occurring and the magnitude of that load. The primary objective of this thesis was to develop a probabilistic framework for assessing debris hazards in extreme coastal flooding events. To achieve this objective, the components of the framework were split into three general categories: debris transport, debris damming, and debris impact. Several physical experimental studies were performed to address each of these components, representing the most comprehensive assessment of debris hazards in extreme flooding events to date. Debris transport was addressed to estimate the likelihood of debris loading occurring on a structure. The studies presented herein examine the different parameters that must be considered in assessing the motion of debris with the flow. The studies showed that the initial configuration of the debris and hydrodynamic conditions were critical in determining the motion of the debris. The stochastic properties of the debris motion were also assessed. It was shown that the lateral displacement of the debris could be approximated by a Gaussian distribution and the debris velocity by a Kumaraswamy (1980) distribution. The study of debris impact was further used to develop the current models used in estimating the impact force. The rigid body impact model was compared to models where the structural response was considered. The analysis showed that the effective stiffness model proposed by Haehnel and Daly (2004) was best suited to provide a conservative estimation of the impact force. Additionally, the impact geometry was taken into consideration examining the influence of various parameters on the impact force. Furthermore, debris damming was examined for the first time in transient loading conditions. This particular study examined the influence of the transient wave condition on the debris dam formation as well as the influence of different debris geometries. The influence of the debris dam geometry was correlated to increases in loading and overtopping conditions at structures. The assessment of debris hazards is critical in the development of accurate design conditions. The probabilistic framework presented within this thesis is expected to provide a basis for estimating debris hazards and inform future studies in the development of hazard assessment models.
CollectionThèses, 2011 - // Theses, 2011 -