Boulder Dislodgement During Coastal Storms and Tsunamis: Insights From a New Ensemble Model

Abstract

Introduction General ContextCoastal boulders are important because they document extreme wave actions, such as those that occur during storms and tsunamis. Furthermore, boulders are a unique type of deposit as they can consist of only few "particles." Nott (2003) demonstrated that information about the pre-transport setting of boulders is important. A variety of different approaches have been developed to consider boulder motion, assuming different pre-transport settings and simplifying assumption about the hydrodynamic forcing to quantify the storm or tsunami that moved boulders. We refer to Cox et al. (2020) for more information about Nott's 2003 approach, the most common approach from which others, such as Benner et al. (2010), Nandasena et al. (2011b), Nandasena et al. (2011a, Buckley et al. (2012), andBarbano et al. (2010), are derived. More fundamentally, if information about the causative events is available, and better yet, data that can be used to calibrate hydrodynamic models, boulder transport can be understood at a much more advanced level (see for example, Watanabe et al., 2019, and references therein). However, very often, especially for historic events or events in the geologic record, such information is not available-including the pre-transport setting of the boulder. None of the existing approaches can accommodate the ambiguity related to the causative process and pre-transport setting in a consistent manner. To address this shortcoming, we present a Monte-Carlo-type ensemble model with the capability to sample parameter distributions that describe the uncertainties around the pre-transport setting as well as how the flow interacts with the boulder.Boulders as heavy as 50 or 80 tonnes have been reported as moved by storms (Cox et al., 2012) and tsunamis (Dewey & Ryan, 2017). Just the fact that hydrodynamic forces during storms and tsunamis are capable of transporting such large particles is in itself astonishing and should narrate a cautionary tale independent of the causative process. However, much of the debate, when it comes to boulder deposits during coastal hazard events, is focused on whether the boulders were moved during a tsunami or a storm. Of course, it is important to delineate the causative boulder movement processes for hazard assessment, as while both storms and tsunamis can have devastating effects, they do vary in their coastal inundation, duration, and the character of how they impact the coastline. However, especially without enhanced knowledge and specific information about the hydrodynamic forcing, it remains difficult to defend a conclusion regarding whether a storm or a tsunami caused the motion of a boulder. Advanced numerical simulations, such as presented in Zainali and Weiss (2015), Kennedy et al. (2016) and Watanabe et al. (2019) are capable, but the wealth of information needed to create the input waves has only been recorded by tide gauges, wave buoys, or other measuring devices in the past few decades. Without these existing constraints, advanced simulations produc...