Estimating the Probability of Compound Floods in Estuarine Regions

Abstract

Abstract. The quantification of flood risk in estuarine regions relies on accurate estimation of flood probability, which is often challenging due to the rareness of flood events and their multi-causal (or compound) nature. Failure to consider the compounding nature of estuarine floods can lead to significant underestimation of flood risk in these regions. This study provides a comparative review of alternative approaches for estuarine flood estimation; namely, traditional univariate flood frequency analysis applied to both observed historical data and simulated data, and multivariate frequency analysis applied to flood events. Three specific implementations of the above approaches are evaluated on a case study – the estuarine portion of Swan River in Western Australia, highlighting the advantages and disadvantages of each approach. The theoretical understanding of the three approaches, combined with findings from the case study, enable generation of guidance on method selection for estuarine flood probability estimation, recognising issues such as data availability, complexity of the application/analysis process, location of interest within the estuarine region, computational demands and whether or not future conditions need to be assessed.

Guidelines for Studying Diverse Types of Compound Weather and Climate Events

“…(Note that all parameters in Equation 1 are significantly different from zero and that, based on the Akaike information criterion (AIC), the two models considering an individual predictor, that is, Q only or (Tastr + S) only, have a lower quality than the model in Equation 1.) More upstream in the estuary the sea's influence diminishes (Bilskie & Hagen, 2018; Gori et al., 2020), hence we expect more pronounced compound effects, that is, discharge and sea levels to have a similar impact on the water levels (orange isolines in Figure 4b) (Eliot, 2012; Helaire et al., 2020; Wu et al., 2021).…”

“…This can affect model accuracy under climate change, when information needs to be extrapolated far beyond the observed range. High‐resolution hydrodynamic models , which include the non‐linear interaction between hydraulic processes, topography, and human interventions (Kumbier et al., 2018; Mohanty et al., 2020; Wu et al., 2021), can provide a valid alternative for a thorough CF analysis. Hydrodynamic modeling can provide a detailed spatial mapping of water levels in the estuary, for example, it can be used to quantify the relative contribution of flood drivers (Mohanty et al., 2020) and the spatial variability of socio‐economic impacts of flooding depending on the level of urbanization within the estuary, which can influence runoff characteristics (Olbert et al., 2017; Sebastian et al., 2019).…”

“…Probabilistic approaches can be applied to assess future flood return levels using simulated multi‐year water‐level time‐series via high‐resolution hydrodynamic modeling forced by an ensemble of climate models. However, running these simulations is computationally expensive, though approaches to circumvent this have been suggested (Parker et al., 2019; Sopelana et al., 2018; Wu et al., 2021). Nevertheless, applying a probabilistic approach for local future risk assessment remains challenging since no probability is attached to anthropogenic forcing scenarios.…”

Modelling the compound flood hydrodynamics under mesh convergence and future storm surge events in Brisbane River Estuary, Australia

“…A storm surge is the rise of water level above the normal sea level along a coast due to reduced atmospheric pressure and/or strong coastal winds (Karim & Mimura, 2008). The storm surge influences may further increase when they coincide with riverine flooding (Zheng, Westra, Leonard, & Sisson, 2014) and the resulting combination is known as compound flood events (Leonard et al, 2014;Wu, Westra, & Leonard, 2020). Initially, the two involved flooding drivers involved were managed individually in coastal flood management (Torres et al, 2015).…”

“…Liu & Lim, 2017;Yu, 2017). However, the combined effect of riverine and tidal flooding was not considered, with the future effect of storm surge (Wu et al, 2020). For flood assessment and inundation mapping both temporal and spatial (flood depth and inundation extent) knowledge is required and can be applied in the flood risk analysis (X.…”