Abstract
Compound flooding is a physical phenomenon that has become more destructive in recent years. Moreover, compound flooding is a broad term that envelops many different physical processes that can range from preconditioned, to multivariate, to temporally compounding, or spatially compounding. This research aims to analyze a specific case of compound flooding related to tropical cyclones where the compounding effect is on coastal flooding due to a combination of storm surge and river discharge. In recent years, such compound flood events have increased in frequency and magnitude, due to a number of factors such as sea-level rise from warming oceans. Therefore, the ability to model such events is of increasing urgency. At present, there is no holistic, integrated modeling system capable of simulating or forecasting compound flooding on a large regional or global scale, leading to the need to couple various existing models. More specifically, two more challenges in such a modeling effort are determining the primary model and accounting for the effect of adjacent watersheds that discharge to the same receiving water body in amplifying the impact of compound flooding from riverine discharge with storm surge when the scale of the model includes an entire coastal line. In this study, we investigated the possibility of using the Advanced Circulation (ADCIRC) model as the primary model to simulate the compounding effects of fluvial flooding and storm surge via loose one-way coupling with gage data through internal time-dependent flux boundary conditions. The performance of the ADCIRC model was compared with the Hydrologic Engineering Center- River Analysis System (HEC-RAS) model both at the watershed and global scales. Furthermore, the importance of including riverine discharges and the interactions among adjacent watersheds were quantified. Results showed that the ADCIRC model could reliably be used to model compound flooding on both a watershed scale and a regional scale. Moreover, accounting for the interaction of river discharge from multiple watersheds is critical in accurately predicting flood patterns when high amounts of riverine flow occur in conjunction with storm surge. Particularly, with storms such as Hurricane Harvey (2017), where river flows were near record levels, inundation patterns and water surface elevations were highly dependent on the incorporation of the discharge input from multiple watersheds. Such an effect caused extra and longer inundations in some areas during Hurricane Harvey. Comparisons with real gauge data show that adding internal flow boundary conditions into ADCIRC to account for river discharge from multiple watersheds significantly improves accuracy in predictions of water surface elevations during coastal flooding events.
Highlights
The observed increasing trend in the destructiveness of coastal storms over the past decades (Emanuel, 1987, 2005; Demaria and Kaplan, 1994; Jongman et al, 2012; Hinkel et al, 2014) has been attributed to the changing climate (Levitus et al, 2000); a trend that does not appear to be slowing down
The main objective of this research is to evaluate the reliability of Advanced Circulation (ADCIRC) as the primary model in simulating compound flooding and comparing its performance with the Hydrologic Engineering Center River Analysis System (HEC-RAS) model
It is important here to see that the Generalized Wave Continuity Equations (GWCE) from ADCIRC was first designed for modeling the open ocean and wind-driven storm surge and so ADCIRC contains many more forcing terms than the HEC-RAS equations
Summary
The observed increasing trend in the destructiveness of coastal storms over the past decades (Emanuel, 1987, 2005; Demaria and Kaplan, 1994; Jongman et al, 2012; Hinkel et al, 2014) has been attributed to the changing climate (Levitus et al, 2000); a trend that does not appear to be slowing down. Part of the increase in damage of more recent storms can be attributed to the higher levels of flooding due to combined effects from riverine flows and storm surge (Bakhtyar et al, 2020a) While oceanic processes such as tides and storm surge impact flooding in low lying coastal areas, meteorological and hydrological mechanisms influence flooding due to rainfall. When these processes interact, they can cause much higher flooding levels than if they were to occur separately. Applying this definition to coastal flooding, a compound flooding event is flooding due to the interaction of the open ocean, meteorological behavior, and hydrological factors (Zscheischler et al, 2018)
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