Compound Flood Modeling of Tropical Cyclone Events in Coastal Bays and Estuaries

Abstract

**Please note that the full text is embargoed until 02/01/2025** Flooding due to tropical cyclones can cause severe impacts on coastal communities, particularly when multiple types of flooding (e.g., coastal and riverine) interact to create a compound flood event. In typical flood hazard assessments, these drivers of flooding are considered separately, even though they may interact and produce more severe flooding. Understanding the interactions between flood drivers is necessary to accurately plan for and respond to future flood events. This dissertation aims to improve the assessment of coupled coastal, fluvial, and pluvial flooding in coastal bays and estuaries impacted by tropical cyclones. To do this, a hydrodynamic model is developed to simulate compound flooding in the Sabine-Neches Estuary in Southeast Texas. In Chapter 2, the model is applied to survey the flooding of three historical hurricanes: Harvey, Ike, and Rita. The results show that interactions between coastal, riverine, and rainfall processes are all possible in the study area, although the relative dominance of each process can vary substantially depending on patterns of rainfall and wind. Storms accompanied by extended periods of rainfall cause the largest extent and longest duration of compound flooding. For these storms, the maximum flood extents and depths can only be simulated accurately using a coupled modeling approach, which provides more than a 70% reduction in error compared to models that simulate coastal, riverine, and rainfall processes separately. This research can inform local planning efforts aimed at reducing vulnerability to future flood events.

Climate change is projected to increase flood hazards due to sea level rise and more intense rainfall. In Chapter 3, the calibrated model is applied to simulate flooding in the Sabine-Neches Estuary during Hurricane Harvey. Future scenarios of sea level rise and rainfall are then applied to estimate how compound flooding will change in the future under 2050 and 2100 climate conditions. The combined effects of future sea level rise and rainfall cause the compound flood zone to expand and shift further inland to areas with higher population density and more development. As a result, the number of residential structures in the compound flood zone increases from 0.6% of all impacted structures in 2017 to 14% in 2100. New strategies for flood mitigation that address multiple types of flooding (e.g., coastal storm surge and high river discharge) may be necessary in these areas to reduce impacts to people and property.

Although damages in coastal areas are anticipated to worsen due to rising sea levels and subsidence, the precise impact of historical sea-level rise and subsidence on compound flooding during tropical cyclones remains uncertain. To address this uncertainty, Chapter 4 of this dissertation investigates the influence of sea-level rise and subsidence on compound flooding during historical hurricanes and assesses how well linear superposition of individual flood drivers can predict compound flooding. The findings indicate that reductions in sea-level rise can shift the location where the riverine-rainfall influence exceeds the storm surge contribution to peak water levels. While sea-level rise typically does not change the nearshore water depth, it does impact flood depths upstream. Land subsidence in the region does not significantly lower water levels but does contribute to deeper inundation in waterways and across the land surface. Predictions of flooding using linear superposition can be inaccurate, sometimes underestimating or overestimating the depth depending on the timing and location.