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We recently developed a dynamically coupled hydrological‐ocean modeling system that provides seamless coverage across the land‐ocean continuum during hurricane‐induced compound flooding. This study introduced a local inertial equation and a diagonal flow algorithm to the overland routing of the coupled system's hydrology model (WRF‐Hydro). Using Hurricane Florence (2018) as a test case, the performance of the coupled model was significantly improved, evidenced by its enhanced capability of capturing backwater and increased water level simulation accuracy and stability. With four model experiments, we present a framework to detangle, define, and quantify compound and nonlinear effects. The results revealed that the flood peaks in the lower Cape Fear River Basin and the coastal waters were contributed by inland flooding and storm surge, respectively. These two processes had comparable contributions to the flooding in the Cape Fear River Estuary. The compound effect was identified when the flood levels resulting from the combination of land and ocean processes surpassed those caused by an individual process alone. The compound effect during Hurricane Florence exhibited limited impact on flood peaks, primarily due to the time lag between the peaks of the storm surge and the inland flooding. In the period between the two peaks, the compound effect was salient and significantly impacted the magnitude and variation of the flood level. The nonlinear effect, defined as the difference between the compound flood level and the superposition of storm surge and inland flooding water levels, reduced flood levels in the river channels while increasing flood levels on the floodplain. Plain Language Summary: This study addresses the phenomenon of hurricane‐induced compound flooding, which arises when inland waters and storm surges coincide at the land‐ocean boundary. We've devised a hydrological‐ocean model that effectively covers such events. This model, enhanced with new algorithms, was tested using Hurricane Florence (2018) data, showing marked improvements in predicting water levels and tide effects. Our research delineates and quantifies the complex interplay between different flooding sources during such events. Key findings include the determination that both inland flooding and storm surges contributed equally to the flooding in the Cape Fear River Estuary. However, the overlapping impact of these processes, termed the "compound effect," was limited in its influence on peak flood levels, mainly due to the time gap between storm surge and inland flooding peaks. Another crucial discovery was the "nonlinear effect," which accounts for discrepancies in predicted flood levels. This effect tended to decrease flood levels in river channels but increased them on floodplains. Key Points: A local inertial equation and a diagonal flow algorithm were introduced to a newly developed dynamically coupled hydrological‐ocean modelStrong compound effect between hydrological and ocean processes occurred between their peaksThe nonlinear effect reduced the flood peaks in the river channels while amplifying them on the floodplains [ABSTRACT FROM AUTHOR]

Hurricane‐induced compound flooding is a combined result of multiple processes, including overland runoff, precipitation, and storm surge. This study presents a dynamical coupling method applied at the boundary of a processes‐based hydrological model (the hydrological modeling extension package of the Weather Research and Forecasting model) and the two‐dimensional Regional Ocean Modeling System on the platform of the Coupled‐Ocean‐Atmosphere‐Wave‐Sediment Transport Modeling System. The coupled model was adapted to the Cape Fear River Basin and adjacent coastal ocean in North Carolina, United States, which suffered severe losses due to the compound flood induced by Hurricane Florence in 2018. The model's robustness was evaluated via comparison against observed water levels in the watershed, estuary, and along the coast. With a series of sensitivity experiments, the contributions from different processes to the water level variations in the estuary were untangled and quantified. Based on the temporal evolution of wind, water flux, water level, and water‐level gradient, compound flooding in the estuary was categorized into four stages: (I) swelling, (II) local‐wind‐dominated, (III) transition, and (IV) overland‐runoff‐dominated. A nonlinear effect was identified between overland runoff and water level in the estuary, which indicated the estuary could serve as a buffer for surges from the ocean side by reducing the maximum surge height. Water budget analysis indicated that water in the estuary was flushed 10 times by overland runoff within 23 days after Florence's landfall. Plain Language Summary: Compound flooding refers to a phenomenon in which two or more flooding sources occur simultaneously or subsequently within a short period of time. In this study, we present a new numerical model that combines hydrological and ocean models to represent the exchange of water levels at the land‐ocean interaction zone. To test the model's robustness, we use this model to simulate the water level changes in Cape Fear River Basin and adjacent coastal ocean in North Carolina, United States, for Hurricane Florence in 2018. The comparison between observed and simulated water level prove that the new model can better resolve the changes in water elevation during a hurricane event than the traditional method where the ocean model utilized the river model's outputs as its boundary condition. We further quantify the contributions from different processes to the water level variations in the estuary. The compound flooding in the estuary was categorized into four stages: (I) swelling, (II) local‐wind‐dominated, (III) transition and (IV) overland‐runoff‐dominated. The estuary could serve as a buffer for surges from the ocean side by reducing the maximum surge height. The water in the estuary was flushed 10 times by overland runoff within 23 days after Florence's landfall. Key Points: A coupled hydrological‐ocean model was developed using hydrological modeling extension package of the Weather Research and Forecasting model (WRF‐Hydro) and two‐dimensional Regional Ocean Modeling System (ROMS 2D) through the Coupled‐Ocean‐Atmosphere‐Wave‐Sediment Transport modeling systemThe dynamical coupling method was applied to the interface boundary of WRF‐Hydro and ROMS 2D to realize a seamless model couplingHurricane Florence‐induced compound flooding event was investigated by analyzing the modeled water level evolution, water budget, and nonlinear effects in the Cape Fear Estuary [ABSTRACT FROM AUTHOR]

Sea level rise and intense hurricane events make the East and Gulf Coasts of the United States increasingly vulnerable to flooding, which necessitates the development of computational models for accurate water level simulation in these areas to safeguard the coastal wellbeing. With this regard, a model framework for water level simulation over coastal transition zone during hurricane events is built in this study. The model takes advantage of the National Water Model's strength in simulating rainfall–runoff process, and D‐Flow Flexible Mesh's ability to support unstructured grid in hydrodynamic processes simulation with storm surges/tides information from the Advanced CIRCulation model. We apply the model on the Delaware Estuary to simulate extreme water level and to investigate the contribution of different physical components to it during Hurricane Isabel (2003). The model shows satisfactory performance with an average Willmott skill of 0.965. Model results suggest that storm surge is the most dominating component of extreme water level with an average contribution of 78.16%, second by the astronomical tide with 19.52%. While the contribution of rivers is mainly restricted to the upper part of the estuary upstream of Schuylkill River, local wind‐induced water level is more pronounced with values larger than 0.2 m over most part of the estuary. [ABSTRACT FROM AUTHOR]