Simulating compound flooding events in a hurricane

Compound flooding is usually induced by the concurrence of coastal storm surge and heavy precipitation induced river flooding, with the former involving oceanic processes and the latter involving hydrological processes. The simulation of these two types of processes is traditionally handled by two different types of models separately, i.e., hydrological models (e.g., NOAA's National Water Model (NWM)) and hydrodynamic models. This dichotomy leaves gaps in simulating the interrelated processes in a holistic fashion. In this paper, we present a creek-to-ocean 3D baroclinic model based on SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) that aims to unite traditional hydrologic and hydrodynamic models in a single modeling platform to simulate compound floods, by taking full advantage of the model polymorphism (i.e., a single model grid can seamlessly morph between full 3D, 2DV, 2DH, and quasi-1D modes). Using Hurricane Irene's impact on Delaware Bay as an example, a seamless 2D-3D model grid is implemented to include the entire US East Coast and Gulf of Mexico with a highly resolved Delaware Bay (down to 20-m resolution). The streamflow from NWM is injected into SCHISM grid at the intersections of NWM's segments and SCHISM's land boundary, and the pluvial and fluvial processes are directly handled by SCHISM. We demonstrate the model's accuracy, stability, and robustness with focus on the compound flooding events. The relative role of different physical processes in such events is examined by a series of sensitivity tests. Our results confirm the occurrence of backwater process into far upstream rivers and creeks and thus demonstrate the need for a dynamic two-way coupling between the hydrodynamic and hydrological models.