Quantifying compound coastal flooding effects in urban regions using a tightly coupled 1D–2D model explicitly resolving flood defense infrastructure

Low-lying coastal areas are highly vulnerable to flooding hazards. High marine water levels may overflow seawalls, render the storm drain system inoperable, and promote pluvial and wave overtopping flooding. Complex interactions between various coastal flooding drivers (marine water levels, precipitation, waves) and urban infrastructure (i.e., the stormwater system, and seawalls) are characterized using a novel, tightly coupled hydrodynamic model. Flood extent field observations of tidal overflow, pluvial flooding, and overtopping flooding, along with storm drain system pressure sensor data are used to evaluate hydrodynamic model performance. High marine water levels, precipitation, and overtopping events are modeled and compared with validation data. Results suggest the hydrodynamic model explicitly resolving both 1D storm drain pipe flow and 2D overland flooding more accurately simulates compound flooding compared to typical 2D overland flow models. Nonlinear compound effects were resolved by comparing combined univariate flood impact to corresponding compound flood impact modeled within a tightly coupled 1D2D infrastructure-resolving model. Projected flood extents were ∼ 50% greater when compound interactions were resolved. Compound effects vary with event type, event magnitude, and site characteristics. Critically, coastal adaptation strategies protecting against high embayment water levels such as elevating seawalls may exacerbate compound flooding effects in low-lying communities. • A novel, tightly coupled Delft-3D model is used to resolve infrastructure impacts. • The compound impacts of rain, tides, and wave overtopping events are examined. • A methodology quantifying nonlinear compound flooding effects is presented. • Wave overtopping uncertainty is explored using bootstrapping methods. • Coastal adaptation strategies may exacerbate compound flooding in low-lying regions.