UCLA

Coastal communities are facing a growing challenge from more frequent and extreme flooding events. Multiple flooding drivers occurring at the same time, or in rapid succession is known as compound flooding. Low-lying urban coastal areas present unique modeling and management challenges associated with the role of infrastructures in managing flood risk. Here, complex interactions between various coastal flooding drivers (i.e., marine water levels, precipitation, waves) and urban infrastructure (i.e., the stormwater system, and seawalls) are characterized using a novel, tightly coupled 1D2D hydrodynamic model. Delft3D-FM hydrodynamic models were developed and validated with survey data at two Southern California urban coastal regions backed by harbors. Model results were evaluated with flood extent field observations of tidal overflow, pluvial flooding, overtopping flooding, and storm drain system pressure sensor data. A novel method quantifying nonlinear compound effects is presented that compares the superposition of two univariate flood events to the corresponding compound event modeled within the tightly coupled 1D2D framework. A copula based multivariate statistical analysis was combined with numerical hydrodynamic modeling to quantify compound flood uncertainties. Results show flood extent and depth uncertainties are dominated by modeling methodology, sampling methods, compound event choices along an isoline. Only minor uncertainties are associated with copula choice. A tightly coupled 1D2D model explicitly resolving the drainage effects accurately simulates compound flood effect in protected urban backshores where storm drain infrastructure is present. Critically, this work suggests coastal adaptation strategies protecting against high embayment water levels such as elevating seawalls may exacerbate compound flooding impacts in low-lying communities.