Predicting storm wave runup at Imperial Beach, California

Accurate modeling of wave runup is essential to relate shoreline impacts (e.g. erosion, wave overtopping and street flooding) to storm waves. Physics-based, dynamical models for runup require specification of shoreward propagating waves (including infragravity and sea-swell frequencies) at the offshore boundary, and of the bottom boundary depth profile h(x). These boundary conditions are well-known in the laboratory, and at a few sites with detailed observations. In these cases, the observed runup is simulated with good accuracy, providing some confidence in the underlying models. However, these boundary conditions are poorly known in many practical applications of estimating runup during storms, both historical and future, at many sites. Observations of wave runup on a steep, rapidly eroding shoreface (~80 cm vertical erosion) during a single high tide storm wave event at Imperial Beach, California in January 2019 illustrate the importance and difficulty of resolving changing h(x) and shoreline beach slopes in runup modeling. Here we develop a runup parameterization specific to Imperial Beach which characterizes runup using an integrated weighted wave energy spectrum and frequency. The numerical model SWASH is used to simulate runup for storm conditions, informed by a 17-year wave hindcast, and 10 years of monthly bathymetry observations. Extreme runup parameterization methods are compared to obtain the best available runup prediction for wave flooding forecasts in Imperial Beach given limited knowledge of precise boundary conditions. Additional error sources, and approaches to error reduction, are discussed.