Preparing for coastal flooding: A warning system for Imperial Beach, CA

Imperial Beach experiences wave overtopping and flooding when energetic winter swell waves and spring high tides coincide. Flooding impacts are increased by beach erosion, low-lying streets and structures, and proximity to estuary wetlands. Wave, water level, runup, and beach sand levels observed during small to moderate floods (winter 2018-19) are being used to inform a warning system for overtopping and flooding. The system is being developed with Imperial Beach city government, and is designed to provide timely warnings customized to the individual street level. Here we assess the critical components of an observing/modeling system for short-term (0-3 day) flood forecasts. Errors in wave forecasts (magnitude and timing relative to high tide) cause errors in flood forecasts, and reliable incident wave conditions are needed. Often, however, the largest forecast errors involve the specification of wave runup on a changing beach profile. A scanning Lidar was used at Imperial Beach to document runup and beach morphology change during a significant flood event, providing valuable information for runup model calibration. Flood forecasts based on typical available information, such as seasonal beach profiles and bulk wave parameters, rather than accurate estimates of waves and bathymetry, can have large uncertainty. Additional observations of waves, bathymetry, and overtopping at Imperial Beach are guiding the development of a more accurate, short-term warning system. The warning system relies heavily on monitoring and detailed understanding of past events. The observation-based approach used at Imperial Beach is viable at other coastal sites. Flood risk at Imperial Beach under future sea-level rise and beach-response scenarios is being assessed.