Observations and modeling of wave overtopping at Imperial Beach, California

Overtopping and other hazardous wave runup events are difficult to observe because they occur infrequently and can be accompanied by logistical challenges (e.g. rain, wind). Understanding and modeling extreme runup is challenging because relatively rapid changes in beach profile (e.g. shoreline erosion) alter runup in a poorly understood nonlinear feedback loop. We present field observations from Imperial Beach, CA during Jan 2019, when energetic sea-swell waves coincided with Spring tides. We collected 5 hours of continuous LiDAR linescans on a transect spanning from the rip rap on the back beach through the surfzone, providing direct observation of short term beach change and wave overtopping. Qualitative evaluation of several overtopping and flooding events were obtained with two GoPro cameras, one of which aimed at an adjacent city street. Two pressure sensors buried either side of the rip rap provided validation of coastal flooding, and an offshore pressure and current meter in ~6 m depth provided wave boundary conditions for the numerical model SWASH. Modeled and observed overtopping are compared, and the effects of wave runup investigated. This study quantifies short term beach erosion and overwash, and identifies key observations to be included in future rapid response studies.