Co-Occurring Coastal Flood Hazards in California: Extreme Waves and Landfalling Atmospheric Rivers

Along California’s coast, open-ocean swell and waves from extratropical storms can raise coastal water levels by up to 4 m, and when combined with astronomic tides and surge, they present the principal hazard for ocean-driven flooding. Additionally, atmospheric rivers (ARs) that make landfall on the California coast can cause fluvially driven flood hazards. Knowing the probability of co-occurrence of these two storm-generated sources of storm flooding can greatly aid hazard modeling and improve risk identification efforts by coastal communities. Instances of land-falling ARs (1989-2010) in California are extracted from a gridded AR database derived from ERA-Interim. The co-occurrence of extreme (>98th percentile) swell and local wave height observations at five buoys along the coast for the same period are characterized in relation to AR intensity and duration. The potential for co-occurring AR-wave hazards varies by latitude, with greater frequency of co-occurrence along the northern section of coast. While cumulative percentile density plots show northernmost land-falling AR intensity may have a proportional relationship to coincident swell and wave height, extreme swell and wave height shows no reciprocal relationship. This asymmetric relationship becomes less apparent further south, where no significant relationship is shown for the co-occurrence of these extreme hazards.