California Institute for Energy and Environment (CIEE)

The change in flooding potential along the California coast over the twenty‐first century was estimated from both ocean wave and sea level rise projections produced from global climate model data. Changes in flooding potential were inferred from changes in wave runup (the vertical height reached by wave‐driven water levels), which depends on the instantaneous sea level (or still water level), beach slope, and wave height and wave period. The still water level is the superposition of regional (or relative) mean sea level, the tide, and non‐tide sea level fluctuations. Non‐tide sea level fluctuations include both the steric response associated with El Niño–related variability that can persist up to a year or more, and local storm–forced variability (storm surge). The potential for greatest coastal flooding occurs when extremes in waves and still water level occur nearly simultaneously. Wave activity provides the primary driving force for coastal flooding. Wave model significant wave height (Hs) and non‐tide sea level projections were generated for a seven global climate model simulations. Comparison of the Hs and non‐ tide projections with wave model hindcast Hs and observed non‐tide fluctuations suggest that the characteristics and incidence of extreme Hs and non‐tide fluctuations will not increase appreciably over the twenty‐first century. Thus, because astronomical tide‐forcing will not increase, increases in relative mean sea level will largely determine the potential for increased ocean‐driven flooding along the California coast. If relative mean sea level along the California coast reaches global mean sea level rise projections associated with climate change, extreme flooding events expected to occur once in about 100 years under stationary relative mean sea levels will occur annually.