Coastal Flooding - Potential Projections: 2000-2100

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 nontide
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.