Even if storminess does not increase in the future, sea-level rise will magnify the adverse impact of storm surges and high waves on the coast. For example, a model using the committee’s sea-level projections predicts that the incidence of extreme high water events (1.4 m above historical mean sea level) in the San Francisco Bay area will increase substantially with sea-level rise, from less than 10 hours per decade today to a few hundred hours per decade by 2050 and to several thousand hours per decade by 2100.
Coastal Responses to Sea-Level Rise and Storminess
The natural shoreline can provide partial protection for coastal development against sea-level rise and storms. Coastal cliffs, beaches, and dunes take the brunt of storm waves and are therefore eroding over the long term. The net result of storms and sea-level rise is coastline retreat, with rates ranging from a few centimeters per year for cliffs made of resistant bedrock to several meters per year for beaches and dunes, which consist primarily of unconsolidated sand. These rates will increase with rising sea level and are likely to further increase if waves become higher. Although seawalls and revetments can make the shoreline more resistant to wave attack, they prevent beaches from migrating landward and will eventually be overwhelmed by sea-level rise.
Marshes and mudflats protect inland areas by storing food waters and damping wave height and energy. To continue providing these services as sea level rises, marshes must be able to maintain their elevation relative to sea level and to move inland in places where they are subject to erosion at the seaward edge. Building elevation requires a sufficient supply of sediment and accumulation of organic material. Most studies of west coast marshes have focused on the supply of sediment. The frequent storms and associated foods in central and southern California potentially provide enough sediment for marshes to keep pace with the sea-level rise projected for 2030 and 2050 by the committee. In Oregon and Washington, rivers also potentially carry enough sediment for marshes to maintain elevation, despite upstream dams, especially because the projections of sea-level rise are lower. For 2100, marshes will need room to migrate, a high sediment supply, and uplift or low subsidence to survive the projected sea-level rise.