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Sea Level Rise  El Niño winter storms—During El Niño winters, atmospheric and oceanographic conditions in the Pacific Ocean produce severe winter storms that bring intense rainfall and storm conditions to the Bay Area. Tides are often elevated 0.5 to 1.0 feet above normal along the coast, and wind setup can elevate water levels even further. Typical impacts include inundation of low-lying roads, boardwalks, and waterfront promenades; storm drain backup; wave damage to coastal structures; and erosion of natural shorelines. El Niño winter conditions prevailed in 1977–1978, 1982–1983, 1997–1998, 2009–2010, and 2015-2016.  Ocean swell and wind-wave events (storm waves)—Pacific Ocean storms and strong thermal gradients can produce strong winds that blow across the ocean and the Bay. When the wind blows over long reaches of open water, large waves can be generated that impact the shoreline and cause damage. Typical impacts include wave damage along the shoreline, particularly to coastal structures such as levees, docks, piers, wharves, and revetments; backshore inundation due to wave overtopping of structures; and erosion of natural shorelines. In the planning area, the potential for new or prolonged flooding as sea level rises will not be confined to the shoreline. Sea level rise will increase the likelihood of major flood events because higher water levels in tidal creeks and flood control channels will reduce capacity to discharge rainfall runoff. While some creeks and coastal infrastructure already flood when rainstorms coincide with high tides, rising sea levels will increasingly cause flooding during smaller, more frequent rainfall events. 13.2.6 Warning Time Sea-level rise is not a hazard that requires near-team advance warning to support response and recovery operations. Programs such as the NOAA sea-level rise program are keeping an active watch on the sea-level rise phenomena to keep communities such as San Mateo County informed of the progression. This stream of information will feed programs to help the County to be prepared for and mitigate the long-term impacts from sea-level rise. 13.3 EXPOSURE A quantitative assessment of exposure to the aggregated sea-level rise inundation area using the ART and OCOF mapping was developed to support the assessment of the sea-level rise hazard. Population exposure was estimated by calculating the number of buildings in each hazard area as a percent of total planning area buildings, and then applying this percentage to the estimated planning area population. 13.3.1 Population and Property Table 13-1 summarizes the estimated citywide population living in the mapped sea level rise risk areas and the estimated property exposure. Figure 13-2 shows the structure type of buildings in the inundation area. See Appendix E for a detailed breakdown of sea level rise exposure by jurisdiction. 13-5