Government agencies and companies have produced a variety of maps that help Californians understand where floods can occur, a critical need as a historically strong El Niño arrives, bringing extremely high tides and likely above-average rainfall this winter. But those maps do not all show the same thing, the San Francisco Chronicle reported in a guide to four maps it found particularly useful.
The official federal maps come from the Federal Emergency Management Agency, which shows places it thinks have at least a 1% chance of flooding in any given year. In addition to places along San Francisco Bay and other inland shorelines, the FEMA map highlights flood-prone areas around inland bodies of water, like the Russian River in the North Bay and creeks in the Santa Cruz Mountains, the Chronicle reported. The California Department of Water Resources has identified additional areas with a similar level of flood risk on maps it prepared for awareness, not for regulatory purposes, including areas along numerous smaller streams throughout the Bay Area.
A privately produced map from First Street, a climate company recently acquired by MSCI Inc., includes most of the coastal and river information in the FEMA maps but also incorporates flooding driven directly by heavy downpours, the Chronicle reported. The company calculates flood risk at the property level, and the Chronicle's summary shows the share of properties in each California census tract with at least a 1% annual chance of flooding. First Street's estimates do not determine federal flood insurance requirements. In parts of Marin and San Mateo counties, First Street estimates that at least 90% of properties in a few census tracts face a 1% or greater annual chance of flooding. Substantial parts of some San Mateo County census tracts, including parts of Redwood Shores and San Mateo east of Highway 101, are shown to be at risk on that map even though they fall outside FEMA's designated high-risk flood zones.
The tract-level view does not show exactly which properties face high flood risk. Some climate experts have questioned how reliably First Street's national-scale models can assess flood risk for individual properties, the Chronicle reported. A fourth map in the guide uses elevation data and modeled water levels to show where water could reach along the region's bay shorelines if coastal and bayshore waters rise three feet above normal high tides — a figure Chronicle meteorologist Anthony Edwards considers plausible, especially if a storm coincides with very high tides. This guide is separate from earlier coverage of November and December king-tide dates. The maps can help residents compare official insurance zones with broader awareness layers; they do not, by themselves, change a property's insurance requirement.