Flood Hazard Mapping for California

Eric Simmons, FEMA Region 9

We’re firmly in the era of big data. Hazard maps are no exception, but what are the differences between various flood products? Five common map types for California are summarized to help clarify their uses. Tsunami hazard area maps are online, cover the state’s populated coastal areas, and are currently being updated. An example around Emma Wood State Park at the mouth of the Ventura River is shown. The California Legislature passed Senate Bill 92 requiring the development of inundation maps for state jurisdictional dams. These maps are posted when approved by Division of Safety of Dams. An example based on a reservoir breach is provided around the same area in Ventura, California. Levee flood protection zone maps from the California Department of Water Resources highlight areas behind State-Federal levees. One can view this using a parcel lookup tool. For the entire nation, the National Levee Database, or NLD, includes maps of ‘leveed areas’ as well as a dashboard to search geographically. The NLD is maintained by the U.S. Army Corps of Engineers.

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Flood Insurance Rate Maps Flood Factor

Purpose of Map Floodplain management and determination structures requiring flood insurance.

Understand current flood hazards and how flood risk changes over time. Map Source Federal Emergency Management Agency First Street Foundation Types of Flood Depicted Riverine, coastal, and alluvial flooding due to storms, and –in some areas– levee failure. 1% annual chance event flood zones and, where available, 0.2% annual chance event. Existing conditions and future flooding from rivers, rainfall and the oceans covering sizes from nuisance flooding to once-in-a-millennium.

Information on Map Flood insurance zones, regulatory floodways, cross section and transect locations, base flood elevations or depths, hydraulic structures, etc. 3-meter resolution grids attributed with inundation probability and flood depths associated with time (year 2020 & 2050).

For active flooding, please consult products issued by the National Weather Service. Future conditions mapping is also available from National Oceanic and Atmospheric Administration and Our Coast, Our Future. FEMA maintains a National Risk Index to identify communities most at risk to 18 hazards. Last and not least, MyHazards by California Office of Emergency Services provides information on hazards from tsunamis, earthquakes, fire, and flooding.

Tsunami Hazard Area Maps Dam Inundation Maps Leveed Areas or Zones

Purpose of Map

Assist response planning by identifying a community’s tsunami hazard.

Emergency planning and a tool to develop emergency action plans.

Map Source California Geological Survey Owners of dams Increase awareness of hazards and risk associated with levees.

Various

Types of Flood Depicted Tsunami – primarily based on inundation limits corresponding to a 975-year average return period tsunami event. Dam breach inundation in California for extremely high, high, and significant hazard dams and their appurtenant structures. Maximum area flooded when a levee fails at maximum capacity that may reasonably be conveyed by flood control system.

Information on Map Area inundated by tsunami; note they are not legal documents and do not meet disclosure requirement for real estate. Inundation boundary, flood wave arrival times, maximum inundation depths, and maximum water velocities. Area inundated during overtopping of a levee, drainage problems, or a levee failure.

Flood Factor is a free, online tool to learn a property’s current flood risk and how that risk changes over time. Using hazard and property data, one can determine depths of flooding reaching buildings and estimate the costs of associated damage. Tsunami hazard area maps are online, cover the state’s populated coastal areas, and are currently being updated. An example around Emma Wood State Park at the mouth of the Ventura River is shown. Flood Insurance Rate Maps, or FIRMs, are hazard maps supporting the National Flood Insurance Program. They show zones of high flood hazard (for example, Zones A, AE, AH, AO, and VE) as well as medium, low, and unknown flood hazard areas. All FIRMs produced digitally are viewable in a National Flood Hazard Layer.