Start by visiting Risk Factor
Risk Factor is a free online tool created by the nonprofit First Street Foundation that makes it easy to check your property's risk of flooding. This offers an independent view of your actual flood risk, and can inform your decisions about what measures you might take to mitigate or insure that risk.
Much of our neighborhood is in a FEMA-designated flood plain. Here's the FEMA Flood Map for
the neighborhood, obtained from the FEMA map
portal, dated July 8, 2015. It is referenced to NAVD88:
Being in the flood plain has two big implications. If you have a loan on your home from a government-insured lender, you are required to buy flood insurance. Also, the flood zone designation puts severe restrictions on property improvements. Major improvements may require that you bring your home up to flood protection standards, which may mean filling in a basement and raising your house a foot or two.
The good news is that it may be possible to remove the flood zone designation from your house. This will simplify remodeling, and either eliminate the insurance requirement or enable you to buy insurance at a very low cost.
Pursuing this means hiring a licensed surveyor, for a typical fee of $650 to $1000.
Check Your Official Flood Zone Designation
The first step in understanding your flood zone situation is to run a property report at the City of Palo Alto Planning Department's Parcel Reports site. If you are in a FEMA flood hazard area, this report will specify the exact data. "Floodzone: AH52.6" (for example) means that, according to FEMA, flood water will reach 52.6 feet above mean sea level (AMSL) at your property in a once-in-a-century event. (FEMA maps are indexed to NAVD88 - see Datums, below).
Next, use the tool below to look up your latitude and longitude (thanks to Google's Javascript Map APIs), and use that data to get your elevation from Google and from the US Geological Survey National 3D Elevation Program. Click on the map to view additional elevations from the Google dataset! The USGS elevation is referenced to the same NAVD88 datum used by FEMA.
Is your USGS 3DEP elevation close to your flood zone designation? Your property might qualify for a Letter of Map Amendment which would release you from the flood insurance requirement and development limitations.
The City of Palo Alto may agree to send out a city worker to measure the elevation at the low point of your curb, at no cost to you. If the city tells you that your curb elevation is 52.0 feet AMSL, you may be able to show that the earth around your house is at least 0.6 feet above the curb (i.e. ≥52.6 feet AMSL). Starting with the curb reference, you can do this measurement yourself with a laser level or even a hose level.
But maybe the city is off by a few inches, or a foot or two… And maybe the city misjudged the low point of your curb.
After looking at all these contradictory data points, I concluded that only a professional surveyor could provide evidence to get a LOMA.
A collection of photos and videos from December 31, 2022
Several different vertical datums are used in published elevation maps. FEMA's FIRMs use the North American Vertical Datum of 1988 (NAVD88) Google uses World Geodetic System 1984 (WGS84) based on the NAD83 ellipsoid. Other systems include the National Geodetic Vertical Datum of 1929 (NGVD29) and NAD27. You can use this tool. to map elevations found above (WGS84) to the FIRM elevation (NAVD88). For more information, see this and this.
The City of Palo Alto has a series of web pages on floodplain management. An excellent FAQ is here,
We're in Flood Zone AH. Here's the description of this zone:
Description: AH Zones are areas of fairly shallow flooding of less than three feet depth. The flooding source of the AH Zones in Palo Alto is overflow from local creeks. The Base Flood Elevation in these areas is expressed as an elevation above sea level. The largest AH Zone results from overflow from San Francisquito Creek. FEMA's hydraulic models predict creek overtopping in the vicinity of Middlefield Road and Chaucer Street in the event of a one percent (100-year) flood. The overflow water flows downhill (one of the characteristics of Palo Alto's creeks is that their banks are actually higher than the adjacent land so that creek overflow water flows away from its source) and the Base Flood Elevation levels are shown by surface contour lines on the flood maps; the Base Flood Elevation for individual properties must be determined by interpolation. The creek overflow water is expected to flow towards the Embarcadero Road/Oregon Expressway/Bayshore Freeway interchange, and then over the Oregon Expressway and through the Greer Park area... The AH Zone was remapped following the February 1998 flood event. During that event, San Francisquito Creek overtopped its banks at multiple locations between Middlefield Road and the Bay, leading to widespread flooding. The San Francisquito Creek Joint Powers Authority (JPA) and its member agencies (Palo Alto, Menlo Park, East Palo Alto, the Santa Clara Valley Water District, and the San Mateo County Flood Control District) are in the process of developing a plan to address the risk of flooding from San Francisquito Creek...The City of Palo Alto has this excellent memo on flood zones, which outlines the procedure for applying for a map amendments, changes or revisions to remove the flood zone designation from your property. It may be possible to do this yourself, but successful applications generally require a report from a licensed surveyor.
FEMA uses elevation above mean sea level to construct the flood plain maps. The result, for our neighborhood, is their expectation that the creek will overtop its banks near Byron and Webster Streets, and flood water will flow east through Downtown North, across Middlefield Road and the 100/200 block of Middlefield to Fulton Street, before continuing east along Fulton and Guinda. This area has never actually flooded, or even come close to flooding. Such flooding will be transient as the water makes its way downhill toward Embarcadero Road. Very small differences in elevation will direct the flowing water. In some cases, the FEMA map will predict several feet of water where actual flooding could never exceed an inch or two.
The vertical references used by FEMA are not accurate. They use two vertical benchmarks in our neighborhood - RM14 at Seneca and Forest, and RM13 in the doorway of the Graduate Hotel at University and Cowper. Datasheets for these reference marks are HT1331, RM14 and HT1251, RM13. (The source was a very hard-to-use Web page, now removed.) RM13 is "OF QUESTIONABLE OR UNKNOWN STABILITY". Neither of these marks is observable by satellite.
Palo Alto Flood Zone Reference
by Jonathan Seder is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.
With gratitude to Google and the US Geological Survey