Flood Zone Descriptions: Zone AH

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.

Because the Greer Park area is also subject to more severe flooding from the Bay, this area is designated as "Zone AE (El. 10.5)" on the flood maps.

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.

Last Updated: May 18, 2009