So-called "100-year" floods

First of all, we need to clarify the meaning of the term "100-year flood". Strictly speaking, a 100-year flood is the flood expected to occur with a probability of one percent per year (i.e. a "one-percent flood"). The term "100-year flood" is commonly applied because over a long, long period of time the one-percent flood will occur, on the average, every one hundred years. Of course, individual occurrences could be twice in one year or several hundred years apart. The 100-year flood is a theoretical event determined using statistical methods, based on local rainfall records. If you took a Statistics course you may remember how to calculate that a 1% per year chance is the same as a 26% chance over thirty years (the life of a typical mortgage).

Special Flood Hazard Areas (SFHA)

The Federal Emergency Management Agency (FEMA) defines flood zones on Flood Insurance Rate Maps (FIRM). Certain types of zones are designated as Special Flood Hazard Areas (SFHA). These are the zones described below that start with an "A", and, roughly speaking, correspond to the limits of the 100-year flood for that area. Although the 100-year flood boundary was chosen as the SFHA limit, a structure located just outside the SFHA boundary is almost equally at risk as one just inside the boundary. And it must be kept in mind that within the 100-year boundary would be boundaries for 50-year, 25-year, and 10-year floods; in other words, many structures inside the SFHA boundary have a much higher than 1% probability of being flooded. For instance, some areas near San Francisquito Creek have been flooded several times in the last 60 years.
Structures within Special Flood Hazard Areas are subject to several special requirements. Most noteworthy are a federal regulation that all equity loans be protected by flood insurance and a requirement that lowest floors of new or substantially improved structures be elevated above the predicted flood level, specified in the zone designation as either an elevation above sea level or a depth above ground.

Special note about the Foothills area: The Flood Insurance Study for Palo Alto covers the area between the San Francisco Bay and Foothill Expressway. The area of Palo Alto further west is considered "ZONE X" by default, with flood insurance available at lower "ZONE X" premiums. Although a structure on a ridge top would appear safe from flooding, owners of structures adjacent to creeks or in swales or valleys in the Foothills area should give serious thought to the probable risk of a flood during a very heavy rainfall in the area. Although the risk of flooding in some Foothills areas may actually be quite high, the default "ZONE X" designation means that flood insurance is available at lower premiums.

How the flood zones are determined

One of the questions frequently asked is how FEMA determines the magnitude of a 100-year flood is if there has never been one recorded in the area. It's a complicated and technical process, involving statistics, historical rainfall records and computer models.

Basically, for ZONES A0 and AH, FEMA's consultants study the data for known stream flows and flooding on the creeks, and by special statistical techniques are able to project what a 100-year condition should be like. There are a lot of variables to take into account; the actual depth of water in the creeks depends on many factors, such as how much it has rained recently, soaking the ground and causing higher runoff, and exactly how fast the rain actually falls. This requires study of actual rainfall records and creek flow data from gauges on the creeks.

Determining the extent of ZONE AE 8 depends on the probability of excessive high tides on the Bay coinciding with storm events. There are normally several very high tides in the winter and summer, but these aren't sufficient to create a 100-year high water situation. The worst-case can occur when severe Pacific winter storms add several feet to the water level, presenting a danger of overtopping of the levees during an otherwise normal high tide.

Flood Zone A

Zone A is a designation for properties that are flood-prone in the event of a 100-year flood, but for which there is insufficient data available to establish a base flood elevation or a flooding depth. Several properties near San Francisquito Creek were mapped on the Flood Insurance Rate Maps as Zone A following the 1998 flood. These properties were subjected to flooding during the 1998 flood even though FEMA's hydrologic models did not predict such flooding.

Flood Zone ACr

Palo Alto is traversed by several creeks. Obviously, these creeks carry rainfall runoff and so, by definition, are flooded after virtually any significant rainfall. The channels, then, are definitely Special Flood Hazard Areas. Where a 100-year flow in the channel would not overflow the banks, FEMA notes this on the flood maps as "ZONE A CONTAINED IN CHANNEL". Even if your land actually extends into the creek channel, you are not affected by the construction and insurance requirements unless your structure actually lies at least partly within the creek channel. Still, if your property does extend into a creek, it can sometimes be a bit difficult to convince a lender that you don't have to have flood insurance. And, if you're that close to a creek, it might not be a bad idea to carry flood insurance in any case.

Flood Zone A0

The A0 Zones involves shallow "sheet-flow" of between one and three feet depth from one of the City's several creeks. The water will flow downhill across a wide area (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). The primary A0 Zones lie between Matadero and Adobe Creeks along Park Boulevard and the Caltrain tracks, and in a strip lying immediately west of El Camino Real. There are also a few other small areas of localized A0 flooding adjacent to some of the creeks. The Santa Clara Valley Water District (District) constructed flood control improvements on Adobe Creek downstream of El Camino Real in the late 1980s and early 1990s, which resulted in the elimination of a portion of the AO Zone. The District also performed work on Barron and Matadero creeks in an effort to address additional flood risks, but further creek modifications must be completed before the all of the AO Zones associated with these creeks are removed from the Flood Insurance Rate Maps.

Flood Zone AE8

There's only one such zone in Palo Alto. Flooding in the AE Zone is due to potential overtopping of the Bayfront levees in the event of an extremely high tide, particularly one that might be associated with a storm front.

This large zone is roughly bounded on the north by Embarcadero Road and on the south by the Mountain View city limits and includes everything easterly from roughly Middlefield, Ross and Louis Roads to San Francisco Bay. Originally, most of this area was tidal marsh and wetlands, but many years ago levees were built in the Baylands to drain the wetlands and allow the development of eastern Palo Alto. Because the levees lack required freeboard (additional height above the estimated high water level)and were not constructed in accordance with current engineering standards, FEMA does not consider these levees to be adequate protection from a high tide event that has a one percent (100-year) probability of occurring. The Flood Insurance Rate Maps were prepared under the assumption that the levees will overtop or fail and that the area in the AE 8 Zone will be flooded by tidal water to an elevation of eight feet above sea level (which is not the same as a depth of eight feet). Much of the residential area immediately west of the Bayshore Freeway is at only about three feet above sea level, meaning that the 100-year flood would reach a height of up to five feet above the ground.

If your property is near Greer Park, it may also be subject to flooding from San Francisquito Creek (see ZONE AH).

Flood 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. 8)" 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.

Another AH Zone lies along Park Boulevard in the area between Matadero and Barron Creeks. This zone is expected to be eliminated when the Santa Clara Valley Water District completes flood control improvements to Matadero Creek.

Flood Zone X

All of Palo Alto that is not in a Special Flood Hazard Area is in "ZONE X", which is described as an area of moderate risk of flooding (roughly speaking, outside the 100-year flood but inside the 500-year flood limits), or, for A0 zones, where the 100-year flood will be less than a foot deep. Thus, all of Palo Alto has been determined to be subject to some risk of flooding, and it is inaccurate to say that a given property is "not in a flood zone" simply because it is not in a Special Flood Hazard Area.

The special floodplain construction rules are not applicable to structures in an "X" Zone, and federal regulations do not require that flood insurance be purchased to protect an equity loan on structures in an "X" Zone. But flood insurance is available for structures in "X" zones, and at rather attractive prices. FEMA states that some one-third of its claims payments for flood damage are for structures that are not within a Special Flood Hazard Area.

A structure can be removed from a Special Flood Hazard Area and changed to an "X" Zone designation by means of a FEMA-issued Letter of Map Amendment or Letter of Map Revision (LOMA or LOMR). For addresses where this has been done, the flood zone is designated as "X" (with the notation "LOMA" or "LOMR" in the comment field) in the look-up lists on this website.

LOMA-LOMR

Note:The Federal Emergency Management Agency (FEMA) frequently makes significant changes to the procedure for obtaining a Letter of Map Amendment (LOMA) or a Letter of Map Revision (LOMR). The process is complicated enough that it is no longer feasible for the City of Palo Alto to provide details. In this handout we will attempt to provide some meaningful general information. For the most part, LOMAs and LOMRs are a process involving the property owner and FEMA; the role of the City of Palo Alto is quite limited. Consequently, the City cannot guarantee the accuracy or currency of the information in this handout. For more detailed information please contact FEMA in San Francisco at (650) 923-7194.

LOMAs and LOMRs have the effect of removing either a structure, a portion of a property, or an entire property from a Special Flood Hazard Area (SFHA), based on the elevation of the ground. In effect, the applicant must prove to FEMA's satisfaction that their structure is located on ground high enough that the structure will be on its own little "island" during the so-called Base Flood. The basic difference between a LOMA and a LOMR is that the LOMR is based on a ground elevation created by the addition of fill to the property since the date of the first Flood Insurance Rate map (February 15, 1980, for Palo Alto).

Please note that floor elevation is not a factor in removing a structure from the SFHA -- only the appropriate ground elevation is relevant.

FEMA now breaks LOMAs and LOMRs into the following categories:
Letter of Map Amendment (LOMA): FEMA verifies that applicant's land, or at least the land immediately adjacent to the structure, is high enough to be above the Base Flood Elevation (BFE).

Conditional Letter of Map Amendment (CLOMA): Same as LOMA, but issued in advance of construction. When construction is complete, appropriate as-built date must be supplied to FEMA for a permanent LOMA to be issued.

Letter of Map Revision Based on Fill (LOMR-F): Similar to a LOMA, but based on fill added since the first FIRM date. LOMRs require that the elevation of the lowest floor, including a basement, be above the Base Flood Elevation.

Conditional Letter of Map Revision Based on Fill (CLOMR-F): An advance LOMR, prior to placement of fill; as-built information must be supplied at completion of construction for issuance of a permanent LOMR.

Letter of Map Revision (LOMR) and Conditional Letter of Map Revision (CLOMR): Used for larger projects involving fill where more general flood plain boundary changes might be involved. These are not usually applicable to individual small property owners.

FEMA will require a number of documents to be submitted as part of the application for a LOMA or LOMR. Among these documents will be:

Recorded plat map or property deed (available from the Santa Clara County Recorder's Office).

Photocopy of the Flood Insurance Rate Map (available from this office).

FEMA Property Information form (to be completed by the property owner).

FEMA Elevation Information form (to be completed by Licensed Surveyor or Registered Professional Engineer).

FEMA Summary of Elevations form (used when more than one lot is involved; to be completed by Licensed Surveyor or Registered Professional Engineer).

FEMA Certification of Fill Compaction (for LOMR applications; to be completed by Registered Professional Engineer).

FEMA Community Acknowledgement of Requests Involving Fill (for LOMR applications; completed by City of Palo Alto Public Works Engineering).

The LOMA/LOMR process is not cheap. It will be necessary to hire a Licensed Surveyor or Registered Professional Engineer to provide some of the required documents. And there are up-front FEMA charges for single-lot CLOMAs and CLOMR-Fs, and multiple-lot LOMR-Fs ranging from $175 to $560.

The FEMA forms are available from FEMA. See above for the number to call.

Once a LOMA or LOMR has been obtained, the applicant is relieved of the federal requirement that loans using the structure as collateral be protected by flood insurance, and a refund can be gotten for some current flood insurance premiums paid. It must be remembered, though, that a LOMA/LOMR only "removes" a structure from the 100-year flood area: there is still some risk of flood. For instance, the structure may still be in a 101-year flood area. and a lender may still consider it prudent to require flood insurance. However, if the lender still requires flood insurance, the premiums will be much lower than before issuance of the LOMA/LOMR.

LOMAs and LOMRs are only valid until the next revision of the FIRM is published. It will then be necessary for the property owner to ask for a "renewal" of the LOMA/LOMR; if the revised map does not document a change in the flood conditions for the property, a new LOMA/LOMR will normally be issued. In some cases FEMA may issue a blanket letter accepting a list of LOMAs and LOMRs that are considered to be valid for the update; if you had a LOMA or LOMR and are uncertain as to its validity after a map update, contact Public Works Engineering at (650) 329-2151.