fldzone = 'AO'

Risk tier: High — inside the Special Flood Hazard Area (SFHA).

Zone AO is a 1%-annual-chance floodplain where flooding takes the form of shallow sheet flow across sloping ground — water running downhill in a broad, shallow sheet, typically 1 to 3 feet deep. It is common on alluvial fans and along the base of hills where there is no defined channel.

Depth, not elevation

The defining quirk of AO is that it is regulated by flood depth, not by a water-surface elevation:

• Instead of a BFE, the FIRM publishes an average depth (usually 1, 2, or 3 feet). On many maps this depth is printed alongside the zone label (e.g. "AO (depth 2)").
• Consequently staticbfe is normally NULL for AO — the regulatory quantity is a depth above the natural grade, and buildings are elevated that many feet above the highest adjacent grade.
• Alluvial-fan areas may also carry a velocity component.

Building and insurance implications

• Flood insurance is federally required for mortgaged structures.
• The lowest floor (or the entire structure on fill) must be elevated above the highest adjacent grade by at least the specified depth (a minimum of 2 feet is used where no depth is given).
• Adequate drainage around the structure is required so sheet flow is not diverted onto neighboring property.

zonesubty values

Zone AO polygons typically carry no subtype; the depth is the key qualifier and is carried on the FIRM.

In the data

SELECT lrid, parceladdr, fldzone
FROM   lr_parcel_us
WHERE  fldzone = 'AO';

See also: Zone AH (shallow ponding with a BFE), Zone AE.