Flood Zones AO and AH Explained: Shallow Flooding Risk

Zones AO and AH are Special Flood Hazard Areas where flooding is typically shallow, usually between 1 and 3 feet, rather than deep. Zone AO describes sheet flow flooding, where water spreads across the land rather than collecting, and the map shows an average depth number instead of a Base Flood Elevation. Zone AH describes ponding, where water collects and stays, and it carries a published BFE much like Zone AE.

This article covers what each zone means, how they differ from each other and from the more familiar A and AE zones, and what shallow flooding means for a flood insurance quote.

What Is Zone AO? (Sheet Flow Flooding)

Zone AO is a Special Flood Hazard Area where floodwater is expected to move across the land as sheet flow rather than through a defined channel. It is common in areas near the base of slopes or alluvial fans, where water spreads out rather than staying in a streambed.

• Depth number instead of a BFE. Instead of a Base Flood Elevation, a Zone AO designation on the map includes an average depth, typically shown in whole feet, such as a 1-, 2-, or 3-foot depth zone.
• Water keeps moving. Unlike ponding, sheet flow flooding does not sit still; it flows across the surface and generally drains once the event passes.
• Still a mapped 1% annual chance flood zone. The "shallow" label describes depth, not the annual chance of flooding, which is the same 1% used across the Special Flood Hazard Area.

What Is Zone AH? (Ponding)

Zone AH is a Special Flood Hazard Area where floodwater is expected to collect and pond rather than flow across the surface. Depths are typically in the same 1-to-3-foot range as Zone AO, but the water behaves differently once it arrives.

• BFE is published. Unlike Zone AO, Zone AH carries a Base Flood Elevation, the same way Zone AE does, because the standing water level can be measured and mapped directly.
• Water stands rather than sheets. AH zones often occur in low-lying areas, depressions, or locations where drainage is limited, in contrast with AO, where water keeps moving.
• Same mandatory purchase treatment as any other SFHA zone. A published BFE puts AH on the same footing as AE for elevation comparison purposes.

How AO and AH Differ From A and AE

A and AE describe flooding that can reach significant depth, with AE's risk measured against a published Base Flood Elevation and A's risk mapped without one. AO and AH describe flooding that is expected to stay shallow, generally 1 to 3 feet, but they split along the same line as A and AE: AH has a published BFE, and AO has a depth number instead.

That makes AO the shallow-flooding version of Zone A, and AH the shallow-flooding version of Zone AE, even though all four sit inside the same 1% annual chance Special Flood Hazard Area.

What This Means for Your Flood Insurance

Shallow does not mean minor. One to three feet of water is enough to reach flooring, drywall, cabinetry, and mechanical equipment in most buildings, and AO and AH carry the same mandatory purchase requirement as any other Special Flood Hazard Area zone when a federally regulated lender is involved.

Sterling does not require an elevation certificate to quote a property in either zone. When a certificate exists for an AH property, it compares the building to the published BFE the same way an AE certificate does. For an AO property, the certificate may reference the mapped depth number instead of a BFE, since no BFE has been published for that zone.

Common Mistakes with Zone AO and AH

• Assuming shallow flooding is low risk. Both zones carry the same 1% annual chance of flooding as A and AE, and the mandatory purchase requirement applies the same way.
• Confusing an AO depth number with a BFE. They are different kinds of measurements, and an elevation certificate for an AO property should reflect that.
• Treating AO and AH as interchangeable. One describes moving sheet flow; the other describes standing water, and they are mapped differently.
• Overlooking low mechanical equipment. Even a shallow flood zone can reach an air handler, water heater, or electrical panel mounted close to the ground.

Pro Tips from Sterling Flood Underwriters

Look for a depth number, not a BFE, on an AO flood map. If the field where you expect a BFE instead shows a number of feet, you are likely looking at Zone AO.

Describe how water behaves on your property, not just the zone letter. Whether water flows through or ponds and sits is useful context for an underwriter alongside the map designation.

Get a Quote

Shallow flooding is still flooding, and AO and AH properties are regularly insured. Get a flood insurance quote from Sterling Flood.

Frequently Asked Questions

What is the difference between Zone AO and Zone AH?

Zone AO describes sheet flow flooding and is mapped with an average depth number instead of a Base Flood Elevation. Zone AH describes ponding, standing water that collects, and carries a published BFE like Zone AE.

Do AO and AH zones require flood insurance?

Both are part of the Special Flood Hazard Area, so a federally regulated lender will require flood insurance as a condition of a mortgage on a building in either zone.

How deep does flooding get in Zone AO?

Zone AO is mapped with an average depth, typically shown in whole feet and generally in the 1-to-3-foot range, rather than a specific elevation.

Does Zone AO have a Base Flood Elevation?

No. Zone AO is mapped with a depth number instead of a BFE. Zone AH, by contrast, does have a published Base Flood Elevation.