Zone AO

The purpose of this page is to define Zone AO, a commonly used term in floodplain management.

Definition/Description

Areas subject to inundation by 1-percent-annual-chance shallow flooding (usually sheet flow on sloping terrain) where average depths are between one and three feet. Average flood depths derived from detailed hydraulic analyses are shown in this zone. Mandatory flood insurance purchase requirements and floodplain management standards apply.

Some Zone AO have been designated in areas with high flood velocities such as alluvial fans and washes. Communities are encouraged to adopt more restrictive rquirements for these areas.

NFIP Requirement

• 60.3
  • (c) (7) - Zone AO Requirements
  • (c)(8) - Zone AO Subdivision Requirements
• 64.3 - Definition

Guidance

• Answers to Questions About the National Flood Insurance Program - Flood Hazard Assessments and Mapping Requirements
• IS-9 Managing Floodplain Development Through The National Flood Insurance Program (NFIP) (page 3-33)

Related Keywords

• Flood Zones
• Flood Map
• Alluvial Fan Flooding