Building in the floodplain can have serious consequences, including:
- Increase in overall flooded area: Buildings and other structures in the floodplain displace flood waters, pushing overflow further out into the watershed and affecting lands and buildings that would not be at risk otherwise.
- Greater financial losses through property damage when flooding occurs.
- Increase in risks to human safety: Deaths and injuries from drowning, being trapped in vulnerable structures, or being struck by flood debris all increase once floodplains are developed.
Regulations for Building in the Floodplain
Flooding is a natural process. Reducing property damage and protecting human safety when flooding does occur are two goals of floodplain management. There are many policies and regulations in place regarding the protection and management of floodplains.
The State, County, and City all have regulations pertaining to floodplains and floodplain development. Some of these regulations are required for participants in the National Flood Insurance Program (NFIP) and are legally enforced under the Michigan Building Code.
General Criteria for floodplain development
- All development in the City require permits from Planning and Development Services.
- Construction within an easement (typically a 30-foot on either side of the centerline of all above ground and underground creeks) that falls within County jurisdiction requires a permit from the Washtenaw County Water Resources Commissioner.
- Construction within the floodplain requires a permit from the Michigan Department of Environment, Great Lakes, and Energy (EGLE) where it has jurisdiction. Flood flow may not be obstructed in a manner that causes a rise in flood elevations at the property line.
- State, County, and City all require no net loss of flood storage capacity; i.e., no fill without hydraulically equivalent compensatory dredging.
Standards for development in the floodway
- New buildings of any kind are prohibited in the floodway, but redevelopment of an existing site may be allowed with certain additional requirements, such as no residential uses.
- The State also prohibits residential uses in the floodway in areas under the jurisdiction of the EGLE.
- For redevelopment in the floodway, the developer must submit an engineering study certifying that the development will not raise the base flood elevation (BFE).
- The lowest floor of any redeveloped non-residential structure must be elevated to 1 foot above the 0.2% annual chance (500-year) flood elevation.
Standards for new buildings in flood fringe
- The NFIP recommends that residential uses in the floodplain be discouraged.
- The lowest floor of any new residential structure must be elevated to 1 foot above the 0.2% annual chance (500-year) flood elevation.
- The lowest floor of any new non-residential structure must be elevated or flood-proofed to 1 foot above the 0.2% annual chance (500-year) flood elevation.
Standards for substantially improved buildings
- All structures that are improved in the floodplain and floodway must meet standards for new buildings if the value of all improvements over the past 10-years exceeds 50% of the market value of the structure.
- Note: historic structures are exempt from the substantial improvement requirement, provided that the historic character of the structure is maintained.
On January 4, 2021 the City Council approved a new Floodplain Management Overlay Zoning District
to add to Chapter 55 (PDF), the Unified Development Code. The ordinance includes provisions that allow for the City's continued participation in the NFIP.
Requirements for fencing in the floodplain
Fences are prohibited in the floodway. Please see our Fence Requirements Publication 2-21.pdf (PDF) for fences in other areas of the floodplain.
A Flood Insurance Survey (FIS) is a compilation and presentation of flood risk data for flood hazard areas. The FIS report contains detailed flood elevation data in flood profiles and data tables.
Washtenaw County Flood Insurance Study (April 3, 2012)
- Vol. 1 (PDF) - FIS Vol. 1 includes discharge and floodway data
- Vol. 2 (PDF) - FIS Vol. 2 includes the flood profiles
The City of Ann Arbor Stormwater and Floodplain Programs Coordinator also maintains elevation and floodproofing certificates for all new buildings and substantial improved structures located in the floodplain. Elevation and floodproofing certificates for these structures can be found on this flood map, or can be provided upon request by contacting Jerry Hancock by email at [email protected].
To learn more about the risk of flooding, visit floodsmart.gov.