Structures in the Floodplain 

There are 622 existing structures built within the 100-year floodplain in Ann Arbor. The majority of these floodplain structures are residential buildings. The City developed and is now implementing a flood mitigation plan to reduce risks in flood-prone areas. Source: Ann Arbor Flood Mitigation Plan, 2007

What is a floodplain?

All watercourses have an associated floodplain consisting of a floodway and a flood fringe. In a large precipitation event it is a natural occurrence for the water levels of streams, rivers, and lakes to rise above their banks onto the adjacent lands.

The floodplain is the land adjacent to a stream or river that experiences occassional or periodic flooding. A 100-year floodplain has a 1% chance of being flooded in any given year. The floodway is the stream or river channel and the adjacent area that is reasonably required to efficiently carry the flood flows. The floodway is the most dangerous part of the floodplain. It is associated with moving water. The flood fringe is the portion of the floodplain that is outside the floodway. It is generally associated with standing, rather than flowing, water.

What happens when structures are built in the floodplain?

When structures are built in the floodplain, the flooded area is increased because rising water does not have anyplace to go. Effectively, the structures act as dams and push the overflow even further out into the watershed, affecting lands and buildings that would not be at risk otherwise.

In addition, the structures themselves have an increased risk of being flooded. Over the course of a 30-year mortgage, a home built in the 100-year floodplain has a 26% chance of being flooded, compared to a 9% chance of being affected by a fire.

Building in the floodplain carries risks of financial losses through property damage when flooding does occur. It also threatens human safety. The risks of drowning, being trapped in vulnerable structures, or of being struck by flood debris all increase once floodplains are developed.

What parts of the City are most at risk for flooding?

Allen Creek, in Ann Arbor's central area, is the most at risk for flooding. Eighty percent of all the structures built in Ann Arbor's floodplains are situated along Allen Creek. Allen Creek also contains all the National Flood Insurance Program (NFIP) claims in the city, suggesting the area is vulnerable to weather events smaller than the 100-year storm.

Flooding on Depot Street, part of Allen Creekshed, in 1999

Flooding along Allen Creek has been a problem since the first settlers in Ann Arbor began living and building along its banks. The creek, basically an open sewer at that time, flooded frequently due to the changes in land use that replaced absorbent vegetation with streets and buildings. In 1926, Allen Creek was piped and submerged under the ground to help improve health conditions and stop flooding in its immediate area. Though this solution may have been an improvement to the conditions that existed in the twenties, nothing was done to curb the development patterns in and around the creek's floodplain. Unfortunately, in the long-term, burying the creek may have had the opposite effect with several buildings subsequently being constructed directly in the center of the watercourse. Burying the stream did not effectively mitigate the flooding experienced by residents in this area and it still continues today. (Source: Flood Mitigation Plan, 2007)

What is the City of Ann Arbor doing to protect and manage floodplains? 

• Participates in the National Flood Insurance Program
• Provides Flood Insurance Rate Map information to the public
• Regulates Floodplains as a natural feature during the site plan approval process (See Chapter 57 of City Code (pdf) and the Guidelines for the Protection and Mitigation of Natural Features)
• Ensures compliance with the flood resistance construction standards in the Michigan Building Codes
• Maintains a extensive park and greenway system in the floodplain along the Huron River, and has numerous other parks in the floodplain areas of the creeks flowing to the Huron River.
• Developed and is in the process of implementing a Flood Mitigation Plan (4 MB pdf)

 Flood Map Modernization

The Federal Emergency Management Agency (FEMA) is currently in the process of revising the Flood Insurance Rate Maps (FIRMS) for all of Washtenaw County.  The final maps and model will be provided to the City in digital format for use within the City's Geographic Information System (GIS).  FEMA released draft maps on July 27, 2007, and held a formal public review process that ended in September of 2008.  FEMA issued the Letter of Final Determination on October 3, 2011.  The City will have to adopt the new maps by ordinance before they go into effect on April 3, 2012.  The revised maps will chang the flood elevations and boundaries in some areas.  In the meantime the City's FIRM dated January 2, 1992 remains the effective FIRM.

For more information

• Floodplain webpage
• Flood Mitigation Plan (4 MB pdf)
• National Flood Insurance Program

Last Updated: December 2011