What is a floodplain?

A floodplain, which consists of a floodway and a flood fringe, is the land adjacent to a stream or river that experiences occasional or periodic flooding.

Floodway: stream or river channel and the adjacent area must be reserved in order to pass the base flood (100-year flood) flow without increasing flood depths. The floodway is the most dangerous part of the floodplain. It is associated with moving water.

Flood fringe: portion of the floodplain that is outside the floodway. It is generally associated with standing, rather than flowing, water.

Many policies and regulations regarding floodplains are based on the 100-year flood. The 100-year flood has a 1% chance of occurring in a given year. The 100-year flood does not refer to a flood that happens once every 100 years. The 100-year flood is also known as the base flood.

Ann Arbor's Floodplains

The Huron River is the largest and most prominent water feature in the City of Ann Arbor. The Huron River originates in Oakland County. Its watershed spans portions of Oakland, Livingston, Washtenaw and Wayne counties. For Ann Arbor residents the river is the primary drinking water source and provides valued recreational opportunities and hydropower.

Within Ann Arbor, five tributary streams have mapped floodplains: Allen, Malletts, Millers, Swift Run, and Traver. The Huron River and tributary floodplains are shown on this map (pdf).

Benefits of Natural Floodplains

Floodplain lands and adjacent waters combine to form a complex, dynamic physical and biological system found nowhere else. When portions of floodplains are preserved in (or restored to) their natural state, they provide many benefits to both human and natural systems. Natural floodplains:

• Have an important impact on flooding - Flood waters can spread out over a large area in undeveloped floodplains. This reduces flood velocities and provides flood storage to reduce peak flows downstream. Natural floodplains reduce wind and wave impacts and their vegetation stabilizes soils during flooding.
• Offer several benefits beyond flood reduction - Water quality is improved in areas where natural cover acts as a filter for runoff and overbank flows; sediment loads, and impurities are also minimized. Natural floodplains moderate water temperature, reducing the possibility of adverse impacts on aquatic plants and animals. Floodplains also act as recharge areas for groundwater and reduce the frequency and duration of shallow flooding.
• Provide wildlife habitat for diverse species of flora and fauna - Some of these species cannot live anywhere else. Floodplains are particularly important as breeding and feeding areas.

Floodplain protection and management in Ann Arbor

The City of Ann Arbor takes a multi-faceted approach to floodplain management, working to protect natural floodplains and to mitigate flooding problems where the floodplain has already been developed. The city:

• 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) 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 an 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)

Last Updated: August 9, 2012