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Impervious surface covers approximately 32% of Ann Arbor. Studies show stream health shows signs of impairment when watershed imperviousness exceeds 10%. In 2007, the City adjusted its stormwater rate system to provide property owners with incentives to reduce impervious surface.
Source: Stormwater Rate System (Impervious Surface Analysis) |
What is impervious surface?
An impervious surface is any surface that does not allow water to soak into the ground. When water from rain and snowmelt washes off a piece of property, it flows into a storm drain system and eventually into the Huron River. Impervious or hard surfaces on the property such as roofs, driveways, and patios, do not absorb the water and contribute to stormwater runoff.
How does impervious surface affect the environment?
There is a strong correlation between impervious surface and measures of water quality including temperature, biodiversity, and pollution. A number of studies have shown that stream health begins to decrease when a watershed's impervious surface exceeds 10%. In the Huron River system, declines in water quality and biodiversity have been observed at 8% watershed imperviousness, with the most severe impacts occurring at 25% watershed imperviousness (HRWC 2004, Wiley and Martin 1999). Key impacts of high watershed imperviousness include:
- Degradation of the stream channel: High stormwater flows cause erosion. Small stream channels in particular cannot accommodate the unnaturally high flows of water that occur as impervious surface increases. As a result of flashy flows, the stream bank and stream bed erode more frequently, degrading the habitat quality for aquatic organisms.
- Decreased groundwater recharge: Groundwater feeds rivers and streams during periods of low flow. Because impervious surfaces prevent rainfall infiltration, groundwater sources are not recharged, and streams are not replenished during low flow periods.
- Increased water temperature: Stormwater runoff that flows over concrete is frequently much warmer than groundwater and will increase the water temperatures. Warm water cannot hold as much oxygen as cold water, so the temperature change is an additional stress to many native aquatic species.
- Impaired water quality: Impervious surfaces accumulate pollutants - fertilizers, automotive fluids, animal waste, deicers, and dirt - that are carried into surface waters by stormwater run-off. The initial half-inch of stormwater tends to carry the most pollution as it washes into the street and down the gutter. These pollutants degrade habitat for aquatic species and can also affect drinking water quality and limit recreational opportunities.
How is the City of Ann Arbor working to manage stormwater?
Federal and state regulations require the City of Ann Arbor to address the amount of runoff and the pollution carried by the water that is deposited, untreated, into the Huron River through the stormwater system. Ann Arbor recently moved to a tiered stormwater rate system through which residents pay a stormwater utility based on the amount of impervious surface on their property. Residents can earn stormwater credits by installing rain barrels or rain gardens, or by becoming part of Wastenaw County's Riversafe Home project. Additional credits are available for commercial properties and residential properties with more than two units. Funds generated through the stormwater utility support a number of stormwater management activities including operation and maintenance of catchment basins and pipes, street sweeping, education and outreach programs, construction run-off inspections, and post-construction stormwater management programs.
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Last Updated August 2009