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How to evaluate your property for green infrastructure

Downspout directing water to storm sewer. Photo by Nick Hagen.

The Detroit Water and Sewerage Department is changing the way it assesses drainage fees so that businesses and residential properties will be charged for impervious surfaces that contribute to stress on the city’s combined sewer system, rather than paying a flat fee. Anything that doesn’t allow water to flow into the ground like a roof, parking lot, garage or driveway is considered impervious. Fortunately, there are several things businesses and homeowners can do to receive credits to offset these charges.

The simple calculation for the stormwater charge is as follows: the city’s monthly rate x the number of impervious acres. The monthly rate is currently $750 per acre.That number is expected to decrease to $614 per acre next year.

You can make a simple measurement of the area you will be billed for by measuring the length and width of any impervious surfaces, including houses and garages on your property,  Then multiply each of these to get the square footage and add them all up for the total square footage. Divide this by 43,560–or the square feet in one acre–and multiply by the city’s rate to find out what your charge will be. You can also view the city's assessment using this parcel viewer tool. If your estimate differs from the city's, you can request an adjustment.

DWSD has been hosting workshops to help property owners find the best ways to receive credits to offset their stormwater fees, the slideshow for these presentations can be found here. Attending one of these is no doubt one of the best ways to figure out exactly what you will be charged for and how to get credits. 

However, some of the strategies worth considering include installing a rain garden–addressed in the previous post–installing pervious pavers on driveways and parking areas, disconnecting downspouts to connect to rainwater catchment devices, rain gardens or bio-retention areas.

Pervious pavers have become increasingly popular. They include various types of concrete-based paving materials that allow water to infiltrate beneath the parking surface rather than running off to surrounding areas and drains. Generally, this is something that has to be done by a professional, although serious do-it-yourselfers may give it a try. A growing number of landscaping companies in Michigan are doing this work. 

Catchment areas installed beneath permeable surfaces and elsewhere have also become popular. These are essentially hollow matrices made from reinforced plastic or other materials that hold onto large quantities of water and slowly release it back into the ground. 

As for disconnecting your downspouts, it’s not enough to cut off your gutter and let it run into your basement our out onto the street. These water sources can either be channeled to rain barrels or other storage tanks, rain gardens, landscaped bio-retention areas or some combination of all three. 

If you’re planning on using rain barrels or other tanks, remember that the average roof sheds a lot of water, so coming up with a plan for the overflow–like a rain garden–is important. Landscaped bio-retention areas are essentially depressions in the soil with well-draining soil that allows water to infiltrate. For the purposes of the credit, all water should be able to move into the soil within twenty-four hours. 

Other more advanced practice would include installing green roofs or bio-retention ponds as a landscape element or for irrigation. 

DWSD has plans to implement a “credit calculator” to help you determine what your offsets could amount to, as well as office hours where you can talk to someone about the best ways forward. 

This story is part of a series on measuring on the role of green infrastructure projects in Detroit's redevelopment. Support for this series is provided by the Erb Family Foundation to Greening of Detroit, Model D, and The Nature Conservancy. Read more articles from the series here.

Read more articles by Brian Allnutt.

Brian Allnutt is a Detroit-based writer and a co-owner of Detroit Farm and Garden.
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