Bioretention - A water quality practice of non-linear, landscaped, shallow depressions that capture and temporarily store stormwater runoff. Runoff intercepted by the practice is then filtered through the soil (often engineered soil filter media). Residential rain gardens, bioswales, and tree-based practices are forms of bioretention, but should be listed under the specific type for the purposes of this project.
GSI in Detroit
In Detroit, the city and the community are increasingly turning to innovative green stormwater infrastructure (GSI) to manage water pollution, reduce flooding, and provide a host of other nature-based benefits.
When rain falls or snow melts in natural, undeveloped areas, the water is absorbed and filtered by soil and plants. But when that same water falls on roofs, streets, and parking lots – the kind of hard surfaces found throughout Detroit – it can’t soak into the ground. Instead, this stormwater runoff collects in gutters and sewers, can lead to flooded roads and basements, and at times can cause the combined sewer system to overflow, pouring untreated sewage into the Detroit and Rouge Rivers.
These sewage discharges, or combined sewer overflows (CSOs) are regulated by the U.S. EPA under the Clean Water Act. Any city with CSOs is required to have a National Pollutant Discharge and Elimination System (NPDES) permit, which sets tailored limits to ensure that cities are managing this problem – including Detroit.
In 2016, The Detroit Water and Sewerage Department (DWSD) was restructured, splitting into DWSD and the Great Lakes Water Authority (GLWA). DWSD manages the water and sewer systems within the city, while GLWA provides drinking water and waste water treatment services for the surrounding region. Under this arrangement, GLWA and DWSD co-manage the City of Detroit’s NPDES permit, and is responsible for meeting Michigan’s zero waste elimination standard for untreated sewer system discharges.
To date, the City of Detroit has made significant investments to address CSOs and flooding. Detroit has invested in infrastructure such as retention and treatment basins (RTBs), that temporarily store and treat combined stormwater and sanitary sewage. With these RTBs and other traditional management methods, DWSD has been able to stop 95% of the untreated combined sewage from entering the Detroit and Rouge Rivers. But, to address the remaining difficult-to-manage 5% and eliminate CSOs completely, the city needed to look to alternative solutions. In 2013, as part of the NPDES permit renewal, DWSD negotiated to include GSI as an approved stormwater management solution.
The City has also implemented policy to help address the issue. DWSD has restructured the way that property owners are charged for drainage, by basing the fee on the amount of hard surfaces that send stormwater runoff into the combined sewer system. The City also passed a post-construction stormwater ordinance to ensure development projects manage the stormwater generated from their project sites. In both cases, green stormwater infrastructure can be a useful and effective management solution to help reduce runoff volume.
A Green Solution
GSI encompasses a wide array of practices, including rain and bioretention gardens, bioswales, living roofs and walls, permeable pavement, rain harvesting, and even certain types of tree planting. Using native plants and soils, GSI helps to reduce runoff volume, remove pollutants, and cut down on flooding by absorbing, delaying, and filtering stormwater runoff before it enters the city’s combined sewer.
GSI is an innovative, cost-effective approach to stormwater management. Rather than transporting water through conventional and costly pipes to wet weather treatment systems, GSI treats stormwater where it falls using nature. And it’s something that all Detroiters can have a hand in. What may appear to simply be a beautiful new garden in your neighbor’s yard could also be managing and filtering stormwater runoff every time it rains.
GSI comes in all shapes and sizes, including:
Bioswale - A linear practice consisting of a modified swale that uses soil filter media to increase water intake at the soil surface. A bioswale helps improves water quality and helps reduce runoff volume and attenuated peak runoff rates for small storms while also providing conveyance of excess runoff. Bioswales are usually long and narrow, have amended soil, and include retention and/or infiltration components.
Direct Discharge - Runoff generated by the impervious areas discharges directly to the Detroit and/or Rouge Rivers.
Disconnected Impervious - A stormwater management practice that directs runoff from impervious surfaces onto properly sized, sloped, and vegetated surfaces. Downspout disconnection is a type of disconnected impervious but should be listed under the specific type for the purposes of this project.
Downspout Disconnection - The process of disconnecting roof downspouts from the sewer system and redirecting the roof runoff onto pervious surfaces, most commonly a lawn.
Green Roof - Conventional rooftops that include a covering of vegetation that intercepts and stores rainfall in the layers of growing media.
Impervious Surface Removal - Removal of impervious surface on a property.
Permeable Pavement - A pavement type that allows rainfall and runoff to pass through the pavement surface and enter the subsurface layer. Permeable pavement is considered a storm water practice and is not considered a pervious surface. Such permeable pavements may be constructed of concrete, asphalt, pavers, and open-graded aggregate.
Rain Barrel - Rain barrels are very small cisterns. Rain barrels commonly collect rain water from a residential roof gutter and downspout and most commonly use the collected water for irrigation needs. Rain barrels are commonly thought as holding less than 100 gallons of water.
Residential Rain Garden - Rain gardens are bioretention practices sized for smaller lots, like residences. They consist of a depressed area with an amended soil or soil filter media capable of infiltrating and filtering stormwater runoff and supporting vegetation.
Stormwater/constructed wetlands - Stormwater wetlands are engineered, shallow-water ecosystems designed to treat stormwater runoff. This only includes engineered GSI wetland solutions, not naturally occurring wetlands.
Subsurface Detention - An underground system consisting of a one or more underground pipes or structures designed to provide the required storage volumes. The system can be designed to allow for infiltration through the bottom.
Subsurface Infiltration - Infiltration practices are designed to encourage percolation and groundwater recharge, which in turn, reduces stormwater runoff from the site along with many of its negative side effects. Infiltration practices come in a variety of configurations that can be fit onto most sites (infiltration basin, infiltration bed, infiltration vault, infiltration swale, infiltration trench, dry well).
Tree-based practices - Tree-based practices are variations on the standard bioretention practice designed to infiltrate runoff and support the growth of trees in highly urbanized areas. The two most common types of tree-based practices are suspended pavement systems and tree boxes. This only includes engineered GSI tree-based solutions.
Water Harvesting - A stormwater management practice where runoff is captured and reused in various processes such as irrigation, toilet flushing, and other graywater uses. This includes above surface cisterns and storage tanks over 100 gallons. Rain barrels (under 100 gallons) are a type of water harvesting but should be listed under the specific practice type for the purposes of this project.
GSI supports a healthy and sustainable Detroit, one with community collaboration and innovative green solutions at the core of its philosophy. In addition to catching and filtering stormwater, GSI adds natural space, beautifies neighborhoods, improves air quality, and adds habitat for local wildlife.
GSI is interwoven into many city and community initiatives including: