Structural Stormwater Best Management Practices
Since stormwater management is a relatively new field, researchers across the world are actively working to invent and improve structural stormwater BMP designs and treatment practices. With new findings, BMP designs are constantly evolving to increase pollutant removal and to further protect our waterways. Below is a list of well-established BMPs utilized by the University of Michigan to treat stormwater runoff.
Bioretention (i.e. Rain Gardens)
Bioretention systems are depressed areas that capture and treat runoff. Most often bioretention includes a gravel layer surrounding an underdrain, a sandy media mix layer atop the gravel to encourage infiltration, vegetation to further increase infiltration and pollutant removal, and a mulch layer to keep in moisture and to prevent internal erosion. Space is left between the underdrain and the bottom of the bioretention basin to promote further storage and thus infiltration.
Figure 1. Profile view of a typical bioretention/rain garden cell
There are several bioretention systems on campus. A great example is found right outside of the Dana Building on Central Campus. Next time you’re walking on campus be sure to check it out!
Constructed stormwater wetlands are designed to include several different water depth zones (shallow marsh zone, deep pool zone, etc.) that allows for the planting of many different wetland species. The carefully selected wetland vegetation aids in the capture and treatment of stormwater runoff. An example of a constructed wetland can be seen on North Campus near the Art and Architecture Building. The southern-most portion of this designed stormwater BMP system is a constructed wetland basin.
Figure 2. The constructed wetland basin included as part of the Art & Architecture stormwater management system
Dry/Wet Detention and Retention Basins
Dry detention basins are basins that remain predominantly dry between storm events. Stormwater runoff is captured during a storm event and is slowly released from the basin to make room for the next storm event. A wet detention basin is very similar, except some of the captured water always stays in the basin, even between storm events.
Dry and wet retention basins are very much like dry and wet detention basins. The only difference is retention basins are not designed to slowly release captured water- all water infiltrates.
There are numerous wet and dry retention and detention systems across campus…. next time you’re passing by the North Campus Grounds Services Building, be sure to look for the retention basin to the southeast of the building, and next time you’re out golfing, be sure to look for the large wet detention basin on the University Golf Course.
Figure 3. Wet detention basin at the University of Michigan Golf Course
Grassed Swales/Vegetated Swales
Grassed or vegetated swales are engineered ditches that promote infiltration while conveying stormwater runoff. These systems can be very simple ditches or very complex vegetated systems. A more complex vegetated swale that contains rock check-dam structures can be seen at the University of Michigan Arboretum (shown below).
Figure 4. Vegetated swale with built in check-dam structures located at the University of Michigan Arboretum
Green roofs are engineered systems that incorporate special media and vegetation to maximize infiltration and evapotranspiration on rooftops. If a building is not specifically designed to include a green roof, it is absolutely critical to confirm that the added weight of a green roof will not impact the structural stability of the building.
Most often, a green roof consists of an impermeable layer over a rooftop (to prevent leaks and structural damage), a drainage layer, a growing media layer, and a vegetation layer. The vegetation must be carefully selected for the local climate and must be able to endure various temperature extremes. Also, when selecting plants, remember that irrigation is discouraged and fertilization is a big no-no! Fertilizers, on or near a BMP, defeat the whole purpose of having the BMP.
Two common green roof types are intensive and extensive green roofs. Intensive green roofs are very intense systems- they have deep growing media for plant growth, large plant types (sometimes trees!) and may even support walking or sitting areas for people. Extensive green roofs have smaller growth media depths, smaller plant species and are generally not built for people to walk on. There are several green roof systems on campus- next time you walk by the Ross Business School or the Kresge Business Library look up and try to find one! Also, be on the lookout for the green roof covering the Mott Children’s Hospital and Von Voigtlander Women’s Hospital.
Figure 5. The green roof installed on top of the Mott Children’s Hospital and Von Voigtlander Women’s Hospital
Hydrodynamic Separators/Swirl Concentrators
Hydrodynamic separators and Swirl Concentrators are manufactured devices designed to remove sediment and/or oil and grease. These devices are often small in size and may be installed underground making them favorable in urban areas where space is limited and larger BMPs are not feasible. Unfortunately, most hydrodynamic separators and swirl concentrators are not suitable for removing nutrients from stormwater runoff, yet when properly designed and maintained they are very effective at removing sediment. Thus, hydrodynamic separators are great for pre-treatment; they are often placed upstream of larger underground or above ground BMPs to prevent the downstream BMP from clogging with sediment. Hydrodynamic separators lower maintenance costs associated with sediment removal and often improve the function of downstream BMPs.
Example: A hydrodynamic device can be placed upstream of a detention basin to reduce the sediment load that enters the detention basin. Essentially the separator is acting as a forebay and is protecting the basin from sediment deposition.
Hydrodynamic separators are used all across campus but may be difficult to see since most are underground!
Figure 6. The manhole cover to an underground hydrodynamic device
Infiltration Devices (Sand filters, Planter Boxes, etc)
Infiltration devices are very similar to bioretention in that they encourage the infiltration of runoff through a media. There are numerous types of infiltration devices with various media types and vegetation types (or no vegetation at all in some cases). Most often sand filters are underground devices that do not include vegetation; whereas, infiltration planter boxes are above-ground planters that incorporate an infiltration/ growth media and vegetation. See if you can spot two infiltration planter boxes outside of the Ross Business School next time you’re on Central Campus!
Level Spreader-Vegetated Filter Strips
Most often level spreaders are long, thin, very evenly laid strips of concrete that receive flow and evenly distribute the flow to a downstream vegetated area. Level spreaders must be very flat in order to be effective, otherwise flow will accumulate in one area and cause erosion. When properly installed, level spreaders evenly disperse flow and greatly improve infiltration.
Figure 7. Profile of a Level Spreader - Vegetated Filter Strip System
There are all sorts of design alternatives to traditional pavement, including: permeable pavement, permeable asphalt, permeable pavers, and permeable concrete and unlike traditional pavement, all of these systems allow for the infiltration of stormwater runoff! Most often a top layer of coarse material (permeable concrete, asphalt, or pavers) is laid over top of a crushed stone mix to allow for maximum infiltration of stormwater runoff. The primary type of pavement is selected depending on the ultimate use, for example, permeable asphalt and concrete types are often used on roadways or in parking lots, whereas permeable pavers are more commonly seen in pedestrian areas or short term parking lots.
While under-drains are commonly installed to ensure adequate drainage, permeable pavement systems are most effective in areas with good draining in-situ soils (i.e.sandy soils)
There are several permeable pavement systems on campus including permeable asphalt, permeable pavement, and permeable pavers.
Figure 8. Examples of porous pavement (left) and permeable pavers (right) used on campus
Underground detention systems are manufactured units designed to capture and hold stormwater runoff. Captured water is allowed to infiltrate into existing soils and slowly release over time. Such devices have been shown to remove nutrients, sediment, and reduce the temperature of water (which is great for trout sensitive waters!) There are plenty of underground detention structures across campus; however, much like the hydrodynamic devices, these systems will be hard to find as they are underground! For example, every time you play soccer at Palmer field, you’re running on-top of an underground detention system!
Structural Stormwater BMP Resources
If you would like further information on BMPs, including: information on design, pollutant removal, regulatory requirements, or current research there are a ton of excellent sources! Below are just a few resources to learn more.