January 2018Bioretention1

Drainage Programwww.detroitmi.gov/drainage

Drainage Program Guide Site Application: Non-ResidentialGSI Practice: BioretentionThe largest impervious surfaces on a non-residential property are typically buildings and parking lots. The average commercial property has 0.27 impervious acres (approximately 11,750 square feet). When rain and snow fall onto impervious surfaces, such as a parking lot or rooftop, the water runs off and into DWSD’s combined sewer system through a catch basin and sewer pipe or sheet flow across the site. Excess stormwater in DWSD’s combined sewer system can result in overflows into the Detroit and Rouge Rivers, which may causing flooding and impact water quality.

This rain and snow can be managed before it reaches DWSD’s sewer system. As a result, a property owner is eligible to receive drainage charge credits for managing the stormwater runoff. One technique that can be used to manage the runoff from a parking lot or rooftop is a bioretention practice.

With a bioretention practice, the runoff is captured in a depressed planting area and then either infiltrates into the soil or flows slowly through to an underdrain or outlet pipe that connects to DWSD’s sewer system. They can be planted with any combination of trees, perennials, shrubs or just seeded with turf grass.

What credits am I eligible for?A bioretention practice that manages stormwater runoff from a parking lot or rooftop can achieve up to an 80% drainage charge credit, depending on the available storage and site conditions. Of the maximum 80% credit available, up to 40% can be attained for controlling how fast water is leaving the property (peak flow credit) and up to 40% can be attained based on how much water can be permanently removed from the sewer system (through infiltration, plant uptake, etc.) (volume credit). See “A Guide to Credits for Commonly Used Stormwater Management Practices” on DWSD’s drainage website (www.detroitmi.gov/drainage) for further explanation.

What is the typical cost of a bioretention practice?The cost of a bioretention practice is dependent on the property’s soil type, size of the impervious area draining into the bioretention, amount of credit desired (i.e., size of the bioretention), and type of plants selected. Bioretention practices typically cost between $15-$45 per square foot.

Linear bioretention along outer edge of parking lot captures the parking lot runoff.

Stormwater from the building’s downspouts is directed into this

bioretention practice

January 2018Bioretention2

Drainage Programwww.detroitmi.gov/drainage

Catch Basin: Existing catch basin structure is modified to divert the runoff to a trench drain.

Trench Drain: A trench drain can be installed to divert stormwater that enters the existing catch basin to the bioretention practice instead of to DWSD’s combined sewer.

Bioretention: Bioretention is installed to manage the stormwater. Use native plants that are salt tolerant if in a location that would receive snowmelt chemicals.

How do I get the water from my impervious surfaces to the bioretention?How the stormwater runoff gets from a parking lot or rooftop to the bioretention will depend on site conditions. One of the first steps is to identify any catch basins that receive runoff from the parking lot or rooftop.

1. Identify all downspouts from the roof, and whether they are directly connected to DWSD’s sewer system. Disconnect downspouts and direct one or more downspouts to a vegetated area or bioretention practice on site.

2. For parking lots, if there is a catch basin in the low point of the parking lot (center), a trench drain may need to be installed to capture the water before it enters a sewer pipe. This will keep the water close to the surface of the parking lot and help keep the bioretention shallower.

3. If there is a catch basin around the outside edge of the parking lot, the catch basin could be removed and the runoff could be directed to the bioretention practice through a shallow stone or vegetated channel.

4. If there are no catch basins in the existing parking lot, a linear bioretention practice is a feasible alteration that extends along the outside edge of the parking lot drain. Be sure the linear bioretention is installed along the edge to where the parking lot drains.

The stormwater in this parking lot flows to the outside edge

where a vegetated bioretention practice was installed to manage

the water before it leaves the property.

Bioretention practices reduce the amount of stormwater runoff that flows into the City’s already burdened sewer system, improve water quality by filtering out pollutants, promote infiltration to the groundwater table, and increase habitat for birds and butterflies.

An existing parking lot in Southfield, Michigan, was restriped and a section of pavement at the low point

was removed and planted for stormwater management.

Catch Basin Trench Drain

Bioretention

Ex. Sewer Pipe

January 2018Bioretention3

Drainage Programwww.detroitmi.gov/drainage

PRACTICE RATIO

VOLUME CREDIT

(%)*

PEAK FLOW CREDIT (%)*

0.1 21 00.2 29 170.3 33 250.4 35 340.5 37 400.6 38 400.7 39 400.8 40 40

Do I have to maintain the bioretention practice?Just like you would maintain a garden or lawn at your home, bioretention practices on non-residential properties require maintenance and pass inspection. Every three years, a property owner will be required to submit a certification that the practice is being maintained. If DWSD does not receive the maintenance certificate once every three years, the credit will expire. Typical maintenance activities include:

6 Removing weeds regularly (more frequently during first couple of years).

6 Periodic watering, especially during the one-to-two season establishment period.

6 Removal of accumulated trash and dirt.

6 Adding mulch when needed.

What is the expected return on my investment?The return on investment will depend on the size of the installed practice and the resulting credit reduction. The curve to the right summarizes the anticipated return on investment based on the practice area, with a bioretention cross-section of 24 inches of engineered soil, 4 inches of mulch, and varying depths of surface storage above the bioretention surface.

Expected Drainage Charge Credit Values

The practice ratio is the size of the stormwater practice relative to the size of the impervious area the practice is collecting.

The expected drainage charge credit values present in the adjacent table are based on a bioretention practice with 12 inches of surface storage and 24 inches of engineered soil.

*Assumes 0.1 inch per hour infiltration rate

An example of a trench drain installed to capture runoff prior to entering the catch

basin connected to DWSD’s sewer system.

Bioretention installed in existing parking lot island to capture and infiltrate

stormwater.

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Drainage Programwww.detroitmi.gov/drainage

6 Sewer Tap (DWSD): Contact DWSD at 313.964.9236

6 Plumbing Permit (BSEED): Any time project work on private property connects to City sewer Contact: BSEED’s Plumbing Inspector at 313.224.3158

6 Parking Lot Requirements: (BSEED) – To confirm City parking requirements, contact BSEED’s Zoning Department at 313.224.1317

6 Complete engineered drawings stamped by a registered Professional Engineer or Landscape Architect must accompany the Drainage Charge Credit Application. Additional required documentation is found on the application and can be downloaded www.detroitmi.gov/drainage.

Guide to Drainage Credits: www.detroitmi.gov/Portals/0/docs/DWSD/A%20Guide%20to%20Drainage%20Charge%20Credits%20-%20Web.pdf?ver=2016-09-06-093601-287

Guide to Credits for Commonly Used Storm Water Practices: www.detroitmi.gov/Portals/0/docs/DWSD/A%20Guide%20for%20Credits%20for%20Commonly%20Used%20Storm%20Water%20MgmtPractices_WEB%20READY.pdf?ver=2016-11-16-173244-490GSI

Green Storm Water Infrastructure Credit Calculator: Add url when available

Low Impact Development Manual for Michigan: www.semcog.org/reports/lid/index.html

Michigan Plant List: www.michigan.gov/documents/deq/wrd-nps-landscape4wq_401217_7.pdf

http://therouge.org/our-wrik/river-restoration

Design Guidelines 6 Avoid installation of bioretention in areas where

the water table is less than 2 feet below the bottom of the practice.

6 Provide a stone channel or other energy dissipation device at the inlet to slow velocities if needed.

Setback Requirements 6 From property line: TBD

6 From parking space: 3-foot minimum

6 From building: 10-foot minimum to edge

6 From pavement: 3-foot minimum

Permits

Additional Resources

Stone infiltration trench installed around the outside perimeter of an existing parking lot to manage the runoff from the parking lot.

Rooftop drainage can be directed to a green stormwater practice, as shown here, before overflowing into the combined sewer system.

NOTE: This GSI practice cannot be self-performed.