Low Impact Development Manual for Michigan - OSEH - University ...

Primary Components of a Bioretention System
1. Pretreatment (may be necessary to help prevent clogging)
• Sediment removal through a vegetated buffer strip, cleanout, stabilized inlet, water quality inlet, or sediment
trap prior to runoff entry into the bioretention area
2. Flow inlet
• Varies with site use (e.g., parking island versus residential lot applications – see Figures 7.11 through 7.14)
• Entering velocities must be non-erosive – use erosion control mats, blankets, or rock where concentrated runoff
enters the bioretention area
3. Ponding area
• Provides temporary surface storage of runoff and allows sediment to settle
• Provides evaporation for a portion of runoff
• Depth no more than 6-18 inches for aesthetics, functionality, and safety
4. Plant material (see Appendix C for recommended plant lists)
• Absorbs stormwater through transpiration
• Root development creates pathways for infiltration
• Bacteria community resides within the root system creating healthy soil structure with water quality benefits
• Can improve aesthetics for site
• Provides habitat for animals and insects
• Reinforces long-term performance of subsurface infiltration
• Ensures plants are salt tolerant if in a location that would receive snowmelt chemicals
• Should be native plant species and placed according to drought and water tolerance
5. Organic layer or mulch
• Acts as a filter for pollutants in runoff
• Protects underlying soil from drying and eroding
• Simulates leaf litter by providing environment for microorganisms to degrade organic material
• Provides a medium for biological growth, decomposition of organic material, adsorption and bonding of
heavy metals
• Wood mulch should be shredded – compost or leaf mulch is preferred
6. Planting soil/volume storage bed
• Provides water/nutrients to plants
• Enhances biological activity and encourages root growth
• Provides storage of stormwater by the voids within the soil particles
• Provides surface for adsorption of nutrients
7. Positive overflow
• Provides for the direct discharge of runoff during large storm events when the subsurface/surface storage
capacity is exceeded
• Examples of outlet controls include domed risers, inlet structures, and weirs
LID Manual for Michigan – Chapter 7 Page 139