Design criteria for bioretention
Design Criteria
- Infiltration requires suitable soils.
- Minimum 10’ setback and located down grade from home foundations.
- Best applied to drainage areas with relatively flat slopes (5%).
Benefits
- Can be very effective for removing fine sediment, trace metals, nutrients, bacteria and organics (Davis et al. 1998).
- Provides many additional environmental (habitat, improves air quality, urban micro-climates), social (creates a unique sense of place) and economic benefits (reduces development and maintenance cost, greater lot yield, increases property values).
- Well suited for high impervious areas.
- Reduces runoff volume.
- Flexible design, affording many opportunities for creativity.
Limitations
- Susceptible to clogging by sediment; therefore maintenance and pre-treatment is necessary to maintain effectiveness.
- Not effective for large drainage areas (use multiple structures, closer to source of runoff).
- Space consumption (5%-10% of drainage area).
Description In general, bioretention systems can be described as shallow, landscaped depressions commonly located in parking lot islands or within small pockets in residential areas that receive stormwater runoff (Metropolitan Council Small Sites BMP Manual, 2001).
Bioretention facilities capture rainwater runoff to be filtered through a prepared soil medium. Once the soil pore space capacity of the medium is exceeded, stormwater begins to pool at the surface of the planting soil. Pollutants are removed by a number of processes including adsorption, filtration, volatilization, ion exchange and decomposition (Prince George’s County, MD, 1993). Filtered runoff can either be allowed to infiltrate into the surrounding soil (functioning as an infiltration basin or rainwater garden), or collected by an under-drain system and discharged to the storm sewer system or directly to receiving waters (functioning like a surface sand filter). Runoff from larger storms is generally diverted past the area to the storm drain system (Metropolitan Council Small Sites BMP Manual, 2001).
Bioretention is a stormwater treatment practice that utilizes the chemical, biological and physical properties of plants, microbes and soils for capturing/reducing stormwater runoff and removing pollutants from runoff. This process is often incorporated into many different types of filtration and infiltration stormwater treatment practices.
Management Suitability
- Med/High Water Quality (Vwq)
- Med Channel Protection (Vcp)
- Low/Med Overbank Flood Protection (Vp10)
- Low Extreme Flood Protection
- High Recharge Volume (Vre)
Mechanisms
- Infiltration with appropriate soils & site conditions
- Filtration
- Temperature Control
- Settling
- Transpiration
- Soil Adsorption
- Biological/ Micro. Uptake
Pollution Removal
- 85% - Total Suspended Solids
- 50%/ 45% - Nutrients - Total Phosphorus/Total Nitrogen
- 95% Metals - Cadmium, Copper, Lead, and Zinc
- 35%* Pathogens - Coliform, Streptococci, E. Coli *less than 5 independent studies
- 80%* Toxins - Hydrocarbon *less than 5 independent studies
Note: Average pollutant removal expected when sizing based on MPCA criteria. Values apply to treated runoff only.
SITE FACTOR
- 2 AC Max; 1 AC Max Impervious - Drainage Area For Filtration Design (Per Practice)
- 2 AC Max; 1/2 AC Max Impervious - Drainage Area For Infiltration Design (Per Practice)
- 20% Max. - Slope of Site
- 3’ Min. - Depth to Bedrock & Seasonally High Water Table
- A,B – Normal NRCS Soil Type *can be used in C&D soil types with modifications (e.g. under-drains) Requires impermeable liner.
- Good - Freeze/ Thaw Suitability
- Suitable - Potential Hotspot Runoff
Storm Sequence
Start of Storm Event - Initial runoff & storage 343x235px
Duration of Storm Event - Storage & filtration/infiltration 343x230px Following Storm Event - Remaining storage drawdown
For regulatory purposes, bioretention practices fall under the “Infiltration / Filtration” category described in Part III.C.2 of the CGP. If used in combination with other practices, credit for combined stormwater treatment can be given as described in Part III.C.4. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the bioretention practice will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.
The following terms are thus used in the text to distinguish various levels of bioretention practice design guidance:
Of course, there are situations, particularly retrofit projects, in which a bioretention practice is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular bioretention practice, depending on where it is situated both jurisdictionally and within the surrounding landscape.
Design Variants
Alternative Names
As bioretention becomes a more common tool in the stormwater management toolbox and as the number of design variants increases, so does the number of names for each of these variants.
As an example of the ongoing evolution of bioretention terminology, the terms “rain garden” and “rainwater garden” have recently caught on with the public and are being used interchangeably with bioretention. In most instances, rain garden designs are utilizing the processes of bioretention, but the term rain garden is also being loosely used to describe BMPs that are operating more as stormwater ponds (or as other BMPs) than as bioretention facilities.
Further confusion stems from the using the terms “process” and “practice” interchangeably. As mentioned earlier, bioretention is not a “practice” per se, but rather a process or group of processes that can be incorporated into many different practices. This section is provided to clarify the more common bioretention terminology being used in the field of stormwater management today