The BMP Landscape Guide for BMPs provides information on vegetating bioretention.
The primary value that vegetation provides is a deep root structure that maintains drainage through the soil media. Vegetation should promote social acceptance: bioretention is a landscape feature, as well as a stormwater BMP.
Construction sediment control
It is best that bioretention areas and sediment control areas be located separately. Unfortunately, this isn’t practicable at some sites. When bioretention and sediment control areas are located in the same place, then:
- Bioretention should never be fully constructed unless sediment can be prevented from entering the filter bed.
- Additional erosion control should be placed within the site and upgradient of the bioretention area. Erosion control should be monitored and maintained throughout all phases of construction. Controlling the amount of sediment that could potentially accumulate within the future bioretention areas will reduce the amount of additional excavation that will be required prior to installing the filter media.
- If bioretention areas are used for capturing construction sediment, then all construction sediment and 1 foot of existing soil beneath the sediment should be removed prior to bioretention construction. Excavator buckets with teeth, tilling, or soil ripping should then be used to loosen the resulting soil subgrade.
Pre-treatment is necessary on larger (>0.5 acre) drainage areas to prevent erosion of the filter bed, as well as capture trash and coarse sediment that can shorten the life of the filter bed. The two most common methods of pretreatment are forebays and sheetflow over vegetated or gravel filter strips.
A forebay is typically used when a stormwater pipe or swale directly discharges into the BMP. The pre-treatment forebay should be sized to store 25% of the water quality volume and to satisfy sedimentation basin minimum surface area requirements (see the Maryland Stormwater Design Manual). The forebay floor should be designed to fully drain to the surface of the bioretention filter bed, and this is typically accomplished by constructing a French drain in the bottom of the forebay.
A filter strip is typically used when stormwater enters the bioretention as overland flow or by curb notches. At a minimum, this approach includes a gravel diaphragm (to slow the runoff velocity) and a grass filter strip between the diaphragm and the filter bed. (The Maryland Manual recommends the grass filter strip be 20′ long.) Bioretention fed by overland flow and curb notches is an “on-line” BMP, and filter strip gravel and vegetation should be designed to withstand flow velocities from the 15-year 20-minute design storm.
Pre-treatment forebays and filter strips are not typically necessary in BMPs draining small tributary drainage areas. When overland flow or curb notches feed the BMP, a gravel diaphragm should be provided along the edge of the BMP.
On-line, Off-line, & Overflow Assessment
On-line BMPs pass all storms either through the BMP filter or the overflow. Bioretention may be placed on-line when fed by overland flow or curb notches, when sized to store the channel protection volume (or greater), or when tributary areas are less than 0.5 acres. All on-line BMPs are required to provide an overflow that safely directs larger storms to other storm drainage features. The overflow and any downstream sewer reaches should be designed for the 15-year 20-minute design storm for separate sewer systems and the 20-year 20-minute design storm for combined sewer systems.
Off-line BMPs pass the water quality flowrate into the BMP, but bypass higher flows into other storm drainage features. This typically is provided using a weir or high-flow/low-flow piping system. Guidance for Design of Flow-Splitting systems is provided. Overflows are recommended for off-line BMPs. The type of overflow selected should consider potential for resuspension of trash/pollution, but also the potential for overtopping and availability of a safe overland flowpath for large storms.
Proximity to Buildings
Bioretention areas whose subgrade is above the foundation or footing should be lined, unless located 10 feet or greater from the foundation edge. The floors and walls of bioretention planter boxes adjacent to building foundations should be made water tight (with the exception of slab-on-grade construction).
Geotechnical evaluation & pre-construction soil testing
Unless pre-construction soil infiltration testing shows that (native) soil subgrade rates exceed 0.52 inches/hour, an underdrain should be provided to ensure the bioretention cell drains. Because of local soil conditions, most bioretention sites within MSD’s jurisdiction have an underdrain. Underdrains are often “perched” or use “internal water storage” to promote infiltration of some stormwater. (Perching the underdrain should be avoided when a liner is placed above the subgrade.)
The Maryland Manual (Appendix D.1) requires one infiltration test and one test pit per 200 sf of subgrade area. MSD will accept either this approach, or a combination of infiltration testing and soil testing. It is strongly recommended that designers discuss soil testing with MSD prior to performing soil infiltration rate field investigations. Soil testing should include grain size, organic matter, and bulk density analysis for input into the Soil Water Characteristics model. The saturated hydraulic conductivity (and infiltration rate) can then be estimated and compared to field infiltration testing results.
Depending on the site, a geotechnical investigation should be conducted to determine the depth to bedrock, depth to ground water, and the presence of any karst topography.
The depth to bedrock will not only influence excavation costs, but will also influence the infiltration capabilities of the facility. If near the underlying infiltration zone (subgrade) of the bioretention, the bedrock will act as an impermeable layer. MSD recommends the bottom of the infiltration component of the bioretention facility be located at least two feet above the bedrock.
When the groundwater table is close to the infiltration zone, this can inhibit the effective infiltration capacity of the facility. Bioretention should be located at least two feet above the seasonal high level of the water table. MSD recommends determining this seasonal high water table elevation from taxonomy (e.g., soil borings that indicate certain types of mottling or coloration).
Groundwater use, site land use, and the potential for impacting groundwater resources should be carefully weighed when evaluating BMP type. Bioretention is typically not recommended for uses that are considered “hotspots.” In addition, the bioretention area should be located at least 100 feet horizontally from any wells.
If karst topography is suspected in an area where bioretention is being considered, the procedures outlined in the 2000 Maryland Stormwater Design Manual, Vol. II, Appendix D.2. Geotechnical Methods for Karst Feasibility Testing should be considered in early preliminary stages of design.
In general, the bioretention shall not result in any of the following:
- A change in the overall surface runoff pattern of the existing sinkhole network
- Artificial routing of stormwater between sinkholes
- Blocking or filling of sinkholes
- Degradation of the quality of the water entering the subsurface through the sinkhole
- Increase in the flow rate or volume of water that enters the subsurface through the sinkhole
In addition, the designer should check with applicable St. Louis County or municipal regulations.