Designers need to carefully think through many factors to choose the most appropriate, effective and feasible practice(s) at a development site that will best meet local and state stormwater objectives. This chapter presents a flexible approach to BMP selection that allows a stormwater manager to select those BMPs most able to address an identified problem. Selecting an inappropriate best management practice (BMP) for a site could lead to adverse resource impacts, friction with regulators if a BMP does not work as anticipated, misperceptions about stormwater control success, and wasted time and money. Careful selection of BMPs will prevent negative impacts from installing the wrong BMP at the wrong location. Regulators can similarly use these matrices to check on the efficiency of proposed BMPs.
The approach used in this Manual is slightly different from many other manuals. The proposed concept uses a “functional components approach” wherein basic BMP components are selected and pieced together to achieve a desired outcome. For example, if a BMP is needed to reduce peak discharge and remove sediment, the actual design components are then assembled based upon the material presented in the design guidance for stormwater ponds. In this case, a pond with a specific outflow rate(s) and sufficient water quality storage is designed to meet both functions according to state design criteria. This approach limits the inclusion of numerous individual BMP sheets in favor of categorical sheets with design variations included on each sheet. This should be a more user-friendly way of defining how BMPs can be designed to solve a particular problem.
BMP lists follow a simple-to-more complex treatment train sequence, one that starts with on-site pollution prevention and works upward in complexity to wetland systems. The list of treatment supplements is a compilation of additional measures that could be used to enhance treatment either before or after more complex BMP use.
Detailed BMP fact sheets can be found in the individual sections for bioretention, filtration (see swales or sand filters), infiltration (see Infiltration trench or Infiltration basin), ponds, wetlands, trees, green roofs, turf, and permeable pavement. Pollution prevention, better site design/LID, runoff minimization (see Stormwater re-use and rainwater harvesting) and temporary construction runoff control practices will include some descriptive language for the numerous practices listed via “fact sheets,” but will not contain engineering details. Sections on treatment supplements will similarly not contain detailed engineering, but will describe a process that designers should follow when considering the use of proprietary devices, inserts and chemical/biological treatment.
The beginning stormwater manager or a designer unfamiliar with the many BMPs available might have some questions on which BMP or group of BMPs to include in a treatment scheme. The figure below is a screening tool to get the user going on BMP selection. It contains the list of BMPs contained in this Manual and a corresponding list of use assessment parameters to help narrow the wide range of potential BMPs for a particular project. A user will need to have some objectives in mind to extract information from the matrix, but once into the matrix, selection of BMPs based on either positive or negative factors will be possible.
Nine factors should be evaluated in the BMP selection process, as follows:
Pollution prevention should be the first consideration during any development or redevelopment project and is the first step in the treatment train. This step involves looking for opportunities to reduce the exposure of soil and other pollutants to rainfall and possible runoff. Examples of pollution prevention practices include keeping urban surfaces clean, proper storage and handling of chemicals, and preventing exposure of unprotected soil and pollutants.
A range of better site design (BSD) techniques can provide non-structural stormwater treatment, improve water quality and reduce the generation of stormwater runoff. These techniques reduce impervious cover and reduce the volume of stormwater runoff at a site, which can save space and reduce the cost of structural BMPs. Designers should understand the comparative benefits and drawbacks of BSD techniques that could potentially be applied to the site. Designers should answer the following questions:
All of the techniques shown in the table below are suitable for cold climate conditions in the State of Minnesota.
This table shows an overview of techniques to reduce runoff during site design and layout
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|Better site design technique||Reduce stormwater runoff volume||Possible stormwater credit||Cost savings||Local feasibilitya||Appropriate land use|
|Natural area conservation||High||Yes||High||Promoted||All|
|Site reforestation and prairie restoration||High||Yes||Medium||Promoted||All|
|Stream and shoreline buffers||High||Yes||High||Required||All|
|Surface impervious cover disconnection||High||yes||High||Experimental||Residential, commercial, industrial; caution with industrial potential stormwater hotspots|
|Open space design||High||No||High||Constrained||Residential, commercial, industrial|
|Grass channels||Low||Yes||High||Constrained||Residential, commercial, industrial; caution with industrial potential stormwater hotspots|
|Reduced street width||High||No||High||Constrained||Residential, commercial|
|Reduced sidewalks||High||No||High||Constrained||Residential, commercial|
|Smaller and vegetated cul-de-sac||High||No||High||Constrained||Residential|
|Green parking lots||Medium||No||Low||Experimental||High density residential, commercial, industrial|
avaries greatly among communities; consult local reviewing authority to determine ease of implementation
Construction sites can be a major source of sediment and nonpoint source pollutants if soils are exposed to erosion. Effective application of temporary sediment controls is an essential element of a stormwater management plan and helps preserve the long-term capacity and function of permanent stormwater BMPs. Designers should recognize that they will need to revisit and refine the erosion and sediment control plan throughout the design and construction period as more information on site layout and the type and location of BMPs becomes available.
This table shows a summary of temporary construction and sediment control techniques
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|Technique||Practice||How it works||When to apply||Comments|
|Pre-construction planning||Site planning and grading||Minimizes soil disturbance and unprotected exposure||Planning||Expose only as much as needed for immediate construction|
|Sequencing||Limits amount of soil exposed||Planning|
|Resource protection||Forest conservation and water resource buffers||Establishes protective zone around valued natural resources||Early||Buffer variable from a few feet to 100 feet depending uopn resource being protected and local regulations|
|Perimeter control||Access and egress control||Minimizes transport of soil off-site||Early||Must be in place prior to commencement of construction activities|
|Inlet protection||Stops movement of soil into drainage collection system||Early|
|Slope stabilization||Grade breaks||Minimizes rill and gully erosion||Early||No unbroken slopes > 75 feet on 3:1 side slopes or greater|
|Silt curtain||Stops sediment from moving||Early|
|Runoff control||Stabilize drainageways||Minimizes increased erosion from channels||All construction phases||Possible to convert these into permanent open channel systems after construction|
|Sediment control basins||Collects sediment that erodes from site before it leaves site or impacts resource||All construction phases||Possible to convert these into post construction practices after construction|
|Rapid stabilization of exposed soils||Seeding and mulch||Immediately established vegetative cover on exposed spoil||All construction phases||Apply seed as soils are exposed|
|Blankets||Provides extra protection for exposed oil or steep slopes||All construction phases||Apply blanket as exposed soil cover until plants established|
|Inspection and maintenance||Formalized I&M program||Assures that BMPs are properly installed and operating in anticipated manner||All construction phases||Essential to proper BMP implementation|
Designers should understand the nature and regulatory status of the waters that will receive runoff from the development site. The type of receiving water strongly influences the preferred BMP to use, and in some cases, may trigger increased treatment requirements. There are many different kinds of Special Waters and other sensitive receiving waters in Minnesota that should be considered (see Sensitive waters and other receiving waters; Regulatory information). For purposes of this Manual, receiving waters fall into five categories: lakes, trout resources, drinking waters, wetlands and impaired waters.
The full spectrum of BMPs can be applied to sites that drain to receiving waters that are not designated as special or sensitive in Minnesota. If the receiving water falls into one of the special or sensitive water categories, the range of BMPs that can be used may be reduced. For example, only BMPs that provide a higher level of phosphorus removal may be encouraged for sensitive lakes. In trout streams, use of ponds may be discouraged due to concerns over stream warming.
The full range of BMP restrictions for the five categories of receiving water are presented in the following table.
This table shows the appropriateness of different BMP groups for different categories of receiving water.
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|Lakes||Trout resources||Drinking waterb||Wetlandsc||Impaired waters|
|General location||Outside of shoreline buffer||Outside of stream buffer||Setbacks from wells, septic systems||Outside of wetland buffer||Selection based on pollutant removal for target pollutants|
|Bioretention||Preferred||Preferred||OK with cautions for potential stormwater hotspots||Preferred||Preferred|
|Filtration||Some variations restricted due to limited P removal; combined with other treatments||Preferred||Preferred||OK||Preferred|
|Infiltration||Preferred||Preferred||Restricted if potential stormwater hotspot||Preferred||Restricted for some target TMDL pollutants|
|Stormwater ponds||Preferred||Some variations restricted due to pool and stream warming concerns||Preferred||Preferred but no use of natural wetlands||Preferred|
|Constructed wetlands||Some variations restricted due to seasonally variable P removal; combined with other treatments||Restricted except for wooded wetlands||Preferred||Preferred but no use of natural wetlands||Preferred|
|Supplemental BMPs||Restricted due to poor P removal; must combine with other treatmentsX||Restricted - must combine with other treatments||Restricted - must combine with other treatments||Restricted - must combine with other treatments||Restricted - must combine with other treatments|
aOutstanding Resource Value Waters (ORVWs) are not included because they fall within one of the receiving water management categories.
bApplies to groundwater drinking water source areas only. Use the sensitive lakes category to define BMP design restrictions for surface water drinking supplies.
cIncluding calcareous fens
Climate and terrain conditions vary widely across the State, and designers need to explicitly consider each of these regional factors in the context of BMP selection. The proposed BMPs for the site should match the prevailing climate and terrain. Specific questions that should be asked include the following:
Preferred BMPs and design modifications are outlined in the following table.
This table shows the summary of climate, soil and terrain factors that affect the selection of Best Management Practices.
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|BMP||Karst||Bedrock and shallow soils||High snowfall - meltwater treatment||Low rainfall|
|Bioretention - no underdrain||Not recommended. Extensive pre-treatment required||Not recommended due to separation distance||OK. Use salt-tolerant vegetationand pre-treatment. Chlorides will move through untreated.||OK. Use appropriate vegetation.|
|Bioretention with underdrain||OK. Under certain conditions use impermeable liner||Recommended.||OK. Use salt-tolerant vegetationand pre-treatment. Chlorides will move through untreated.||OK. Use appropriate vegetation.|
|Filtrationa||OK. Under certain conditions use impermeable liner||Recommended||OK. Place below frost line. Use pre-treatment. Chlorides will move through untreated.||Recommended.|
|Infiltrationb||Not recommended. Extensive pre-treatment required||Not recommended due to separation distance||OK but could be limited. Active management needed to prevent infiltration of chlorides and soluble toxics.||Recommended|
|Stormwater ponds||OK. Under certain conditions use impermeable liner; limit depth; geotechnical investigation needed.||Limited due to available depth and large surface area requirement.||Recommended. Limit depth to avoid stratification. Adapt outlet structure.||Limited. Water budget calculations may show this to be unsuitable.|
|Constructed wetlands||OK. Under certain conditions use impermeable liner; limit depth; geotechnical investigation needed.||OK. Large surface area.||OK. Use salt-tolerant vegetation.||Limited. Water budget calculations may show this to be unsuitable.|
Not all BMPs work over the wide range of storm events that need to be managed at a site. Designers first need to determine which of the recommended unified sizing criteria apply to the development site (i.e., recharge, water quality, channel protection, peak discharge), and then choose the type or combination of BMPs that can achieve them.
This is the stage in BMP selection process where designers often find that a single BMP may not satisfy all stormwater treatment requirements. The alternative is to use a combination of BMPs arranged in a series or treatment train, or add supplemental practices to the primary BMP that provide additional pre- or post-treatment. Some questions that designers should ask include the following:
The following table provides guidance on BMP selection for different management scenarios.
This table shows guidance on BMP suitability for different stormwater strategies.
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|BMP group||Rechargea||Water qualitya||Channel protectiona||Peak dischargea||Hot spot runoff|
|Bioretention||Varies||Yes||Possibleb||No||Yes. Needs underdrain|
|Filtration - media||No||Yes||No||No||Yes|
|Filtration - vegetative||Variesc||Yes||Possibleb||No||Yes|
|Constructed wetlands||Variesd||Yes||Yes||Yes||Yes. Needs pre-treatment|
aSee section on unified sizing criteria for more information
bCan be incorporated into the structural control in certain situations
cMay be provided by infiltration
dWhen impermeable liners are required or pool intercepts groundwater
eCan be included as part of the treatment train
By this point, the list of possible BMPs has been narrowed and now physical factors at the site are assessed to whittle it down even further. Table 7.8 indicates eight physical factors at the site that can constrain, restrict or eliminate BMPs from further consideration.
Some BMPs can provide positive economic and environmental benefits for the community, while others can have drawbacks or create nuisances. Table 7.9 presents general guidance on how to choose the most economically and environmentally sustainable BMPs for the community. Readers should note that rankings in this table are fairly subjective, and may vary according to community perceptions and values. A poor score should not mean the BMP is discarded; rather, it signals that attention should be focused on improving that element of the BMP during the design phase.
The last step in BMP selection checks to see if any environmental resources or infrastructure are present that will influence where a BMP can be located on the site (i.e., setback or similar restriction). Table 7.10 presents an overview of ten site-specific conditions that impact where a BMP can be located on a site. A more extensive discussion of the relevant Minnesota rules and regulations that influence BMP design can be found in Chapter 5 and Appendix G.
Stormwater managers are reluctant to make a final BMP selection without having some basic information on the construction and maintenance costs. Chapter 12 and Appendix D contain guidance on the preparation of construction and maintenance costs for specific BMPs. However, this technique is not always practical or even feasible at the BMP selection stage. Stormwater managers who wish to learn the relative cost effectiveness between two specific BMPs are encouraged to use information prepared by the Minnesota Department of Transportation in a May, 2005 report titled The Cost Effectiveness of Stormwater Management Practices. As part of their research, the authors incorporated both historical construction costs and 20 years of expected annual maintenance costs. The result is a series of graphs that present total present cost (construction plus maintenance) plotted against water quality volume. Figure 6.2 can be used to determine the total present worth value of construction plus maintenance costs for wet basins. Similar graphs are available for dry detention basins, constructed wetlands, infiltration trenches, bioinfiltration filters, sand filters, and 1,000-foot long vegetated swales in the Mn/DOT report. This simple technique can then be used to estimate the total present cost of a BMP under consideration. For purposes of establishing a specific budget for construction and maintenance, stormwater managers are encouraged to follow the procedures outlined in Chapter 12.