Process for selecting Best Management Practices

The Interstate Technology and Regulatory Council has created a Stormwater BMP Performance Evaluation Team. The team is tasked with identifying best methods for evaluating the pollution-reduction capabilities and verifying the performance of stormwater best management practices (BMPs) for Clean Water Act compliance purposes.

The Minnesota Local Road Research Board produced a BMP Selection Manual that provides useful information on selecting best management practices. They also produced a YouTube Video that provides additional information. Note: The video states that ponds have 70 percent removal of nutrients; the MPCA believes the number is closer to 50 percent.

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 page presents a flexible approach to Best Management Practices (BMP) selection that allows a stormwater manager to select those BMPs most able to address an identified problem. Selecting an inappropriate 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 resulting from installing the wrong BMP at the wrong location. Regulators can similarly use these matrices to check on the efficiency of proposed BMPs.

Using the manual to select BMPs

This Manual 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, stormwater ponds can be selected as the BMP and 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 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.

Information on BMPs can be found in the individual sections for bioretention, filtration (see Swales or Sand filters), infiltration (see Infiltration trench or Infiltration basin), stormwater ponds, stormwater wetlands, trees, green roofs, turf, and permeable pavement. Sections on pollution prevention, better site design/LID, runoff minimization (see Stormwater re-use and rainwater harvesting) and temporary construction runoff control practices include some descriptive language but do not include 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. A matrix can be developed to serve as a screening tool to get the user going on BMP selection. The matrix contains a 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.

Information: The 2006 Stormwater Manual contained a table that provided considerable information about different stormwater BMPs. That table can be accessed at this link. Images of the table, taken from the 2006 Minnesota Stormwater Manual, are shown to the right. The information from the table has also been placed into a File:BMP recommended practices.xls.

The four images above show tables illustrating recommended and non-recommended practices associated with BMPs for different use assessments (e.g. volume reduction, cold climate suitability, appropriateness for lakes, etc.).

Factors to consider in selecting BMPs

Nine factors should be evaluated in the BMP selection process, as follows:

1. Investigate pollution prevention opportunities. Evaluate the site to look for opportunities to prevent pollution sources on the land from becoming mobilized by runoff.
2. Design site to minimize runoff. Assess whether any better site design techniques can be applied at the site to minimize runoff and therefore reduce the size of structural BMPs.
3. Select temporary construction erosion and sediment control techniques. Check to see what set of temporary sediment control techniques will prevent erosion and minimize site disturbance during construction.
4. Identify receiving water issues. Understand the regulatory status of the receiving water to which the site drains. Depending on the nature of the receiving water, certain BMPs may be promoted, restricted or prohibited, or special design or sizing criteria may apply.
5. Identify climate and terrain factors. Climate and terrain conditions vary widely across the state, and designers need to explicitly consider how each regional factor will influence the BMPs proposed for the site.
6. Evaluate stormwater treatment suitability. Not all BMPs work over the wide range of storm events that need to be managed at the site, so designers need to choose the type or combination of BMPs that will provide the desired level of treatment.
7. Assess physical feasibility at the site. Each development site has many physical constraints that influence the feasibility of different kinds of BMPs. Designers confirm feasibility by assessing eight physical factors at the site.
8. Investigate community and environmental factors. Each group of BMPs provides different economic, community, and environmental benefits and drawbacks. Designers need to carefully weigh these factors when choosing BMPs for the site.
9. Determine any site restrictions and setbacks. Check to see if any environmental resources or infrastructure are present that will influence where a BMP can be located at the development site.

Investigate pollution prevention opportunities

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.

Design site to minimize runoff

A range of better site design (BSD) techniques can provide non-structural stormwater treatment, improve water quality and reduce the generation of stormwater runoff

Green Infrastructure: 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:

1. How well does the technique reduce stormwater runoff volume? Each BSD technique is rated as having a high, medium, or low capability to reduce the volume of stormwater runoff generated at a development site. The ability to promote infiltration of runoff, preserve natural hydrology or filter pollutants are main reasons why these techniques vary in their volume reduction capability.
2. Is the technique eligible for a possible stormwater credit? While all better site design techniques can reduce the size and cost of structural BMPs needed at the site, 6 techniques may be eligible as a stormwater credit during the design phase. Check with your local review authority to see which credits may be offered in your community. Stormwater credits can reduce required water quality volumes by as much as 10 to 40 percent, and even more if multiple credits are applied.
3. What are the potential cost savings for developers? Many BSD techniques can result in significant cost savings for developers during construction in the form of reduced infrastructure costs, more available land for development, higher and faster sales, and lower long-term maintenance costs.
4. How easy is it to implement the technique in most communities? Some BSD techniques are standard practices in many communities, while others are newer and more difficult to adopt. Some techniques are considered experimental and are not included in current local design guidelines and may involve a time-consuming and uncertain approval process. Required techniques are allowed under most local design guidelines, whereas promoted techniques are actively encouraged in most communities. Constrained techniques are harder to implement since current local codes impede or even prohibit their use in some communities. Designers should always check with their local reviewing authority to confirm which techniques can be used.
5. What is the most appropriate land use for the technique? The nature of the proposed land use at a site often influences the kinds of BSD techniques that can be applied. Land uses include residential development, high density residential development, commercial/office including institutional uses, and industrial development. Care must be take to ensure the land use is not a stormwater hotspot that restricts the use of certain BMPs.

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
Link to this table

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
Soil amendments	High	Yes	Low	Experimental	All
Surface impervious cover disconnection	High	yes	High	Experimental	Residential, commercial, industrial; caution with industrial potential stormwater hotspots
Rooftop disconnection	High	Yes	High	Experimental	All
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
Shorter driveways	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

Select temporary construction sediment control techniques

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
Link to this table

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

Identify receiving water issues

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.

1. Does the site drain to a sensitive lake? BMPs differ in their ability to remove phosphorus, which is the key stormwater pollutant managed to protect sensitive lakes (Note: this category also includes trout lakes and surface water drinking supplies). Communities may require greater water quality treatment, a specific phosphorus removal rate or even load reduction at the development site to protect their most sensitive lakes. In general, higher phosphorus removal requirements result in shorter list of acceptable BMP designs that can be used at the site.
2. Does the site drain to a trout stream protection? Trout streams merit special protection, which strongly influences the choice of BMPs. Some BMPs are preferred because they promote baseflow, protect channels from erosion or achieve high rates of sediment removal. Other BMPs, such as ponds, may be discouraged because they cause stream warming.
3. Is the site within a groundwater drinking water source area? Sites located in aquifers used for drinking water supply require BMPs that can recharge aquifers at the same time they prevent groundwater contamination from polluted stormwater, particularly when it is generated from stormwater hotspots.
4. Does the site drain to a wetland? Wetlands can be indirectly impacted by upland development sites, so designers should choose BMPs that can maintain wetland hydroperiods and limit phosphorus loads. Several BMPs provide infiltration and extended detention storage that protect natural wetlands from increased stormwater runoff and nutrient loads from upland development.
5. Does the site drain to an “impaired water”? BMP selection becomes very important when a development site drains to a receiving water that is not meeting water quality standards and is subject to a TMDL. The designer may need to choose BMPs that achieve a more stringent level of removal for the listed pollutant(s) of concern.

Caution: The literature contains a large amount of information concerning BMP removal capability for a range of common pollutants. When using this information be aware of the assumptions and limitations of the data. Some of this data has been compiled for this Manual. See data on BMP removal for the following pollutants:

• phosphorus
• total suspended solids
• total nitrogen
• hydrocarbons
• bacteria
• metals

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.
Link to this table

BMP group	Receiving water management categorya
	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

Identify climate and terrain factors

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:

1. Is the site within an active karst region? Active karst is defined as karst features within 50 feet of the surface of the site and poses many challenges to BMP design. It is safe to assume that any treated or untreated runoff that is infiltrated will reach the drinking water supply in karst areas. In addition, some BMPs can promote sinkhole formation that may threaten the integrity of the practice. Certain BMPs are more feasible than others in active karst regions. Certain types of geotechnical investigations are needed in karst terrain.
2. Does the site have exposed bedrock or shallow soils? Portions of the State have exposed bedrock or extremely shallow soils that may preclude the use of some BMPs (see statewide maps of these features). For example, infiltration practices may be impractical in shallow soils due to the limited soil separation distance between the bottom of the practice and bedrock. Other BMPs, such as ponds and wetlands may be feasible, but may be more difficult or costly to design and construct (e.g., may require liners to prevent rapid drawdown).
3. Will the site experience high snowfall or require melt water treatment? BMP selection should consider high snowfall areas and should withstand snow and ice cover (see climate information). Frozen conditions will inhibit performance throughout the winter and generate a significant volume of melt water and pollutant loads in the spring.
4. Is the site located in a region with low annual rainfall? Development sites in the southwest part of the State get much less annual rainfall, which plays a strong role in BMP selection. Frequent rainfall is often important to maintain water balance in ponds and wetlands. BMP function in these BMPs could decline when there is not enough runoff to sustain a normal pool elevation.

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.
Link to this table

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.

^aSee sand filters, Dry swale (Grass swale), High-gradient stormwater step-pool swale, or Wet swale (wetland channel)
^bSee infiltration trench or infiltration basin

Evaluate stormwater treatment suitability

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:

1. Can the BMP provide groundwater recharge? BMPs that infiltrate runoff into the soil are needed when a site is subject to a recharge requirement. If infiltration is impractical, designers may want to use some of the better site design techniques to make up the difference and provide full treatment.
2. Can the BMP treat the water quality volume? All of the BMPs in this Manual, with the exception of supplemental BMPs, can meet the water quality volume (V_wq) requirement stipulated in construction general permit, so this is seldom a major factor in BMP selection.
3. Can the BMP provide channel protection? BMPs must provide extended detention for long periods at sites where channel protection (V_cp) is required to protect streams, which means that only a short list of BMPs can meet this criterion. BMPs that cannot meet the channel protection requirement as stand alone practices should not be discarded, as they may still be needed to meet other sizing criteria (e.g., water quality).
4. Can the BMP effectively control peak discharges from overbank floods? Generally, only ponds, wetlands and infiltration basins have the capacity to control peak discharge events that cause flooding at the site (e.g., V_p10 and V_p100 storm events). Once again, if a BMP cannot meet peak discharge requirements, it can be used in combination with one that does to meet all sizing criteria.
5. Can the BMP accept runoff from potential stormwater hotspots? Designers need to be careful choosing BMPs at sites determined to be stormwater hotspots to minimize the risk of groundwater contamination. BMPs that rely on infiltration should be avoided at stormwater hotspots and other design modifications may be needed for other practices that send runoff into the soil.

The following table provides guidance on BMP selection for different management scenarios.

This table shows guidance on BMP suitability for different stormwater strategies.
Link to this table

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
Infiltration trench	Yes	Yes	No	No	No
Infiltration basin	Yes	Yes	Yes	Yes	No
Stormwater ponds	Nod	Yes	Yes	Yes	Yes
Constructed wetlands	Variesd	Yes	Yes	Yes	Yes. Needs pre-treatment
Supplemental BMPs	Varies	Nob	Possibleb	No	Noe

^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

Assess physical feasibility at the site

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. There are several physical factors at the site that can constrain, restrict or eliminate BMPs from further consideration.

For linear projects, a flowchart will help SWPPP designers to identify several areas for Best Management Practices (BMPs) during the planning phase and take each one individually through the decision process. Link here: File:Linear permanent stormwater management flow chart.pdf.

1. Is there enough space available for the BMP at the site? BMPs vary widely in the amount of surface area of the site they consume, which can be an important factor at intensively developed sites where space may be limiting and land prices are at a premium. In some instances, underground BMPs may be an attractive option in highly urban areas.
2. Is the drainage area at the site suitable for the proposed BMP? If the drainage area of the site exceeds the maximum, designers can always use multiple smaller BMPs of the same type, or modify the design. The minimum drainage area thresholds for ponds and wetlands are not quite as flexible, although smaller drainage areas can work if designers can confirm the presence of ground water or baseflow that can sustain a normal pool and incorporate design features to prevent clogging.
3. Will soils limit BMP options at the site? Low infiltration rates limit or preclude the use of infiltration practices and certain kinds of bioretention designs. By contrast, soils with low infiltration rates are preferred for ponds and wetlands since they help to maintain permanent pools without need for a liner. Designers should consult the design guidance for appropriate BMPs to determine minimum soil infiltration rates and testing procedures for each kind of BMP (see design infiltration rates for information on infiltration rates for Hydrologic soil groups). Further geotechnical testing may be needed to confirm soil permeability and ground water depth.
4. Is enough head present at the site to drive the BMP? Head is defined as the elevation difference between the inflow and outflow point of a BMP that enables gravity to drive the BMP. BMP choices are constrained at flatter sites that have less than three or four feet of available head.
5. Will depth to bedrock or the water table constrain the proposed BMP? Bioretention, infiltration and some filtering practices need a minimum separation distance from the bottom of the practice to bedrock (or the water table) to function properly. The Minnesota Pollution Control Agency’s Construction General Permit (CGP) requires a minimum distance of three feet between the bottom of an infiltrating BMP and the seasonally saturated water table. Other BMPs do not require as much separation distance, although the cost and complexity of construction of most BMPs increases sharply at development sites where the bedrock or water table are close to the surface.
6. Is the slope at the proposed BMP site a design constraint? Sites with extremely steep slopes can make it hard to locate suitable areas for BMPs. Maximum slope recommendations for BMPs refers to the gradient where the BMP will actually be installed. Designers will need to carefully scrutinize site topographic and grading plans to find suitable locations, and if this does not work, the grading plan may need to be changed to meet slope thresholds.
7. Is the BMP suitable for ultra-urban sites? BMP selection for ultra-urban development and redevelopment sites is challenging, since space is extremely limited, land is expensive, soils are disturbed, and runoff volumes and pollutant loadings are great. These sites do, however, present a great opportunity for making progress in stormwater management where it has not previously existed.

The following table provides guidance for BMP selection considering physical feasibility at the site.

This table shows guidance on BMP selection based on physical feasibility characteristics of a site.
Link to this table.

BMP group	Surface areaa	Drainage area	Soil infiltration rate	Head	Separation from bedrock	Depth to seasonally high water table	Maximum slopec	Ultra-urban
Bioretention	7-10%; Minimum 200 ft2	5 acre maximum; 0.5-2.0 acre preferred	Any soil; use underdrain for C and D soilsd		3 feet	3 feet	20%	Yes
Filtration - media	Negligible, except for access	5 acre maximum; 0.5-2.0 acre preferred	Media part of designd	2-6 feet	0 feet if enclosed	3 feet for vegetated; 0 feet if enclosed	20%	Yes
Filtration - vegetative	Varies based on depth	10 acre maximum	Media part of design	2-6 feet	0 feet if enclosed	3 feet for vegetated; 0 feet if enclosed	20%	Possible
Infiltration trench	Varies based on depth	5 to 10 acre maximum	Native soils with i >= 0.2 inches/hour	2-12 feet	3 feet	3 feet	15%	Possible
Infiltration basin	Varies based on depth	5 to 50 acre maximum	Native soils with i >= 0.2 inches/hour	2-12 feet	3 feet	3 feet	15%	No
Stormwater ponds	1-3%	10 to 25 acres recommendedb	A or B soils may require liner	3-10 feet	0 feet (shallow soil limits design)	0 feet (except if hotspot or aquifer)	25%	No
Constructed wetlands	2-4%	25 acre minimumb	A or B soils may require liner	3-10 feet	0 feet	0 feet (except if hotspot or aquifer)	25%	No

^aSurface area as a function of contributing surface area, except for ponds and wetlands, where it is a function of entire drainage area
^b10 acres or less may be feasible if groundwater is intercepted and/or if water balance calculations indicate a wet pool can be sustained
^cSlope is defined as the slope across the proposed location of the practice
^dInfiltration gallery could be designed to provide limited recharge

Investigate community and environmental factors

Some BMPs can provide positive economic and environmental benefits for the community, while others can have drawbacks or create nuisances.

1. Ease of Maintenance. All BMPs require routine inspection and maintenance throughout their life cycle, although some are easier to maintain than others. This screening factor looks at each major BMP from the standpoint of the frequency and cost of scheduled maintenance, chronic maintenance problems, reported failure rates, and inspection needs. Designers should try to prevent or reduce maintenance problems during the design phase for BMPs that are rated as difficult to maintain.
2. Community Acceptance. Community acceptance involves a great deal of subjective perception, but a general sense can be gleaned from market surveys, reported nuisance problems, visual preference, and vegetative management. BMPs rated as having low or medium community acceptance can often be improved through better landscaping or more creative design. Note that while underground BMPs enjoy high community acceptance, this is solely due to the fact they are “out of sight, out of mind,” which substantially reduces their ease of maintenance.
3. Construction Cost. Designers should very generally compare BMP construction costs, based on the average cost per impervious acre treated.
4. Habitat Quality. BMPs have the potential to create aquatic and terrestrial habitat for wildlife and waterfowl, which can be an important community amenity. Potential habitat quality is ranked as low, medium or high depending on BMP-specific factors such as surface area, water and wetland features, vegetative cover, and buffers. Habitat quality is not automatic, and requires proper installation, landscaping, and vegetative management at the BMP.
5. Nuisances. Nearly all BMPs can create nuisance conditions, particularly if they are poorly designed or maintained. BMP nuisances reduce community acceptance and generate complaints, but seldom affect the pollutant removal performance of the BMP. Common nuisances include mosquitoes, geese, overgrown vegetation, floatable debris and odors. If a BMP is prone to nuisance conditions, designers should focus attention on preventing or minimizing the problem. For example, distance to residences could be a factor in determining the impact of mosquito breeding, so an analysis of BMP placement relative to residences could result in some impact mitigation.

The following table provides guidance on assessing community and environmental factors in selecting BMPs.

Information: 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.

This table shows guidance on selecting BMPs based on community and environmental factors.
Link to this table

BMP group	Ease of maintenancea	Community acceptance	Construction cost	Habitat quality	Nuisances
Bioretention	Medium	High	Medium	Medium	Mosquitoes Overgrown vegetation
Filtration - media	Difficult	High	High	Low	Filter media replacement Underground practices not seen and maintained
Filtration - vegetative	Medium	Medium	High	Low	Filter media replacement Underground practices not seen and maintained
Infiltration trench	Difficult	High	High	Low	Susceptible to failure if poorly installed or maintained
Infiltration basin	Medium	Low	Medium	Low	Susceptible to failure if poorly installed or maintained
Stormwater ponds	Easy to medium	Medium to high	Low	Medium	Geese odors Mosquitoes Floatables
Constructed wetlands	Medium	Medium to high	Medium	Medium	Overgrown vegetation Mosquitoes
Hydrodynamic devices	Medium	High	High	Low	Underground practices not seen or maintained
Filtration devices	Difficult (expensive)	High	High	Low	Underground practices not seen or maintained

Determine any site restrictions and setbacks

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). Below is an overview of 10 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 the regulatory part of this manual.

Jurisdictional wetland

• Authorities and regulations
  • U.S. Army Corps of Engineers (USACE) Section 404 permit
  • Minnesota Department of Natural Resources Public Waters permit
  • Minnesota Pollution Control Agency 401 Water Quality Certification and NPDES permit
  • Local governments under the Wetland Conservation Act
• Considerations
  • Wetlands should be delineated prior to siting stormwater BMPs
  • Demonstrate that the impact to a wetland complies with all of the following principles in descending order of priority: avoid direct or indirect impacts, minimize impact by limiting the degree or magnitude of activity,mitigate unavoidable impacts through restoration or creation.
  • Always check with local, state and federal jurisdictions for appropriate regulations.
  • Natural wetlands should not be used for stormwater treatment, unless they are severely impaired, and construction would enhance or restore wetland functions.
  • Direct pipe outfalls to wetlands should be restricted. The discharge of untreated stormwater to a wetland should be avoided. BMPs are restricted in the wetland buffer.
  • For sensitive bogs and fens, BMPs should be designed for site-based nutrient load reduction.

Stream channel

• Authorities and regulations
  • U.S. Army Corps of Engineers (USACE) Section 404 permit
  • Minnesota Department of Natural Resources Public Waters permit
  • Minnesota Pollution Control Agency 401 Water Quality Certification and NPDES permit
• Considerations
  • All waterways (including streams, ponds, lakes, etc) should be delineated

prior to design.

• • Use of any Waters of the U.S. for stormwater quality treatment is contrary to the goals of the Clean Water Act and should be avoided. BMPs should not be placed on-line (in-stream) under most conditions.
  • If in-stream BMPs are used, justification of no existing practical upland treatment alternatives must be made. Implement measures that reduce downstream warming.
  • Activities such as excavation, shore protection, structures, dams, and water level controls are regulated.
  • State water quality standards apply.

Shoreland management

• Authorities and regulations
  • DNR and local governments state Floodplain Management Act
• Considerations
  • Check state and local shoreland development ordinances regarding BMP setbacks from the shoreline and any required buffers.

Stream buffer

• Authorities and regulations - consult local authority for stormwater policy regarding buffers.
• Considerations
  • Outstanding Resource Value Waters (ORVWs) require a 100-foot buffer.
  • Structural BMPs are strongly discouraged in the stream-side zone (within 25 feet of streambank). BMP may be allowed within the outer portion of a buffer.
  • Consider how the outfall channel will cross the buffer to the stream.

Sinkholes

• Considerations
  • Existing known sinkholes should be identified with a 100 foot buffer and delineated on site plans.
  • Sinkholes should be remediated and stormwater directed away from these areas during and after construction. Whenever possible, discharges from BMPs or impervious surfaces should not be routed within 100 feet of the edge of any existing un-remediated sinkhole, and runoff should not be directed to an area underlain by known active karst conditions.
  • Sinkholes occurring within BMPs should be repaired as soon as feasible after the first observation.
  • BMPs should be designed off-line to limit volumes and flow rates managed by individual practices. Sinkhole formation is less likely when practices such as swales, bioretention, and vegetated filters are used.

100-year floodplain

• Considerations
  • Grading and fill for BMP construction is strongly discouraged within the ultimate 100-year floodplain, as delineated on FEMA flood insurance rate maps, FEMA flood boundary and floodway, or more stringent local maps.
  • Floodplain fill cannot raise the 100-year water surface elevation by more than 0.5 feet (local regulations may be more stringent).

Water wells - private and public

• Considerations
  • Observe local wellhead protection zones and minimum setbacks.
  • Consult the Minnesota Department of Health (MDH), County health department and local water utility.
  • Mn.Rule 4725.4350 requires a 50-foot setback between stormwater ponds and water supply wells
  • If not otherwise regulated, a similar 50-foot setback for infiltration BMPs is advisable
  • No infiltration of confirmed stormwater hotspot runoff. Infiltration of runoff from potential stormwater hotspots should have suitable pre-treatment.

Septic systems

• Considerations
  • Recommended setback is 35 feet minimum from a drain field.
  • Consult the MDH and County health department.

Utilities

• Considerations
  • Call Gopher State One Call (800-252-1166) to locate existing utilities prior to design.
  • Consider the location of proposed utilities to serve the development.
  • Structural controls are discouraged within utility easements or the right of

way for public or private utilities.

Roads

• Considerations
  • Consult local/county highway or public works department for any setback requirement from local/county roads.
  • Consult Mn/DOT guidelines for setback from State roads.
  • Approval may be needed to discharge stormwater to a local, county or state owned storm drain or channel.

Structures

• Considerations
  • Consult the local review authority for the BMP setback from structures.
  • See individual BMP sections within this Manual for recommended setbacks.

Plot of total cost of construction and maintenance per water quality volume for wet basins. The dashed line represents average cost and the two solids lines represent the range of costs. Source: Mn/DOT, 2005

Using cost factors to select BMPs

Stormwater managers are reluctant to make a final BMP selection without having some basic information on the construction and maintenance costs. Cost information can be found for most BMPs in this Manual within individual BMP sections of the Manual. 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. Graphs of total present worth value of construction plus maintenance costs are available for wet basins, 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 within the individual BMP sections of this Manual.

References

• Caraco, D. 2001. Managing Phosphorus Inputs Into Lakes III: Evaluating the Impact of Watershed Treatment. Watershed Protection Techniques. 3 (4): 791-796. Center for Watershed Protection. Ellicott City, MD.
• Maryland Department of the Environment] (MDE). 2000. 2000 Maryland Stormwater Design Manual. MDE. Baltimore, MD.
• Walker Jr., W. W., 2000. P8 Urban Catchment Modal (Version 2.4), IEP, Inc. and Narrangansett Bay Project USEPA/RIDEM. wwwalker.net/p8/
• Winer, R. 2000. National Pollutant Removal Performance Database for Stormwater Treatment Practices. 2nd Edition. Center for Watershed Protection. Ellicott City, MD.