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The basic premise for selection of a Best Management Practice (BMP) or group of BMPs is to follow the treatment train approach. Under the treatment train approach, stormwater management begins with simple methods that minimize the amount of runoff that occurs from a site and methods that prevent pollution from accumulating on the land surface and becoming available for wash-off. Even though we know that we will never be able to fully accomplish either of these goals, we can make substantial progress using the better site design, Low Impact Development (LID), pollution prevention, volume minimization, and temporary construction erosion and sediment control techniques.
After all of the efforts possible are made to minimize runoff and surface wash-off, we must recognize that some potential for runoff will occur. The next major BMP then becomes collection and treatment of runoff locally and regionally, either as stand-alone practices or in treatment train combinations. Some of the available BMPs are best used to reduce runoff volume, while others focus on water quality improvement. Some BMPs will be easy to implement, while others involve serious engineering and sophisticated design. Other BMPs used in a treatment train include bioretention devices, filtration practices, infiltration practices, stormwater ponds and stormwater wetlands.
When evaluating the benefits of various BMPs, it is essential to account for the amount of water that will enter the system versus the amount that will be by-passed or diverted. Water that does not fall within the design parameters of a BMP will be sent either to another down-gradient BMP or simply routed to a receiving water untreated (not recommended). Although some BMPs, such as ponds and wetlands, will minimally treat excess water because it is routed through the BMP, other such as filtration and infiltration systems, cannot operate properly if excess water flows into them. This is an important distinction that must be evaluated for each BMP installation.
The design recommendations and expected BMP performance contained within this Manual assume that only the amount of water contained within the design will actually be treated. It is not acceptable to assume that all water falling in any event and within the area draining to a BMP will, in fact, be treated by that BMP. An analysis of every BMP installation should include an identification of where by-passed water will flow and how it could be treated.
The key to proper selection of a single or series of BMPs is to match the pollutant to be controlled against the pollutant removal mechanism of a specific BMP. For example, it is not appropriate to use a stormwater pond when temperature control is necessary; however it is very appropriate to use a pond for purposes of rate control. The definition of pollutant being utilized by the Minnesota Stormwater Manual includes both the traditional pollutants (nutrients, solids, etc.) plus the negative effects caused by thermal increases and excessive rate/speed of stormwater flows. Stormwater planners and designers will first need to understand the pollutant(s) of concern that may be generated at their sites. At the early stages of design, stormwater managers should be contacting local water management agencies (watershed districts, watershed management organizations, soil and water conservation districts, counties and/or cities) to learn which pollutants are necessary to control prior to discharge of new stormwater runoff to local waterbodies.
The guidance fact sheets contained in Chapter 12 discuss the pollutant removal mechanisms of each BMP in greater detail. However, the key mechanisms for each group of structural BMPs are presented in Table 6.1 and can be used by stormwater managers as a preliminary screening tool. Further tools for BMP selection are presented in Chapter 7.
The following sections describe the BMPs that are recommended for Minnesota. The selection criteria are included in Chapter 7 and the specific information on each category is included in Chapter 12. Some additional support information occurs in Appendix D. Note that the order of the BMP presentation follows the treatment train sequence illustrated in Figure 6.1.
The first level of BMP application occurs at the planning stage and is intended to minimize the impact of development. These practices are intended to prevent pollution and minimize the increase in stormwater volume and are considered prior to initiation of construction or land altering activity.
Residential, municipal and industrial/commercial practice categories (Water Quality Focus). Specific recommended practices include such things as:
(See Chapter 4 and Chapter 12)
(Water Quantity Focus; see also Table 7.1) Note that many typical runoff volume reduction techniques are included in the Chapter 4 discussion of better site design and Chapter 12 Fact Sheets, including:
(Water Quality Focus; see Also Table 7.2; Reference MS4, NPDES, and Local References and Ordinances) These practices are described in terms of perimeter, slope, drainageway and “other” criteria, and include:
These BMPs have design guidance describing the engineering details for the BMP category. This design guidance is used, for example, to determine storage volume and physical configuration that best meet the objectives of the BMP application. Also note that some of these BMPs, such as filtration, can be either a primary treatment technique or used for pre-treatment into another BMP.
This BMP suite includes vegetated systems that provide a combination of filtration and infiltration into a bio-system consisting of plants and soil, including:
Design based upon components needed to fulfill the desired function.
Selection criteria are similar to stormwater ponds.
The final category of BMPs present those that are generally, but not always, included in the stormwater treatment train as a supplement to the primary treatment device. Although this is not generally recommended, there is the possibility that these devices could be the only BMP used. These are described in less detail than the previous sections. The designer will be guided through a process of determining the function a generic device serves within the treatment train and evaluating the proposed device against the needed function and manufacturer claims. Proprietary devices are generically described rather than listed as individual companies to avoid risking some omissions and claims of certification in the Manual.