Stormwater treatment trains combine multiple stormwater treatment processes and/or practices in a manner that ensures management of all pollutants that could affect a receiving water. To provide guidance for stormwater managers, hypothetical treatment trains were developed for five common stormwater management scenarios. This page provides information on implementing a stormwater Best Management Practice (BMP) treatment train for a site with a parking lot. To see other scenarios, see related articles at the bottom of this page.

Step 1: Review project goals and site conditions

schematic illustrating Parking Lot Scenario Base
Site layout for a treatment train scenario for a site with a parking lot. Blue arrows and lines depict the existing storm sewer system and drainage at the site. (source: CDM Smith).

The parking lot scenario consists of a parking lot on a commercial property currently draining to an existing stream directly south of the site. Bank erosion of the existing stream has become evident as well as high pollutant and sediment loads from the site’s runoff. The goal of the project is to implement BMPs onsite to help control the volume of runoff and pollutant load contribution to the stream for the Water Quality Event, without disturbing the existing commercial function of the site. The site conditions for this scenario are summarized in the table below, and the existing site layout can be seen in the figure to the right.

Parking lot scenario existing site conditions
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Site Parking lot of commercial property with limited green space along the east, west, and south perimeters
Drainage Area 3.0 Acres
Impervious Area 64%
Soil Conditions Hydrologic Soil Group ‘B’, silty sands and silty gravelly sands with moderate infiltration rates. Permeability ranges between 0.30- 0.45 inches per hour at a depth of 3 feet. Bedrock was not observed in borings conducted on site.
Topography and Drainage Site generally drains from north to south with relatively shallow slopes (2%-4%), collecting direct site runoff only. Building downspouts discharge to grass areas on either side of parking lot. Parking area is inversely crowned draining to center parking isle.
Project Goals Improve conditions to stream directly south of site by reducing volume of runoff and pollutant loads entering stream for the Water Quality Event.


Step 2: Review pollutant removal processes and identify potential practices

Because the goals of the project are to both decrease runoff volume and pollutant loading, the BMPs selected for this site should offer both pollutant removal and volume reduction processes. The table below summarizes the BMP practice categories and their applicability to this site.

Parking Lot Scenario BMP Practice Selection
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Practice Applicability Reason
Pollution Prevention and Public Education Not relevant practices for this private commercial parking lot scenario.
Source Controls The main source of pollutants is currently from build- up of contaminants from vehicular traffic throughout the parking lot. Because the land use for the site is not changing and the property owner does not intend to perform additional maintenance measures for source control, this is not an applicable practice.
Infiltrators X The site has well-draining existing underlying soils, making it a good candidate for infiltrator BMPs.
Swales and Strips X Swales and strips may serve as a good form of pretreatment and conveyance to BMPs downstream in the treatment train.
Filters X Allow for reduction of stormwater volume and pollutant loads and easy diversion to other BMPs. May serve as an alternative to infiltrator BMPs if unable to manage the entire Water Quality Volume.
Constructed Basins Too large to meet space constraints.
Manufactured Devices The site is not connected to an existing storm sewer system; this is not an applicable practice.
Storage, Harvesting and Reuse Applicable depending on available space and building function. However, stormwater storage for re-use is not necessary for this scenario.


Infiltrators, filters, and swales and strips are determined to be the practices to best address the goals for this scenario. The process of selecting and placing BMPs on a site is typically iterative, working between the site constraints, project goals, and available budget. The approach and considerations for this scenario are discussed in the following sections.

Step 3: Determine site constraints and BMP placement

schematic illustrating Parking Lot Scenario BMP Placement & Site Constraints
Proposed site layout for a hypothetical site with a parking lot (Source: CDM Smith).

Existing site conditions cause several constraints regarding placement of BMPs. The most significant of these constraints are available space, existing topography, and regulatory requirements:

  • Available Space - The majority of the drainage area is impervious, consisting of building and parking lot space that is necessary for the commercial property to function. BMP selection that will minimize loss of these amenities is a key consideration.
  • Topography - Currently runoff generally drains from north to south. More specifically, the parking lot drains to the center aisle, and the building drains to the grassed area on either side of the parking lot before continuing south and offsite. Ideal locations for BMPs to maximize stormwater capture include the middle of the parking lot and the open space on the south side of the property.
  • Regulatory Requirements - All local, state, and federal regulatory requirements must be met. The design criteria and recommendations from the Minnesota Stormwater Manual will be followed.

The viable locations for BMP placement are the center parking aisles and the open space on the perimeter of the parking lot. The optimal placement to maximum site runoff capture is in the open space along the southern property edge. Building foundations and property lines are both considered structures of concern when adjacent to infiltration BMPs. It is important to maintain the recommended 10 foot minimum setback from each when placing the BMPs. The schematic to the right displays the recommended placement of the BMPs for this site.

Step 4: Select individual BMPs and evaluate range of performance

As discussed in Step 2, the practices that are most applicable to this site are infiltrators, swales and strips, and filters. In general, filters remove pollutants by physical, chemical, or biological processes that occur while the stormwater moves through the filter media. Depending on the type of filter used, some volume reduction may be recognized through infiltration below an underdrain, or transpiration through vegetation. Filter’s typically have volume and pollutant removal efficiencies of 100 percent for portions of water that are infiltrated, and have a reduced percentage reduction for stormwater that is filtered and released from the system. Infiltration BMPs however, assume 100 percent volume reduction and pollutant removal efficiency for all stormwater treated by the BMP.

Because the site has both volume reduction and pollutant removal goals, it is recommended to use infiltration BMPs to more effectively reduce the volume of stormwater runoff and pollutant loads. Given the sites well-draining soils, infiltrators are a viable option for this scenario.

Infiltrator: Permeable pavement
  • Replacing a portion of the parking lot with permeable pavement allows for a reduction in impervious area as well as a potential for reduction of stormwater runoff and pollutants through infiltration. For permeable pavement designs with underdrains, the volume of water intercepted by the underdrain can expect a pollutant removal performance of 45 percent of total phosphorous (TP) and 74 percent TSS. If the permeable pavement is designed without an underdrain, the entire volume of runoff entering the system will ultimately be infiltrated, providing 100 percent TP and TSS removal for treated stormwater.
Infiltrator: Infiltration basin
  • Placement of the secondary treatment train BMP determined to be on the south side of the property backing up directly to the creek is not a highly visible location. For this reason, a highly aesthetic infiltration BMP is not necessary. While bioinfiltration and tree trenches could offer additional performance benefits through biological and microbiological processes, they are not necessary to meet the project goals and could include higher construction and maintenance costs. Additionally, because erosion of the streambed is a concern, utilizing a BMP that offers channel protection through peak discharge attenuation is ideal. An infiltration basin can provide this type of benefit as well as volume reduction and pollutant removal.
schematic illustrating Parking Lot Scenario BMP Layout
BMP layout for a parking lot scenario (Source: CDM Smith).
Swales and strips: Grass channel
  • Infiltration basins require use of pretreatment of stormwater before being directed to the practice. Using a swale would both serve as pretreatment, and offer some pollutant removal benefits. Because the infiltration basin will treat the majority of the stormwater, the main purpose of the swale will be to serve as pretreatment and to convey stormwater to the basin. For this reason, a simple grass channel is sufficient.

While multiple infiltrator, filter, or swale and strip BMPs could potentially work for this site, it is determined that permeable pavement, grass channels, and an infiltration basin will best meet the goals of the project, while working within site constraints and meeting client budget expectations. The figure to the right displays the proposed BMP treatment train layout for this parking lot scenario.

Step 5: Size BMPs and assess performance

BMP sizing can also be an iterative process between meeting project goals, working within site constraints, meeting regulatory requirements, and meeting client expectations. The MIDS calculator was used to evaluate the performance goals for the site given the drainage area, soil conditions, impervious area, average annual rainfall for the area, and a Water Quality Event retention goal of 1.1 inches. Assumptions included an annual total phosphorus event mean concentration (EMC) of 0.3 milligrams per liter and an annual TSS EMC of 54.5 milligrams per liter. The following table summarizes the existing site runoff, volume and pollutant retention goals.

Parking lot scenario performance goal
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Annual Runoff Volume (acre-ft.) Annual Particulate Phosphorous Load (lbs.) Annual Dissolved Phosphorous Load (lbs.) Annual Total Suspended Solids Load (lbs.) Performance Goal Volume Retention Requirement (ft3)
4.86 2.18 1.79 721 7600


The BMPs were sized to best meet the project performance goals within the available space on site.

Permeable pavement

schematic illustratingTypical Permeable Pavement Configuration
Typical permeable pavement configuration (Source: CDM Smith).

The main sizing considerations for permeable pavement infiltration BMP are the surface area of the permeable pavement, the depth the aggregate reservoir, the porosity of the aggregate media, and the infiltration rate of the underlying soils. The figure to the right shows the typical configuration of a permeable pavement section. For information on using the MIDS calculator for permeable pavement, link here.

The amount of impervious area draining from the parking lot is about 0.95 acres. The amount of at-grade contributing impervious drainage area into permeable pavement should not exceed a 5:1 ratio to help reduce the rate of sedimentation. In order to stay within this parameter, an additional permeable pavement parking lane was added on either side of the center aisle, keeping about a 3.2:1 impervious drainage area to permeable pavement area ratio. The permeable pavement BMP is designed to store the entire treatment volume for the Water Quality Event while meeting the MPCA design criteria, requirements and recommendations. The following table displays the remaining sizing input parameters used for the permeable pavement design.

Permeable pavement sizing input parameters
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Permeable Pavement Design Permeable Pavement Surface Area (ft2) Surface Area at Subgrade (ft2) Storage Depth (ft.) Storage Media Porosity Drawdown Time (hrs.)
Permeable Asphalt 1,900 1,900 1 0.42 48
Permeable Concrete 1,900 1,900 0.8 0.42 48


The infiltration rate of the underlying soils is 0.45 inches per hour, and the porosity of the aggregate was assumed to be 0.35 cubic feet per cubic foot. These parameters were entered into the MIDS calculator to evaluate performance of the permeable pavement BMP for runoff volume and pollutant load reductions. These results are summarized below.

Parking lot permeable pavement performance summary
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Event Runoff Volume Performance Summary
Runoff Volume from Direct Watershed (ft3) 3,7761
Runoff Volume from Upstream BMPs (ft3) 1
Retention Volume Provided by BMP (ft3) 3,761
Outflow Volume from BMP (ft3) 0
Percent Performance Goal Achieved (%) 100
Annual Performance Goal Summary
Volume Runoff from Direct Watershed (acre-ft.) 2.15
Runoff from Upstream BMPs (acre-ft.) 0
Retention Volume Provided by BMP (acre-ft.) 2.02
Outflow Volume from BMP (acre-ft.) 0.13
Percent Performance Goal Achieved (%) 94
Phosphorus (P) Particulate P from Watershed (lbs.) 0.96
Particulate P from Upstream BMPs (lbs.) 0
Particulate P Load Reduction (lbs.) 0.95
Particulate P Outflow 0.01
Total Percent Particulate P Reduction 99
Dissolved P from Watershed (lbs.) 0.79
Dissolved P from Upstream BMPs (lbs.) 0
Dissolved P Load Reduction (lbs.) 0.74
Dissolved P Outflow (lbs.) 0.05
Total Percent Dissolved P Reduction (%) 94
Total Suspended Solids (TSS) TSS from Watershed (lbs.) 318
TSS from Upstream BMPs (lbs.) 0
TSS Load Reduction (lbs.) 313
TSS Outflow (lbs.) 5
Total Percent TSS Reduction (%) 98


Grass Channel

A grass channel, or swale, can have a multitude of different configurations and components depending on its desired functions and site topography. As discussed in Step 4, the primary purpose for the swale at this site is to serve as conveyance and pretreatment for the downstream infiltration basin BMP. For this reason, the main design parameters will include main channel length, width, depth, and slope as well as the swale side slope length. No check dams, soil media, or gravel and underdrain layers are included in this design.

schematic illustrating Typical Grass Channel Configuration
Typical grass channel configuration (Source:CDM Smith).

The swale will be placed at the outlet point of the building downspouts to maximize capture of runoff from impervious area. The slope of the main channel is set at 2 percent to match existing grades of the site. The channel length extends from the downspout outlet to the grassed area south of the parking lot. The grass channels were design to meet the MPCA design criteria, requirements and recommendations. The table below displays the sizing input parameters used for the grass channel design

Grass Channel Sizing Input Parameters
Side Slope (V:H) Channel Length (ft.) Bottom Width (ft.) Slope (%)
1.4 216 4 2

These parameters were entered into the MIDS calculator to evaluate performance of each of the grass channel BMPs for runoff volume and pollutant load reductions, as well as to gain an understanding of the outflow that will be directed to the infiltration basin BMP downstream in the treatment train. The MIDS calculator evaluates swale side slopes as an upstream BMP to swale main channel. However, virtually no volume or pollutant removal benefits were recognized for the swale side slope. Results for the grass channel BMPs are summarized in the table below.

Grass channels performance summary
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Event Runoff Volume Performance Summary
Runoff Volume from Direct Watershed (ft3) 3,336
Runoff Volume from Upstream BMPs (ft3) 0
Retention Volume Provided by BMP (ft3) 17
Outflow Volume from BMP (ft3) 3,158
Percent Performance Goal Achieved (%) 5
Annual Performance Goal Summary
Volume Runoff from Direct Watershed (acre-ft.) 2.3
Runoff from Upstream BMPs (acre-ft.) 0
Retention Volume Provided by BMP (acre-ft.) 0.34
Outflow Volume from BMP (acre-ft.) 1.96
Percent Performance Goal Achieved (%) 15
Phosphorous (P) Particulate P from Watershed (lbs.) 1.04
Particulate P from Upstream BMPs (lbs.) 0
Particulate P Load Reduction (lbs.) 0.8
Particulate P Outflow 0.24
Total Percent Particulate P Reduction 77
Dissolved P from Watershed (lbs.) 0.84
Dissolved P from Upstream BMPs (lbs.) 0
Dissolved P Load Reduction (lbs.) 0.12
Dissolved P Outflow (lbs.) 0.72
Total Percent Dissolved P Reduction (%) 14
Total Suspended Solids (TSS) TSS from Watershed (lbs.) 342
TSS from Upstream BMPs (lbs.) 0
TSS Load Reduction (lbs.) 248
TSS Outflow (lbs.) 94
Total Percent TSS Reduction (%) 73


Infiltration basin

schematic illustrating Typical Infiltration Basin Configuration
Typical Infiltration Basin Configuration

The primary goal when sizing the infiltration basin is to treat the stormwater runoff from the site not previously removed by upstream treatment train BMPs. The figure to the right shows a typical configuration and components of an infiltration basin. The inflow will be directed from the grass channels on the east and west side of the parking lot directly upstream of the basin. The basin itself will rely solely on infiltration into existing soils in the ponding area of the BMP within the required 48 hour drawdown time. Flows in excess will be directed to the overflow and released at non-erosive rates. No additional sand layer is included for this site.

The major constraints for sizing the basin include the ponding area, the basin area, and the depth of the ponding area (for information on using the MIDS calculator for infiltration trench/basin, link here). The available space in the open area south of the parking limits the maximum ponding area of the basin. This is controlled by a required 10 foot setback from the property line, as well as a desired offset form the parking lot to avoid the infiltration basin being directly connected to structure or pavement foundations to avoid seepage and frost heave concerns, per MPCA design criteria. This additionally provides pretreatment of any direct runoff not routed through the upstream BMPs, allowing for impervious surface disconnection over a grass strip between the parking lot and the infiltration basin. The basin itself will extend the entire width of the parcel, less the required 10 foot offset in order to maximize stormwater capture from the site. The sizing parameters for the infiltration basin are described in the table below.

Infiltration basin sizing input parameters
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Surface Area at Overflow (ft2) Media Surface Area (ft2) (ft2) Drawdown Time (hrs.)
5525 2707 1 48


These parameters were entered into the MIDS calculator to evaluate the performance of the BMP. The results are summarized in the table below.

Infiltration basin performance summary
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Event Runoff Volume Performance Summary
Runoff Volume from Direct Watershed (ft3) 502
Runoff Volume from Upstream BMPs (ft3) 3,158
Retention Volume Provided by BMP (ft3) 3,660
Outflow Volume from BMP (ft3) 0
Percent Performance Goal Achieved (%) 100
Annual Performance Goal Summary
Volume Runoff from Direct Watershed (acre-ft.) 0.41
Runoff from Upstream BMPs (acre-ft.) 2.07
Retention Volume Provided by BMP (acre-ft.) 2.28
Outflow Volume from BMP (acre-ft.) 0.2
Percent Performance Goal Achieved (%) 92
Phosphorous (P) Particulate P from Watershed (lbs.) 0.18
Particulate P from Upstream BMPs (lbs.) 0.25
Particulate P Load Reduction (lbs.) 0.4
Particulate P Outflow 0.03
Total Percent Particulate P Reduction 92
Dissolved P from Watershed (lbs.) 0.15
Dissolved P from Upstream BMPs (lbs.) 0.76
Dissolved P Load Reduction (lbs.) 84
Dissolved P Outflow (lbs.) 0.07
Total Percent Dissolved P Reduction (%) 92
Total Suspended Solids (TSS) TSS from Watershed (lbs.) 60
TSS from Upstream BMPs (lbs.) 98
TSS Load Reduction (lbs.) 145
TSS Outflow (lbs.) 13
Total Percent TSS Reduction (%) 92


Overall parking lot BMP treatment train performance

The treatment train as a whole was evaluated using the MIDS calculator. The swale side slopes were assumed to drain to the swale main channel and then to the infiltration trench, while the permeable pavement acts as stand-alone BMP able to treat the entire volume of stormwater collected for the 1.1 inch event. This configuration is shown in the schematic to the right.

The treatment train provides an annual removal efficiency of 96 percent runoff volume for the site as well as 97 percent total phosphorus and 98 percent TSS. The MIDS calculator annual performance results of the BMP treatment train as a whole are summarized in the table below.

schematic illustrating Parking Lot BMP Treatment Train MIDS Calculator Schematic
Parking lot BMP treatment train MIDS calculator schematic (Source: CDM Smith).

Parking Lot Annual BMP Treatment Train Performance Summary
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Parking Lot Annual BMP Treatment Train Performance Summary
Volume Performance Goal Runoff Volume (acre-ft.) 4.86
Retention Volume Provided by BMPs (acre-ft.) 4.66
Percent Runoff Volume Removed (%) 96
Total Phosphorous (TP) Performance Goal Particulate P Load (lbs.) 2.18
Performance Goal Dissolved P Load (lbs.) 1.79
Particulate P Load Removed by BMPs (lbs.) 2.15
Dissolved P Load Removed by BMPs (lbs.) 1.71
Percent TP Removed (%) 97
Total Suspended Solids (TSS) Performance Goal TSS Loads (lbs.) 721
TSS Load Removed by BMPs (lbs.) 707
Percent TSS Removed (%) 98


Step 6: Review construction and operations criteria

Each of the BMPs included in the treatment train have unique criteria for construction and operations. Information regarding construction and operations should be reviewed in detail before design and construction of the BMPs take place. Available information for each of the BMPs can be found by using the links indicated below.

Construction and Operations Guidance for Ultra Urban BMPs
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Permeable Pavement Grass Channel Infiltration Basin
Construction Construction Construction
Operations Operations Operations


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This page was last modified on 28 September 2015, at 14:07.

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