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Minnesota Stormwater Manual

Calculating credits for dry swale (grass swale)
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Recommended pollutant removal efficiencies, in percent, for dry swale BMPs. Sources. NOTE: removal efficiencies are 100 percent for water that is infiltrated.

TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen

TSS TP PP DP TN Metals Bacteria Hydrocarbons
68 link to table link to table link to table 35 80 0 80

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual Best Management Practice BMP or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

This page provides a discussion of how dry swales can achieve stormwater credits. Swales with and without underdrains are both discussed, with separate sections for each type of system as appropriate. Note that wet swales achieve no volume reduction and have limited pollutant removal capability. Wet swales are therefore not included in the following discussion.

Overview

File:Dry swale credit picture 2.png
Schematic showing characteristics of a dry swale.

Dry Swales are vegetative stormwater quality practices that are primarily used to convey and filter stormwater runoff through a combination of soil media and vegetation filtration. Swales are often used as pretreatment to stormwater prior to entering downstream BMPs. Check dams may also be used in dry swales to reduce velocities of stormwater and further promote filtration and infiltration.

Pollutant Removal Mechanisms

Dry swales primarily remove pollutants through filtration during conveyance of stormwater runoff. Dry swales may also provide some volume reduction benefits through infiltration and evapotranspiration during conveyance. Water quality treatment is also recognized through biological and microbiological uptake, and soil adsorption.

Location in the Treatment Train

Stormwater Treatment Trains are comprised of multiple BMPs that work together to minimize the volume of stormwater runoff, remove pollutants, and reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes and streams. Dry Swales are typically installed near the start of the treatment train as a method that directs stormwater runoff through the engineered grass channel in order to filter pollutants from the stormwater. Dry Swales serve as an effective pretreatment to other BMPs by providing conveyance through elongated flow paths and therefor reducing times of concentration and detention requirements downstream.

Methodology for calculating credits

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 This section is under construction. Completion date is uncertain


Methods for calculating credits

This section provides specific information on generating and calculating credits from bioretention BMPS for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:

  • Quantifying volume and pollution reductions based on accepted hydrologic models
  • The Simple Method and MPCA Estimator
  • MIDS Calculator
  • Quantifying volume and pollution reductions based on values reported in literature
  • Quantifying volume and pollution reductions based on field monitoring

Credits based on models

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits for dry swales. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

  1. Model name and version
  2. Date of analysis
  3. Person or organization conducting analysis
  4. Detailed summary of input data
  5. Calibration and verification information
  6. Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal for constructed basin BMPs. In using this table to identify models appropriate for constructed ponds and wetlands, use the sort arrow on the table and sort by Constructed Basin BMPs. Models identified with an X may be appropriate for using with constructed basins.

Comparison of stormwater models and calculators. Additional information and descriptions for some of the models listed in this table can be found at this link. Note that the Construction Stormwater General Permit requires the water quality volume to be calculated as an instantaneous volume, meaning several of these models cannot be used to determine compliance with the permit.
Link to this table
Access this table as a Microsoft Word document: File:Stormwater Model and Calculator Comparisons table.docx.

Model name BMP Category Assess TP removal? Assess TSS removal? Assess volume reduction? Comments
Constructed basin BMPs Filter BMPs Infiltrator BMPs Swale or strip BMPs Reuse Manu-
factured devices
Center for Neighborhood Technology Green Values National Stormwater Management Calculator X X X X No No Yes Does not compute volume reduction for some BMPs, including cisterns and tree trenches.
CivilStorm Yes Yes Yes CivilStorm has an engineering library with many different types of BMPs to choose from. This list changes as new information becomes available.
EPA National Stormwater Calculator X X X No No Yes Primary purpose is to assess reductions in stormwater volume.
EPA SWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
HydroCAD X X X No No Yes Will assess hydraulics, volumes, and pollutant loading, but not pollutant reduction.
infoSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
infoWorks ICM X X X X Yes Yes Yes
i-Tree-Hydro X No No Yes Includes simple calculator for rain gardens.
i-Tree-Streets No No Yes Computes volume reduction for trees, only.
LSPC X X X Yes Yes Yes Though developed for HSPF, the USEPA BMP Web Toolkit can be used with LSPC to model structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
MapShed X X X X Yes Yes Yes Region-specific input data not available for Minnesota but user can create this data for any region.
MCWD/MWMO Stormwater Reuse Calculator X Yes No Yes Computes storage volume for stormwater reuse systems
Metropolitan Council Stormwater Reuse Guide Excel Spreadsheet X No No Yes Computes storage volume for stormwater reuse systems. Uses 30-year precipitation data specific to Twin Cites region of Minnesota.
MIDS Calculator X X X X X X Yes Yes Yes Includes user-defined feature that can be used for manufactured devices and other BMPs.
MIKE URBAN (SWMM or MOUSE) X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
P8 X X X X Yes Yes Yes
PCSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.
PLOAD X X X X X Yes Yes No User-defined practices with user-specified removal percentages.
PondNet X Yes No Yes Flow and phosphorus routing in pond networks.
PondPack X [ No No Yes PondPack can calculate first-flush volume, but does not model pollutants. It can be used to calculate pond infiltration.
RECARGA X No No Yes
SHSAM X No Yes No Several flow-through structures including standard sumps, and proprietary systems such as CDS, Stormceptors, and Vortechs systems
SUSTAIN X X X X X Yes Yes Yes Categorizes BMPs into Point BMPs, Linear BMPs, and Area BMPs
SWAT X X X Yes Yes Yes Model offers many agricultural BMPs and practices, but limited urban BMPs at this time.
Virginia Runoff Reduction Method X X X X X X Yes No Yes Users input Event Mean Concentration (EMC) pollutant removal percentages for manufactured devices.
WARMF X X Yes Yes Yes Includes agriculture BMP assessment tools. Compatible with USEPA Basins
WinHSPF X X X Yes Yes Yes USEPA BMP Web Toolkit available to assist with implementing structural BMPs such as detention basins, or infiltration BMPs that represent source control facilities, which capture runoff from small impervious areas (e.g., parking lots or rooftops).
WinSLAMM X X X X Yes Yes Yes
XPSWMM X X X Yes Yes Yes User defines parameter that can be used to simulate generalized constituents.


MIDS Calculator

Users should refer to the MIDS Calculator section of the WIKI for additional information and guidance on credit calculation using this approach.

Credits Based on Reported Literature Values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or event mean concentration (EMC) of the dry swale. A more detailed explanation of the differences between mass load reductions and EMC reductions can be found here.

Designers may use the pollutant reduction values reported here or may research values from other databases and published literature.

Designers who opt for this approach should:

  • Select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database.
  • Select a pollutant removal reduction from literature that studied a dry swale device with site characteristics and climate similar to the device being considered for credits.
  • When using data from an individual study, review the article to determine that the design principles of the studied dry swale are close to the design recommendations for Minnesota, as described here, and/or by a local permitting agency.
  • Preference should be given to literature that has been published in a peer-reviewed publication.

The following references summarize pollutant reduction values from multiple studies or sources that could be used to determine credits. Users should note that there is a wide range of monitored pollutant removal effectiveness in the literature. Before selecting a literature value, users should compare the characteristics of the monitored site in the literature against the characteristics of the proposed dry swale, considering such conditions as watershed characteristics, swale sizing, and climate factors.

  • Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon.====
    • Appendix M contains Excel spreadsheet of structural and non-structural BMP performance evaluations.
    • Provides values for sediment, nutrients, pathogens, metals, quantity, air purification, carbon sequestration, flood storage, avian habitat, aquatics habitat and aesthetics.
    • Applicable to Filters, Wet Ponds, Porous Pavements, Soakage Trenches, Flow through Stormwater Planters, Infiltration Stormwater Planters, Vegetated Infiltration Basins, Swales, and Treatment Wetlands.
  • The Illinois Green Infrastructure Study.
    • Figure ES-1 summarizes BMP effectiveness
    • Provides values for TN, TSS, peak flows / runoff volumes
    • Applicable to Permeable Pavements, Constructed Wetlands, Infiltration, Detention, Filtration, and Green Roofs
  • New Hampshire Stormwater Manual.
    • Volume 2, Appendix B summarizes BMP effectiveness
    • Provides values for TSS, TN, and TP removal
    • Applicable to basins and wetlands, stormwater wetlands, infiltration practices, filtering practices, treatment swales, vegetated buffers, and pre-treatment practices
  • BMP Performance Analysis. Prepared for US EPA Region 1, Boston MA.
    • Appendix B provides pollutant removal performance curves
    • Provides values for TP, TSS, and Zn.
    • Pollutant removal broken down according to land use.
    • Applicable to Infiltration Trench, Infiltration Basin, Bioretention, Grass Swale, Wet Pond, and Porous Pavement.
  • Weiss, P.T., J.S. Gulliver and A.J. Erickson. 2005. The Cost and Effectiveness of Stormwater Management Practices: Final Report.
    • Table 8 and Appendix B provides pollutant removal efficiencies for TSS and P
    • Applicable to Wet Basins, Stormwater Wetlands, Bioretention Filter, Sand Filter, Infiltration Trench, and Filter Strips/Grass Swales.

Credits Based on Field Monitoring

Other pollutants

According to the International BMP Database, studies have shown dry swales are effective at reducing concentration of other pollutants as well including solids, bacteria, metals, and nutrients. This database provides an overview of BMP performance in relation to various pollutant categories and constituents that were monitored in BMP studies within the database. The report notes that effectiveness and range of unit treatment processes can vary greatly depending on BMP design and location. Table 3-4 shows a list of the constituents and associated pollutant category for the monitored “media filters” data. The constituents shown all had data representing decreases in effluent pollutant loads for the median of the data points and the 95% confidence interval about the median. If dry swale design utilizes a bioretention base, additional pollutant removals may be applicable as well (For more information see the bioretention credit article ). Pollutant removal percentages for dry swale BMPs can also be found on the WIKI page.

Dry swale pollutant load reduction
Link to this table

Pollutant Category Constituent Treatment Capabilities

(Low = < 30%; Medium = 30-65%;

High = 65 -100%)
Metals1 Cd, Cr, Cu, Zn Medium
As2,Fe, Ni, Pb Medium/High
Nutrients Total Nitrogen, TKN Low
Bacteria Fecal Coliform, E. coli Low
Organics Medium

1Results are for total metals only
2Information on As was found only in the International Stormwater Database where removal was found to be low


1Results are for total metals only

2Information on As was found only in the International Stormwater Database where removal was found to be low

References and suggested reading

  • Ahearn, Dylan, and Richard Tveten. "Legacy LID: Stormwater Treatment in Unimproved Embankments along Highway Shoulders in Western Washington." In Proceedings of the 2008 International Low Impact Development (LID) Conference, pp. 16-19. 2008.
  • Barrett, Michael E., Michael Vincent Keblin, Patrick M. Walsh, Joseph F. Malina Jr, and Randall J. Charbeneau. Evaluation of the performance of permanent runoff controls: summary and conclusions. No. TX-99/2954-3F,. 1998.
  • Barrett, Michael E., Patrick M. Walsh, Joseph F. Malina Jr, and Randall J. Charbeneau. "Performance of vegetative controls for treating highway runoff." Journal of environmental engineering 124, no. 11 (1998): 1121-1128.
  • Barrett, Michael, Anna Lantin, and Steve Austrheim-Smith. "Storm water pollutant removal in roadside vegetated buffer strips." Transportation Research Record: Journal of the Transportation Research Board 1890, no. 1 (2004): 129-140.
  • Bureau of Environmental Services. 2006. Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon. Bureau of Environmental Services, Portland, Oregon.
  • California Stormwater Quality Association. "California Stormwater BMP Handbook-New Development and Redevelopment." California Stormwater Quality Association, Menlo Park, CA (2003).
  • Caltrans. 2004. BMP Retrofit Pilot Program Final Report, Report No., CTSW-RT-01-050. Division of Environmental Analysis, California Dept. of Transportation, Sacramento, CA
  • CDM Smith. 2012. Omaha Regional Stormwater Design Manual Chapter 8 Stormwater Best Management Practices. Kansas City, MO.
  • Dorman, M. E., H. Hartigan, F. Johnson, and B. Maestri. Retention, detention, and overland flow for pollutant removal from highway stormwater runoff: interim guidelines for management measures. Final report, September 1985-June 1987. No. PB-89-133292/XAB.
  • Consultants, Geosyntec, and Wright Water Engineers. "Urban stormwater BMP performance monitoring." (2002).
  • Leisenring, M., J. Clary, and P. Hobson. "International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals July 2012." (2012): 1-31.
  • Gulliver, J. S., A. J. Erickson, and PTe Weiss. "Stormwater treatment: Assessment and maintenance." University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN. http://stormwaterbook. safl. umn. edu (2010).
  • Guo, James CY, Gerald E. Blackler, T. Andrew Earles, and Ken MacKenzie. "Incentive index developed to evaluate storm-water low-impact designs." Journal of Environmental Engineering 136, no. 12 (2010): 1341-1346.
  • Harper, Harvey H. "Effects of stormwater management systems on groundwater quality." FDEP Project# WM190. Florida Department of Environmental Regulation, Tallahassee, FL (1988).
  • Jaffe, et. al. 2010. The Illinois Green Infrastructure Study. Prepared by the University of Illinois at Chicago, Chicago Metropolitan Agency for Planning, Center for Neighborhood Technology, Illinois-Indiana Sea Grant.
  • Jurries, Dennis. "Biofilters (Bioswales, Vegetative Buffers, & Constructed Wetlands) for Storm Water Discharge Pollution Removal." Quality, S. o. OD o. E.(Ed.).
  • Kearfott, Pamela J., Michael E. Barrett, and Joseph F. Malina. Stormwater quality documentation of roadside shoulders borrow ditches. Center for Research in Water Resources, University of Texas at Austin, 2005.
  • Kim, Yun Ki, and Seung Rae Lee. "Field infiltration characteristics of natural rainfall in compacted roadside slopes." Journal of geotechnical and geoenvironmental engineering 136, no. 1 (2009): 248-252.
  • Leisenring, M., J. Clary, and P. Hobson. "International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals July 2012." (2012): 1-31.
  • New Hampshire Department of Environmental Services. 2008. New Hampshire Stormwater Manual. Volume 2 Appendix B. Concord, NH.
  • Transportation Officials, Oregon State University. Dept. of Civil, Environmental Engineering, University of Florida. Dept. of Environmental Engineering Sciences, GeoSyntec Consultants, and Low Impact Development Center, Inc. Evaluation of Best Management Practices for Highway Runoff Control. No. 565. Transportation Research Board, 2006.
  • State of California, Department of Transportation. 2013. Caltrans Stormwater Monitoring Guidance Manual. Sacramento, CA.
  • TetraTech. 2008. BMP Performance Analysis. Prepared for US EPA Region 1, Boston, MA.
  • Torres, Camilo. "Characterization and Pollutant Loading Estimation for Highway Runoff in Omaha, Nebraska." (2010).
  • Water Environment Federation. 2014. Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices. A White Paper by the National Stormwater Testing and Evaluation of Products and Practices (STEPP) Workgroup Steering Committee.
  • WEF, ASCE/EWRI. 2012. Design of Urban Stormwater Controls, WEF Manual of Practice No. 23, ASCE/EWRI Manuals and Reports on Engineering Practice No. 87. Prepared by the Design of Urban Stormwater Controls Task Forces of the Water Environment Federation and the American Society of Civil Engineers/Environmental & Water Resources Institute.
  • Weiss, Peter T., John S. Gulliver, and Andrew J. Erickson. "The Cost and Effectiveness of Stormwater Management Practices Final Report." (2005).


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