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Minnesota Stormwater Manual

Calculating credits for stormwater ponds
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Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual 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 constructed basins (constructed ponds and constructed wetlands) can achieve stormwater credits.

Overview

schematic of constructed pond/wetland
Schematic showing characteristics of a constructed pond or constructed wetland.

Stormwater ponds and stormwater wetlands are the most common types of constructed basins. Constructed basins have a permanent pool of water and are built for the purpose of capturing and storing stormwater runoff. These basins are constructed, either temporarily or in a permanent installation, to prevent or mitigate downstream water quantity and/or quality impacts. Several types of constructed basins and wetlands (stormwater basins, constructed stormwater ponds, wet ponds, forebays, wet sedimentation basins, wet detention ponds, constructed wetlands, stormwater wetlands, etc) are included in this general category. Generally stormwater ponds do not have a significant area of vegetation. Stormwater wetlands do have significant vegetation that enhances the nutrient removal of the basin. Not included in this BMP category are dry basins without a permanent pool. Also not included are oil/water separators, swirl concentrators, and other manufactured devices with a permanent pool of water in the device.

Pollutant Removal Mechanisms

Constructed basins rely on physical, biological, and chemical processes to remove pollutants from incoming stormwater runoff. The primary treatment mechanism is gravitational settling of particulates and their associated pollutants as stormwater runoff resides in the permanent pool. Stormwater wetlands provide an additional mechanism for the removal of nutrient and other pollutants through the uptake by algae and aquatic vegetation. Volatilization and chemical activity can also occur in both ponds and wetlands, breaking down and assimilating a number of other stormwater contaminants such as hydrocarbons (WEF, ASCE/EWRI, 2012).

The longer stormwater runoff remains in the permanent pool, the more settling (and associated pollutant removal) and other treatment will occur. After the particulates settle to the bottom of a pond, a permanent pool provides protection from re-suspension when additional runoff enters the pond during and after a rain event (WEF, ASCE/EWRI, 2012).

Location in the Treatment Train

Stormwater treatment trains are comprised of multiple Best Management Practices 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. Under the treatment train approach, stormwater management begins with simple methods that prevent pollution from accumulating on the land surface, followed by methods that minimize the volume of runoff generated and is completed by Best Management Practices that reduce the pollutant concentration and/or volume of stormwater runoff. Constructed basins are typically located at the end of the stormwater treatment train, capturing all the runoff from the site.

Methodology for calculating credits

This section describes the basic concepts used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific methods for calculating credits are discussed later in this article.

Constructed basins generate credits for Total Suspended Solids (TSS) and Total Phosphorus (TP). They do not substantially reduce the volume of runoff. Constructed basins are effective at reducing concentrations of other pollutants associated with sediment, including metals and hydrocarbons. This article does not provide information on calculating credits for pollutants other than TSS and TP, but references are provided that may be useful for calculating credits for other pollutants.

Assumptions and Approach

In developing the credit calculations, it is assumed the constructed basin is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions is not valid, the BMP may not qualify for credits or credits should be reduced based on reduced ability of the BMP to achieve volume or pollutant reductions. For guidance on design, construction, and maintenance, see the appropriate article within the Manual (pond design, construction, maintenance; wetland design, construction, maintenance).

The approach in the following sections is based on the following general design considerations:

  • It is REQUIRED in the CGP that the Vwq is discharged at no more than 5.66 cubic feet per second per acre surface area of the pond.
  • The REQUIRED total storage volume (Vts) equals the sum of the volume in the permanent pool (Vpp below the outlet elevation) plus live storage allocation for water quality volume (Vwq). Vwq equals 1.0 inch of runoff per impervious acre.
  • If the pond is being designed as a wet detention pond for new construction under the MPCA CGP Permit, then a permanent pool volume (Vpp) equal to 1,800 cubic feet for each acre draining to the pond is REQUIRED.
  • It is REQUIRED in the CGP that permanent pool depths be a minimum of 3 feet and maximum of 10 feet at the deepest points.
  • It is REQUIRED in the CGP that the riser be located so that short-circuiting between inflow points and the riser does not occur.
  • If the pond will be used for temporary sediment control during construction, the associated permanent pool volume REQUIRED is either the 2 year, 24 hour storm runoff volume draining to the pond (with minimum 1800 cubic feet for each acre draining to the basin), or in the absence of such a calculation, 3600 cubic feet for each acre draining to the basin.

If any of these assumptions are not valid, the credit will be reduced.

Volume credit calculations

Constructed basins provide pollutant removal associated with settling of particulates normally present in stormwater runoff, and serve the purpose of reducing peak stormwater flows for channel protection and overbank flood control. Pollutant removal is accomplished by the maintenance of a permanent pool of water that serves to both settle and store the particulates. The necessity of the permanent pool negates the ability to infiltrate runoff; therefore no volume credit can be obtained for basins and wetlands.

Total suspended solids (TSS) calculations

Constructed basins provide pollutant removal associated with settling of particulates normally present in stormwater runoff. No credits associated with volume reduction are available. TSS removal for constructed ponds and wetlands varies with the design. Median removal rates in this Manual are 84 percent for constructed ponds and 73 percent for constructed wetlands.

Total phosphorus (TP) calculations

Constructed basins provide pollutant removal associated with settling of particulates normally present in stormwater runoff. No credits associated with volume reduction are available. TP removal for constructed ponds and wetlands varies with the design. Median removal rates in this Manual are 50 percent for constructed ponds and 38 percent for constructed wetlands. All of the phosphorus removal is assumed to be associated with removal of particulate phosphorus, with no removal credit for dissolved phosphorus.

Methods for calculating credits

This section provides specific information on generating and calculating credits from constructed basins for total suspended solids (TSS) and total phosphorus (TP). Stormwater runoff pollution reductions (“credits”) may be calculated using one of the following methods:

  1. Quantifying volume and pollution reductions based on accepted hydrologic/hydraulic models
  2. The Simple Method and MPCA Estimator
  3. MIDS Calculator
  4. Quantifying volume and pollution reductions based on values reported in literature
  5. Quantifying volume and pollution reductions based on field monitoring

The techniques described in this article assume that volume credit cannot be obtained for stormwater ponds and wetlands. This is based on an overall assumption that ponds and wetlands have insignificant losses related to seepage, evaporation, and transpiration. Stormwater pond and wetland designers that suspect significant volume losses from a specific BMP are encouraged to quantify these volume losses through field measurements.

Ponds and wetlands are also effective at reducing concentrations of other pollutants including nitrogen and metals. This article does not provide information on calculating credits for pollutants other than TSS and phosphorus, but references are provided that may be useful for calculating credits for other pollutants; see Other Pollutants and References for more information.

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 stormwater ponds and wetlands. The available models described in this section 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.

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.


The Simple Method and MPCA Estimator

The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. Pollutant loads are estimated as the product of mean pollutant concentrations and runoff depths over specified periods of time (usually annual or seasonal). The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. Ohrel (2000) states: "In general, the Simple Method is most appropriate for small watersheds (<640 acres) and when quick and reasonable stormwater pollutant load estimates are required". Rainfall data, land use (runoff coefficients), land area, and pollutant concentration are needed to use the Simple Method. For more information on the Simple Method, see The Simple method to Calculate Urban Stormwater Loads or The Simple Method for estimating phosphorus export.

Some simple stormwater calculators utilize the Simple Method (STEPL, Watershed Treatment Model). The MPCA developed a simple calculator for estimating load reductions for TSS, total phosphorus, and bacteria. Called the MPCA Estimator, this tool was developed specifically for complying with the MS4 General Permit TMDL annual reporting requirement. The MPCA Estimator provides default values for pollutant concentration, runoff coefficients for different land uses, and precipitation, although the user can modify these and is encouraged to do so when local data exist. The user is required to enter area for different land uses and area treated by BMPs within each of the land uses. BMPs include infiltrators (e.g. bioinfiltration, infiltration basin, tree trench, permeable pavement, etc.), filters (biofiltration, sand filter, green roof), constructed ponds and wetlands, and swales/filters. The MPCA Estimator includes standard removal efficiencies for these BMPs, but the user can modify those values if better data are available. Output from the calculator is given as a load reduction (percent, mass, or number of bacteria) from the original estimated load.

Warning: The MPCA Estimator should not be used for modeling a stormwater system or selecting BMPs.

Because the MPCA Estimator does not consider BMPs in series, makes simplifying assumptions about runoff and pollutant removal processes, and uses generalized default information, it should only be used for estimating pollutant reductions from an estimated load. It is not intended as a decision-making tool.

Download MPCA Estimator here: File:MPCA Estimator.xlsx

A quick guide for the estimator is available Quick Guide: MPCA Estimator tab.

Credits Computed by the MIDS Calculator

Users should refer to the MIDS Calculator section of the WIKI for additional information and guidance on credit calculation using this approach. For specific MIDS calculator applications to constructed basins, see ponds and wetlands.

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 concentration (EMC) of the pond or wetland device. A more detailed explanation of the differences between mass load reductions and concentration (EMC) reductions can be found on the pollutant removal page.

Designers may use the pollutant reduction values in the Minnesota Stormwater Manual 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 stormwater pond or wetland 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 stormwater pond or wetland are close to the design recommendations for Minnesota 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 stormwater pond, considering such conditions as watershed characteristics, pond sizing, and climate factors.

  • International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals.
    • Compilation of BMP performance studies published through 2011.
    • Provides values for TSS, Bacteria, Nutrients, and Metals
    • Applicable to grass strips, bioretention, bioswales, detention basins, green roofs, manufactured devices, media filters, porous pavements, wetland basins, and wetland channels.
  • Effectiveness Evaluation of Best Management Practices for Stormwater Management in Portland, Oregon.
    • Appendices L and M contain Excel spreadsheets 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 (page 9) 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.
  • Watershed Protection Techniques, Technical Note #114. Pollutant Removal Dynamics of Three Wet Ponds in Canada. 2000.
    • Provides values for TSS, Phosphorus, Nitrogen, Metals, Bacteria, Pentachlorophenol and Oil/Grease.
    • Applicable to Wet Ponds.
  • 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.
  • Semadeni‐Davies, Annette. "Winter performance of an urban stormwater pond in southern Sweden." Hydrological processes 20.1 (2006): 165-182.
    • Provides removal efficiencies in cold-weather climates for TSS and metals, and reports influent/effluent vales of pH
    • Applicable to stormwater ponds.

Credits Based on Field Monitoring

In the event that a credit is being calculated for an existing stormwater pond or wetland installation, field monitoring may be made in lieu of desktop calculations or models/calculators as described. Careful planning is HIGHLY RECOMMENDED before commencing a program to monitor the performance of a BMP. The general steps involved in planning and implementing BMP monitoring include the following.

  1. Establish the objectives and goals of the monitoring. When monitoring BMP performance, typical objectives may include the following.
    1. Which pollutants will be measured?
    2. Will the monitoring study the performance of a single BMP or multiple BMPs?
    3. Are there any variables that will affect the BMP performance? Variables could include design approaches, maintenance activities, rainfall events, rainfall intensity, etc.
    4. Will the results be compared to other BMP performance studies?
    5. What should be the duration of the monitoring period? Is there a need to look at the annual performance vs the performance during a single rain event? Is there a need to assess the seasonal variation of BMP performance?
  2. Plan the field activities. Field considerations include
    1. equipment selection and placement;
    2. sampling protocols including selection, storage, and delivery to the laboratory;
    3. laboratory services;
    4. health and Safety plans for field personnel;
    5. record keeping protocols and forms; and
    6. quality control and quality assurance protocols
  3. Execute the field monitoring
  4. Analyze the results

The following guidance manuals have been developed to assist BMP owners and operators on how to plan and implement BMP performance monitoring.

Urban Stormwater BMP Performance Monitoring

Geosyntec Consultants and Wright Water Engineers prepared this guide in 2009 with support from the USEPA, Water Environment Research Foundation, Federal Highway Administration, and the Environment and Water Resource Institute of the American Society of Civil Engineers. This guide was developed to improve and standardize the protocols for all BMP monitoring and to provide additional guidance for Low Impact Development (LID) BMP monitoring. Highlighted chapters in this manual include:

Evaluation of Best Management Practices for Highway Runoff Control (NCHRP Report 565)

AASHTO (American Association of State Highway and Transportation Officials) and the FHWA (Federal Highway Administration) sponsored this 2006 research report, which was authored by Oregon State University, Geosyntec Consultants, the University of Florida, and the Low Impact Development Center. The primary purpose of this report is to advise on the selection and design of BMPs that are best suited for highway runoff. The document includes chapters on performance monitoring that may be a useful reference for BMP performance monitoring, especially for the performance assessment of a highway BMP.

  • Chapter 4: Stormwater Characterization
    • 4.2: General Characteristics and Pollutant Sources
    • 4.3: Sources of Stormwater Quality data
  • Chapter 8: Performance Evaluation
    • 8.1: Methodology Options
    • 8.5: Evaluation of Quality Performance for Individual BMPs
    • 8.6: Overall Hydrologic and Water Quality Performance Evaluation
  • Chapter 10: Hydrologic Evaluation
    • 10.5: Performance Verification and Design Optimization
Investigation into the Feasibility of a National Testing and Evaluation Program for Stormwater Products and Practices
  • In 2014 the Water Environment Federation released this White Paper that investigates the feasibility of a national program for the testing of stormwater products and practices. The report does not include any specific guidance on the monitoring of a BMP, but it does include a summary of the existing technical evaluation programs that could be consulted for testing results for specific products (see Table 1 on page 8).
Caltrans Stormwater Monitoring Guidance Manual (Document No. CTSW-OT-13-999.43.01)

The most current version of this manual was released by the State of California, Department of Transportation in November 2013. As with the other monitoring manuals described, this manual does include guidance on planning a stormwater monitoring program. However, this manual is among the most thorough for field activities. Relevant chapters include.

  • Chapter 4: Monitoring Methods and Equipment
  • Chapter 5: Analytical Methods and Laboratory Selection
  • Chapter 6: Monitoring Site Selection
  • Chapter 8: Equipment Installation and Maintenance
  • Chapter 10: Pre-Storm Preparation
  • Chapter 11: Sample Collection and Handling
  • Chapter 12: Quality Assurance / Quality Control
  • Chapter 13: Laboratory Reports and Data Review
  • Chapter 15: Gross Solids Monitoring
Optimizing Stormwater Treatment Practices: A Handbook of Assessment and Maintenance

This online manual was developed in 2010 by Andrew Erickson, Peter Weiss, and John Gulliver from the University of Minnesota and St. Anthony Falls Hydraulic Laboratory with funding provided by the Minnesota Pollution Control Agency. The manual advises on a four-level process to assess the performance of a Best Management Practice.

Level 1 activities do not produce numerical performance data that could be used to obtain a stormwater management credit. BMP owners and operators who are interested in using data obtained from Levels 2 and 3 should consult with the MPCA or other regulatory agency to determine if the results are appropriate for credit calculations. Level 4, Monitoring, is the method most frequently used for assessment of the performance of a BMP.

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Other Pollutants

In addition to TSS and phosphorus, constructed basins can reduce loading of other pollutants. According to the International Stormwater Database, studies have shown that constructed basins are effective at reducing concentration of pollutants, including nutrients, metals, bacteria, cyanide, oils and grease, Volatile Organic Compounds (VOC), and Biological Oxygen Demand (BOD). A compilation of the pollutant removal capabilities from a review of literature are summarized below.

Other Pollutants Reduced by Constructed Basins: Stormwater Ponds
Link to this table

Pollutant Category Constituent Treatment Capabilities (Low = < 30%;

Medium = 30-65%; High = 65 -100%)

Metals1, 2 Cd, Cr, Cu, Zn Medium/High
As, Fe, Ni, Pb
Nutrients Total Nitrogen, Medium
TKN Low
Organics High

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


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

References

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