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 infiltration practices can achieve stormwater credits. Infiltration practices include infiltration basins, infiltration trenches, and underground infiltration systems.
An Infiltration basin is a large earthen structure designed to capture, store, and infiltrate stormwater water runoff. Infiltration basins rely on naturally permeable soils to fully infiltrate the designed water quality volume. Infiltration basins are typically off-line practices utilizing an emergency spillway or outlet structure to capture the volume of stormwater runoff for which the basin is designed. Volumes that exceed the rate or volume of the infiltration basin are allowed to bypass the BMP.
Infiltration basins reduce stormwater volume and pollutant loads through infiltration of the stormwater runoff into the native soil. Infiltration basins also can remove a wide variety of stormwater pollutants through secondary removal mechanisms including filtration, biological uptake, and soil adsorption through plantings and soil media (WEF Design of Urban Stormwater Controls, 2012). See Other Pollutants, for a complete list of other pollutants addressed by infiltration basins.
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 followed by Best Management Practices that reduce the pollutant concentration and/or volume of stormwater runoff. Because Infiltration basins are designed to be off-line, they may either be located at the end of the treatment train, or used as off-line configurations to divert the water quality volume from the on-line system.
This section describes the basic concepts and equations 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. Specific methods for calculating credits are discussed later in this article. Infiltration basins are also effective at reducing concentrations of other pollutants including nitrogen, metals, bacteria, 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.
Infiltration basins generate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP).
In developing the credit calculations, it is assumed the infiltration 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 infiltration basin of the Manual.
In the following discussion, the kerplunk method is assumed in calculating volume and pollutant reductions. This method assumes the water quality volume (WQV) is delivered instantaneously to the BMP. The WQV is stored as water ponded above the soil or engineered media and below the overflow elevation. The WQV can vary depending on the stormwater management objective(s). For construction stormwater, the water quality volume is 1 inch off new impervious surface. For MIDS, the WQV is 1.1 inches.
In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds. The kerplunk method therefore may underestimate actual volume and pollutant losses.
The approach in the following sections is based on the following general design considerations:
Volume credits are calculated based on the capacity of the BMP to permanently remove stormwater runoff from the existing stormwater collection system via infiltration into the underlying soil. These credits are assumed to be instantaneous values entirely based on the capacity of the infiltration basin for any storm event.
Volume reduction credits are dependent on the time required for drawdown, the infiltration rate of the underlying soil, and the area at the bottom of the BMP. The event-based volume credit, Vi in cubic feet, is given by
\( V_i = A_B\ DDT\ (I_R/12) \)
where
The required drawdown time for the Construction Stormwater General Permit is 48 hours.
The annual volume reduction credit can be calculated using the MIDS calculator or other appropriate modeling techniques. The MIDS calculator obtains the annual volume reduction through performance curves developed from multiple modeling scenarios using the volume reduction capacity for the infiltration basin, the infiltration rate of the underlying soils, and the contributing watershed size and imperviousness.
Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, MTSSi in pounds, is given by
\( M_{TSS_i} = 0.0000624\ V_i\ EMC_{TSS} \)
where
The EMCTSS entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. MTSSi can be converted to an annual load reduction if the annual volume of infiltrated water is known and if the event mean concentration represents an average annual concentration.
Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, in pounds, is given by
\( M_{TP_i} = 0.0000624\ V_i\ EMC_{TP} \)
where
The EMCTP entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs. MTPi can be converted to an annual load reduction if the annual volume of infiltrated water is known and if the event mean concentration represents an average annual concentration.
This section provides specific information on generating and calculating credits from infiltration practices for volume, TSS and Total Phosphorus (TP). Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:
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 infiltration basins. 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:
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 biofiltration 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 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.
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.
The Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various low impact development (LID) BMPs. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.
The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.
Detailed guidance has been developed for all BMPs in the calculator, including permeable pavement. An overview of individual input parameters and workflows is presented in the MIDS Calculator User Documentation.
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 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:
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.
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 in Table 3-1.
Relative pollutant reduction from infiltration systems for metals, nitrogen, bacteria, and organics.
Link to this table
Pollutant Category | Constituent | Treatment Capabilities
(Low = < 30%; Medium = 30-65%; High = 65 -100%) |
---|---|---|
Metals1 | Cr, Cu, Zn | High2 |
Ni, Pb | ||
Nutrients | Total Nitrogen, TKN | Medium/High |
Bacteria | Fecal Coliform, E. coli | High |
Organics | High |
1 Results are for total metals only
2 Treatment capabilities are based mainly on information from sources that referenced only metals as a category and did not provide individual efficiency for specific metals