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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 volume credit, V, is given by | 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 volume credit, V, is given by | ||
+ | <math> A = 25\ B\ (C + D)\ /E </math> | ||
<math> V = A_B\ DDT\ (I_R/12) </math> | <math> V = A_B\ DDT\ (I_R/12) </math> |
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: •Credit calculations presented in this article are for both event and annual volume and pollutant load removals. •Stormwater volume credit equates to the volume of runoff that will ultimately be infiltrated into the soil subgrade. •TSS and TP credits are achieved for the volume of runoff that is infiltrated.
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. Instantaneous volume reduction, or event based volume reduction, can be converted to annual volume reduction percentages using the MIDS calculator or other appropriate modeling tools.
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 volume credit, V, is given by \( A = 25\ B\ (C + D)\ /E \)
\( V = A_B\ DDT\ (I_R/12) \)
where
The required drawdown time for the Construction Stormwater General Permit is 48 hours.
The volume reduction credit (V) can be converted to annual volume reduction percentage (VA%) if the annual volume reduction quantity is desired. This conversion can be generated using the MIDS calculator or other appropriate modeling techniques. The MIDS calculator obtains the percentage 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.
Information about Infiltration Basins
Overview of Stormwater Credits
Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:
This section provides specific information on generating and calculating credits from infiltration basins for volume, TSS, and phosphorus. Infiltration basins 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.
Alternative techniques for calculating credits associated with volume and pollutant reductions may be proposed to the Minnesota Pollution Control Agency or other permitting agency for their consideration and approval.
Quality credits applied to infiltration basins can be calculated per rain event or based on total annual rainfall. Though there is little available data to demonstrate load reductions in infiltration basins, when properly designed, constructed, and maintained, the entire volume of stormwater entering the basin, and the pollutant loads carried by that runoff, should be removed entirely. This does not include any stormwater in excess of the capacity of the BMP that ultimately bypasses the system.
TSS reduction credit corresponds directly with the volume reduction capacity of the infiltration basin. Because infiltration basins are designed entirely offline, 100% TSS removal is assumed for infiltrated stormwater.
The annual TSS credit (MTSS) for infiltration basins is given by
\( M_TSS=2.72*V_Annual*〖EMC〗_TSS \)
where: MTSS =Annual or event TSS removal (lb/yr or lb/event).
Vannual = Annual volume reduction credit calculated above (acre-ft).
EMCTSS = Event Mean Concentration, concentration of TSS in the runoff. (mg/L). Note: if infiltration basin is not the upstream most BMP in the treatment train, EMCTSS should be dependent on the MTSS effluent (mg/L) from the next upstream tributary BMP.
Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds.
The storm event based TSS credit (MTSS) for infiltration basins is given by \( M_TSS=2.72*V/43,560*〖EMC〗_TSS \)
where: MTSS =Annual or event TSS removal (lb/yr or lb/event).
V = Event volume reduction credit calculated above (cf).
EMCTSS = Event Mean Concentration of TSS in the runoff. (mg/L). Note: if infiltration basin is not the upstream most BMP in the treatment train, EMCTSS should be dependent on the MTSS effluent (mg/L) from the next upstream tributary BMP.
Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds. A factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.
Similar to TSS, TP reduction credits correspond with volume reduction through infiltration of water captured by the infiltration basin. 100% removal of TP in captured stormwater is also assumed.
The annual TP credit (MTP) for infiltration basins is given by
\( M_TP=V_annual*〖EMC〗_TP*2.72 \)
where:
MTP =Annual or event TP removal (lb/yr or lb/event).
Vannual = Annual volume reduction credit calculated above (acre-ft).
EMCTP = Event Mean Concentration of TP in runoff. (mg/L). Note: if infiltration basin is not the upstream most BMP in the treatment train, EMCTP should be dependent on the MTP effluent (mg/L) from the next upstream tributary BMP.
Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds.
The storm event based TP credit (MTP-I) for infiltration basins is given by
\( M_TP=2.72*V/43,560*EMC_TP \)
where: MTP =Annual or event TP removal (lb/yr or lb/event).
V = Event volume reduction credit calculated above (cf).
EMCTP = Event Mean Concentration of TP in the runoff. (mg/L). Note: if infiltration basin is not the upstream most BMP in the treatment train, EMCTP should be dependent on the MTP effluent (mg/L) from the next upstream tributary BMP.
Factor of 2.72 used for conversion of acre-feet to liters and milligrams to pounds. A factor of 43,560 is used for conversion of volume from cubic feet to acre-ft.
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:
Users should refer to the MIDS Calculator section of the WIKI for additional information and guidance on credit calculation using this approach.
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