Calculating credits for infiltration basin

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Overview

Schematic showing Infiltration Basin Detailed Cross Section

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.

Pollutant Removal Mechanisms

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 Section 3, Other Pollutants, for a complete list of other pollutants addressed by infiltration basins.

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 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.

Related Articles within the Minnesota Stormwater Manual

Information about Infiltration Basins

Overview of Stormwater Credits

Credit Calculation Methods

Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:

1. Quantifying volume and pollution reductions based on volume reduction and BMP parameters presented in this credit article
2. Quantifying volume and pollution reductions based on accepted hydrologic/hydraulic models
3. MIDS Calculator approach
4. Quantifying volume and pollution reductions based on values reported in literature
5. Quantifying volume and pollution reductions based on field measurements

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 Section 3, Other Pollutants, and Section 4, Resources, 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.

Assumptions and Approach

For this section, several key assumptions were necessary in developing the credit calculations. These assumptions include that the infiltration basin:

• Is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual.

• Follow the REQUIRED, HIGHLY RECOMMENDED, and RECOMMENDED design criteria.

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

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

• The credit for volume reduction represents the volume of runoff that can be retained and ultimately infiltrated between the basin bottom and the overflow elevation. Credit is also given for infiltration and evapotranspiration occurring within the drawdown time of the basin.

• Native soils of the site should be evaluated to achieve representative infiltration rates either through field soil measurements or by using a county soil survey or the NRCS Web Soil Survey. Design infiltration rates should use the infiltration rate of the least permeable soil within the first 5 feet below the bottom elevation of the proposed infiltration practice. Infiltration rates can be related to either tested values or the typical design infiltration rates for A, B, C, and D soil groups. It is REQUIRED that the measured infiltration rates shall be divided by a minimum safety factor of 2, though a higher factor of safety between 2 and 10 is recommended.

• Water Quality credits for infiltration basins are calculated assuming all stormwater entering the basin is infiltrated and all pollutants associated with this volume are removed.

Volume Reduction and Water Quality Credits

Volume Credit Calculations

Volume credits for infiltration basins are calculated based on the volume capture ability to permanently remove stormwater runoff from the existing stormwater collection system. 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 volume capacity of the basin between the overflow elevation, and the basin bottom, infiltration through the bottom of the basin, and evapotranspiration. Credit is only obtainable for evapotranspiration and infiltration values if there is available space for water within the ponding area to be infiltrated or evapotranspirated within the drawdown time. If the ponding volume is greater than the infiltration and evapotranspiration capacities of the native soils and plantings, then the volume credit for the infiltration pond is limited to the ponding area volume.

Volume credits (V) for infiltration basins are given by

If \( V_p<V_(inf⁡_b)+V_ET, V=V_P+V_(inf⁡_b)+V_ET \)

If \( V_p>V_(inf⁡_b)+V_ET, V=V_P \)

where:

V = Total event based volume credit for BMP (cf).

V_P = Volume reduction credit for ponding area storage capacity (cf).

V_{inf_b} = Volume reduction of infiltration through bottom of basin (cf).

V_ET = Volume reduction through evapotranspiration (cf).

Volume capacity of the ponding area (V_P) is given by

\( V_P=(A_O+A_B)/2*D_O \)

where:

A_O = Overflow surface area at elevation of outlet or overflow, or ponding surface area (sf).

A_B = Surface area at the bottom of the infiltration basin (sf).

D_O = Overflow depth, or depth from top of the filter bed to the top of the ponding area (ft).

Volume credits for infiltration through the bottom of the basin (Vinf_b) are given by

\( V_(inf⁡_b⁡)=I_R/12*A_B*DDT_calc \)

where:

I_R = Design infiltration rate of underlying soil group (in/hr).

DDT_calc = Required drawdown time (hrs). A conservative approach would be to assume a maximum of 24 hour drawdown time for credit calculations so as not to overestimate volume reduction capacity.

The volume of water lost through ET is assumed to be the smaller of two calculated values: potential ET (ETpot) and measured ET (ETmea). Potential ET is equal to the amount of water stored in the basin between field capacity and the wilting point. Potential ET is converted to ET by multiplying by a factor of 0.5. Measured ET is the amount of water lost to ET as measured using available data and is assumed to be 0.2 inches/day. ET is considered to occur over a period equal to the drawdown time of the basin.

Volume credit for evapotranspiration (VET) is given by the lesser of

\( ET_mea=0.2/12*A_B*0.5*DDT_calc \)

or

\( ET_pot=D_B*A_B*(FC-WP) \)

where: ET_mea = The amount of water lost to ET as measured using available data (cf).

ET_pot = The amount of water stored between field capacity and the wilting point in the media (cf).

D_B= Depth below infiltration basin to seasonal high groundwater or bedrock (ft).

FC-WP = Water holding capacity of soils, equal to field capacity minus the wilting point of soils (cf/cf).

The volume reduction credit (V) can be converted to annual volume reduction percentage (V_A%) 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. The annual volume reduction (V_annual) credit is then given by

\( V_annual=V_(A%)*V_AR \)

where: V_annual = Annual volume reduction credit (acre-ft).

V_A% = Annual volume reduction percentage. Value calculated using MIDS calculator or other appropriate modeling techniques using the volume reduction (V) calculated above.

V_AR=Annual runoff volume (acre-ft).= \((DA*P_j*P*R_V)/12\)

DA = Total drainage area to BMP (acre).

P_j = Annual rainfall correction factor. Fraction of annual rainfall volume that produces runoff.

P = Annual rainfall precipitation (in).

R_V = Site runoff coefficient. Weighted average of area of land cover and associated runoff coefficient values.

Water Quality Credit Calculations

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.

Total Suspended Solids

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: M_TSS =Annual or event TSS removal (lb/yr or lb/event).

V_annual = Annual volume reduction credit calculated above (acre-ft).

EMC_TSS = 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: M_TSS =Annual or event TSS removal (lb/yr or lb/event).

V = Event volume reduction credit calculated above (cf).

EMC_TSS = 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.

Total Phosphorus

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:

M_TP =Annual or event TP removal (lb/yr or lb/event).

V_annual = Annual volume reduction credit calculated above (acre-ft).

EMC_TP = 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: M_TP =Annual or event TP removal (lb/yr or lb/event).

V = Event volume reduction credit calculated above (cf).

EMC_TP = Event Mean Concentration of TP in the runoff. (mg/L). Note: if infiltration basin is not the upstream most BMP in the treatment train, EMC_TP 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.