Calculating credits for infiltration basin

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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.

Credit Calculation Methods

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

Quantifying volume and pollution reductions based on volume reduction and BMP parameters presented in this credit article
Quantifying volume and pollution reductions based on accepted hydrologic/hydraulic models
MIDS Calculator approach
Quantifying volume and pollution reductions based on values reported in literature
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 (VP) is given by

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

where: AO = Overflow surface area at elevation of outlet or overflow, or ponding surface area (sf). AB = Surface area at the bottom of the infiltration basin (sf). DO = 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: IR = Design infiltration rate of underlying soil group (in/hr). DDTcalc = 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.

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