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Minnesota Stormwater Manual

Calculating credits for stormwater and rainwater harvest and use/reuse
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Recommended pollutant removal efficiencies, in percent, for infiltration BMPs. Sources.

TSS=total suspended solids; TP=total phosphorus; PP=particulate phosphorus; DP=dissolved phosphorus; TN=total nitrogen

TSS TP PP DP TN Metals Bacteria Hydrocarbons
Pollutant removal is 100 percent for the volume that is captured and infiltrated

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual Best Management Practice (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 (including dry wells), and underground infiltration systems. The discussion does not include bioinfiltration and permeable pavement systems, unless specifically mentioned. To view the credit articles for other BMPs, see the Related pages section.

Overview

This schematic shows Example Stormwater Harvesting and Use System Schematic
Schematic illustrating the basic concepts for a harvest and use-reuse system. stormwater or rainwater is stored in a cistern or pond and delivered outdoors or indoors through a distribution system.

Stormwater and rainwater harvest and use/reuse systems capture and store runoff. The stored water is typically utilized for irrigation. This water is assumed to infiltrate. Credits for these BMPs are therefore similar to credits for other infiltration practices in that all water applied for irrigation and pollutants in that water are credited. The methodology differs, however, in that the water is captured instantaneously, but use of the water is dependent on the irrigation rate rather than the soil infiltration rate, as is the case with infiltration BMPs. The period of use is also during the growing season, meaning the generated credits only apply at that time. If harvested water is used indoors, it may be discharged to a sewer system, to a septic drainfield, or to another stormwater BMP. Credits for these vary and are discussed below.

Pollutant removal mechanisms

Infiltration practices reduce stormwater volume and pollutant loads through infiltration of the stormwater runoff into the native soil. Infiltration practices 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 practices.

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. The position of a harvest and use/reuse system in a treatment train is a function of the surface from which the water is being collected. Rainwater harvest systems, which are designed to collect water from rooftops, will generally be located near the beginning of the treatment train, while systems that store water in ponds will be located near the end of treatment trains.

Methodology for calculating credits

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. If harvest water is being infiltrated, this practice is 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.

Assumptions and approach

In developing the credit calculations, it is assumed the harvest and use/reuse system 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 Stormwater and rainwater harvest and use/reuse

Warning: Pre-treatment is required for all infiltration practices

In the following discussion, the water quality volume (VWQ) is delivered instantaneously to the BMP. VWQ is stored in a cistern or a pond. VWQ can vary depending on the stormwater management objective(s). For construction stormwater, VWQ is 1 inch off new impervious surface. For MIDS, VWQ is 1.1 inches.

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.
  • TSS and TP credits are achieved for the volume of runoff that is infiltrated.

Volume Credit Calculations

Volume credits are calculated based on the capacity of the BMP and its ability to permanently remove stormwater runoff via infiltration into the underlying soil from the existing stormwater collection system. These credits are assumed to be instantaneous values. However, unlike other stormwater infiltration practices, for an irrigation system, the volume credit is a function of both the water available for storage, the rate at which water is applied, and the area over which the water is applied.

If we assume that on average there are 3 days between rain events, then the volume V retained toward a performance goal can be given by

<math> V = I_r/12 * A * 3/7

where

Ir is the weekly irrigation rate (in/week); and
A is the irrigated area (ft2.

This calculation assumes the storage device is sized to hold the water quality volume. If the storage device holds less than the water quality volume, then the above calculation must be adjusted accordingly.

This credit can only be applied during the time of year when the irrigation system is in practice. To determine compliance with a performance goal throughout the year, we need to know the annual volume of runoff and the volume of water applied as irrigation. The annual volume captured and infiltrated by the BMP can be determined with appropriate modeling tools, including the MIDS calculator. Example values are shown below for a scenario using the MIDS calculator. For example, a permeable pavement system designed to capture 1 inch of runoff from impervious surfaces will capture 89 percent of annual runoff from a site with B (SM) soils.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume. See footnote1 for how these were determined.
Link to this table

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.



Related pages


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