Inver Grove Heights Stormwater Manual Oakdale, MN. Kayhanian, M. Characterization and prediction of highway runoff constituent event mean concentration. Maestre, A. University of Alabama.
Center for Watershed Protection. Massachusetts Department of Environmental Protection, Massachusetts Stormwater Handbook , Volume 2.
Maniquiz-Redillas, M. Geronimo, and L. Journal of Environmental Sciences, Vol. New Jersey Department of Environmental Protection.
Chapter 9. New Jersey Corporation for Advanced Technology. Marsalek, D. Doede, and C. Taylor, G. Fletcher, T. Wong, P. Breen, and H. Water Research, Vol. Tahoe Regional Planning Agency. Best Management Practices Handbook. Trash and Debris Management. Virginia Department of Ecology. Volumes 1 and 2. Virginia Department of Environmental Quality. In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds.
The instantaneous volume method therefore may underestimate actual volume and pollutant losses. 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 entirely based on the capacity of the BMP to capture, store, and transmit water in any storm event.
Because the volume is calculated as an instantaneous volume, the water quality volume V WQ is assumed to pond below the overflow elevation and above the bioretention media. This entire volume is assumed to infiltrate through the bottom of the BMP. The volume credit V inf b for infiltration through the bottom of the BMP into the underlying soil, in cubic feet, is given by.
If native soils are used rather than engineered media, the term A M may be substituted by A B , as shown in the above schematic and in the schematics for the MIDS calculator. To comply with the Construction Stormwater General Permit , V WQ must infiltrate within 48 hours 24 hours is recommended if discharges are to a trout stream.
Some of the V WQ will be lost to evapotranspiration rather than all being lost to infiltration. In terms of a water quantity credit, this differentiation is unimportant, but it may be important if attempting to calculate actual infiltration into the underlying soil.
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 footnote 1 for how these were determined. Link to this table. 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 Pollutant removal for infiltrated water is assumed to be percent. The mass of pollutant removed through infiltration, M TSS i in pounds, is given by.
The above calculation may be applied on an annual basis and is given by. The mass of pollutant removed through infiltration, in pounds, is given by. This section provides specific information on generating and calculating credits from infiltration practices for volume, TSS and TP.
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. Furthermore, many of the models listed below cannot be used to determine compliance with the Construction Stormwater General permit since the permit requires the water quality volume to be calculated as an instantaneous volume. Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:.
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. 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 event mean concentration 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. Rainfall data, land use runoff coefficients , land area, and pollutant concentration are needed to use the Simple Method. 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.
Output from the calculator is given as a load reduction percent, mass, or number of bacteria from the original estimated load. Default TSS removal fractions are 0. Due to vegetation, Biofiltration Strips and Swales look like a landscaped roadside, which makes these devices more acceptable than Treatment BMPs that make use of concrete-lined vaults. They were also determined to be very cost effective and among the least expensive Treatment BMP per volume of runoff treated. Limitations Must meet maximum longitudinal slope and maximum slope requirements for Biofiltration Swales.
Vegetated Biofiltration Strips and Swales should not be used in arid regions. Instead, consider using non vegetated swales that rely upon gravel mulch or similar inert materials to protect disturbed soil areas. Discuss with District Design NPDES Coordinator if Strips and Swales are under consideration above contaminated soils or groundwater plumes, although the infiltration that might occur is usually considered incidental.
Biofiltration Strips are not generally subject to the same setback restrictions as Infiltration Devices; however, if unusual geotechnical conditions exist, or if it is proposed above a retaining wall and the soils are known to be especially erodible or permeable, consult District Geotechnical Design.
Nearby fill slopes should be observed for signs that the embankment soils are highly erodible, and the District Landscape Architect should be consulted about soil amendments, the use of fiber rolls, or other methods to reduce the erosion potential.
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