Stormwater disconnection/Impervious surface disconnection spreads runoff generated from parking lots, driveways, rooftops, sidewalks and other impervious surfaces onto adjacent pervious areas where it can be infiltrated. All pollutants in the infiltrated water are credited as being reduced. Pollutants in the stormwater that bypasses the best management practice (BMP) receive 68 percent removal for total suspended solids (TSS) and 0 percent reduction for both dissolved and particulate phosphorus.
For stormwater disconnection systems, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP.
The following are requirements or recommendations for inputs into the MIDS calculator. If the following are not met, an error message will inform the user to change the input to meet the requirement.
The methodology for deriving this credit, including equations, summary of modeling, and examples, are described in the turf section of this manual.
“Required treatment volume,” or the volume of stormwater runoff delivered to the BMP, equals the performance goal (1.1 inches or user-specified performance goal) times the impervious area draining to the BMP, plus any water routed to the BMP from an upstream BMP. This stormwater is delivered to the BMP instantaneously.
The volume reduction achieved by a BMP compares the capacity of the BMP to the required treatment volume. The “Volume reduction capacity of BMP [V]” is calculated using BMP inputs provided by the user. A stormwater disconnection BMP does not have storage capacity similar to other BMPs in the MIDS calculator. Volume reduction occurs through infiltration as the stormwater travels over the effective pervious area. To obtain an instantaneous stormwater volume credit modeling was conducted. A modeling analysis was performed to quantify the runoff reduction achieved from redirecting runoff from impervious areas to pervious areas of varying size and soil type. The long-term, 35-year continuous simulation XP-SWMM model developed in support of the MIDS performance goal development (Barr, 2011) was modified to represent watersheds with I/P ratios ranging from 0.2:1 to 50:1 and hydrologic soil types of A, B, C, and D. See the Assessment of MIDS Performance Goal Alternatives: Runoff Volumes, Runoff Rates, and Pollutant Removal Efficiencies report for additional information on the hydrologic model input parameters utilized for this model. The models results are used with a series of calculations to obtain the “Volume reduction capacity of the BMP [V]. The calculations are summarized here.
Pollutant load reductions are calculated on an annual basis. Therefore, the first step in calculating annual pollutant load reductions is converting the “Volume reduction capacity of BMP,” which is an instantaneous volume reduction, to an annual volume reduction percentage. This is accomplished through the use of performance curves developed from multiple modeling scenarios. The performance curves use the “Volume reduction capacity of BMP”, the infiltration rate of the effective pervious area underlying soils, the contributing watershed percent impervious area, and the size of the contributing watershed to calculate a percent annual volume reduction.
A 100 percent removal is credited for all pollutants associated with the reduced volume of stormwater since these pollutants are either attenuated within the media or pass into the underlying soil with infiltrating water. For water routed to the effective pervious area but that does not infiltrate, pollutant removal occurs through filtration. Removal rates for this water are 68 percent for total suspended solids (TSS), 0 percent for particulate phosphorus, and 0 percent for dissolved phosphorus. A schematic of the removal rates can be seen in the sidebar. NOTE: The user can modify event mean concentrations (EMCs) on the Site Information tab in the calculator. Default concentrations are 54.5 milligrams per liter for total suspended solids (TSS) and 0.3 milligrams per liter for total phosphorus (particulate plus dissolved). The calculator will notify the user if the default is changed. Changing the default EMC will result in changes to the total pounds of pollutant reduced.
A stormwater disconnection BMP can be routed to any other BMP, except for a green roof and a swale side slope or any BMP that would cause stormwater to be rerouted back to the stormwater disconnection already in the stormwater runoff treatment sequence. No BMPs can be routed to the stormwater disconnection BMP.
The initial screen in the calculator requires the user to specify if they are using the calculator to determine compliance with the Minnesota Construction Stormwater general permit. If the user answers YES, the disconnection BMP cannot be used for determination of volume reductions. This is because this BMP does not capture an instantaneous volume of water, as required in section III.D.1.e of the permit. If the user chooses no, the BMP is available and the Output page in the calculator will indicate that the user has chosen not to use the calculator for determining compliance with the permit.
An impervious area (e.g. parking lot) that is 435.6 feet long and 100 feet wide is redirected to a pervious area. The pervious area is on A soils. Although the pervious area is more 100 feet wide, only 100 feet of the pervious area is considered effective because channelized flow is assumed beyond 100 feet. The effective pervious area is therefore 1 acre.
Step 1: Determine the watershed characteristics of your entire site. For this example, we have a 2 acre site that includes 1 acre of impervious area and 1 acre of pervious turf area in type A soils. The pervious area includes the turf area and therefore does not contribute runoff to the BMP.
Step 2: Fill in the site specific information into the Site Information tab. This includes entering a ZIP Code (55105 for this example) and the watershed information from Step 1. ZIP code and impervious area must be filled in or an error message will be generated. The user must also indicate whether the calculator is being used for permit compliance. To use the disconnection BMP the user must select No for this question, otherwise the disconnection BMP will not be available to the user. Other fields on this screen are optional.
Step 3: Go to the Schematic tab and drag and drop the Stormwater disconnection (impervious disconnection) icon into the Schematic window.
Step 4: Open the BMP properties for underground infiltration by right clicking on the Stormwater disconnection (impervious disconnection) icon and selecting Edit BMP Properties, or by double clicking on the Stormwater disconnection (impervious disconnection) icon.
Step 5: If help is needed, click on the Minnesota Stormwater Manual Wiki link or the Help button to review input parameter specifications and calculations pertinent to Stormwater disconnection (impervious disconnection).
Step 6: Determine the watershed characteristics for the Stormwater disconnection (impervious disconnection) practice. The watershed parameters include 1 acre of redirected impervious surface draining to 1 acre of pervious turf on A soil. There is no routing/downstream BMP for this BMP. Fill in this BMP-specific watershed information in the Watershed tab (1 acres of Impervious Cover and 1 acre of Managed Turf in A soils).
Step 7: Click on the BMP Parameters tab and enter the BMP design parameters.