For a green roof system, stormwater that is captured and stored within the soil is removed from the system through evapotranspiration. The green roof does not provide phosphorus (P) reduction and reduces outflow total suspended solid (TSS) concentrations to 2.25 milligrams per liter.
For green roof 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.
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. 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 (or volume reduction credit) is calculated using BMP inputs provided by the user. For this BMP, the “Volume reduction capacity” is equal to the amount of stormwater that can be instantaneously captured by the BMP in the soils of the green roof. The capture volume (V) is given by
\(V = A_M * D_M * 0.33\)
The Volume of retention provided by BMP is the amount of volume credit the BMP provides toward the performance goal. This value is equal to the lesser of the Volume reduction capacity of BMP calculated using the above method or the Required treatment volume. This check makes sure that the BMP is not getting more credit than necessary to meet the performance goal. For example, if the BMP is oversized the user will only receive credit for the Required treatment volume routed to the BMP, which corresponds with meeting the performance goal for the site.
Pollutant load reductions are calculated on an annual basis. For this BMP, the annual total phosphorus load reduction is restricted to 0 percent. This means that the total load of phosphorus (both particulate and dissolved) calculated to enter the green roof will exit the green roof without being removed. The total suspended solids (TSS) removal is based on the outflow event mean concentration (EMC). The outflow concentration of TSS from the green roof is set at 2.25 milligrams per liter. The remaining TSS load is filtered in the green roof and removed from the system. This results in an outflow load from the green roof equal to the outflow water volume times a concentration of 2.25 milligrams per liter. For more information see the water quality treatment section in the green roofs overview and the literature review of pollutant removal by green roofs.
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 green roof can be routed to any BMP except for a swale side slope. Only another green roof BMP can be routed to a green roof BMP unless it results in water being rerouted back to the original green roof.
The following general assumptions apply in calculating the credit for a green roof. If these assumptions are not followed the volume and pollutant reduction credits cannot be applied.
A green roof is going to be constructed on an existing 0.5 (21,780 square feet) acre roof. The green roof will cover half (10,890 square feet) of the existing roof and contain media with a depth of 4 inches above the drainage system. Runoff from the entire roof will drain through the green roof. The following steps detail how this system would be set up in the MIDS calculator.
Step 1: Determine the watershed characteristics of your entire site. For this example we have a 0.5 acre site with all 0.5 acres being impervious (this is because the green roof portion is included in the impervious surface area).
Step 2: Fill in the site specific information into the Site Information tab. This includes entering a Zip Code (55414 for this example) and the watershed information from Step 1. Zip code and impervious area must be filled in. Other fields on this screen are optional.
Step 3: Go to the Schematic tab and drag and drop the “Green Roof” icon into the Schematic Window.
Step 4: Open the BMP properties for the green roof by right clicking on the green roof icon and selecting Edit BMP properties, or by double clicking on the green roof 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 calculation specific to the Green roof BMP.
Step 6: Determine the watershed characteristic for the green roof. For this example the entire site is draining to the green roof therefore the watershed parameters include a 0.5 acre site with all 0.5 acres of impervious area. There is no routing for this BMP. Fill in the BMP specific watershed information, which is 0.5 acres of impervious cover.
Step 7: Click on the BMP Parameters tab and enter in the BMP design parameters. Green roof requires two entries.
Step 8: Click on BMP Summary tab to view results for this BMP.
Step 9: Click on the OK button to exit the BMP properties screen.
Step 10: Click on Results tab to see overall results for the site.