A Constructed wetland BMP does not reduce stormwater volume. While this BMP does not provide stormwater volume reduction toward the performance goal or on an annual basis, it does provide annual pollutant load reductions.
For the constructed wetland BMP, BMP watershed areas is the only required design parameter.
The 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.
This BMP does not provide volume reduction toward the performance goal.
Pollutant load reductions are calculated on an annual basis. A constructed wetland BMP does not provide annual volume reduction. Fixed removal rates of 68 percent total suspended solids (TSS), 55 percent particulate phosphorus, and 0 percent dissolved phosphorus are applied to stormwater runoff that enters the constructed wetland.
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 constructed wetland can be routed to any other BMP except for a green roof and a swale side slope or any BMP in a stormwater treatment sequence that would cause stormwater to be rerouted back to the constructed wetland already in the sequence. All BMPs can be routed to a constructed wetland except for a swale side slope.
The following general assumption applies in calculating the credits for a constructed wetland. If this assumption is not followed, the pollutant reduction credits cannot be applied.
On the watershed tab, the following warning is given: “The calculator does not require sizing inputs for non-volume reducing BMPs. This BMP should be sized according to the guidelines in the MN stormwater manual.” Design and construction criteria can be found at this link
A wetland is to be constructed in a watershed that contains a 1.4 acre parking lot surrounded by 0.8 acres of pervious turf area. All of the runoff from the watershed will be treated by the wetland. The soils across the area have a unified soils classification of SM (HSG type B soil). 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 2.2 acre site with 1.4 acres of impervious area and 0.8 acre of pervious turf area in type B soils. The pervious area or impervious area does not include the wetland 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 or an error message will be generated. Other fields on this screen are optional.
Step 3: Go to the Schematic tab and drag and drop the “Wetland” icon into the “Schematic Window”
Step 4: Open the BMP properties for the wetland by right clicking on the “Constructed wetland” icon and selecting “Edit BMP properties”, or by double clicking on the “Constructed wetland” icon.
Step 5: Click on the “Minnesota Stormwater Manual Wiki” link or the “Help” button to review input parameter specifications and calculation specific to the “Constructed Wetland” BMP.
Step 6: Determine the watershed characteristics for the wetland. For this example the entire site is draining to the wetland. The watershed parameters therefore include a 2.2 acre site with 1.4 acres of impervious area and 0.8 acres of pervious turf area in type B soils. There is no routing for this BMP. Fill in the BMP specific watershed information (1.4 acres on impervious cover and 0.8 acres of Managed Turf in B soils).
Step 7: Design parameters are not required for the wetland BMP. Click on “BMP Summary” tab to view results for this BMP.
Step 8: Click on the “OK” button to exit the BMP properties screen.
Step 9: Click on “Results” tab to see overall results for the site.