Requirements, recommendations and information for using wet swale as a BMP in the MIDS calculator Revision as of 14:39, 7 August 2014 by PLeegar (talk | contribs) (→Swale Side Slope and Wet Swale Example)
The anticipated construction period for this page is through July, 2014
Contents
Wet Swale
A wet swale does not reduce stormwater volume. While this BMP does not provide stormwater volume reduction towards the performance goal or on an annual basis, it does provide annual pollutant load reductions. If runoff to the wet swale flows over a side slope through sheet flow, a swale side slope BMP should be used in combination with the wet swale BMP.
MIDS calculator user inputs for wet swale
For the wet swale BMP, BMP watershed areas is the only required design parameter.
- Watershed tab
- BMP Name: this cell is auto-filled but can be changed by the user.
- 'Routing/downstream BMP: if this BMP is part of a treatment train and water is being routed from this BMP to another BMP, the user selects the name of the BMP from the dropdown box to which water is being routed. All water must be routed to a single downstream BMP. Note that the user must include the BMP receiving the routed water in the Schematic or the BMP will not appear in the dropdown box.
- BMP Watershed Area: BMP watershed areas are the areas draining directly to the BMP. Values can be added for four soil types (Hydrologic Soil Groups (HSG) A, B, C, D) and for three Land Cover types (Forest/Open Space, Managed Turf and impervious). The surface area of the BMP should be included as a managed turf land cover under the hydrologic soils group of the native soils located under the BMP. Units are in acres.
- BMP Summary Tab: The BMP Summary tab summarizes the volume and pollutant reductions provided by the specific BMP. It details the performance goal volume reductions and annual average volume, dissolved P, particulate P, and TSS load reductions. Included in the summary are the total volume and pollutant loads received by the BMP from its direct watershed, from upstream BMPs and a combined value of the two. Also included in the summary, are the volume and pollutant load reductions provided by the BMP, in addition to the volume and pollutant loads that exit the BMP through the outflow. This outflow load and volume is what is routed to the downstream BMP if one is defined in the Watershed tab. Finally, percent reductions are provided for the percent of the performance goal achieved, percent annual runoff volume retained, total percent annual particulate phosphorus reduction, total percent annual dissolved phosphorus reduction, total percent annual TP reduction, and total percent annual TSS reduction.
Methodology
Required Treatment Volume
“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 following the Kerplunk method.
Volume Reduction
This BMP does not provide volume reduction towards the performance goal.
Pollutant Reduction
Pollutant load reductions are calculated on an annual basis. A wet swale does not provide annual volume reduction. Fixed removal rates of 68 percent TSS, 0 percent particulate phosphorus, and 0 percent dissolved phosphorus are applied to water that travels through the wet swale.
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.
Routing
A wet swale can be routed to any other BMP except for a green roof and a swale side slope or any BMP that would cause water to be rerouted back to the wet swale. All BMPs can be routed to a wet swale.
=Assumptions for swale main channel
The following general assumption applies in calculating the credits for a wet swale. If this assumption is not followed the pollutant reduction credits cannot be applied.
- The wet swale has been properly designed, constructed and will be properly maintained according to specifications for filtration systems.
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.
Swale Side Slope and Wet Swale Example
The runoff from a 1.4 acre parking lot surrounded by 0.359 acre of pervious turf area flows through sheet flow over one of the side slopes of a swale and into the main channel of the swale, which consists of a wet swale. The soils across the area have a unified soils classification of SW (HSG type A soil). A second side slope associated with the wet swale does not receive runoff from impervious surfaces. Each of the swale side slopes are 800 feet long by 10 feet wide with a side slope of 5H:1V. The maintenance on the swale side slopes call for mowing once a year. 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 (parking lot) and 0.8 acre of pervious area in type A soils. The pervious area includes the turf area and the area of the swale side slopes and main channel (wet swale).
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. The Managed Turf area includes the turf area, the area of the side slopes and the area of the main channel. 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 a “Swale Side Slope” and a “Wet Swale” icon into the “Schematic Window”
Step 4: Determine the watershed characteristic for each of the BMP components. For this example the swale side slope watershed includes 1.4 acres of impervious area and 0.543 acre of pervious area (0.359 acre of turf area plus 0.184 acre of swale side slope. The watershed of the wet swale includes the pervious area of the wet swale (0.073 acre) and the pervious area of the other swale side slope (0.184) for a total pervious area of 0.257 acre. Since no impervious area is being routed to the second side slope, the area can be included in the direct watershed area of the wet swale. However, the second swale side slope could be also placed in the calculator as a separate BMP.
Step 5: Open the BMP properties for the swale side slope by right clicking on the “Swale Side Slope” icon and selecting “Edit BMP properties”, or by double clicking on the “Swale Side Slope” icon.
Step 6: Click on the “Minnesota Stormwater Manual Wiki” link or the “Help” button to review input parameter specifications and calculation specific to the “Swale Side Slope” BMP.
Step 7: Fill in the specific BMP watershed information (1.4 acres of impervious and 0.543 acre of Managed Turf on A Soils. Route the side slope BMP to the wet swale BMP.
Step 8: Enter in the BMP design parameters into the “BMP parameters” tab. Swale Side Slope requires the following entries. • Side slope [H:V] which is 5:1 • Flow path length which is 10 feet • Channel length, which is 800 feet • Underlying soil – Hydrologic Soil Group, which is 4 SW (HSG A, 1.63 in/hr) • Manning’s n (vegetation), which is Native grass
Step 9: Click on “BMP Summary” tab to view results for this BMP.
Step 10: Click on the “OK” button to exit the BMP properties screen. An arrow will appear showing that the swale side slope has been routed to the wet swale.
Step 11: Open the BMP properties window for the swale main channel by right clicking on the “Wet Swale” icon and selecting “Edit BMP properties”, or by double clicking on the “Wet Swale” icon.
Step 12: Click on the “Minnesota Stormwater Manual Wiki” link or the “Help” button to review input parameter specifications and calculation specific to the “Wet Swale.”
Step 13: Enter in the watershed information for the swale main channel in the “Watershed” tab (0.257 acre for Pervious Turf on A Soil which includes the area of the main channel and the other side slope).
Step 14: Design parameters are not required for the wet swale BMP. Click on “BMP Summary” tab to view results for this BMP.
Step 15: Click on the “OK” button to exit the BMP properties screen.
Step 16: Click on “Results” tab to see overall results for the site.
