Requirements, recommendations and information for using swale without an underdrain as a BMP in the MIDS calculator

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Swale Main Channel

For a swale main channel BMP (no underdrain), stormwater can be retained through three separate methods. Stormwater can infiltrate into the soils as it travels through the main channel to the outflow, stormwater can pond behind a check dam and infiltrate into the underlying soils, or stormwater can be stored in the pore spaces of an engineered bioretention base and infiltrate into the underlying soils. If stormwater runoff to the main channel flows over a side slope through sheet flow, a swale side slope BMP should be used in combination with the swale main channel BMP. All pollutants in the infiltrated water are credited as being reduced. Pollutants in the stormwater that flows through the channel outflow are removed through filtration and sedimentation.

MIDS calculator user inputs for swale main channel

For swale main channel systems, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP.

• 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 tab 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.
  • If a swale side slope is routed to this BMP, do not include the side slope watershed areas for the swale main channel since this would be double counting the contributing area.
• BMP Parameters tab
  • Swale bottom width (WB): This is the average bottom width of the swale main channel. Units are in feet. Based on design restrictions, the bottom width cannot be less than 2 feet.
  • Channel length (LC): This is the length of the swale channel from the furthest upstream point to the furthest downstream point. Units are in feet.

• • Channel slope (S): This is the slope of the channel. Units are in percent. The slope is calculated by taking the difference between vertical elevations at the upstream and downstream points of the swale and dividing by the horizontal distance between the two locations. The slope therefore represents an average slope over the length of the swale. If the slope varies over the length of a swale, the user should take an average slope over the entire length rather than break the swale into separate smaller lengths. Based on design restrictions, the channel slope cannot be less than 0.5 percent or greater than 4 percent.
  • Underlying soil - Hydrologic Soil Group: The user selects the most restrictive soil (lowest hydraulic conductivity) within 3 feet of the soil surface or the soil/media interface (if a bioretention base is utilized) of the swale main channel. There are 14 soil options that fall into 4 different Hydrologic Soil Groups (Hydrologic Soil Group (HSG) A, B, C, or D) for the user. These correspond with soils and infiltration rates contained in this Manual. Once a soil type is selected, the corresponding infiltration rate will populate the “Infiltration rate of underlying soils” field. The user may also select “User Defined.” This selection will activate the “User Defined Infiltration Rate” cell, allowing the user to enter a different value from the value in the predefined selection list. The maximum allowable infiltration rate is 1.63 inches per hour.
  • Manning’s n (Vegetation): The user selects a type of vegetation cover on the swale main channel, which populates a corresponding Manning’s n. The vegetation types are mowed turf or native grasses. Mowed turf would be selected if the main channel is mowed on a consistent basis. Native grasses would be selected if the main channel is left to grow or is mowed infrequently. Once a vegetation cover is selected, the corresponding Manning’s n will populate in the “Manning’s n” field. The user may also select “User Defined.” This selection will activate the “User Defined Manning’s n” cell allowing the user to enter a value different from the values in the predefined selection list.
  • Does the swale have a check dam?: This is a YES/NO question. The user selects YES if one or more check dams have been installed in the swale main channel. By selecting YES, the “Is check dam permeable?” input box will become active.
  • Is check dam permeable?: This is a YES/NO questions. The user selects YES if the check dams are permeable, allowing stormwater to filter through the media and NO if they are impermeable, acting as a dam and providing extended detention in the swale main channel. If NO is selected, the design parameters associated with the check dam will become active. If YES is selected, the design parameters will remain inactive as no credit toward the performance goal is obtained for permeable check dams.
  • Top width at check dam (WT): This is the width of the check dam at the overflow elevation (top of check dam) and is used to calculate the storage volume behind the check dam. Units are in feet.

Swale Side Slope and Main Channel Example (Version 2)

The runoff from a 1.4 acre parking lot surrounded by 0.359 acres of pervious turf area flows through sheet flow over one of the side slopes of a swale and into a swale main channel. The soils across the area have a unified soils classification of SW (HSG type A soil). A second side slope associated with the main channel 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 main channel of the swale is 800 feet long by 4 feet wide with a 2 percent slope. The swale main channel does not have an underdrain, bioretention base or check dams. The maintenance on the swale calls 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 acres 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.

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 “Swale Main Channel” 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 swale main channel includes the pervious area of the main channel (0.073 acre) and the pervious area of the other swale side slope (0.184 acre) for a total pervious area of 0.257 acre. Since no impervious area is being routed to the second swale side slope, the area can be included in the direct watershed area of the main channel. However, the second swale side slope could be placed in the calculator as an additional BMP. Including it as a separate BMP provides a slightly greater annual volume reduction and more closely represents the true system.

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 main channel 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 swale main channel.

Step 11: Open the BMP properties window for the swale main channel by right clicking on the “Swale main channel” icon and selecting “Edit BMP properties”, or by double clicking on the “Swale main channel” 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 “Swale main channel” BMP.

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: Enter in the BMP design parameters into the “BMP parameters” tab. Swale main channel requires the following entries.

• Channel length, which is 800 feet
• Swale bottom width, which is 4 feet
• Channel slope, which is 2 percent (%)
• Underlying soil – Hydrologic Soil Group, which is 4 SW (HSG A, 1.63 in/hr)
• Manning’s n (vegetation), which is native grass
• Does the swale have a check dam, which is no
• Does the swale have a bioretention base, which is no

Step 15: Click on “BMP Summary” tab to view results for this BMP.

Step 16: Click on the “OK” button to exit the BMP properties screen.

Step 17: Click on “Results” tab to see overall results for the site.