Stormwater Treatment Practice Calculator

The STP Calculator is a tool developed by the Department of Environmental Conservation (DEC) to estimate total phosphorus load reductions achieved by stormwater treatment practices (STPs). The user enters STP data into the tool and the tool calculates the estimated annual average total phosphorus load reduction. Calculations are based on the same methods DEC will use to track progress reducing phosphorus pollution loading into Lake Champlain and Lake Memphremagog. The tool can currently only be applied to estimate total phosphorus reductions in the Lake Champlain and Lake Memphremagog watersheds, as pollutant loading rates are currently unavailable outside these basins in Vermont. The calculator tool focuses on STPs treating runoff from developed lands, only, and should not be used for agricultural, forested, or other types of land use. The calculator may not be suited for complex STP systems or retrofit projects. The STP calculator should only be used for planning purposes to understand pollutant reduction potential for STPs. Data entered in the STP calculator will not be stored in a database. DEC retains the right to verify the data input and will provide final phosphorus load reduction crediting based on data reported to and stored in DEC’s Watershed Projects Database.

Instructions can be found Here.

Please direct any questions to Claire.Madden@vermont.gov or 802-636-7536.

STP Calculator

Loading Information

Drainage Area 1 - Mettawee River 1 - Poultney River 1 - South Lake B Direct Drainage 2 - South Lake A Direct Drainage 3 - Port Henry Direct Drainage 4 - Lewis Creek 4 - Little Otter Creek 4 - Otter Creek 4 - Otter Creek Direct Drainage 5 - Main Lake Direct Drainage 5 - Winooski River 6 - Laplatte River 7 - Burlington Bay - CSO 7 - Burlington Bay Direct Drainage 9 - Lamoille River 9 - Malletts Bay Direct Drainage 10 - Northeast Arm Direct Drainage 11 - St. Albans Bay Direct Drainage 12 - Missisquoi Bay Direct Drainage 12 - Missisquoi River 13 - Isle La Motte Direct Drainage M - Black River-headwaters to Seaver Branch M - Black River-Seaver Branch to Lords Creek M - Lords Creek M - Black River-Lords Creek to mouth M - Barton River-headwaters to Roaring Brook M - Barton River-Roaring Branch to Willoughby River M - Willoughby River M - Barton River-Willoughby River to mouth M - Clyde River-headwaters to Echo Lake stream M - Seymour and Echo Lakes M - Clyde River-Echo Lake stream to mouth M - Direct drainage-south end of Lake Memphremagog

The Hydrological region of the state that is draining into this STP. If you are unsure of your Drainage area you can find it on the Natural Resource Atlas , with these instructions.

Impervious Area acres

Enter the acres of impervious surface draining to the practice. Acres should include all types of impervious area (i.e., rooftop, road, gravel, parking lot, driveway, etc.). This is used to calculate the total phosphorus load from impervious surface within the area draining to the practice.

Value must be number greater than zero Impervious or Pervious Area Required

Pervious Area acres

Enter the acres of pervious surface draining to the practice. Acres should include all types of pervious area (i.e., vegetation, trees, grass, landscaped areas, etc.). This is used to calculate the total phosphorus load from pervious surface within the area draining to the practice.

Value must be number greater than zero Impervious or Pervious Area Required

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STP Information

STP Type Extended Dry Detention Basin Grass Conveyance Swale Gravel Wetland Hydrodynamic (swirl) Separator Structure Infiltration Trench Porous Pavement w/ underdrain Porous Pavement with infiltration Rain Garden / Bioretention (w/ underdrain) Rain Garden / Bioretention (no underdrains) Sand Filter w/ underdrain Subsurface Infiltration Surface Infiltration Wet Pond

Storage Volume ft³

The Volume of water a STP can store from a rain event. Help for calculating the Storage Volume can be found here here.

Value must be number greater than zero Required

Infiltration Rate 0.17 (Sandy Clay Loam, HSG - C) 0.27 (Silt Loam, HSG - C) 0.52 (Loam, HSG - B) 1.02 (Sandy Loam, HSG - B) 2.41 (Loamy Sand, HSG - A) 8.27 (Sand, HSG - A) in/hr

Choose the infiltration rate that most closely matches the infiltration rate, without going over, of the surrounding soils of the site in question. If you do not know the exact infiltration rate of the surrounding soils this rate can be estimated based on the soil type. Soil type can be estimated using the Hydrologic Soil Group layer on the Natural Resources Atlas. Determine the HSG type for the practice site and use the table below to estimate the site’s infiltration rate. Note: field verified infiltration rates are necessary to properly design STPs and to accurately estimate pollutant reductions achieved by constructed STPs.

Filter Course Depth in

The “Filter Course” component of a porous pavement system includes the depth of the filtering layer. It does not include the depth of pavement itself, nor does it include the gravel reservoir layer. The depth is usually between 12 and 36 inches.

Value must be number greater than zero Required

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Estimated Phosphorus Reduction

Load kg/year

This field indicates the modeled total Phosphorus load coming off the land based on the land use drainage area in kg per year.

STP Capacity in

This field indicates the inches of rain the STP is capable of treating (treatment depth)

Efficiency %

This field displays the efficiency of the STP at removing Phosphorus based on the parameters entered above.

Reduction kg/year

This field indicates the modeled total Phosphorus reduction based on the STP type and storage volume of the practice in kg per year.

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Identification

WPD ID

STP Name