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*Download the draft Version 3 of the estimator here: [[File:MPCA simple estimator version 3 October 10.xlsx]] - '''NOTE; Version 3 is draft and currently undergoing review and testing. Note the date on the file if you download it. | *Download the draft Version 3 of the estimator here: [[File:MPCA simple estimator version 3 October 10.xlsx]] - '''NOTE; Version 3 is draft and currently undergoing review and testing. Note the date on the file if you download it. | ||
− | A quick guide for the | + | A quick guide for the Estimator is available [[Quick Guide: MPCA Estimator tab]]. |
'''NOTE: This page is a User's Guide that explains the various cells and worksheets in the Estimator. Guidance for using the Estimator for permit compliance and case studies are found at the following links''' | '''NOTE: This page is a User's Guide that explains the various cells and worksheets in the Estimator. Guidance for using the Estimator for permit compliance and case studies are found at the following links''' | ||
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==Overview of the MPCA Simple Estimator== | ==Overview of the MPCA Simple Estimator== | ||
− | The MPCA | + | The MPCA Estimator worksheet presents an optional calculator approach to computing the pollutant load reduction for [https://stormwater.pca.state.mn.us/index.php?title=Phosphorus total phosphorus] (TP) and [https://stormwater.pca.state.mn.us/index.php?title=Total_Suspended_Solids_(TSS)_in_stormwater total suspended solids] (TSS)(note: the Estimator may not be used for any other pollutants). Results from the Estimator can be used in the ''Cumulative reductions'' tab of the [https://stormwater.pca.state.mn.us/index.php?title=Guidance_for_completing_the_TMDL_reporting_form Annual Report form]. The Estimator applies only to specific <span title="a stationary and permanent BMP that is designed, constructed and operated to prevent or reduce the discharge of pollutants in stormwater"> '''structural stormwater BMP'''</span> and is a simplistic tool that provides estimates of loading and load reductions. It should not be used for modeling a stormwater system or selecting BMPs. The following color coding applies to the Estimator spreadsheet. |
*Grey cells are column and row headings and cannot be edited | *Grey cells are column and row headings and cannot be edited | ||
*White cells are User input (some cells contain default values that can be edited) | *White cells are User input (some cells contain default values that can be edited) | ||
*Yellow cells contain formulas that calculate loads. These cells cannot be edited but values for some cells can be changed by editing values in other parts of the spreadsheet. | *Yellow cells contain formulas that calculate loads. These cells cannot be edited but values for some cells can be changed by editing values in other parts of the spreadsheet. | ||
*Red cells contain formulas that contain results for reductions and cannot be edited | *Red cells contain formulas that contain results for reductions and cannot be edited | ||
− | The Estimator utilizes the [http://www.stormwatercenter.net/monitoring%20and%20assessment/simple%20meth/simple.htm Simple Method] to estimate land use based pollutant loads for total phosphorus and total suspended solids (TSS). BMPs included in the | + | The Estimator utilizes the [http://www.stormwatercenter.net/monitoring%20and%20assessment/simple%20meth/simple.htm Simple Method] to estimate land use based pollutant loads for total phosphorus and total suspended solids (TSS). BMPs included in the Estimator are biofiltration (bioretention with an underdrain), bioinfiltration (bioretention with no underdrain), filter strip, landscaped roof (green roof), permeable pavement with an underdrain, sand filter, swale, wet basin, and constructed wetland. Users may include other BMPs if they have reliable data on pollutant <span title="Pollutant removal efficiency, usually represented by a percentage, specifically refers to the pollutant reduction from the inflow to the outflow of a system"> '''removal efficiency'''</span>. |
− | The Estimator computes pollutant reduction using BMP performance data as published in the [http://www.bmpdatabase.org/performance-summaries.html 2012 International BMP Database]. The database compiles <span title="the average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L)"> '''event mean concentration'''</span> (EMC) performance data and reports in terms of 25th Percentile, Median, and 75th Percentile. The | + | The Estimator computes pollutant reduction using BMP performance data as published in the [http://www.bmpdatabase.org/performance-summaries.html 2012 International BMP Database]. The database compiles <span title="the average pollutant concentration for a given stormwater event, expressed in units of mass per volume (e.g., mg/L)"> '''event mean concentration'''</span> (EMC) performance data and reports in terms of 25th Percentile, Median, and 75th Percentile. The Estimator computes the load reduction according to the formula |
:Load Reduction for a BMP = Load in – Load out = (EMC in * Volume in) – (EMC out * Volume out). | :Load Reduction for a BMP = Load in – Load out = (EMC in * Volume in) – (EMC out * Volume out). | ||
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The Estimator can only be used for one TMDL at a time. If a Permittee has multiple TMDLs and chooses to use the Estimator, separate calculations must be made for each TMDL. | The Estimator can only be used for one TMDL at a time. If a Permittee has multiple TMDLs and chooses to use the Estimator, separate calculations must be made for each TMDL. | ||
− | {{alert|The results from the MPCA Estimator are intended to show a generalized value that represents approximate progress towards meeting TMDL WLAs. The MPCA | + | {{alert|The results from the MPCA Estimator are intended to show a generalized value that represents approximate progress towards meeting TMDL WLAs. The MPCA Estimator should not be used for other modeling or compliance purposes. Instead, Permittees should use field investigations combined with more detailed models.|alert-warning}} |
==Structure of the MPCA Estimator spreadsheet== | ==Structure of the MPCA Estimator spreadsheet== | ||
Line 100: | Line 100: | ||
*Description: Standard runoff coefficient for each type of land use. | *Description: Standard runoff coefficient for each type of land use. | ||
*Input: Default value with an option for the User to enter a different value | *Input: Default value with an option for the User to enter a different value | ||
− | *Source of Information: MPCA input values based on literature. The default is set at the median. The User may enter a value in the ''Input values for MPCA | + | *Source of Information: MPCA input values based on literature. The default is set at the median. The User may enter a value in the ''Input values for MPCA Estimator'' tab if available data supports the value. For a discussion of runoff coefficients, see [http://www.brighthubengineering.com/hydraulics-civil-engineering/93173-runoff-coefficients-for-use-in-rational-method-calculations/#imgn_1]. |
'''Columns G and H, Rows 6 through 24 - Pollutant loads''' | '''Columns G and H, Rows 6 through 24 - Pollutant loads''' | ||
Line 168: | Line 168: | ||
*Description: The fraction of the treated water that is infiltrated by the BMP. Defaults are 100 percent for all infiltration BMPs and 0 percent for the rest. All of the water captured by an infiltration BMP is assumed to infiltrate into the soil underlying the BMP. The assumption that no water infiltrates in the remaining BMPs is a conservative assumption, since some water always infiltrates through the bottom unless the BMP is lined. The [[MIDS calculator]] can be used to estimate volume loss in BMPs with an underdrain. Typical infiltration in a BMP with an underdrain and an underlying D soil will be about 20 percent (0.20). | *Description: The fraction of the treated water that is infiltrated by the BMP. Defaults are 100 percent for all infiltration BMPs and 0 percent for the rest. All of the water captured by an infiltration BMP is assumed to infiltrate into the soil underlying the BMP. The assumption that no water infiltrates in the remaining BMPs is a conservative assumption, since some water always infiltrates through the bottom unless the BMP is lined. The [[MIDS calculator]] can be used to estimate volume loss in BMPs with an underdrain. Typical infiltration in a BMP with an underdrain and an underlying D soil will be about 20 percent (0.20). | ||
*Input: None or User input | *Input: None or User input | ||
− | *Source of Information: MPCA Input, but User can change the value in the ''Input values for MPCA | + | *Source of Information: MPCA Input, but User can change the value in the ''Input values for MPCA Estimator'' tab. |
'''Row 69''' | '''Row 69''' | ||
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*Description: The fraction of the treated water that is infiltrated by the BMP. Defaults are 100 percent for all infiltration BMPs and 0 percent for the rest. All of the water captured by an infiltration BMP is assumed to infiltrate into the soil underlying the BMP. The assumption that no water infiltrates in the remaining BMPs is a conservative assumption, since some water always infiltrates through the bottom unless the BMP is lined. The [[MIDS calculator]] can be used to estimate volume loss in BMPs with an underdrain. Typical infiltration in a BMP with an underdrain and an underlying D soil will be about 20 percent (0.20). | *Description: The fraction of the treated water that is infiltrated by the BMP. Defaults are 100 percent for all infiltration BMPs and 0 percent for the rest. All of the water captured by an infiltration BMP is assumed to infiltrate into the soil underlying the BMP. The assumption that no water infiltrates in the remaining BMPs is a conservative assumption, since some water always infiltrates through the bottom unless the BMP is lined. The [[MIDS calculator]] can be used to estimate volume loss in BMPs with an underdrain. Typical infiltration in a BMP with an underdrain and an underlying D soil will be about 20 percent (0.20). | ||
*Input: None or User input | *Input: None or User input | ||
− | *Source of Information: MPCA Input, but User can change the value in the ''Input values for MPCA | + | *Source of Information: MPCA Input, but User can change the value in the ''Input values for MPCA Estimator'' tab. |
'''Row 94''' | '''Row 94''' |
This User Guide is intended to assist MS4 (Municipal Separate Storm Sewer System) permittees in the completion of the Commissioner-approved TMDL Annual Reporting Form (TMDL Form).
A quick guide for the Estimator is available Quick Guide: MPCA Estimator tab.
NOTE: This page is a User's Guide that explains the various cells and worksheets in the Estimator. Guidance for using the Estimator for permit compliance and case studies are found at the following links
The MPCA Estimator worksheet presents an optional calculator approach to computing the pollutant load reduction for total phosphorus (TP) and total suspended solids (TSS)(note: the Estimator may not be used for any other pollutants). Results from the Estimator can be used in the Cumulative reductions tab of the Annual Report form. The Estimator applies only to specific structural stormwater BMP and is a simplistic tool that provides estimates of loading and load reductions. It should not be used for modeling a stormwater system or selecting BMPs. The following color coding applies to the Estimator spreadsheet.
The Estimator utilizes the Simple Method to estimate land use based pollutant loads for total phosphorus and total suspended solids (TSS). BMPs included in the Estimator are biofiltration (bioretention with an underdrain), bioinfiltration (bioretention with no underdrain), filter strip, landscaped roof (green roof), permeable pavement with an underdrain, sand filter, swale, wet basin, and constructed wetland. Users may include other BMPs if they have reliable data on pollutant removal efficiency.
The Estimator computes pollutant reduction using BMP performance data as published in the 2012 International BMP Database. The database compiles event mean concentration (EMC) performance data and reports in terms of 25th Percentile, Median, and 75th Percentile. The Estimator computes the load reduction according to the formula
This is then converted to a percentage reduction
The cumulative reduction is the sum of load reduced for all BMPs. This computed reduction can be input into the Cumulative reductions tab of the TMDL Annual Report form.
The Estimator can only be used for one TMDL at a time. If a Permittee has multiple TMDLs and chooses to use the Estimator, separate calculations must be made for each TMDL.
The MPCA Simple Estimator (Estimator) spreadsheet contains 12 worksheets.
Each of the 10 calculation worksheets represents a sub-watershed. The user is not obligated to use 10 worksheets for their study area. We encourage the user to use separate worksheets for the following conditions.
The 10 calculation worksheets are identical and each contains 5 sections, described below.
Each of these sections is described below.
In this section, the total unadjusted load, in pounds, is calculated for the area considered. Unadjusted means there is no consideration of reductions associated with practices such as street sweeping, pollution prevention, changes in land use, etc.
This section comprises Cells A5 through L24 of each of the 10 calculation worksheets. In this section, the user inputs land area, in acres, associated with different land uses within the area being considered. The user inputs annual precipitation, in inches. A link provides access to precipitation information if the user does not know the precipitation for the sub-watershed. The section contains default values for TP and TSS event mean concentrations (mg/L) and default values for runoff coefficients. EMCs and runoff coefficients can be changed by the user in the section containing BMP and land use inputs.
Column A - Land use
Columns B and C, Rows 6 through 23 - Event mean concentrations
Column D, Rows 6 through 23 - Area of specified land use
Column E, Rows 6 through 23 - Annual precipitation
Note: The default value is 30.65 inches per year, which is the average annual precipitation at the Minneapolis-St. Paul International airport. The User should input the appropriate value for their location. The references below can be used to determine this value.
Column F, Rows 6 through 23 - runoff coefficients
Columns G and H, Rows 6 through 24 - Pollutant loads
This section of the 10 calculation worksheets contains information and calculations for adjustments to the total loads calculated in the previous section (Unadjusted total loads). It comprises Cells A26 through L45.
Adjusted total loads account for changes in event mean concentration or runoff coefficient values used in the section "Unadjusted total loads". Examples of practices or actions that result in changes in EMC or runoff coefficients include but are not limited to the following.
The user will adjust appropriate emcs in Cells B29 through C44, and/or adjust the appropriate runoff coefficients in cells F29 through F44. The user should provide a description or rationale in the appropriate cell in Columns K-L.
{alert|The user should provide a description or rationale for changed values in the appropriate cell in Columns K-L|alert-warning}}
In this section of the worksheet the user enters acreages for BMPs implemented within each land use in the sub-watershed. The section comprises Cells A47 through L70. The area treated by a BMP cannot exceed the total acreage within a specific land use (i.e. an error message is returned if the user attempts to enter an acreage greater than the total acres in a land use). If the cumulative acreage for multiple BMPs exceeds the total acreage within a land use, Column L returns a message stating the treated acres exceed the total acres for the land use. Note: it is possible for treated acres to exceed total acres when multiple BMPs are employed. The warning alerts the user to this situation, in which case the user should ensure the entered values are correct. If treated acres do exceed total acres, the user should consider adjusting removal efficiencies for downstream BMPs that are receiving water treated by upstream BMPs.
Rows 48 through 65
Row 66
Row 67
Annual runoff as a function of precipitation at Minneapolis-St. Paul International airport. Knowing how a BMP is sized, this table can be used to estimate the annual volume treated by the BMP.
Link to this table
Daily precipitation | Cumulative annual rainfall |
---|---|
0.25 | 45% |
0.50 | 65% |
0.75 | 82% |
1.00 | 90% |
1.25 | 93% |
1.50 | 95% |
3.00 | 99% |
9.00 | 100% |
Row 68
Row 69
Row 70
In this section of the worksheet the user enters acreages for BMPs implemented within each land use in the sub-watershed. The section comprises Cells A72 through L95. The area treated by a BMP cannot exceed the total acreage within a specific land use (i.e. an error message is returned if the user attempts to enter an acreage greater than the total acres in a land use). If the cumulative acreage for multiple BMPs exceeds the total acreage within a land use, Column L returns a message stating the treated acres exceed the total acres for the land use. Note: it is possible for treated acres to exceed total acres when multiple BMPs are employed. The warning alerts the user to this situation, in which case the user should ensure the entered values are correct. If treated acres do exceed total acres, the user should consider adjusting removal efficiencies for downstream BMPs that are receiving water treated by upstream BMPs.
Rows 72 through 90
Row 91
Row 92
Annual runoff as a function of precipitation at Minneapolis-St. Paul International airport. Knowing how a BMP is sized, this table can be used to estimate the annual volume treated by the BMP.
Link to this table
Daily precipitation | Cumulative annual rainfall |
---|---|
0.25 | 45% |
0.50 | 65% |
0.75 | 82% |
1.00 | 90% |
1.25 | 93% |
1.50 | 95% |
3.00 | 99% |
9.00 | 100% |
Row 93
Row 94
Row 95
This section of each of the 10 calculation worksheets contains default values for filter BMP performance, event mean concentrations (EMCs), and runoff coefficients. The values for BMP performance are only for the water that is filtered and are expressed as fractions, which represents the fraction of pollutant that is removed by the BMP. A value of 0.50, for example, means the BMP removes half of the pollutant. Note that the values for infiltration BMPs is 0 because it is assumed that all pollutant in infiltrated water is removed. The equations built in to the Estimator account for this complete removal for infiltrated water. Removal values are shown for TP and TSS.
EMCs are given for several different land uses and for TP and TSS. Runoff coefficients are given for several different land uses.
The user can change the default values in this tab. Once changed, the default values are lost, although they can be re-entered.
Download MPCA Estimator here: File:MPCA Estimator.xlsx
Example calculations were made for reductions in pollutant loading for the map shown to the left. There are 4 MS4s (City A, City B, City C, and the MS4 College) and 4 impaired waters (Lake 1, Lake 2, Lake 3, and Stream 1). The lakes are impaired for phosphorus and the stream is impaired for TSS and E. coli bacteria. Below is a summary of MS4 contributions to each impaired water.
City A comprises 585 acres within the Lake 1 watershed. The breakdown by specific land use is shown in the summary table above. The City has implemented 5 BMPs within the watershed that can be included in the MPCA Estimator.
Using the defaults in the worksheet, reductions in phosphorus loading are shown in Rows 37 (total pounds reduced) and 38 (Percent load reduced). The total load reduction for all BMPs is 5.875 pounds, or about 1.00 percent of the original loading of 586.45 pounds. The User can enter either the pounds reduced (5.875) or the percent reduced (1.00) in the Cumulative reductions tab.
The estimator only allows the user to estimate loads for one impaired water at a time. The User should either save the worksheet as a separate Excel file, keep a record of the inputs used for this impairment, or insert additional estimator worksheets into the spreadsheet. To create additional estimator worksheets within the spreadsheet, on the Home tab in Excel, click on Insert and select Insert Sheet. Then Copy the contents of the MPCA estimator tab into the new worksheet. Tabs within the spreadsheet can be renamed by double clicking on the tab at the bottom of the screen and then typing in a new name. See the image to the left and Excel Help for more information.
A new version of the estimator is run for the Lake 2 watershed. City A comprises 84 acres of this watershed and has implemented just one BMP that can be used in the estimator. This is a bioinfiltration BMP that drains 2 acres of medium density residential area. this is an infiltration BMP, so the User enters 2 in row 30, Column C (Infiltration (BMP with no underdrain), Residential - medium density). this BMP results in a reduction of 2.004 pounds of phosphorus, or 2.46 percent of the original load of 81.33 pounds.
City A may conduct additional BMPs that decrease pollutant load. If these BMPs can be quantified, they can be entered as Other BMPs in the worksheet.
City C comprises 297 acres of the Lake 2 watershed. The City has implemented one BMP that can be used in the estimator. This is a dry pond that drains 8 acres of medium density residential land use. The User enters 8 in Row 30, Column I (Wet basin, Residential - medium density). The resulting decrease in phosphorus loading is 4.453 pounds or 1.39 percent of the initial load of 319.67 pounds. This example again utilizes the default values in the worksheet.
As with City A, the estimator can only be used for one impaired water at a time. The User must therefore run the estimator separately for Lake 3 and Stream 1.
City C comprises 366 acres of the Lake 3 watershed. The City has implemented two BMPs, an underground sand filter that serves 5 acres of medium density residential land use, and a 12 acre dry swale that serves 10 acres of commercial development and 2 acres of transportation. The User enters 5 in Row 30, Column G (Sand filter, residential - medium density), 10 in Row 22 Column H (Swale, Commercial), and 2 in Row 31, Column H (Swale, Transportation). The resulting total decrease in phosphorus load is 9.087 pounds or 2.24 percent of the total load of 405.62 pounds.
City C comprises 427 acres of the Stream 1 watershed (127 acres in the Oasis Lake watershed and 300 acres in the Lake 3 watershed). The City has implemented a dry swale that drains 10 acres of commercial land and 2 acres of transportation land use, and a tree trench that drains 8 acres of commercial land and 7 acres of industrial land. For the tree trench, the user enters 8 in Row 22, Column C (Commercial; Infiltration) and 7 in Row 23, Column C (industrial; infiltration). For the dry swale the User enters 10 in Row 22, Column H (Commercial; Swale) and 2 in Row 31, Column H (transportation; Swale). The resulting decrease in TSS load is 21,885.4 pounds or 9.78 percent of the initial load of 223,855 pounds. The decrease in E. coli load is 3.12 percent of the initial load. Note the swale does not reduce any of the E. coli load.
The MS4 College is a non-traditional regulated MS4. The area of the college is 84 acres and the college owns and operates the stormwater conveyance within the campus boundaries. The entire acreage of the college lies within the Lake 1 watershed. Land use within the campus consists of 15 acres of park and 69 acres of institutional land use.
The college has constructed and operates one BMP (FIL1). The BMP is a perimeter sand filter that accepts runoff from a 3 acre area. The land use contributing runoff to the BMP is institutional. The User therefore enters 3 in row 24, Column G of the estimator (Sand filter, Institutional). The BMP decreases phosphorus loading by 1.391 pounds, or 1.77 percent from the initial load of 78.75 pounds.
Results of the MPCA estimator can be entered into the Cumulative reductions tab. This is illustrated in the image to the right for City A. In this example, for Lake 1 a value of 5.875 is entered into Row 4, Column F. In Column D the units are selected from the dropdown box, In this case the units are pounds reduced. For Lake 2 a value of 2.46 is entered in row 5, column F and the units are % load reduction. The calculation method, which is an optional field, is entered as MPCA estimator.
The Estimator is a simple tool based on simple assumptions. This section contains some tips for applying the tool to different situations that you may encounter.
The Estimator does not explicitly address stormwater treatment trains. It will therefore underestimate pollutant removal for BMPs that are in series. For example, in a treatment train consisting solely of ponds, the first pond removes the greatest fraction of pollutant concentration. Each pond in succession will treat less polluted water but will further the reduction in pollutant concentration flowing downstream. The Estimator doesn’t recognize the total number of BMPs on the ground so it cannot recognize BMPs as being connected. Therefore each pond is assumed to treat the same degree of polluted water regardless of how many are connected in a treatment train.
We recommend that BMPs in a treatment train be considered as possible separate sub-watersheds. Using this approach, only the area draining to a single BMP is considered in a worksheet.
If BMPs in a treatment train are not treated separately, adjusting the Estimator to more closely simulate pollutant removal for treatment trains can be challenging since the Estimator uses a lumped BMP approach in which all similar BMPs are lumped as a single BMP. For example, permeable pavement with no underdrain, bioinfiltration, and infiltration basins are all lumped together as infiltrator BMPs. Another complication is that each treatment train differs and attempting to model them as a single system creates inaccuracies.
To get an idea of how to adjust the Estimator to account for treatment trains, we ran a series of treatment train scenarios through the MIDS calculator and compared them to results for the Estimator. A more detailed description of this exercise, including results, is in a Word document (File:Treatment trains.docx. Recommendations are summarized below.
Tables with recommended values for pollutant removal fractions are in a Word document (File:Treatment trains.docx).
If you are calculating reductions in loading as a percent, there is no value in modifying the EMCs in the Estimator since the only factors affecting the percent removal are the BMP removal fraction and the fraction of runoff being treated by and/or infiltrating through the BMP. If you are calculating reductions in pounds or number of bacteria, the EMC affects the initial pollutant load. The higher the initial load the greater the reduction when the BMPs are applied. Values in the Estimator are on the high end of concentrations for the different land uses. For more information on EMCs, see tabled values in the manual.
If you have a land use not shown in the Estimator, values should be obtained from the literature. Recommended values for agriculture are shown below.
The default value for the fraction of runoff treated by BMPs is 0.9, except for wet basins and wetlands, where the value is 1.0. The 0.9 corresponds with treatment of the first 1 inch of runoff on B soils, while a value of 1.0 assumes all runoff is treated by the BMP. This value can be changed in the Input values for MPCA Estimator tab if your BMPs are sized for a different water quality volume or if you have different soils. The Estimator uses the values from this tab The table below can be used to determine the appropriate number. For example, if your soils were A rather than B, you should enter a value ranging from 0.92 to 0.96, depending on the specific soil type. If you had B soils but the water quality volume was 0.75 inches, the value should be changed to 0.81.
Similarly, the fraction of runoff that is infiltrated into an infiltrator BMP is 0.9. Again, this value should be adjusted if the water quality volume or soils differ from 1 inch and B soils. The only other BMP in the Estimator that infiltrates water as the default is biofiltration. The infiltration fraction for this BMP is 0.2, which is based on data generated from MIDS calculator runs. Infiltration may occur in other BMPs, in particular permeable pavement with an underdrain and swales. A value of 0.2 can be entered for permeable pavement with underdrains to make it similar to biofiltration. An infiltration value for swales is difficult to generate because of the many potential swale configurations. The MIDS calculator is one tool that can be used to generate a value for fraction of water infiltrated in swales.
Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and water quality volume. See footnote^{1} for how these were determined.
Link to this table
Soil | Water quality volume (V_{WQ}) (inches) | ||||
---|---|---|---|---|---|
0.5 | 0.75 | 1.00 | 1.25 | 1.50 | |
A (GW) | 84 | 92 | 96 | 98 | 99 |
A (SP) | 75 | 86 | 92 | 95 | 97 |
B (SM) | 68 | 81 | 89 | 93 | 95 |
B (MH) | 65 | 78 | 86 | 91 | 94 |
C | 63 | 76 | 85 | 90 | 93 |
^{1}Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.
The pollutant removal fraction is the most important factor affecting the calculations in the Estimator. The default values correspond with recommended values in the manual and represent well-defended data from the literature. The user can change the default, but any change should be supported with data. For further information on pollutant removal by BMPs, see the appropriate page in this manual on BMP pollutant credits. Each of the credit articles contains information to help determine the most appropriate value for pollutant removal for a specific BMP.
The Estimator allows the user to enter an additional BMP beyond the default BMPs (called Other in the Estimator). Most urban BMPs fit into one of the default BMPs in the Estimator. There may be exceptions however. If you can determine values for pollutant removal fraction, fraction of water that is treated, and fraction of water that is infiltrated for the BMP, you can include an additional BMP. If these values cannot be generated for the BMP, calculate pollutant removal independently and add that value to the value generated by the Estimator when reporting cumulative reductions on the Annual Report form. For example, assume you had an in-line treatment system that treated stormwater runoff in a part of your conveyance system. This BMP is not easily incorporated into the Estimator, but if you monitor the BMP and have pollutant removal information, you can simply add the removal amount to the amount calculated by the Estimator for the remainder of your system.
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