m (→Example) |
m (→Example) |
||
Line 73: | Line 73: | ||
*View results on the Summary Sheet in the Estimator | *View results on the Summary Sheet in the Estimator | ||
− | === | + | ===Examples=== |
− | [[File:TMDL example 1a.png|400px|thumb|alt=screenshot of Estimator|<font size=3>Screenshot of Estimator showing loading scenario for Group 1 with implementation of biofiltration.</font size>]] | + | [[File:TMDL example 1a.png|400px|thumb|alt=screenshot of Estimator|<font size=3>Screenshot of Estimator showing loading scenario for Group 1 with implementation of biofiltration. Click on image to enlarge.</font size>]] |
− | [[File:TMDL example 2.png|400px|thumb|alt=screenshot of Estimator|<font size=3>Screenshot of Estimator showing loading scenario for Group 1 with adjusted acreages and implementation of biofiltration.</font size>]] | + | [[File:TMDL example 2.png|400px|thumb|alt=screenshot of Estimator|<font size=3>Screenshot of Estimator showing loading scenario for Group 1 with adjusted acreages and implementation of biofiltration. Click on image to enlarge.</font size>]] |
+ | |||
+ | The following examples illustrate how the default Estimator files can be used to determine pollutant loading reductions associated with BMP implementation. The Group 1 Excel file was used for these examples. | ||
'''Example 1'''</br> | '''Example 1'''</br> |
This page provides default values derived from the MPCA Simple Estimator that may be used by permittees or practitioners to estimate TP and TSS loading from three land use types.
These values may be used as a base loading from which pollutant reductions, as a percent, pounds reduced, or final loading rate, may be calculated by including BMPs in the MPCA Simple Estimator. The corresponding Estimator files are at the following links.
|
This page provides default estimates for annual loads of total phosphorus (TP) and total suspended solids (TSS), in pounds per acre, from stormwater runoff in Minnesota cities. The estimates were derived using the MPCA Simple Estimator. Cities may use the estimates when their land use matches or approximates one of the land use scenarios from this analysis. The results represent estimated loads with no implemented best management practices (BMPs). Cities can then use the appropriate default Estimator spreadsheet to determine the TP and TSS reductions associated with BMP implementation.
We used the MPCA Simple Estimator (Estimator) to calculate average annual loading, in pounds per acre, for total phosphorus (TP) and total suspended solids (TSS). We used the default values in the Estimator for event mean concentration and runoff coefficient and assumed an annual average precipitation of 30.65 inches (Twin Cities Metro Area (TCMA)). These values are discussed in the Minnesota Stormwater Manual, as well as guidance for the MPCA Simple Estimator. Appropriate links include the following.
We gathered land use data for several TCMA cities using Metropolitan Council land use data. The land use categories for the Met Council data do not match those used in the Estimator, so we converted the Met Council data into land uses used in the Estimator. These are summarized in the following table. Because this is subjective, we include a column describing the relative significance of a specific land use to total acres in a city. For example, farmstead was assigned to agricultural land use. Farmsteads may not be associated with typical agricultural practices, but farmstead is a minor land use and therefore has a minor impact on overall pollutant loading.
Met Council and Manual land use categories
Link to this table
Met Council land use data | Land use assigned in the Simple Estimator | Relative contribution of land use to total acres in a city |
---|---|---|
Agriculture | Agriculture | Low to high |
Farmstead | Agriculture | Low |
Golf course | Golf course | Low |
Industrial and utility | Indutrial | Low to moderate |
Institutional | Institutional | Low to moderate |
Major highway | Transportation | Low to moderate |
Airport | Transportation | Low |
Railroad | Transportation | Low |
Mixed use commercial and other | Commercial | Low |
Mixed use industrial | Industrial | Low |
Mixed use residential | Residential | Low |
Multifamily | Residential | Low |
Office | Commercial | Low to moderate |
Open water | Water | Moderate |
Park, recreational, or preserve | Park | High |
Retail and other commercial | Commercial | Low to moderate |
Single family attached | Residential | Low |
Single family detached | Residential | High |
Undeveloped land | Open space | High |
Manufactured housing park | Residential | Low |
Retail and other commercial | Commercial | Low to moderate |
Extractive | Extractive | Low |
The Estimator does not have default emcs and runoff coefficients for golf courses and extractive land use. We conducted a brief literature review to determine values for these two land uses. Default values for Estimator inputs are summarized in the following table.
Land use event mean concentrations (emcs) for the MPCA Simple Estimator
Link to this table
Land use | Phosphorus emc (mg/L) | TSS emc | Annual precipitation (inches) | Runoff coefficient |
---|---|---|---|---|
Commercial | 0.20 | 75 | 30.65 | 0.71 |
Industrial | 0.235 | 93 | 30.65 | 0.68 |
Institutional | 0.25 | 80 | 30.65 | 0.3 |
Open space | 0.19 | 21 | 30.65 | 0.08 |
Residential | 0.325 | 73 | 30.65 | 0.27 |
Park | 0.19 | 21 | 30.65 | 0.08 |
Agriculture | 0.50 | 100 | 30.65 | 0.11 |
Transportation | 0.28 | 87 | 30.65 | 0.80 |
Water | 0 | 0 | 30.65 | 0 |
Extractive | 0.15 | 94 | 30.65 | 0.15 |
Golf course | 0.45 | 49 | 30.65 | 0.27 |
We divided the selected cities into three groups, as described below.
Using the land uses from the Estimator and Met Council, we entered percentages for each land use for each city and converted these to fractions. If the fractions did not add to 1.0, we adjusted the fractions so that the total equaled 1.0. Thus, the data represent land use across one (1) acre within a specific city. We calculated means, standard deviations, and associated coefficients of variation (CV). We selected a sufficient number of cities to ensure the CV was less than 1 for each land use. CVs exceeded tolerance limits for agriculture in Group 1 and extraction in Group 3. Since these land uses make up a small percent of the total land use for these two groups, we assumed the high variability would not significantly affect the results.
The distribution of land uses for the three groups is shown in an adjacent figure. Residential land use decreases in importance from Groups 1 through 3, while open space and agriculture increase in importance. Commercial, industrial, institutional, and transportation decrease as a percent of total acreage from Groups 1 through 3, although these land uses each comprise a relatively small percent of total land use. Parkland is similar across the three groups, while water comprises a greater percent of land use in Group 3 compared to Groups 1 and 2.
These land use distributions seem logical and have a significant impact on TP and TSS loading. The adjacent figure, which utilizes a log scale, shows that TP and TSS loading are much higher for developed land uses compared to open space and park. Also note that loading from agricultural land use is lower than from urban land uses. The default emcs and curve numbers used in the Estimator are the result of an extensive literature review, with the exception of agriculture, golf courses, and extraction. Golf courses and extraction are not important land uses, but agriculture is for Groups 2 and 3 and the Estimator may underestimate or overestimate contributions from agriculture.
Average annual pollutant loads are shown for each Group in the adjacent table. The table includes values for a scenario where water was included in the calculation and a scenario where water was excluded. Typically the values excluding water should be used in calculations, as loading is generally considered to be from surfaces that contribute runoff and associated pollutants. Water may be included when using monitoring data at outfalls to receiving waters, as this reflects contributions from the land use contributing to the outfall.
Average TP and TSS loading rates for three land use groups
Link to this table
Group | Includes water | Excludes water | ||
---|---|---|---|---|
TP (lb/ac/yr) | TSS (lb/ac/yr) | TP (lb/ac/yr) | TSS (lb/ac/yr) | |
1 | 0.611 | 165.8 | 0.643 | 174.5 |
2 | 0.472 | 121.2 | 0.497 | 127.6 |
3 | 0.345 | 78.4 | 0.379 | 86.2 |
The pollutant loads for each Group represent average annual loads, in pounds per acre, for TP and TSS. These are loads with no BMPs implemented and are based on default emcs, curve numbers, and precipitation values. The numbers may be used directly by cities, but cities should carefully read the section on Notes to ensure these default conditions match the areas being modeled by the city. If the assumptions used in this analysis do not match conditions in the city or the area being modeled, appropriate values should be used in the Estimator to derive a value for the area being modeled. This manual contains a page with guidance, including a video, on using the Estimator.
The files for the three groups can be accessed at the following links.
To estimate pollutant reductions associated with implementation of best management practices (BMPs), follow these steps.
The following examples illustrate how the default Estimator files can be used to determine pollutant loading reductions associated with BMP implementation. The Group 1 Excel file was used for these examples.
Example 1
See the adjacent figure for screenshots for this example.
Example 2
See the adjacent figure for screenshots for this example.
Note in this second example that the biofiltration practices achieve the same reduction in pounds but the percent pollutant reduction is lower because the initial loading rates, in lb/ac/yr, are higher.
There are several considerations when using the above information as a surrogate for pollutant loading from a city. These are summarized below. Cities should carefully review these to ensure the conditions in their area of interest meet these conditions.
To see Met Council definitions for different land uses, link here.