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*Drainage area is one (1) acre of impervious surface | *Drainage area is one (1) acre of impervious surface | ||
*Each practice is sized to meet the MIDS performance goal of 1.1 inches (3000 square feet areas and 1.35 feet deep | *Each practice is sized to meet the MIDS performance goal of 1.1 inches (3000 square feet areas and 1.35 feet deep | ||
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+ | <gallery caption="Screen shots for Example 1. Click on an image for enlarged view." widths="200px"> | ||
+ | File:Example 1 calculator inputs.png|alt=Screen shot of calculator inputs for Example 1|Screen shot of calculator inputs for Example 1 | ||
+ | File:Example 1 Excel screen shot.png|alt=Screen shot of where to change values for Example 1|Screen shot of where to change values for Example 1 | ||
+ | File:Example 1 summary.png|alt=Screen shot of summary information for Example 1|Screen shot of summary information for Example 1 | ||
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==Example 2 - leaf drop versus non-leaf drop== | ==Example 2 - leaf drop versus non-leaf drop== |
Phosphorus in stormwater runoff occurs in particulate and dissolved forms. Many stormwater practices are effective at removing particulate phosphorus, but many are ineffective for removing dissolved phosphorus. In addition, the dissolved phosphorus (DP) and total phosphorous (TP) loads for a site vary depending on several factors, such as land use (residential, commercial, and industrial), time of year, and precipitation amounts and patterns. To accurately model pollutant loading at a site, it may therefore be important to accurately assign these ratios. See this page for more information on phosphorus.
This page describes how to adjust the dissolved phosphorus to total phosphorus (DP:TP and the particulate phosphorus to total phosphorus (PP:TP) ratios in the MIDS Calculator.
The Minimal Impact Design Standards (MIDS) calculator assigns a single value across a site for fractions of total phosphorus that are in dissolved and particulate forms (45 and 55 percent, respectively). These values can be adjusted by changing the value fields within the Excel spreadsheet that the MIDS calculator creates. To change the dissolved phosphorous to total phosphorous ratio for a site (DP:TP) follow these instructions:
When you open the MIDS calculator GUI, the Results tab will show results for the site and for the individual bmps. The Summary Information accurately reflects phosphorus on the site. However, the individual BMP information under BMP Summary will not be accurate as you did not change the fractions for these. If you want accurate information for the individual bmps, you must adjust the individual bmps as described below in Case 2.
DP:TP and PP:TP ratios can be adjusted for any bmp. Changing the ratio only affects the bmp for which the change was made. Thus, when evaluating results for the site, you must use the BMP summary rather than the site summary information.
Two land uses with differing phosphorus runoff characteristics are being modeled. Land use 1 is an industrial area and land use 2 is in a residential area. Per guidance in the Minnesota Stormwater Manual, we adjust the dissolved P to total P ratio to 0.25, thus giving a particulate P to total P ratio of 0.75.
Within each land use there is one biofiltration practice. Both practices are sized as follows.
A residential land use is being modeled and we want to know the differences in phosphorus loading during fall leaf drop compared to other times of the year. For the fall leaf drop we change the dissolved P to total P ratio to 0.50, the particulate P to total P ratio to 0.50, and change the event mean concentration to 1.0 mg/L. For the remainder of the year we leave the emc at the default of 0.35 and change the DP:TP ratio to 0.40 and the PP:TP ratio to 0.60.
Note: The term “leaf drop” implies fall when the leaves are falling.