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#To change the DP:TP ratio for the same bmp, navigate to the “MIDS BMP Calculator” tab, Column AU and in the row corresponding to your bmp, change the PP:TP ratio and emc to desired values. The defaults are 0.45 for the ratio and cell F6 for the emc (0.3 mg/L). Column AU is named “Annual Particulate P Load from Direct Drainage Area (lbs)”. See the corresponding screen shot. | #To change the DP:TP ratio for the same bmp, navigate to the “MIDS BMP Calculator” tab, Column AU and in the row corresponding to your bmp, change the PP:TP ratio and emc to desired values. The defaults are 0.45 for the ratio and cell F6 for the emc (0.3 mg/L). Column AU is named “Annual Particulate P Load from Direct Drainage Area (lbs)”. See the corresponding screen shot. | ||
− | ==Example 1 - | + | ==Example 1 - Adjusting phosphorus ratios and emcs across an entire site== |
− | 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. [https://stormwater.pca.state.mn.us/index.php?title=Event_mean_concentrations_of_total_and_dissolved_phosphorus_in_stormwater_runoff#Ratios_of_particulate_to_dissolved_phosphorus Per guidance in the Minnesota Stormwater Manual], we adjust the dissolved P to total P ratio | + | Data suggests that DP:TP and PP:TP ratios differ by land use. Assume we are modeling a commercial site that is likely to have limited green space and tree canopy cover. [https://stormwater.pca.state.mn.us/index.php?title=Event_mean_concentrations_of_total_and_dissolved_phosphorus_in_stormwater_runoff#Ratios_of_particulate_to_dissolved_phosphorus Per guidance in the Minnesota Stormwater Manual], we decide to change the PP:TP ratio to 0.75, the DP:TP ratio to 0.25, and the emc to 0.200 mg/L. There are two biofiltration practices planned for the site. Both practices are sized as follows. |
+ | *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, on D soil) | ||
+ | *Media Mix C is used with no amendment | ||
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+ | ==Example 2 - Adjusting phosphorus ratios and emcs for individual bmpx== | ||
+ | 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. [https://stormwater.pca.state.mn.us/index.php?title=Event_mean_concentrations_of_total_and_dissolved_phosphorus_in_stormwater_runoff#Ratios_of_particulate_to_dissolved_phosphorus Per guidance in the Minnesota Stormwater Manual], we adjust the dissolved P to total P ratio, the particulate P to total P ratio, and the event mean concentrations. | ||
Within each land use there is one biofiltration practice. Both practices are sized as follows. | Within each land use there is one biofiltration practice. Both practices are sized as follows. |
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.
Data suggests that DP:TP and PP:TP ratios differ by land use. Assume we are modeling a commercial site that is likely to have limited green space and tree canopy cover. Per guidance in the Minnesota Stormwater Manual, we decide to change the PP:TP ratio to 0.75, the DP:TP ratio to 0.25, and the emc to 0.200 mg/L. There are two biofiltration practices planned for the site. Both practices are sized as follows.
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, the particulate P to total P ratio, and the event mean concentrations.
Within each land use there is one biofiltration practice. Both practices are sized as follows.
The adjacent image gallery provides screen shots of the calculator inputs, where to change the particulate and dissolved phosphorus ratios, and the resulting summary. The summary shows increased particulate loads for the commercial land use and increased dissolved loads for the residential land use. Because the bmp is more efficient at retaining particulate phosphorus, a greater amount of total phosphorus is removed in the commercial land use setting.