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*What practices or conditions might result in a change in emc? | *What practices or conditions might result in a change in emc? | ||
**There are many practices and conditions that affect emc. Examples of site conditions that likely will increase an emc include increased tree canopy, greater connectivity of impervious surface, small watersheds with highly connected impervious surface, greater runoff from pervious surfaces (e.g. compacted soils), increased construction activity, and winter application of road salt and sand. We suggest reviewing information on the pages [[Event mean concentrations of total and dissolved phosphorus in stormwater runoff]] and [[Event mean concentrations of total suspended solids in stormwater runoff]] | **There are many practices and conditions that affect emc. Examples of site conditions that likely will increase an emc include increased tree canopy, greater connectivity of impervious surface, small watersheds with highly connected impervious surface, greater runoff from pervious surfaces (e.g. compacted soils), increased construction activity, and winter application of road salt and sand. We suggest reviewing information on the pages [[Event mean concentrations of total and dissolved phosphorus in stormwater runoff]] and [[Event mean concentrations of total suspended solids in stormwater runoff]] | ||
− | *Does the MPCA recommend how to adjust emcs or runoff coefficients? | + | *Does the MPCA recommend how to adjust emcs or runoff coefficients? Practices that affect emcs in runoff include a wide variety of [https://stormwater.pca.state.mn.us/index.php?title=Non-structural_stormwater_Best_Management_Practices non-structural practices], such as street sweeping, lawn yard debris collection, and removal of illicit discharges. Structural practices that return treated water to the runoff system also decrease emcs in runoff. |
**The MPCA provides information on adjusting emcs at the two links provided above. We do not provide guidance on adjusting runoff coefficients. | **The MPCA provides information on adjusting emcs at the two links provided above. We do not provide guidance on adjusting runoff coefficients. | ||
+ | *What practices might result in a change in runoff coefficient? | ||
+ | **The runoff coefficient should be modified for any practice that affects runoff volume. The default coefficients are based on B soils on flat slopes (0-2%). The coefficient should be increased for C and D soils and steeper slopes, and decreased for A soils. Other conditions that may affect runoff include, increased or decreased compaction of soils in pervious areas, disconnecting impervious surfaces, runoff collection (e.g. rain barrels), and increased or decreased tree canopy. See [[Runoff coefficients for different soil groups and slopes]], [[Event mean concentrations of total and dissolved phosphorus in stormwater runoff]] and [[Event mean concentrations of total suspended solids in stormwater runoff]]. | ||
*Is there information on pollutant removal for individual BMPs? | *Is there information on pollutant removal for individual BMPs? | ||
**Yes. Click on the links to individual BMPs in the Estimator. You may also find the information at [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_pollutant_removal,_stormwater_credits this link]. | **Yes. Click on the links to individual BMPs in the Estimator. You may also find the information at [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_pollutant_removal,_stormwater_credits this link]. | ||
− | * | + | *When should I adjust pollutant removal efficiencies? |
− | **We recommend using the values in the Minnesota Stormwater Manual unless you have data to suggest a different value or a designed BMP to which values in the manual don't apply. For different designs, the user would have to make this determination. Adjustments to removal efficiencies are entered in Row 66 for TP and Row 91 for TSS. | + | **We recommend using the values in the Minnesota Stormwater Manual unless you have data to suggest a different value or a designed BMP to which values in the manual don't apply. For different designs, the user would have to make this determination. Adjustments to removal efficiencies are entered in Row 66 for TP and Row 91 for TSS. Examples of conditions where a removal efficiency could be changed include adding an amendment to a BMP to improve treatment (e.g. iron), using engineered media mixes that do not effectively retain phosphorus (e.g. mixes A, B, E, F), structurally enhanced practices (e.g. increased forebay size for a constructed pond), and increased pretreatement. |
*What removal efficiency should I use for modified BMPs? | *What removal efficiency should I use for modified BMPs? | ||
− | **The pollutant removals in the Minnesota Stormwater Manual apply to the designs described in the manual. If the user has a modified design, they will have to determine the pollutant removal efficiency. | + | **The pollutant removals in the Minnesota Stormwater Manual apply to the designs described in the manual. If the user has a modified design, they will have to determine the pollutant removal efficiency. There may be information in the literature, or contact us. |
+ | *What conditions apply when fraction treated or infiltrated changes? | ||
+ | **The default treatment fraction is 0.90, which roughly corresponds with retention of 1 inch of runoff from impervious surfaces. With increased precipitation and increased intensity of precipitation, the default of 0.90 is likely to overestimate the fraction of annual runoff captured and treated. Consider adjusting the value down for areas with higher annual precipitation. Consider adjusting the value down as the contributing acreage from C and D soils increases. The value should be adjusted down for undersized bmps. The default infiltration values for filtration practices are based on infiltration into C soils. Adjust the value down in D soils. Certain designs, such as an upturned elbow in an underdrain, may increase the fraction of water infiltrating. Raising an underdrain may increase infiltration on C soils. | ||
==Land use== | ==Land use== |
The MPCA Simple Estimator is a spreadsheet tool that calculates pollutant loading and load reductions associated with stormwater best management practices (BMPs). The spreadsheet utilizes the Simple Method.
This page provides answers to some questions about the Estimator. Like any calculation tool or model, there is a certain amount of user expertise and best professional judgement involved in calculating stormwater loads and load reductions. Consequently, some of the information on this page reflects recommendations for how to use the Estimator rather than specific answers and approaches.
Link to more information on the MPCA Simple Estimator