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The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. This information is needed by planners and engineers to make rational nonpoint source pollution decisions at the site level.  The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. This information is needed by planners and engineers to make rational nonpoint source pollution decisions at the site level.  
−  The Simple Method  +  The Simple Method calculation is intended for use on development sites less than a square mile in area. As with any simple model, the method to some degree sacrifices precision for the sake of simplicity and generality. Even so, the Simple Method is still reliable enough to use as a basis for making nonpoint pollution management decisions at the site level. Phosphorus pollutant loading (L, in pounds per year) from a development site can be determined by solving equation 1, shown below. 
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−  Phosphorus pollutant loading (L, in pounds per year) from a development site can be determined by solving  
==Factors used in calculating phosphorus pollutant loading==  ==Factors used in calculating phosphorus pollutant loading==  
−  ==='''Depth of  +  ==='''Depth of rainfall (P)'''=== 
The value of P represents the number of inches of precipitation that falls during the course of a normal year of rainfall. Longterm weather records around the state of Minnesota suggest that the average annual rainfall depth is about 26 inches. This can be used to estimate P or a user can substitute the average annual rainfall depth from the closest National Weather Service longterm weather station or other suitable locations for which a reliable record can be demonstrated (> 10 years).  The value of P represents the number of inches of precipitation that falls during the course of a normal year of rainfall. Longterm weather records around the state of Minnesota suggest that the average annual rainfall depth is about 26 inches. This can be used to estimate P or a user can substitute the average annual rainfall depth from the closest National Weather Service longterm weather station or other suitable locations for which a reliable record can be demonstrated (> 10 years).  
−  ==='''Correction  +  ==='''Correction factor (P)'''=== 
−  The  +  The P<sub>j</sub> factor is used to account for the fraction of the annual rainfall that does not produce any measurable runoff. Many of the storms that occur during the year are so minor that all of the rainfall is stored in surface depressions and eventually evaporates. As a consequence, no runoff is produced. An analysis of regional rainfall/runoff patterns indicates that only 90 percent of the annual rainfall volume produces any runoff at all. Therefore, P<sub>j</sub> should be set at 0.9. 
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−  The  +  ==='''Runoff coefficient (R<sub>v</sub>)'''=== 
+  The runoff coefficient (R<sub>v</sub>) is a measure of the site response to rainfall events, and in theory is calculated as R<sub>v</sub> = r/p, where ''r'' and ''p'' are the volume of storm runoff and storm rainfall, respectively, expressed as inches. The R<sub>v</sub> for the site depends on the nature of the soils, topography, and cover. However, the primary influence on the R<sub>v</sub> in urban areas is the amount of imperviousness of the site. Impervious area is defined as those surfaces in the landscape that cannot infiltrate rainfall consisting of building rooftops, pavement, sidewalks, driveways, etc. In the equation R<sub>v</sub> = 0.05 + 0.009(I), ''I'' represents the percentage of impervious cover expressed as a whole number. A site that is 75% impervious would use ''I'' = 75 for the purposes of calculating R<sub>v</sub>. To see runoff coefficients for different land uses, [http://water.me.vccs.edu/courses/civ246/table2.htm link here].  
−  +  ==='''Site area (A)'''===  
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−  ==='''Site  
The total area of the site (in acres) can be directly obtained from site plans. If the total area of the site is greater than one square mile (640 acres), the Simple Method may not be appropriate and applicants should consider utilizing other approaches, such as modeling or monitoring.  The total area of the site (in acres) can be directly obtained from site plans. If the total area of the site is greater than one square mile (640 acres), the Simple Method may not be appropriate and applicants should consider utilizing other approaches, such as modeling or monitoring.  
−  ==='''Pollutant  +  ==='''Pollutant concentration (C)'''=== 
−  Statistical analysis of several urban runoff monitoring datasets has shown that the average storm concentrations for total phosphorus do not significantly differ between new and existing development sites. Therefore, a pollutant concentration, C, of 0.30 mg/l should be used in this equation as a default. However, if good local data are available or an adjustment is needed, this factor can be customized for local condition.  +  Statistical analysis of several urban runoff monitoring datasets has shown that the average storm concentrations for total phosphorus do not significantly differ between new and existing development sites. Therefore, a pollutant concentration, ''C'', of 0.30 milligrams per liter (mg/l) should be used in this equation as a default. However, if good local data are available or an adjustment is needed, this factor can be customized for local condition. 
−  +  The phosphorus pollutant export calculation is described by  
−  +  <math>L = 0.227 P P_j R_v C A</math>  
+  where  
+  :L = Load of a pollutant in pounds per year;  
+  :P = Rainfall depth per year (inches);  
+  :P<sub>j</sub> = Fraction of rainfall events that produce runoff;  
+  :R<sub>v</sub> = Runoff coefficient, which expresses the fraction of rainfall which is converted into runoff. Rv = 0.05 + 0.009(I);  
+  :I = Site imperviousness (i.e., I = 75 if site is 75% impervious);  
+  :C = Flowweighted mean concentration of the pollutant in urban runoff (mg/l); and  
+  :A = Area of the development site (acres).  
+  The above equation can be simplified to  
−  +  <math>L = 0.20 P R_v C A</math>  
−  '''  +  =='''Calculating predevelopment and postdevelopment phosphorus load'''== 
−  The methodology for comparing annual predevelopment pollutant loads to postdevelopment pollutant loads is a sixstep process  +  The methodology for comparing annual predevelopment pollutant loads to postdevelopment pollutant loads is a sixstep process: 
−  +  #Calculate site imperviousness;  
−  +  #Calculate the predevelopment phosphorus load;  
+  #calculate postdevelopment pollutant load;  
+  #Calculate the pollutant removal requirement;  
+  #Identify feasible BMPs; and  
+  #Select offsite mitigation option.  
+  ===Step 1: Calculate site imperviousness===  
In this step, the applicant calculates the impervious cover of the predevelopment (existing) and postdevelopment (proposed) site conditions.  In this step, the applicant calculates the impervious cover of the predevelopment (existing) and postdevelopment (proposed) site conditions.  
−  Impervious cover is defined as those surfaces in the landscape that impede the infiltration of rainfall and result in an increased volume of surface runoff. As a simple rule, humanmade surfaces that are not vegetated will be considered impervious. Impervious surfaces include roofs, buildings, paved streets and parking areas and any concrete, asphalt, compacted dirt or compacted  +  Impervious cover is defined as those surfaces in the landscape that impede the infiltration of rainfall and result in an increased volume of surface runoff. As a simple rule, humanmade surfaces that are not vegetated will be considered impervious. Impervious surfaces include roofs, buildings, paved streets and parking areas and any concrete, asphalt, compacted dirt or compacted gravel surface. 
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−  +  ===Step 2: Calculate predevelopment phosphorus load===  
+  {{alertThe following equations use default values for phosphorus loading. It is best to use sitespecific data if possible. If sitespecific data are not available, values from the literature can be used for loading from specific land uses. For more information and phosphorus load information for different land uses, see [[Phosphorus in stormwater]].alertwarning}}  
−  +  In this step, the applicant calculates stormwater phosphorus loadings from the site prior to development. Loading estimates in a new development situation utilizes a benchmark load for undeveloped areas based on average phosphorus loadings for a typical mix of undeveloped land uses and is given by  
−  <  +  <math>L_{pre} = 0.5 A</math> 
−  <  +  where 
+  :L<sub>pre</sub> = Average annual load of total phosphorus exported from the site prior to development (lbs/year);  
+  :0.5 = Annual total phosphorus load from undeveloped lands (lbs/acre/year); and  
+  :A = Area of the site (acres).  
−  +  The equation to determine phosphorus loading in a redevelopment situation is based on the Simple Method and is given by  
−  The  
+  <math>L_{pre} = 0.20 P R_v C A</math>  
+  where;  
+  :L<sub>pre</sub> = Average annual load of total phosphorus exported from the site prior to development (lbs/year);  
+  :P = Rainfall depth over the desired time interval (inches);  
+  :R<sub>v</sub> = Runoff coefficient, which expresses the fraction of rainfall which is converted into runoff = 0.05 + 0.009(Ipre);  
+  :I<sub>pre</sub> = Predevelopment (existing) site imperviousness (i.e., I = 75 if site is 75% impervious);  
+  :C = Flowweighted mean concentration of the pollutant (total P);  
+  :A = Area of the development site (acres); and  
+  :0.20 is a regional constant and unit conversion factor  
−  +  ===Step 3: Calculate postdevelopment pollutant load===  
−  +  In this step, the applicant calculates stormwater phosphorus loadings from the postdevelopment, or proposed, site. Again, an abbreviated version of the Simple Method is used for the calculations, and the equation is the same for both new development and redevelopment sites.  
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+  <math>L_{post} = 0.20 P R_v C A</math>  
−  +  where:  
+  :L<sub>post</sub> = Average annual load of total phosphorus exported from the postdevelopment site (lbs/year);  
+  :P = Rainfall depth over the desired time interval (inches);  
+  :R<sub>v</sub> = Runoff coefficient, which expresses the fraction of rainfall which is converted into runoff = 0.05 + 0.009(I<sub>post</sub>);  
+  :I<sub>post</sub> = Postdevelopment (proposed) site imperviousness (i.e., I = 75 if site is 75% impervious);  
+  :C = Flowweighted mean concentration of the pollutant (total phosphorus) in urban runoff (mg/l)= 0.30 mg/l;  
+  :A = Area of the development site (acres); and  
+  :0.20 is a regional constant and unit conversion factor.  
−  +  ===Step 4: Calculate the pollutant removal requirement===  
+  The phosphorus load generated from the postdevelopment site must be reduced so that it is 90 percent or less of the load generated prior to development. In this example, a 10 percent reduction in phosphorus loading from predevelopment conditions is used. This should not be construed as a recommended reduction for the State of Minnesota. Applicants should check with local stormwater authorities to determine if specific pre to postdevelopment phosphorus reduction requirements exist. The amount of phosphorus that must be removed through the use of stormwater BMPs is called the Pollutant Removal Requirement (RR) and is given by  
−  +  <math>RR = L_{post}  0.9 L_{pre}</math>  
−  +  where  
+  :RR= Pollutant removal requirement (lbs/year);  
+  :L<sub>post</sub> = Average annual load of total phosphorus exported from the postdevelopment site (lbs/year);  
+  :L<sub>pre</sub> = Average annual load of total phosphorus exported from the site prior to development (lbs/year); and  
+  :0.90 is suggested postdevelopment phosphorus load reduction. Local requirements may vary.  
−  +  ===Step 5: Identify feasible BMPs===  
+  Step 5 looks at the ability of the chosen BMP to meet the site’s pollutant removal requirements. The pollutant load removed by each BMP is calculated using the average BMP removal rate, the computed postdevelopment load, and the drainage area served. If the load removed is equal to or greater than the pollutant removal requirement computed in Step 4, then the onsite BMP complies. If not, the designer must evaluate alternative BMP designs to achieve higher removal efficiencies, add additional BMPs, design the project so that more of the site is treated by the proposed BMPs, or design the BMP to treat runoff from an offsite area.  
−  +  <math>LR = L_{post} BMP_{RE} DA</math>  
−  +  where  
−  +  :LR = Annual total phosphorus load removed by the proposed BMP (lbs/year);  
−  +  :L<sub>post</sub> = Average annual load of total phosphorus exported from the postdevelopment site prior to development (lbs/year);  
−  +  :BMP<sub>RE</sub> = BMP removal efficiency for total phosphorus (%); and  
−  +  :DA = Fraction of the drainage area served by the BMP (%)  
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+  ===Step 6: Select offSite mitigation option===  
If the pollutant removal requirement has been met through the application of onsite stormwater BMPs, the process is complete.  If the pollutant removal requirement has been met through the application of onsite stormwater BMPs, the process is complete.  
−  In the event that onsite BMPs cannot fully meet the pollutant removal requirement and onsite design cannot be changed, an offset fee should be charge (e.g.  +  In the event that onsite BMPs cannot fully meet the pollutant removal requirement and onsite design cannot be changed, an offset fee should be charge (e.g. $X per pound of phosphorus). 
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−  +  {{:Comparative BMP phosphorus removal}}  
−  +  <noinclude>  
+  =='''References'''==  
+  *Caraco, D. 2001. ''Managing Phosphorus Inputs Into Lakes III: Evaluating the Impact of Watershed Treatment.'' Watershed Protection Techniques. 3 (4): 791796. Center for Watershed Protection. Ellicott City, MD.  
+  *Maryland Department of the Environment (MDE). 2000. 2000 [http://www.mde.state.md.us/programs/Water/StormwaterManagementProgram/MarylandStormwaterDesignManual/Pages/programs/waterprograms/sedimentandstormwater/stormwater_design/index.aspx Maryland Stormwater Design Manual]. MDE. Baltimore, MD.  
+  *Winer, R. 2000. [http://www.stormwatercenter.net/Library/STPPollutantRemovalDatabase.pdf National Pollutant Removal Performance Database for Stormwater Treatment Practices]. 2nd Edition. Center for Watershed Protection. Ellicott City, MD.  
−  +  [[category:Models and calculators]]  
−  +  </noinclude>  
−  
− 
The Simple Method is a technique used for estimating storm pollutant export delivered from urban development sites. The method was developed to provide an easy yet reasonably accurate means of predicting the change in pollutant loadings in response to development. This information is needed by planners and engineers to make rational nonpoint source pollution decisions at the site level.
The Simple Method calculation is intended for use on development sites less than a square mile in area. As with any simple model, the method to some degree sacrifices precision for the sake of simplicity and generality. Even so, the Simple Method is still reliable enough to use as a basis for making nonpoint pollution management decisions at the site level. Phosphorus pollutant loading (L, in pounds per year) from a development site can be determined by solving equation 1, shown below.
The value of P represents the number of inches of precipitation that falls during the course of a normal year of rainfall. Longterm weather records around the state of Minnesota suggest that the average annual rainfall depth is about 26 inches. This can be used to estimate P or a user can substitute the average annual rainfall depth from the closest National Weather Service longterm weather station or other suitable locations for which a reliable record can be demonstrated (> 10 years).
The P_{j} factor is used to account for the fraction of the annual rainfall that does not produce any measurable runoff. Many of the storms that occur during the year are so minor that all of the rainfall is stored in surface depressions and eventually evaporates. As a consequence, no runoff is produced. An analysis of regional rainfall/runoff patterns indicates that only 90 percent of the annual rainfall volume produces any runoff at all. Therefore, P_{j} should be set at 0.9.
The runoff coefficient (R_{v}) is a measure of the site response to rainfall events, and in theory is calculated as R_{v} = r/p, where r and p are the volume of storm runoff and storm rainfall, respectively, expressed as inches. The R_{v} for the site depends on the nature of the soils, topography, and cover. However, the primary influence on the R_{v} in urban areas is the amount of imperviousness of the site. Impervious area is defined as those surfaces in the landscape that cannot infiltrate rainfall consisting of building rooftops, pavement, sidewalks, driveways, etc. In the equation R_{v} = 0.05 + 0.009(I), I represents the percentage of impervious cover expressed as a whole number. A site that is 75% impervious would use I = 75 for the purposes of calculating R_{v}. To see runoff coefficients for different land uses, link here.
The total area of the site (in acres) can be directly obtained from site plans. If the total area of the site is greater than one square mile (640 acres), the Simple Method may not be appropriate and applicants should consider utilizing other approaches, such as modeling or monitoring.
Statistical analysis of several urban runoff monitoring datasets has shown that the average storm concentrations for total phosphorus do not significantly differ between new and existing development sites. Therefore, a pollutant concentration, C, of 0.30 milligrams per liter (mg/l) should be used in this equation as a default. However, if good local data are available or an adjustment is needed, this factor can be customized for local condition.
The phosphorus pollutant export calculation is described by
\(L = 0.227 P P_j R_v C A\)
where
The above equation can be simplified to
\(L = 0.20 P R_v C A\)
The methodology for comparing annual predevelopment pollutant loads to postdevelopment pollutant loads is a sixstep process:
In this step, the applicant calculates the impervious cover of the predevelopment (existing) and postdevelopment (proposed) site conditions.
Impervious cover is defined as those surfaces in the landscape that impede the infiltration of rainfall and result in an increased volume of surface runoff. As a simple rule, humanmade surfaces that are not vegetated will be considered impervious. Impervious surfaces include roofs, buildings, paved streets and parking areas and any concrete, asphalt, compacted dirt or compacted gravel surface.
In this step, the applicant calculates stormwater phosphorus loadings from the site prior to development. Loading estimates in a new development situation utilizes a benchmark load for undeveloped areas based on average phosphorus loadings for a typical mix of undeveloped land uses and is given by
\(L_{pre} = 0.5 A\)
where
The equation to determine phosphorus loading in a redevelopment situation is based on the Simple Method and is given by
\(L_{pre} = 0.20 P R_v C A\)
where;
In this step, the applicant calculates stormwater phosphorus loadings from the postdevelopment, or proposed, site. Again, an abbreviated version of the Simple Method is used for the calculations, and the equation is the same for both new development and redevelopment sites.
\(L_{post} = 0.20 P R_v C A\)
where:
The phosphorus load generated from the postdevelopment site must be reduced so that it is 90 percent or less of the load generated prior to development. In this example, a 10 percent reduction in phosphorus loading from predevelopment conditions is used. This should not be construed as a recommended reduction for the State of Minnesota. Applicants should check with local stormwater authorities to determine if specific pre to postdevelopment phosphorus reduction requirements exist. The amount of phosphorus that must be removed through the use of stormwater BMPs is called the Pollutant Removal Requirement (RR) and is given by
\(RR = L_{post}  0.9 L_{pre}\)
where
Step 5 looks at the ability of the chosen BMP to meet the site’s pollutant removal requirements. The pollutant load removed by each BMP is calculated using the average BMP removal rate, the computed postdevelopment load, and the drainage area served. If the load removed is equal to or greater than the pollutant removal requirement computed in Step 4, then the onsite BMP complies. If not, the designer must evaluate alternative BMP designs to achieve higher removal efficiencies, add additional BMPs, design the project so that more of the site is treated by the proposed BMPs, or design the BMP to treat runoff from an offsite area.
\(LR = L_{post} BMP_{RE} DA\)
where
If the pollutant removal requirement has been met through the application of onsite stormwater BMPs, the process is complete.
In the event that onsite BMPs cannot fully meet the pollutant removal requirement and onsite design cannot be changed, an offset fee should be charge (e.g. $X per pound of phosphorus).
General summary of comparative BMP phosphorus removal performance^{a,e,f}
Link to this table
BMP Group  BMP Design Variation  Average TP Removal Rate (%)^{b}  Maximum TP Removal Rate (%)^{c}  Average Soluble P Removal Rate (%)^{dg} 

Bioretention  Underdrain  see Phosphorus credits for bioretention systems with an underdrain  see Phosphorus credits for bioretention systems with an underdrain  see Phosphorus credits for bioretention systems with an underdrain 
Infiltration 




Filtration  Media Filter  50  55  0 
Vegetative Filters (dry)  50  55  0  
Wet Swale  0  35  0  
Infiltration^{f,i}  Infiltration Trench 



Infiltration Basin 




Stormwater Ponds  Wet Pond  50  75  0 
Multiple Pond  60  75  0  
Stormwater Wetlands  Shallow Wetland  40  45  0 
Pond/Wetland  0 
^{a} Removal rates shown in table are a composite of five sources: ASCE/EPA International BMP Database; Caraco (CWP), 2001; MDE, 2000; Winer (CWP), 2000; and Issue Paper D P8 modeling
^{b} Average removal efficiency expected under MPCA Construction General Permit sizing requirements
^{c} Upper limit on phosphorus removal with increased sizing and design features, based on national review
^{d} Average rate of soluble phosphorus removal in literature
^{e} See also Calculating stormwater volume and pollutant reductions and credits
^{f} Note that the performance numbers apply only to that portion of total flow actually being treated; it does not include any runoff that bypasses the BMP
^{g} Note that soluble P can transfer from surface water to ground water, but this column refers only to surface water
^{h} Note that 100% is assumed for all infiltration, but only for that portion of the flow fully treated in theinfiltration facility; bypassed runoff or runoff diverted via underdrain does not receive this level of treatment
This page was last edited on 8 April 2020, at 12:23.
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