Two case studies are included on this page. The first was developed by the City of Rochester using Version 2 of the Simple Estimator. Version 2 did not allow calculations for sub-watersheds, thus requiring all best management practices (BMPs) to be grouped. The second case study is a hypothetical study area where Version 3 of the Estimator is used.

Version 2 example: Rochester case study

image of allocations for Zumbro TMDL
Example of TSS TMDL (Zumbro River, West Indian Cr to Mississippi River, 07040004-501)

The Zumbro River Watershed Turbidity total maximum daily load (TMDL) (MCPA, 2012) is a turbidity TMDL completed for seventeen (17) stream reaches in the Zumbro River watershed. Although the Zumbro TMDL addresses a turbidity impairment, by correlating turbidity to total suspended solids (TSS) loading, individual TMDLs are expressed as TSS loading (tons/day). The TSS TMDL for one reach (Zumbro River, West Indian Creek to Mississippi River, 07040004-501), is shown on the right.

Note: in 2015, the U.S. Environmental Protection Agency (USEPA) and Minnesota Pollution Control Agency (MPCA) issued and approved an amendment to replace existing turbidity water quality standards with TSS water quality standards.

The TMDL is one of 17 TMDLs included in the Zumbro River Watershed TMDL. The wasteload allocations (WLAs) and all other TMDLs developed in the report are categorical. In addition to being categorical, all TMDLs developed in the report separate allocations into flow regimes (or “flow zones”). Municipal Separate Storm Sewer System (MS4) compliance related to categorical and flow regime TMDLS is discussed below.

MS4 Compliance: Rochester

The following subsections provide an overview of reporting related to TMDL requirements and how the City of Rochester uses the MPCA Simple Estimator to evaluate and demonstrate progress towards TMDL compliance.

General TMDL reporting information can be found at the following links.

Information related to the City of Rochester and the Zumbro Watershed Turbidity TMDL can be found at the following links.

Using the MPCA Simple Estimator to Demonstrate WLA Compliance: Rochester

City of Rochester BMP cumulative TSS reduction, in lbs/year, from the 2017 City of Rochester TMDL Annual Report Form
Permitee TMDL Project 2014 2015 2016 2017
Rochester Zumbro River Watershed TMDL for Turbidity Impairments - TSS 946,501 1,119,278 1,182,284 1,331,304

The Zumbro River Watershed Turbidity TMDL (MCPA, 2012) establishes that the City of Rochester is required to reduce daily TSS loading to several stream reaches in the Zumbro River watershed. An overview of general steps which should be taken by MS4s not meeting WLAs for approved TMDLs is included below. More detailed information related to each step can be found in the Model documentation.

Note: the case study example developed in this section is provided as an example of how an individual MS4 can demonstrate WLA compliance using the MPCA Simple Estimator, and was not developed with input or coordination with the MS4 permittees.

1. Review applicable WLAs developed in the TMDL. TSS TMDLs within the Zumbro River Watershed Turbidity TMDL (MPCA, 2012) are developed for individual stream reaches. Eleven (11) of the seventeen (17) TSS TMDLs developed in the Zumbro River Watershed Turbidity TMDL assign WLAs to the City of Rochester.

  • Zumbro River; West Indian Cr to Mississippi River (see Table 32, 07040004-501)
  • Zumbro River, South Fork; Cascade Cr to Zumbro Lk (07040004-507)
  • Zumbro River, South Fork; Salem Cr to Bear Cr (07040004-536)
  • Bear Creek; Willow Cr to S Fk Zumbro R. (07040004-538)
  • Bear Creek; Headwaters to Willow Cr (07040004-539)
  • Willow Creek; Headwaters to Bear Cr (07040004-540)
  • Silver Creek; Unnamed Cr to Unnamed Cr (07040004-552)
  • Silver Creek; Unnamed Cr to Silver Lk S Fk Zumbro R. (07040004-553)
  • Cascade Creek; Unnamed Cr to S Fk Zumbro R. (07040004-581)
  • Kings Run; Unnamed Cr to Unnamed Cr (07040004-601)
  • Cascade Creek; Headwaters to Unnamed Cr (07040004-639)

TMDLS developed in the Zumbro River Watershed Turbidity TMDL are categorical, meaning that a single WLA is assigned to all tributary MS4s, rather than assigning individual WLAs to each MS4. Additionally, TMDLs developed in the report separate allocations into flow regimes (or “flow zones”). TMDLs developed for streams and rivers are often developed for five (5) flow regimes shown, where “high” represents the top 10 percent of highest flows and pollutant loading observed, “low” represents the lowest 10 percent of flows and pollutant loading, etc. TMDLs structured in this way pose challenges to individual MS4s attempting to comply or evaluate compliance with the assigned MS4 wasteload allocation(s), including

  • separating the categorical wasteload allocation into allocations for individual MS4s, and
  • evaluating and demonstrating WLA compliance for the five (5) flow regimes.

The MPCA provides guidance on how to separate categorical WLAs into individual MS4 WLAs (see “Guidance for developing a TMDL implementation plan for MS4 storm water - “Target loads for each MS4”).

While continuing to develop their individual WLA status assessment, the City of Rochester is demonstrating progress towards WLA compliance through implementation of water quality BMPs and adaptive management (see the SWPPP Reauthorization Form and TMDL Annual Report Form).

Planned BMP implementation activities, from the 2017 City of Rochester TMDL Annual Report Form
BMP description Status Reporting year
Create a new storm water ordinance that will include provisions to meet the requirements of the MS4 permit; ordinances will be developed for IDDE, ESC and post construction stormwater management Funded 2018
Complete waste load allocation status assessments for the turbidity and fecal coliform bacteria TMDLs Funded 2018
Assess open channels (swales) for determination of water quality treatment benefits; assign BMP IDs to them and incorporate into TMDL assessment Discontinued 2014
Implement BMPs for public and private development projects according to MPCA standards Planned 2018
Manorwoods Outlet Channel Restoration and Water Quality Improvement Project Funded 2019
Modification/Construction of Centurion Ridge (South) Wet Sedimentation Basin Planned 2020
Spring Brook Valley Management Plan and Stabilization Projects Funded 2020
Cascade Lake Stormwater BMP Implementation Funded 2019
Section 7 Stormwater Management Plan Update for Northwest Rochester Under construction 2018
Section 7 Stormwater Management Plan Update for Northwest Rochester Under construction 2018
Quarry Hill Creek Bank Failure Repair Funded 2019
Stormwater Management Plan Update - Citywide and Downtown District Funded 2020
Rocky Creek Stabilization Project (Phase III) Under construction 2018
Elton Hills Stabilization Planned 2019
Pond 128 Outlet Modification Project Funded 2019
NE River Road/37th St Infiltration Basin Funded 2019
Implement adopt-a-drain program Funded 2020
West River Parkway NW Storm Sewer Outfall Stabilization Funded 2019
image of Zumbro River watershed
Zumbro River Watershed and MS4s (from the Zumbro River Watershed Turbidity TMDL (MPCA, 2012)

2. Review the drainage area used to develop the TMDL. The City of Rochester and the impaired reaches within the Zumbro River Watershed are shown in the map. For some of the impaired reaches (e.g., Bear Creek, 07040004-538; Silver Creek, 07040004-553) only portions of the City are tributary, while for others (e.g. Zumbro River, South Fork; Cascade Cr to Zumbro Lk, 07040004-507) the entire municipal area is tributary to the impaired reach. It is critical to determine the drainage area the TMDL applies to, as the WLA and required pollutant reduction apply only to the area tributary to the impaired waterbody. Because the WLA applies to the impaired water, pollutant reduction achieved within the MS4 but not within the tributary area to the impaired water does not apply to the required reduction targets developed by the WLA.

Note: if the applicable MS4 area is not clearly outlined within the TMDL, mapping and associated digital files can be requested directly from the MPCA. The MPCA maintains a database of drainage area files which can be downloaded from the source: PLACEHOLDER FOR LINK.

3. Review the TMDL modeling period and existing BMPs incorporated into the TMDL (i.e., determine the baseline condition used to establish the WLA). In addition to determining existing loading and required pollutant reduction, it is critical to determine (a) what modeling period was used the establish the TMDL and (b) what BMPs were incorporated into the TMDL model used to develop WLA reduction targets. Before selecting an annualized model (i.e., models that produce annualized, rather than continuous results), it is critical to first make sure that applicable TMDLs are also developed on an annualized basis (e.g., required pounds of TP reduction per year). If the TMDL was developed for a non-annualized period (e.g., the April to October growing season), annualized models like the MIDS Calculator, MPCA Simple Estimator, etc. may not be the best choice for evaluating WLA compliance. Note: if desired, the permittee can contact and work with MPCA stormwater staff to adjust the non annualized WLAs (e.g., growing-season) to annualized WLAs.

Stream reach TMDLs in the Zumbro River Watershed Turbidity TMDL were developed using the load duration curve approach (i.e., the TSS water quality standard was applied to flow duration curves developed from long term stream flow monitoring data). Because stream TMDLs developed using this approach are based on long-term flow monitoring datasets, the developed TMDL is representative of observed flow conditions during the monitoring period and implicitly includes any and all constructed and existing BMPs in the watershed throughout the monitoring dataset used. As discussed in Appendix D of the Zumbro River Watershed Turbidity TMDL (MPCA, 2012), continuous flow and turbidity monitoring data collected from 2007 through 2008 was used to develop TMDLs for each reach. For this reason, 2008 is considered the baseline year for the TMDL, meaning that any BMPs constructed post-2008 within the City of Rochester can be incorporated into the City’s MPCA Simple Estimator model and counted towards achieving WLA reduction goals.

4. Develop an inventory of existing water quality BMPs related to the TMDL pollutant. As outlined in the 2017 City of Rochester TMDL Annual Report Form, the City began an inventory or structural and non-structural BMPs in 2014. At that time, there were a total of 51 TSS reduction BMPs within the City. As of 2017 report, there are now 89 TSS reduction BMPs indicating that the City of Rochester has implemented 38 structural and non-structural BMPs since developing the inventory in 2014.

Per the TMDL modeling period discussion outlined in step #3, TMDLs developed within the Zumbro River Watershed Turbidity TMDL are reflective of watershed conditions in the period of 2007 to 2008. This means that all structural (e.g., wet detention pond) and non-structural (e.g., street-sweeping) BMPs constructed or implemented post-2008 can be counted towards the City of Rochester’s WLA reduction targets. The City of Rochester’s 2017 TMDL Annual Report Form inventory of BMPs tracks the “year when BMP was implemented”, making it easy to determine which BMPs can be counted towards WLA reduction targets.

image from estimator
Screen shots of the Simple Estimator. The top image shows the acreages for this TMDL and the resulting pollutant loads. The middle image shows BMPs implemented by Rochester and reported in the 2017 Annual Report Form, including pollutant pounds and percent reduced. The bottom image shows projected load reductions associated with a proposed biofiltration practice. Click on image to enlarge.

5. Develop a MPCA Simple Estimator model to evaluate WLA compliance. After review of the TMDL and development of the existing BMP inventory, the next step is to develop a MPCA Simple Estimator model for the drainage area outlined in the TMDL (see step #3; for this TMDL, the study area is the entire municipal area). Note: a number of methods (e.g., monitoring) and models (e.g., P8) can be used to evaluate TMDL compliance, however, because this section focuses on the MPCA Simple Estimator, the following steps reference model development and analysis using the MPCA Simple Estimator.

a Develop land use and existing BMP treatment area inputs and incorporate into the model. The MPCA Simple Estimator is a simplified pollutant reduction estimation tool that does not require any dimensional inputs related to modeled BMPs (e.g., permanent pool volume). The City of Rochester’s existing MPCA Simple Estimator model is shown on the right. The tool estimates pollutant loading based on the total study area assigned to ten (10) different land use types and estimates pollutant reduction based on tributary land use area assigned to ten (10) structural BMP types. Because the MPCA Simple Estimator does not account for pollutant routing through BMPs, the engineer or designer must take special precautions to endure the tool does not over-estimate or incorrectly calculate pollutant removal. See Recommendations and guidance for utilizing the MPCA Simple Estimator to meet TMDL permit requirements.
b. Determine existing and WLA load reduction. After generating area inputs for land use and all modeled BMPs, area values can be entered into the MPCA Simple Estimator to evaluate existing load reduction. In the case of the Zumbro River Watershed Turbidity TMDL, it is recommended that a version of the existing conditions model be retained which only include BMPS constructed or existing in the watershed prior to 2008 (the TMDL modeling period, see discussion in step #3).

Updated versions of the model with BMPs constructed post-2008 and proposed BMPs can then be compared back to the pre-2008 existing conditions model. Using this methodology, any additional TSS reduction beyond the pre-2008 existing conditions model represent progress towards the target WLA load reduction.

Using the MPCA Simple Estimator, the City of Rochester has tracked year-to-year TSS reduction from 2014 through 2017. 2014 is the first year of reporting included in City of Rochester’s 2017 TMDL Annual Report Form and includes removal from BMPs existing or constructed prior to 2014.

Note: cumulative reduction shown for 2017 matches the 2017 cumulative reduction value calculated using the MPCA Simple Estimator.

Example showing cumulative TSS reduction tracking for the City of Rochester
Year/Condition Cumulative TSS Reduction (lbs/yr) WLA Reduction Progress (lbs/yr)
Pre-2008 (assumed) 800,000 -
2014 946,501 146,501
2015 1,119,278 319,278
2016 1,182,284 382,284
2017 1,331,304 531,304
2017 with proposed biofiltration implementation plan 1,412,651 612,651
1 Assumed cumulative reduction for TMDL condition (pre-2008) for purposes of this example.

2 WLA reduction progress tracks the cumulative WLA reduction achieved.

6. Review the TMDL implementation plan, Watershed Restoration and Protection Strategies report, Use Attainability Analysis (UAA) report, or other relevant watershed planning documents (if applicable). The Zumbro Watershed Comprehensive Management Plan: Sediment Reduction Component was published by the Zumbro Watershed Partnership in 2012. The comprehensive management plan developed a four part strategy to achieve TMDL TSS loading reduction goals: (1) watershed-wide strategies, (2) headwaters region strategies, (3) major tributary strategies, and (4) regulated point source strategies. Because the majority of the Zumbro watershed is rural and agricultural areas, many of the strategies focus on watershed-wide implementation of land-use based strategies that do not directly apply to the City of Rochester (e.g., conservation tillage and residue management, adopt a soil loss limits ordinance, etc.).

The management plan does discuss strategies for reducing TSS loading from municipal stormwater sources (e.g., Rochester), but does not discuss implementation of specific BMPs (e.g., construct XX wet detention ponds treating XX% developed portions of Rochester). Instead, the movement document provides guidance on how regulated MS4s should incorporate WLA reduction goals, implementation staging, and interim milestones into their SWPPP reauthorizations.

Following guidance developed in the management plan, the City of Rochester developed and incorporated a detailed BMP implementation compliance schedule into their 2013 SWPPP Reauthorization Form, including a Gantt chart outlining when specific implementation tasks will be completed. The City of Rochester continues to develop interim milestones and track BMP implementation, developed from the 2017 TMDL Annual Report Form.

7. Utilize model to track progress towards meeting the WLA (e.g., use the model to evaluate BMPs and incorporate BMPs as they are implemented). As discussed in step #5, above, the existing conditions MPCA Simple Estimator model can be updated as BMPs are constructed to track progress towards attaining WLA compliance. Because the MPCA Simple Estimator does not require BMP dimensional information and only requires that the user input the cumulative tributary area to ten (10) BMP types, the model can be updated as BMPs are implemented throughout the watershed, and updated cumulative TSS reduction values can be calculated and compared back to previous model versions to evaluate progress towards achieving target WLA reductions., the example updated as BMPs are implanted.

To illustrate how the MPCA Simple Estimator model can be used to track TSS reduction and progress towards WLA reduction targets, it is assumed that several campuses of institutional land use enact a biofiltration implementation plan. After completion, it is assumed that a total of 200 acres of previously untreated institutional land use is now treated by biofiltration BMPs. Values in bold text in the lower portion of the image on the left highlight values that have been updated in the City of Rochester’s 2017 MPCA Simple Estimator Model. The table on the right shows how cumulative reduction can be tracked year-to-year.

Version 3.0 example - hypothetical example

The Rochester case study illustrates how Version 2 of the Simple Estimator was used. The MPCA updated the Estimator to Version 3 in summer of 2019. There were four primary enhancements to the estimator.

  1. The Estimator includes ten worksheets where calculations can be made, compared to just one sheet in Version 2. This allows users to break their project area into smaller units for analysis.
  2. The Estimator allows for reductions based on factors that affect the concentration of pollutants in runoff. For example, a change in land use or implementation of enhanced street sweeping will alter the concentration of a pollutant in runoff. Version 2 only allowed reductions for structural BMPs.
  3. Version 3 allows more entries for different land uses and provides event mean concentration (emcs) and runoff coefficients for a broader range of land uses
  4. Version 3 of the Estimator includes a summary worksheet where data from the ten worksheets is compiled

The following example is hypothetical. For general guidance, read the 7 steps from the Rochester case study above, since they are applicable to this case study except for specific differences between Versions 2 and 3 of the Estimator. This example utilizes a site with three subwatersheds and proceeds through a series of implemented practices. To access the spreadsheet used for this example, click here File:MPCA simple estimator version 3 subwatershed example.xlsx.

Step 1. Unadjusted load calculation and entering data for land uses

image used for example
Schematic used for subwatershed example.

The adjacent image is used for this example and will be modified as adjustments are made and BMPs added. The entire project area consists of three subwatersheds. Land uses within the project area include the following.

  • Residential with <25% tree canopy coverage
  • Residential with >50% tree canopy coverage
  • Industrial
  • Commercial
  • Park
  • Transportation
  • Mixed (multi-use)
  • Agricultural

The entire area drains to a lake. The goal is to reduce phosphorus loading to the lake by 30 percent.

In this first step, we enter acreages for each of the three subwatersheds. These are shown in the adjacent image gallery. To account for the two different residential areas in subwatershed A, we change the default emc for residential to 0.350 mg/L for the high canopy area and add a residential land use with an emc of 0.275 for the low canopy area. For subwatersheds B and C, we change the default residential value to 0.35 mg/L to account for higher phosphorus inputs from trees in the high canopy areas. Alternatively, we could have made these adjustments for residential areas in the second section of the three worksheets (Adjusted loads). The total phosphorus load to the lake from all three watersheds is 2699.54 pounds. With a phosphorus reduction goal of 30 percent, this requires a 809.86 pound reduction in phosphorus loading.

Step 2. Calculating adjusted loads

schematic for example
Actions implemented and accounted for in calculating adjusted loads. The implemented actions include enhanced street sweeping in the residential area with >50% tree canopy, an impervious disconnection program in the commercial area of subwatershed A, and conversion of agricultural land to mixed land use.

Section 2 of each calculation worksheet allows the user to enter data that results in adjustments to the total load. In this example, the following actions were implemented (see adjacent image).

  • In the residential areas with >50% tree canopy coverage, an enhanced street sweeping program was implemented. This consisted of street sweeping at the time of fall leaf drop, rather than the once-a-fall sweeping program previously implemented. Measurements of street sweeping material collected during this time indicate this effort reduced annual loading by about 17 percent. Since the Estimator does not allow direct changes in loading, we adjusted the emc downward until we achieved a 17 percent reduction in loading. To accomplish this, we adjusted the emc from 0.35 mg/L to 0.30 mg/L. This resulted in a phosphorus decrease of 18.79 pounds in subwatershed A, 18.79 pounds in subwatershed B, and 7.51 pounds in Subwatersdhed C, for a total reduction of 45.09 pounds.
  • In the commercial area of subwatershed A, a rooftop and parking lot disconnection effort was implemented. Runoff from these areas was diverted to pervious surfaces. The runoff coefficient was adjusted downward from 0.71 to 0.50 based on the calculated decrease in connected impervious surface. This resulted in a phosphorus decrease of 21.91 pounds.
  • The agricultural areas in subwatersheds B and C were converted to mixed land use. We changed the emc and runoff coefficients from agricultural land use to the default values for mixed land use. Phosphorus loads increased by 137.76 pounds in subwatershed B and 150.28 pounds in subwatershed C. BMPs were implemented during the land use conversion, but these calculations are made in Section 3.

After these adjustments, the phosphorus load increased by 221.04 pounds, assuming no further BMPs were implemented. The adjacent photo gallery provides screen shots for each of the three subwatersheds.

Step 3. Entering structural best management practices (BMPs)

schematic for Estimator example
Schematic illustrating where structural BMPs are implemented. Not shown are swales associated with transportation corridors.

In this section, structural BMPs are entered for each of the subwatersheds. The adjacent schematic illustrates where BMPs are implemented, except for swales associated with transportation corridors. A summary of the implemented practices is provided below.

  • Subwatershed A. This subwatershed consists primarily of hydrologic soil group (HSG) C and D soils, making infiltration impractical.
    • Stormwater runoff in much of the area is diverted to wet pond (constructed ponds). A total of 450 acres of mixed land use (multi-use) is treated by ponds and 400 acres of low canopy residential area is treated by ponds. This results in a phosphorus reduction of 330.31 pounds.
    • Biofiltration practices (rain gardens, with an underdrain) are implemented for 100 acres of the residential, high canopy area, resulting in a phosphorus reduction of 28.63 pounds
    • Swales treat 75 acres of runoff from transportation areas, resulting in a reduction of 42.08 pounds of phosphorus
The total phosphorus reduction from these BMPs is 401.02 pounds
  • Subwatershed B. This subwatershed includes HSG A, B, and C soils. Infiltration is therefore feasible in some of the area.
    • Underground infiltration is utilized for 290 acres in the mixed land use areas, including the entire area where agricultural land was converted to mixed land use. This results in a reduction of 263.31 pounds of phosphorus.
    • Biofiltration is utilized for 100 acres of residential land, resulting in a reduction of 28.63 pounds.
The total phosphorus reduction in this subwatershed is 291.94 pounds.
  • Subwatershed C. This subwatershed includes HSG A, B, andC soils. Infiltration is therefore feasible in some of the area.
    • Underground infiltration is utilized for 280 acres in the mixed land use areas, including the entire area where agricultural land was converted to mixed land use. This results in a reduction of 254.23 pounds of phosphorus.
    • Biofiltration is utilized for 40 acres of residential land, resulting in a reduction of 11.45 pounds.
    • Swales are utilized in transportation corridors, treating 60 acres for a phosphorus reduction of 33.66 pounds
    • iron-enhanced sand filters treat 60 acres of runoff in the industrial area. Note the phosphorus removal efficiency of this practice was change from 0.47 to 0.80 due to the use of iron in the treatment. The total phosphorus reduction for this practice is 48.03 pounds.
The total phosphorus reduction in this subwatershed is 347.37 pounds.

The total phosphorus reduction for the three subwatersheds is 819.29 pounds, which meets the target of 809.86 pounds. The adjacent image gallery provides screenshots from the Estimator for the three subwatersheds.

This example illustrates the following.

  • Infiltration is a very effective practice for removing phosphorus
  • Biofiltration in residential areas is only somewhat effective due to the low runoff coefficient; i.e. the practice does not treat a lot of runoff. For example, swales in subwatershed A remove more phosphorus even though swales are not particularly effective at removing phosphorus and less area is treated compared to residential land use. The higher removal is associated with the higher runoff coefficient for transportation, resulting in greater runoff volumes being treated.
  • We assume the engineered media mixes for biofiltration do not leach phosphorus. These would be Mixes C or D, or use of some material in the mix that does not leach phosphorus, such as coir or biochar.
  • Iron enhanced treatment is an effective practice if large volumes of water can be treated in this manner. One concern with sand filters, however, is that large systems are needed to treat large runoff volumes.
  • Additional practices, such impervious disconnection and use of permeable pavement, could further reduce phosphorus loading. These are practices that can be used at the individual homeowner scale.

NOTE: In Section 3, a total reduction of 1080.4 pounds is achieved with BMP implementation. However, phosphorus loading would increase with the land use conversion if no BMPs were implemented. This explains why the load reduction in Section 3 is greater than the net reduction of 819.29 pounds.

Review Summary Sheet

screen shot of Summary tab from Estimator
Screen shot of the Summary worksheet from this Estimator example.

The adjacent image provides a screen shot of the Summary worksheet for this example. Note the following in the image.

  • Data are shown for each subwatershed and for the entire site
  • Initial and final loads are shown, as well as the percent reduction achieved
  • Loading rate, in pounds per acre, are included (both initial and final)
  • Cumulative load reductions for the different BMPs are illustrated

Note that we did not address total suspended solids in our calculations. The summary suggests TSS loads increased, but this is because we ignored calculations for TSS as it was not a pollutant of concern. To accurately reflect TSS, we would enter the BMP data for each subwatershed in Section 4 of each worksheet in the Estimator.

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