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*A sand filter and an iron-enhanced sand filter in watershed 4
 
*A sand filter and an iron-enhanced sand filter in watershed 4
  
===Defining the sub-watersheds===
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===Defining the subwatersheds===
 
Each of the watersheds in the study area could be addressed on a single worksheet or on separate worksheets. Splitting the study area into multiple watersheds is desirable. In addition to the four watersheds, note the bioretention practices in watershed 3 belong to treatment trains. Because the Estimator cannot address reductions in pollutant loads from upstream BMPs, we can treat the furthest upstream bioretention practices as a separate sub-watershed and the downstream bioretention practices as a separate sub-watershed (each of the four bioretention basins could also be entered on separate worksheets). In watershed 4, there are two sand filters in two land use settings. One of the land uses is currently agricultural land that will be converted to urban land. It should be treated as a separate sub-watershed.
 
Each of the watersheds in the study area could be addressed on a single worksheet or on separate worksheets. Splitting the study area into multiple watersheds is desirable. In addition to the four watersheds, note the bioretention practices in watershed 3 belong to treatment trains. Because the Estimator cannot address reductions in pollutant loads from upstream BMPs, we can treat the furthest upstream bioretention practices as a separate sub-watershed and the downstream bioretention practices as a separate sub-watershed (each of the four bioretention basins could also be entered on separate worksheets). In watershed 4, there are two sand filters in two land use settings. One of the land uses is currently agricultural land that will be converted to urban land. It should be treated as a separate sub-watershed.
  

Revision as of 16:26, 31 March 2020

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This page is under review during the MS4 Phase 2 permit reissuance period
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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.

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.

Case study 2 - hypothetical example

image of hypothetical example
Schematic of hypothetical study area, showing four sub-watersheds drainign to an impaired river, and planned BMPs.

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 and the information is general. For specific details, see the Rochester case study above.

Study area and wasteload allocations

image from estimator
Screen shots of loading for sub-watersheds 1, 2, and 3. Note the reduction for street sweeping in sub-watershed 1. Also note sub-watershed 3 includes only the area draining to the two northern bioretention BMPs. For higher resolution, click on the image and then click again on the image that appears.
image from estimator
Screen shots of loading for Sub-watersheds 4, 5, and 6. Note the reduction for conversion of agricultural land in sub-watershed 5. Also note sub-watershed 4 includes only the area draining to the two southern bioretention BMPs. For higher resolution, click on the image and then click again on the image that appears.

A hypothetical study area (see figure) includes four watersheds that drain to a river impaired for TSS. Each watershed is 500 acres, giving a total acreage for the study area of 2000 acres. Prior to completion of a TMDL, no BMPs were implemented. The Estimator was used to develop the TMDL allocations and the TMDL report provides the following information.

  • Current load = 888,225 pounds
  • Wasteload allocation = 799,402 pounds
  • Required reduction = 10% or 88,823 pounds

Planned BMPs to meet the wasteload allocation

To reach the target load reduction, several BMPs are planned (see figure). These include the following.

  • A stormwater pond in a residential area of watershed 1
  • An infiltration BMP and a stormwater pond in watershed 2
  • Several biofiltration BMPs (rain gardens) in watershed 3
  • A sand filter and an iron-enhanced sand filter in watershed 4

Defining the subwatersheds

Each of the watersheds in the study area could be addressed on a single worksheet or on separate worksheets. Splitting the study area into multiple watersheds is desirable. In addition to the four watersheds, note the bioretention practices in watershed 3 belong to treatment trains. Because the Estimator cannot address reductions in pollutant loads from upstream BMPs, we can treat the furthest upstream bioretention practices as a separate sub-watershed and the downstream bioretention practices as a separate sub-watershed (each of the four bioretention basins could also be entered on separate worksheets). In watershed 4, there are two sand filters in two land use settings. One of the land uses is currently agricultural land that will be converted to urban land. It should be treated as a separate sub-watershed.

Thus there are six sub-watersheds for this study area.

  • Watershed 1
  • Watershed 2
  • Watershed 3 - northern rain gardens
  • Watershed 3 - southern rain gardens
  • Watershed 4 - land use change
  • Watershed 4 - called Sub-watershed 6 in the Estimator spreadsheet

Calculating initial and adjusted loads

screen shot estimator
Screen shot of BMP inputs for sub-watersheds 1 (top), 2 (middle), and 3 (bottom). Acreages for BMPs are shown in white cells. Pounds and percent reduced are shown in red cells. For higher resolution, click on the image and then click again on the image that appears.

[File:Estimator bmps 2a.png|300px|thumb|alt=screen shot estimator|Screen shot of BMP inputs for sub-watersheds 4 (top), 5 (middle), and 6 (bottom). Acreages for BMPs are shown in white cells. Pounds and percent reduced are shown in red cells. For higher resolution, click on the image and then click again on the image that appears.]]

Each worksheet in the Estimator, except the summary worksheet, contains two areas in which loads are calculated. The top area (rows 5-24), called The total load, is the initial load prior to implementation of any practices. For each of the 6 sub-watersheds in this example, the user enters the area (acres) for each land use and the annual rainfall. There are defaults for emcs and runoff coefficients, but the user may modify these. Links to guidance on adjusting these is included in the spreadsheet and shown below.

Below the initial load is a section where loads can be adjusted (rows 26-45). Adjusted loads account for changes in loading associated with certain practices, such as street sweeping, or land use changes. These changes must occur after completion of the TMDL. By default, the cells in this section of a worksheet are auto-populated with values from the initial load. The user must change the emc and/or the runoff coefficient based on changes in practices or changed land use. In this example, enhanced street sweeping occurred in residential areas in watershed 1 (Sheet 1), lowering the TSS emc by 24.2 percent (based on monitoring data) and reducing TSS loads by 32,328 pounds. In watershed 3, in the area where agricultural land converted to urban land, the user entered a higher TSS emc but a lower runoff coefficient, reducing TSS loads by 3479 pounds (Sheet 5). When a user modifies the emc or runoff coefficient, they can enter a description of the change in Columns K and L (which are merged). See Sheets 1 and 5 for the examples discussed above.

Note: These adjusted loads are implemented after completion of the TMDL and therefore qualify as pollutant reduction credits. Any changes in loading associated with activities prior to completion of the TMDL do not qualify for credit.

Calculating load reductions associated with permanent structural bmps

Once any changes in load are entered (load adjustments), the user enters acreages treated by permanent structural bmps (see figure). For a detailed description of and guidance for using the Simple Estimator, including entering bmp information, link here. For this example, the bmps to be implemented are summarized below.

  • Sub-watershed 1: 20 acres treated by a wet basin in a high density residential area
  • Sub-watershed 2: 10 acres treat by an infiltration basin in a commercial area and 20 acres treated by a wet basin in a multi-use area
  • Sub-watershed 3 (northern rain gardens): 30 acres treated by the two northern bioretention basins in a high density residential area
  • Sub-watershed 3 (southern rain gardens): 20 acres treated by the two southern bioretention basins in a high density residential area
  • Sub-watershed 4 (land use change): 20 acres treated by an iron-enhanced sand filter in a high density residential area
  • Sub-watershed 6: 20 acres treated by a sand filter in a high density residential area

The acreages associated with each practice and land use are shown in the figures. For example, the wet basin in sub-watershed 1 treats runoff from 20 acres in high density residential land use, as shown in Cell I81 in worksheet 1. The infiltration basin and wet basin in sub-watershed 2 are shown in cells C75 and I78, respectively. Load reductions and percent load reduced as a result of structural BMPs are shown for the entire sub-watershed in cells L94 and L95, respectively. In sub-watershed 1, the wet basin will reduce loading by 5,143 pounds or 2.92%. Note these reductions pertain only to sub-watershed 1 (i.e. the wet basin does not reduce loading for the entire study area by 2.92%).

Interpreting the results

screen shot of summary sheet
Screen shot of the summary tab from the Estimator. The sheet shows total initial and adjusted loads, pounds and percent load reduced, and final loading rates. Click on image and then click on image again to enlarge.

The summary worksheet in the Estimator provides the following information.

  • Sub-watershed name and associated worksheet
  • Total acres for each sub-watershed
  • Initial load, final load (after load adjustments and implementation of structural BMPs), pounds reduced, percent reduced, initial and final loading rates for TP and TSS for each sub-watershed.
  • Summation of loads across all sub-watersheds (i.e. for the entire study area)
  • Final loading rate for the study area (lb/ac/yr)

The initial TSS load for the study area was 888,225 pounds and the final load is 799,368 pounds, for a reduction of 88,857 pounds or 10 percent from the initial load. This reduction meets the TMDL requirement.

Note that the most effective practices for this case study were enhanced street sweeping and implementation of an infiltration basin. The Estimator is highly sensitive to the emc value. Infiltration is a highly effective practice, since all captured pollutant is considered to be removed. Thus, a combination of implementing infiltration practices in areas with high pollutant loads is most effective at reducing loads, although under these conditions, pretreatment and BMP operation and maintenance would be important.


Related pages