Over the past several years we've heard from people suggesting they be notified when updates are made to the Minnesota Stormwater Manual. We have also identified several stormwater management concerns and felt that focused communication on these specific issues might be useful. We therefore decided that periodic emails to subscribers would be one way of notifying practitioners about updates to the Manual and focus on specific stormwater issues. Emails are sent periodically, roughly every 6-12 weeks.

The emails contain only a short description of updates and other information. This page provides more detailed information. It is organized by the approximate dates when emails are sent to subscribers. We welcome recommendations for featured topics and links to case studies and other items of information. Please contact Mike Trojan at the MPCA..

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June 2021

image of street sweeper
Image courtesy Sarah Hobbie, University of Minnesota

Updates to the Manual (June 2021)

What are we working on (June 2021)

  • Street sweeping: We are developing guidance on street sweeping (e.g. when and where to sweep, costs, managing sweeping wastes, etc.) and that should be available in early summer. We also expect to provide information on how to incorporate sweeping into the MIDS Calculator.
  • Engineered media: We continue to populate the Manual with information on engineered media, including amendments and materials in engineered media. This is a significant project and will continue through the summer. We continue to work closely with University of Minnesota researchers working on bioretention media. Most recently we have added information on wood chips and calcium-based water treatment residuals. Click on this link for more information.
  • Green infrastructure: A long-term project to improve information on Green Stormwater Infrastructure in the Manual started earlier this year. We recently added some case studies and anticipate having information on O&M this summer.
  • Vegetation: Another long-term project to improve information in the Manual, this work has only recently started and is in a scoping phase to identify information needs on the topic of vegetation in stormwater practices.
  • Managing stormwater media waste: By this summer we expect to have information on disposal and management of engineered media used in stormwater applications, focused primarily on bioretention media.

Featured article - Pretreatment (June 2021)

The Minnesota Construction Stormwater Permit requires pretreatment for filtration and infiltration practices. Forebays or other pretreatment practices are highly recommended for constructed stormwater ponds. The permit, however, does not specify the type of or sizing for pretreatment practices. We are discovering that many stormwater best management practices (bmps, also often called stormwater control measures or scms) are not performing as designed, often due to heavy sediment loads to the bmp and to poor design.

Proper pretreatment can extend the life and improve performance of downstream bmps. But we frequently hear stories about inadequate or improperly designed pretreatment. This is unfortunate, since the Manual contains a wealth of information on pretreatment. For example, did you know the Manual contains the following?

Below are examples of some design issues that have been brought to our attention.

  • Pre-treatment baffles with shallow sumps showing up on plans (2 or 3 foot deep sumps regardless of inlet/outlet pipe size diameters and/or manhole housing diameter). Questions have been raised whether shallow sumps would function the same for large (24 inch +) inlet pipes as they would for smaller inlet pipes. Same issue goes for diameter of the structure. Much deeper sumps would seem to be required for larger inlet pipes.
  • Hoods or skimmer devices are being placed over the outlet pipe with baffles, which results in a change to the flow path and likely performance of the baffle. Stormwater should flow “unimpeded” from inlet to outlet as tested in past studies. In addition to effecting the hydraulic flow, trash and floatables will build up and can negatively impact the baffle function.
  • Questions specific to the use baffles with the following designs have been raised.
    • Multiple inlet pipes entering a manhole with a baffle. Observations have been made where the design shows the flows short-circuit the baffle, undermining expected treatment and promoting premature bypass.
    • Grated inlets which does not direct flows to one side of baffle, resulting in flows and pollutants prematurely bypassing the pretreatment device

The information in the manual could be better organized and made more accessible. So, we hope to execute a work order this summer to better organize the information on pretreatment. You can help by providing the following (NOTE: We cannot endorse or promote specific commercial products, processes, or services).

  • Examples (case studies) of good and bad pretreatment practices you are finding
  • Photos
  • Comments and suggestions for improving information on pretreatment

Some other stuff of interest (June 2021)

Take the pretreatment quiz

September 2021

Updates to the Manual (September 2021)

What are we working on (September 2021)

  • Street sweeping
    • Case studies. We will be conducting interviews with a variety of entities that conduct street sweeping. An important goal of this is to develop guidance for how to establish sweeping programs targeted to water quality improvement.
    • Communication plan. We will complete a communication plan designed to disseminate information on street sweeping and application of the recently developed sweeping credit and sweeping guidance.
  • Vegetation. We have a work order in place and have communicated with a stakeholder team about improving information on vegetation in the manual. The team identified the following as top priorities in moving forward.
    • Site assessment, preparation, design considerations, and recommendations for vegetation in stormwater management
    • Plant and seed selection based on project goals and site conditions
    • Design phase O&M
  • Manufactured treatment devices (mtds). MTDs are pre-fabricated stormwater treatment structures utilizing settling (sedimentation), filtration, absorptive/adsorptive materials, vortex separation, vegetative components, and/or other appropriate technology to remove pollutants from stormwater runoff. We are in the process of developing phosphorus and TSS pollutant removal credits for these practices. A workgroup will be reviewing the draft credits in the coming weeks.
  • Continue working on soil information and media.

Featured article (September 2021)

The MIDS Calculator and MPCA Simple Estimator

The Minimal Impacts Design Standards (MIDS) Calculator and MPCA's Simple Estimator (Estimator) are two tools used to estimate volume and pollutant load reductions associated with implementation of stormwater control measures, also known as best management practices (BMPs). Each of these tools is widely used. The MIDS Calculator, for example, has had over 8000 downloads in the past 3 years, while the Estimator is used by many MS4 permittees to assess progress toward or determine if they are meeting a TMDL Wasteload Allocation. The advantage of these tools is they are relatively easy to use and understand. However, with simplicity comes the potential for inaccuracy. This article provides a high level discussion of these two tools, including their advantages and potential pitfalls. The closing paragraph in this article provides a discussion of how MPCA hopes to move forward in providing a better understanding and application of these tools.

MIDS Calculator
The MIDS Calculator was originally designed as a tool for determining how to meet the MIDS performance goal of 1.1 inches at development sites. Detailed discussion of the Calculator and MIDS performance goal are found on the following pages.

Despite the original intent, the Calculator is being used for purposes other than assessing volume reduction at individual sites. It was and continues to be widely used for water quality determinations, including at sub-watershed scales. This presents a number of challenges since the Calculator is not easily modified or is inflexible in addressing water quality at these larger scales. Examples of some limitations of the Calculator include the following.

  • The Calculator uses a single value for event mean concentration that must be applied across the entire project.
  • The Calculator has default ratios for particulate phosphorus (PP) and dissolved phosphorus (DP) as a fraction of total phosphorus (0.55 for PP and 0.45 for DP).
  • Pollutant removal efficiencies of BMPs cannot be modified.
  • Bypass is not easily addressed.

These are just examples of limitations.

Fundamentally, the MIDS Calculator is limited by it's platform. It is built in Excel and addressing these various issues would be challenging. Additionally, as these issues get addressed, the tool becomes more complex to use, defeating one of its original intentions.

MPCA Simple Estimator
The 2013 MS4 permit required quantification of pollutant reductions to demonstrate a permittee was making progress toward achieving or had achieved a TMDL ( total maximum daily load) wasteload allocation. The tools available, such as P8, WinSLAMM, SWMM, and even the MIDS Calculator, were accessible only to "modeling experts". As a result, the MPCA developed a simple Excel spreadsheet that employed the Simple Method. This spreadsheet was readily accessible to most practitioners and could be used to demonstrate progress toward meeting a WLA.

The tool was intended to be used for demonstrating progress toward reducing pollutant loads, since it only considered a single watershed and there was limited guidance on modifying defaults in the spreadsheet. To make the tool more robust and rigorous, the Excel workbook was updated to include up to 10 subwatersheds (as separate worksheets), default values were updated based on literature reviews, and guidance for using the tool was expanded. The tool was again widely used by permittees during the 2020-21 permit cycle. Questions and submittals we received indicated the tool continues to have limitations and is not being correctly applied in some situations. Some of the limitations in the tool include the following.

  • The Estimator does not easily address treatment trains. The user must manipulate the model inputs to account for treatment within treatment trains.
  • While the Estimator allows the user to account for reductions from non-structural practices, there is limited guidance on how reductions for these practices should be addressed in the workbook.
  • Guidance on adjusting event mean concentrations and runoff coefficients is limited.
  • Clearer guidance and training is needed for adjusting pollutant removal within individual practices.

Addressing these issues
MPCA hopes to conduct a more rigorous analysis of how these tools are being used and develop appropriate guidance. Additional training is likely. The Estimator may undergo modifications based on this analysis. Unfortunately, changes to the MIDS Calculator are unlikely as the Excel platform is limiting in terms of adjusting the Calculator for water quality modeling. We are in the process of documenting some of these issues and hope to have more detailed guidance and training in place in 2022.

Alternative models may be used. The Minnesota Stormwater Manual provides a detailed summary of available water quality models. These models typically require greater modeling expertise and lack extensive documentation in the Manual, though many have user guides. The MPCA will also accept other modeling approaches, but it is recommended that permittees consult the MPCA prior to using alternative models.

Some other stuff of interest (September 2021)

Take the green infrastructure O&M quiz