Minnesota Stormwater Manual email Updates

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 courtesy Sarah Hobbie, University of Minnesota

Updates to the Manual (June 2021)

• Street sweeping: For the past two years we have been working with U of MN researchers and our contractor to develop a calculator to estimate phosphorus reductions associated with street sweeping. The calculator, a simple Excel workbook, is now available, along with a User Guide. Phosphorus crediting is based on measurements of sweeping material collected (wet or dry mass, or organic matter content). Standard Operating Procedures have also been developed. On May 13, we held a webinar that included an overview of the calculator, guidance, and presentations from nationally recognized researchers.
• MIDS Calculator: The Minimal Impact Design Standards (MIDS) Calculator has been updated to Version 4. The Manual contains a page summarizing updates to the calculator. The updates, with links, are summarized below.
  • Constructed ponds with an iron bench
  • Intensive green roofs
  • Updated pollutant removals
  • Bioretention media mix option
  • Media amendments
  • Tree rooting depth
  • Tree canopy interception
  • Viewing phosphorus results
  • Restrictions on infiltration in D soils
  • Accounting for bypass in non-infiltration and undersized BMPs
  • Sand filter design levels and updated phosphorus removal
  • Swale configuration and routing
• Reuse project list: Information on more than 50 reuse projects in Minnesota was added to the Manual. This includes information on source water type, end use, treatment, and more. If you know of reuse projects not included in the table and would like them included, please contact mike.trojan@state.mn.us.

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?

• A pretreatment selection tool
• Tables with information and links to information on design, sizing, pollutant removal, installation, and maintenance
• Sizing guidelines
• Guidance for managing wastes from pretreatment practices
• Case studies

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)

• Stormwater is a major pathway for microplastics to receiving waters. Bioretention is an effective practice for retaining microplastics. ES&T Water, May, 2021.
• Stormwater Receives a ‘D’ in First-Ever ASCE Report Card Appearance - ASCE’s D+ for stormwater infrastructure results from decades of chronic underinvestment. Stormwater Report, March 2021.
• Nevada becomes the first state to formally ban “non-essential” grass. Digital Journal
• Stormwater newsletters
• On-going webinar series
• Stormwater research