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Minnesota Stormwater Manual

Minnesota Stormwater Manual test page 5
Revision as of 14:08, 3 October 2019 by Mtrojan (talk | contribs)

Warning: This page is an edit and testing page use by the wiki authors. It is not a content page for the Manual. Information on this page may not be accurate and should not be used as guidance in managing stormwater.


Initial research was conducted to determine whether an approach for sizing pretreatment practices was consistently applied across the country. Stormwater programs for every state were reviewed to determine the approaches being used, as well as a consensus in the approach. The results of this review indicated that a consistent approach to stormwater pretreatment across the country was not used, and some states do not require pretreatment of stormwater runoff at all. Because of the lack of consensus and research-backed sizing criteria for pretreatment, stormwater practitioners in Minnesota determined that guidelines were needed to define specific sizing criteria to provide consistent, effective results.

Given the lack of a consistent approach in stormwater pretreatment sizing, analysis was conducted to determine the general guidelines for pretreatment sizing in Minnesota based on the settling velocity of particles in stormwater. Many sizes of suspended solids enter a treatment practice–from clays below 2 μm (2 micrometers or 2000 mm) to silts between 2 μm and 80 μm to sands between 80 μm and 4 mm and beyond. A four order of magnitude difference between the smallest clay particles and largest sand sizes particles entering the practice is noted with a greater order of magnitude difference in settling velocity. A pretreatment practice is designed to settle the largest of these (primarily inorganic) particles in a location where they will be relatively simple to clean out and dispose of. As a result of this pretreatment, required maintenance will be reduced on the primary treatment practice (also called a structural stormwater BMP) because a large portion of the suspended solids will settle in the pretreatment practice, which will prolong the life span and maintain the design pollutant removal efficiency of the primary treatment.

Quick Guide for pretreatment sizing of basins
  • Determine, vS, the settling velocity for the particle size targeted (recommend 80 microns for particle size. Determine vs from table on this page)
  • Determine FR, the target percent removal (recommend 50-70%)
  • Determine A, the area of directly connected impervious draining to the pretreatment practice
  • Determine I, the peak rain intensity (0.5 in/hr for a 1-inch event, Type 2 distribution)
  • Calculate the area of the pretreatment basin from AS = (FR * I * A * FR) / (vS * (1 - FR))
  • Determine the basin depth as the lesser of sqrt(AS) or 6 feet

Initial research was conducted to determine whether an approach for sizing pretreatment practices was consistently applied across the country. Stormwater programs for every state were reviewed to determine the approaches being used, as well as a consensus in the approach. The results of this review indicated that a consistent approach to stormwater pretreatment across the country was not used, and some states do not require pretreatment of stormwater runoff at all. Because of the lack of consensus and research-backed sizing criteria for pretreatment, stormwater practitioners in Minnesota determined that guidelines were needed to define specific sizing criteria to provide consistent, effective results.

Given the lack of a consistent approach in stormwater pretreatment sizing, analysis was conducted to determine the general guidelines for pretreatment sizing in Minnesota based on the settling velocity of particles in stormwater. Many sizes of suspended solids enter a treatment practice–from clays below 2 μm (2 micrometers or 2000 mm) to silts between 2 μm and 80 μm to sands between 80 μm and 4 mm and beyond. A four order of magnitude difference between the smallest clay particles and largest sand sizes particles entering the practice is noted with a greater order of magnitude difference in settling velocity. A pretreatment practice is designed to settle the largest of these (primarily inorganic) particles in a location where they will be relatively simple to clean out and dispose of. As a result of this pretreatment, required maintenance will be reduced on the primary treatment practice (also called a structural stormwater BMP) because a large portion of the suspended solids will settle in the pretreatment practice, which will prolong the life span and maintain the design pollutant removal efficiency of the primary treatment.







Stormwater and soil, engineered (bioretention) media, and media amendments

image bioretention
Engineered media in a bioretention practice. Image from MPCA's Flickr website.
compost image
Compost is an important component of most engineered media mixes. It is also commonly used as an amendment to improve soil properties, such as infiltration rate, fertility, and structure. Image from MPCA's Flickr website.
Information: Engineered media is manufactured from soil (sand, silt, clay) and other components (e.g. compost, iron, etc.), in specific proportions, for a specific application (e.g. green roof, bioretention, tree box). Because engineered media are widely used in bioretention practices, the term "bioretention media" is widely used. We prefer the term "engineered media" as it more accurately describes the applicability of these media.

Soil and engineered media, often referred to as bioretention media, are fundamental design characteristics of most post-construction stormwater practices. In some applications, soil or media amendments are utilized to improve soil conditions or enhance treatment effectiveness of a BMP.

This page provides links to pages that address topics related to soil, engineered media, and soil/media amendments.


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