Engineering Process | Preliminary Design | Blowing and Drifting Snow Control
Snow: Blowing and Drifting Snow Control process
Purpose
Blowing and drifting snow on Minnesota’s roadways affects transportation efficiency and safety. Deploying blowing control measures, such as grading, structural snow fences, and living snow fences, minimizes the negative impacts blowing snow can have on our highways helping Minnesota’s economy and the well-being of its citizens. Between the winters of 2015-2016 through 2019-2020 on average 14 fatalities annually occurred with over 7,000 crashes due to blowing and drifting snow on state highways. Implementing proactive, preventative blowing and drifting snow control measures will:
- Improve public safety
- Reduce salt use helping to protect the environment from chlorides that harms vegetation and water resources
- Reduce Greenhouse Gas Emissions
- Reduce snow removal and road maintenance costs
- Provide economic benefits by reducing travel delays and road closures
Using blowing and drifting snow control measures
Address blowing and drifting snow control needs in project documentation if the project is located on a section of roadway that the district has identified with a snow trap problem. Call your District Maintenance Snow Fence Coordinator for information about existing show trap problems. Contact MnDOT’s Blowing Snow Control Shared Service for assistance determining whether the proposed road project is growing to create a snow trap problem.
Ensure that all mitigation commitments are in the plans, specifications, and estimate (PS&E) package for all Class I, II, and III actions (as appropriate) during final design.
If there are no problems with blowing or drifting snow on a project, use the following statement in the environmental document:
No snow trap problems are identified on this project or will be created as a result of this project.
If the project has issues with blowing and drifting snow, use the following statement in the environmental document:
There are snow trap problems on this project, construction of this project will deploy preventative blowing and drifting snow control measures to solve the problem.
or
There are snow trap problems on this project, upon review of the preventative blowing and drifting snow control strategies it was determined not to pursue them due to lack of community support and/or funding.
See the snowcontroltools.umn.edu for more information on raising the road profile, flattening backslopes & widening ditches, and use of snow fencing.
Raising the road profile in fill sections
The equation He=0.4S + 0.6 (where S is the mean seasonal snowfall in meters) gives the guideline for the minimum height of the edge of pavement above the surrounding terrain. The purpose of raising the grade is to facilitate the movement of blowing snow over the roadway to minimize drift formation on the road. It works best in open areas where the wind has enough force to blow the road clear of snowdrifts. Driver visibility is typically not improved. Raising the grade is not effective against blow ice but is very effective at preventing snow drifts on the road.
Flattening backslopes & widening ditch bottoms
Most of the time MnDOT’s typical eight-foot wide ditch bottom provides enough protection against drifting snow; however, when the unsheltered fetch distance exceeds 1,000 feet to the north and west side of the road, blowing snow can be a problem. Contact MnDOT’s Blowing Snow Control Shared Service for assistance with measuring the unsheltered fetch distance and calculating the tons per lineal foot of blowing snow transport.
The backslope should be as steep as possible, 3:1 is preferred, to create an eddy trip point causing the drift formation to form further from the road increasing the volume of snow stored in the ditch.
4:1 in slopes from the shoulder PI optimizes the efficiency for snow plows to cast snow into the ditch minimizing the formation of plow berms of snow which during snow events; creates visibility concerns, reduces travel lane width, and interferes with site corners.
In general, when blowing snow transport is between 7 to 13 tons per lineal foot an enhanced ditch bottom width of 30 feet is needed with a ditch depth of 4 feet or greater below the road shoulder.
When blowing snow transport exceed 14 tons per lineal foot the ditch bottom width will require further enhancement to 67 feet wide with a ditch depth of 4 feet or greater below the road shoulder.
In the case of sections with a cut on both sides of the road, the cross-section geometry on the downwind side also affects snowdrift formation on the road, and the same guidelines must be applied on both sides of the road in order to achieve a snowdrift-free section.
Driver visibility is typically not improved with flattening backslopes and widening ditch bottoms. Flattening backslopes and widening ditch bottoms are typically not effective against blow ice but are very effective at preventing snow drifts on the road.
There may be sections where it is not possible to construct the maximum enhanced-ditch bottom of 67 feet due to environmental and/or cost effectiveness concerns. In those cases, snow fencing needs to be explored to solve the blowing snow problem.
Snow fences
Contact MnDOT’s Blowing Snow Control Shared Service for assistance with snow fence design. MnDOT uses both living and structural snow fencing. Snow fences are the only blowing snow control tool that effectively controls blow ice and prevents snow drift formations on roads. Snow fences improve driver visibility during blizzard conditions.
Use of snow fencing can be a contentious issue because it often involves acquiring additional highway right of way or entering into a rental agreement for snow fence placement and snow storage on private lands. MnDOT’s Blowing Snow Control Shared Service assists MnDOT project managers with landowner engagement to help landowners understand why blowing snow is a problem and their treatment options available to correct the problem.
Snow fence height and setback from the road shoulder are determined by:
- Prevailing winter wind direction and the unsheltered fetch distance along that alignment to determine potential blowing snow transport (tons per lineal foot) based on winter climate data for snow fall, snow water equivalent, and relocation coefficient for any given site in Minnesota
- Fence porosity
- Snow storage capacity in the road ditch
- Prevailing winter wind attack angle on the road alignment
- If the prevailing winter wind hits the road alignment at a 90° attack angle is the worst-case scenario and lead to further snow fence setback from the road shoulder than a 40° attack angle
Agencies involved
Projects involving snow trap removal require coordination. Coordination needs to start with landowner engagement with the snow traps being corrected as a stand-alone project, incorporated into the scope of a larger road construction project, or completed in partnership with the United States Department of Agriculture and County Soil and Water Conservation Districts. The extent of that coordination can vary from simply submitting a signed project document to conducting field reviews, formal correspondence, and reviews of draft reports. The type and timing of coordination depends on the magnitude of the snow trap impact, landowner engagement findings, and the partnering agency interest. The Blowing Snow Control Shared Service is available for assistance in determining the appropriate level of coordination on each project involving snow traps.
United States Department of Agriculture (USDA) Natural Resources Conservation Service
The USDA Natural Resources Conservation Service is statutorily responsible for certifying that each living snow fence enrolled in the federal Conservation Reserve Program is technically correct.
USDA Farm Services Agency (FSA)
The USDA Farm Services Agency is statutorily responsible for administering the federal Conservation Reserve Program.
Soil and water conservation districts
Each soil and water conservation district provide design-build assistance for the private landowners about implementing living snow fence practices.
Permits and approvals
The USDA Farm Services Agency approves the CP17A-Living Snow Fence Practice.
Glossary
Snow trap: A section of roadway that experiences problems associated with blowing and drifting snow
- Within snow trap areas motorists experience reduced visibility and/or icy, slippery road surface conditions resulting from "blow ice”
- In extreme cases, large snowdrift formations make the highway impassable, resulting in road closures
- MnDOT spends extra money on snow traps in the form of heavy equipment, extra labor and salt used to keep the highway open or even reopen it after a blowing snow event
- Historically, snow traps have a higher frequency of lane departures or crashes because of blowing and drifting snow
Blow ice: A slippery road-surface condition where slush/snow is compacted by vehicle tires. It occurs typically on sunny days when an existing snowpack is dislodged by wind speeds greater than 18 mph. The sun warms the pavement, causing the snowy/slush to be compacted by vehicle tires, creating icy driving conditions.
Blowing snow: Blowing snow that reduces visibility causing travel delays and can lead to the formation of slush and ice on the road
Snowdrift: Drifts of snow that can cause loss of vehicle control, reduce sight distance on curves and at intersections, obscure signs, cause ice formation, reduce effective road width and render safety barriers ineffective. Drifts directly contribute to pavement damage by blocking ditches, drains, and culverts and can cause water to infiltrate beneath the pavement.
Structural snow fence: Fences that prevent snow blowing and drifting. Structural snow fencing is best suited for sites that are not conducive for tree and shrub plantings. The following factors may preclude the use of living snow fences:
- Herbicide concerns
- Unavoidable tile lines
- Soil pH above 8.0
- Soil salinity and/or salt spray
- Soils types that are too compacted, wet, dry, and/or rocky to permit normal root development
- Presence of a deer wintering area that raises the threat that a living snow fence will be browsed to the point that it never reaches the required fence height
Living snow fence: Plantings of trees, shrubs, and native grasses located along roads or around communities and farmsteads to prevent snow blowing and drifting. Properly designed and placed, these living barriers trap snow as it blows across fields, piling it up before it reaches a road, waterway, farmstead, or community.
Wind attack angle: The angle between the prevailing winter-wind direction and the road alignment.