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Low Salt Design principle: horizontal drainage considerations

This section of the manual is currently under construction.

Snow falls across an entire landscape filled with roads, buildings, parking lots, sidewalks, lawns, fields, gardens, sport fields, and it all melts. The meltwater must go somewhere, and saltable surfaces should be protected from meltwater intrusion. Wet pavement turns to icy pavement. By restricting the entry of snowmelt onto saltable surfaces, a safer winter surface emerges. Unless you are in a very windy area, this is likely to be the single most important concept in Low Salt Design.

The problem occurs when temperatures are above 32°F long enough that snow located adjacent to saltable surfaces starts melting. Snowmelt moves downhill, often spreading across the saltable surface. Wet pavements freeze as temperatures drop, becoming ice covered and unsafe, and are often salted. 

Horizontal drainage is the near lateral movement of meltwater. Vertical drainage, described in the next section has a more extreme elevation change (i.e., roof to sidewalk).

Figure with description of Low Salt Design strategy, Horizontal Drainage

Snow storage that obstructs storm drains can inhibit the flow of meltwater into drains resulting in icy, heavily salted areas when refreeze occurs. 

Applicability

Stormwater runoff from rain is typically captured or controlled at a low spot in the landscape. Management of snowmelt requires additional considerations. To avoid repeated salting during freeze/thaw cycles, meltwater must be intercepted before it starts crossing pavement, which may require control at higher elevations. 

Horizontal drainage: stop meltwater sprawl as close to the source as possible. Graphic courtesy of Bolton & Menk.

Contours of the land should be exaggerated (swale or ditch deeper and/or wider) in order to control the movement of meltwater. The lower layers of snow may fill in and block a swale and the snowmelt will not follow its contours.

A swale adequate for rain may not be adequate to accommodate winter snow with meltwater management. Graphic courtesy of Bolton & Menk

To reduce meltwater sprawl in critical areas:

  • Avoid storing snow upgradient of saltable surfaces
  • Consider meltwater drainage patterns when locating storm drains.
  • Vegetation selection and placement
  • Vegetation management  

Benefits and limitations

Meltwater sprawl onto saltable surfaces can threaten public safety, drive up salt use, and shorten the lifespan of infrastructure. To get the best return on investment, restrict meltwater sprawl in critical areas. Potential benefits of eliminating meltwater sprawl include:

  • Improved safety for pedestrians
  • Cost savings - both maintenance crew staff time and deicing chemicals
  • Maintain property value by limiting infrastructure and vegetation damage associated with salt application
  • Maintain surface and ground water quality by limiting chloride loads in runoff/meltwater
Snowmelt and sprawl problem that can be controlled in design. Image courtesy of Bolton & Menk.

Design criteria and considerations

  • Understand where the snow will fall
    • Lawn, landscape, natural areas, unplowed areas, saltable surfaces
  • Understand where the snow will be pushed to
    • Along the perimeter of saltable surfaces
    • Roads, parking lots, sidewalks, trails, ramps
  • Design the snowmelt path so it avoids reentry onto saltable surfaces
  • Do not locate storm drains or curb cuts in ADA parking spots
  • Avoid routing meltwater across ADA zone (i.e., crosswalk)

Retrofit suitability

High: Snow storage areas can often be adjusted to limit interference with meltwater drainage.

Permits and regulations

Currently there are no requirements to design for winter performance beyond that of summer performance. It is logical that controlling/reducing/eliminating meltwater sprawl will be a future performance standard especially in critical pavement friction areas. 

Potential conflicts

Winter design considerations versus summer design considerations can create conflict. Both must be considered to move ahead with the most optimal four-season design.

Winter design: Key objective is to reduce meltwater sprawl.  

  • Once the surface recovers from a snow event (plowed/shoveled), do not let the meltwater sprawl across impervious surfaces, thus reducing the need to apply salt.
  • Snow on the perimeter of a paved surface (parking lot, sidewalk, etc.) encounters above freezing temperatures long enough so that the snow starts melting.  
  • Snow meltwater drains downhill and often cascades across impervious surfaces.
  • These wet pavements freeze as temperatures drop, become ice covered, unsafe and often get salted.   
  • A key strategy in winter design and safety is to keep the pavement dry.

Summer design: Key objective is to meet requirements for stormwater management (volume and rate control). 

  • Spreading runoff over the pavement is a strategy that distributes the volume across a greater surface area, reducing infrastructure installation costs (i.e., additional storm drains, curb and gutter, etc.).
  • There is little concern about wet pavement as long as water does not pool since stormwater will drain to the nearest curb/catch basin, ditch, or turf/planting area.
  • Temperatures are not cold, so there is no concern about ice. 

Summer/Winter design conflict: 

Reducing meltwater sprawl may require including practices in upgradient portions of paved surfaces that are not essential for summer performance.

Research needed

Research needs for the Horizontal Drainage strategy include:

  • Study and document the average meltwater sprawl of current designs. This will inform design goals for the future to reduce meltwater sprawl and deicer use.
  • Split research between critical area meltwater sprawl and standard area meltwater sprawl.
  • Tool to predict and measure meltwater sprawl of designs.