Snow Control with Shelterbelts

Introduction

Properly designed shelterbelts can control snow distribution patterns for specific uses. Shelterbelts can provide an additional water source for dugouts and field crops, or prevent snow accumulation on roads and driveways or in farmyards.

Snow Distribution Patterns

The design of shelterbelts will affect snow distribution patterns. By adjusting the design, snowdrift size and placement can be manipulated. Such features as height, density, presence of a bottom gap, upwind vegetation, and fetch distance will all affect snow distribution.

Height: Shelterbelts reduce wind velocity by forcing the wind to go up and over the shelterbelt and reducing the speed of the wind coming through the shelterbelt. By reducing wind speed, the ability of the wind to pick up and transport snow particles is also reduced. The greatest wind speed reduction occurs at a distance of 2.5 times the height of the trees (2.5H) downwind and decreases gradually from there.

Density: Dense shelterbelts give a greater wind reduction within 10H but more open shelterbelts will give protection for a greater distance. Snowdrifts reflect this with drifts behind dense belts usually less than 10H in length.

Bottom gap: A gap at the bottom of the shelterbelt will affect the location of the snow bank. Belts with a naturally occurring gap at the bottom, or that have been pruned, allow more wind to move through the lower part of the canopy which reduces the wind turbulence and causes the snow to be carried further beyond the tree row before being deposited. Figure 2.

Upwind Vegetation and Fetch Distance: The vegetation and the unsheltered distance upwind will affect the amount of snow trapped by the shelterbelt. The distance that the wind has to pick up and transport snow particles before being intercepted by a shelterbelt is called the fetch distance. PFRA research found that a fetch distance greater than 400 m made no difference in the amount of snow trapped by caragana belts, while distances shorter than 400 m reduced the amount of snow trapped, reducing the size of the snowdrift.Figure 3.

Field shelterbelts

Objective: In areas where snowfall provides a significant portion of the total annual precipitation, well-designed field shelterbelts will provide uniform snow distribution across the field and thereby, additional moisture. In dry years, this additional moisture may be important for crop growth.

Location and Design: In areas with high amounts of snowfall, a single tree row with a high porosity will distribute the snow evenly across the field and reduce the formation of deep snow drifts. A gap at the bottom of the belt inceases this affect. A multiple row shelterbelt can also be planted, trapping snow within its rows with added benefits such as providing wildlife habitat, and wood and fruit products. Areas with only moderate amounts of snow may require a dense shelterbelt to trap as much snow as possible.

A series of shelterbelts distributes snow uniformly over the entire field. Shelterbelts should be spaced to provide the best snow distribution. Parallel rows should be perpendicular to prevailing winds and a distance of 20H from each other.

Figure 4:A typical high-efficiency field shleterbelt design

Species and Spacing: In drier areas, shrubs like caragana have proven to be effective in trapping snow and providing extra moisture for crops. In areas with more snowfall, green ash has been recommended at a spacing of 6 ft to give a more uniform snow distribution across the field. Multiple row, multiple species shelterbelts trap snow, provide wildlife habitat, protect soils and provide biodiversity in the shelterbelt. A minimum of three rows are suggested for such shelterbelts.

For more information on field shelterbelts see Planning Field Shelterbelts.

Roadside Shelterbelts

Objective: Properly designed roadside shelterbelts will trap blowing snow and reduce the incidence of blizzard-like conditions, making for safer winter driving.

Figure 5: The effect of snow fences on accident rates during blizzards in Wyoming ( from Tabler and Furnish, 1982).

Design: The shelterbelt should be designed so as to trap blowing snow and deposit it as close to the belt as possible. The belt should therefore be fairly dense with no bottom gap.

The amount of snow that needs to be trapped will affect the number of rows needed. If the fetch distance is short or if only moderate amounts of blowing snow occur, one row of tall, dense evergreens or one row of shrubs is usually enough. But if more snow storage capacity is required, multiple rows of shrubs and/or conifers may be needed. Since two rows planted close together store practically no more snow than one row, the rows should be at least 50 ft apart to trap the most snow.

Figure 6. To trap the most snow the rows should be at least 50 ft apart

Location: Roadside belts should be placed as close to the road as possible yet far enough away so that snow drift edges do not touch the road. They should also be placed far enough away so that the microclimate effect produced by the shelterbelts does not affect road conditions. This is an important consideration since trees planted too close may affect temperatures on the road, resulting in icy patches.

Since the length of the snow bank depends on the height and density, of the shelterbelt the belt should be placed parallel to the road at a distance of 10 times the height of the shelterbelt. In open areas with large fetch distances, this distance may have to be increased.

Species and Spacing: Choose species that do well in local soils, climate and growing conditions. Shrubs available through the Shelterbelt Centre which could be used include caragana, lilac, willow, chokecherry, buffaloberry, etc. Trees include green ash, Manitoba maple and the conifers available are white spruce, Colorado spruce, Scots pine and Siberian larch. Tree and row spacings for these species are given in Tables 1 and 2.

Shelterbelts for dugouts

Objective: Spring runoff is a source of water for dugouts. Shelterbelts around a dugout can trap large amounts of snow. This can be an important source of water in times of drought. Generally, every 100 m of a caragana belt is capable of capturing up to 300,000 L of meltwater.

Design and Location: Multiple rows of trees placed to the north and west of the dugout will trap the most snow. Deciduous trees should be planted at least 50 m from the dugout to prevent leaves and branches from contaminating the dugout. Conifers and small-leaved shrubs can be planted as close as 20 m from the dugout as the small leaves or needles are not carried into the dugout by the wind.

Figure 7. Shelterbelts trap snow and increase the amount of runoff water stored in the dugout.

To avoid erosion during spring runoff, trees should not be planted on land with a slope greater than 10% unless adjacent land is in forage. Grassed waterays or drainage channels can be constructed to control the flow of surface water.

Species and Spacing: Dense, drought tolerant trees and shrubs are recommended for planting near dugouts. Caragana should not be placed where they will experience flooding. Recommended spacings are given in Tables 1 and 2.

For more information on planting trees around dugouts see Shelterbelts for Dugouts.

Farmstead Shelterbelts

Objective: In terms of snow control, the farmstead belt should trap blowing snow to prevent snow buildup in the yard and on driveways.

Design and Location: Since prevailing winds are usually from the north and west, up to 5 rows are recommended for these sides of the yard. The inside row should be no closer than 30 m (100 ft) from the main buildings and driveways to prevent snow buildup problems. If space is limited, the number of tree rows should be reduced rather than decreasing row spacing or planting closer to buildings.

The outside row of the shelterbelt should be planted to shrubs to act as a snow trap and the inside rows should be planted to tall, dense, long-lived trees.

Species and Spacing: The outside row is the most important snow trap in the farmstead belt and should be planted to shrubs like caragana and lilac. Tree and row spacings are listed in Tables 1 and 2.

For more detailed description of tree species, refer to Shelterbelt Species.

Note:

Table 2 Recommended Minimum Spacings Within Rows

Deciduous shrubs	Spacings
	m	ft
Caragana	0.3	1
Villosa lilac	1	3
Choke Cherry	1	3
Buffaloberry	1	3
Sea-buckthorn	1	3
Deciduous Trees	Spacings
	m	ft
Green ash	2.5	8
Manitoba maple	2.5	8
Poplars	2.5	8
Siberian elm	2.5	8
Willows	2.5	8
Coniferous Trees	Spacings
	m	ft.
Colorado spruce	3.5	12
White spruce	3.5	12
Scots pine	3.5	12
Siberian larch	2.0	6

Recommended minimum spacings between rows

5m (16 ft) between adjacent rows
6m (20 ft) between adjacent deciduous and conferous rows
5m (16 ft) between adjacent coniferous rows

For more information on farmstead shelterbelt design see Planning Farmstead Shelterbelts.