Snow storage

Introduction

Storing snow over the summer has become more common the last few years, both for Cross-Country and alpine venues. More research data and information on the topic is now available. Miscellanous research- and other articles from projects in Sweden, Norway and Switzerland, as well as a few Norwegian Master- and Bachelor theses can be found under the main menu selection “Litterature”.

The snow that is being stored should be homogeneous and optimally with a density of about 600 – 700 kg/m³ to not collaps too much through the summer. A grooming machine should be used to achieve the optimal density, shape and size of the stored snow pile; the machine can mix and push/doze the snow into a pile as well as compact it. If the snow is too dry or not dense enough, water should be added. Man-made snow is better than natural snow.

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Melting

Even though some smaller venues both produce and store the snow indoors in large garages or warehouses, most snow storage takes place outdoors in large snow piles from a few thousand m³ up to 100 000 m³. The following factors influence the storing- and melting process: 

• Geography and weather
• The ground surface 
• Snow type, size and shape of the snow pile 
• Type, thickness and age of the cover material

Research and experiences the last 20 years show that the main melting happens (in average) at the snow surface (80%), then due to rain (15%) and to heat from the ground (5%).

Geography and weather

Melting will depend on a few natural factors that are not easy to control or influence:

• Sun (hours per day) 

Both radiation (short waves) and heat (long waves) will increase the rate of melting. A cold and shady area with less sun exposure is therefore preferred.

• Geographic direction 

North and east exposure is preferred, while south and west should be avoided.

• Precipitation and humidity 

High humidity and heavy rain will both increase the rate of melting, so a dry climate is positive.

• Wind 

Since wind will increase the rate of melting, the snow pile should be placed in a sheltered location.

Ground surface

Melting at the bottom of the snow pile will happen due to the ground heat. It is therefore important to ensure a proper surface with good drainage before building the snow pile. Ice will be formed if the drainage is poor or the snow pile too big (up to 1 meter ice per year may be formed, which is equivalent to several hundred meters of Cross-Country track if the snow pile is large). 

• The following photos from Canmore show the ice buildup over several years – from snowpile with sawdust, the floor of the storage site after the snow has been distributed, and the ice layers per year clearly visible (all photos: Dave Rees)
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Studies have been done to learn how different surfaces influence the temperature in the snow pile. At Beitostølen, Norway, the influence of asphalt, gravel, saw dust, wood chips and grass on the temperature in the snow pile was studied (see photos and examples from the 2016/17 study below).

Based on comparing the temperatures, using saw dust as a surface gave the best result.

Asphalt (blue), 5 cm saw dust (red): 

Size and shape of the snow pile

Both natural snow and man-made (artificial) snow can be used for storage, but the qualities of man-made snow are better suited for storage.   

The density of man-made snow (round crystals) are four times higher than natural snow (hexagonal crystals), and makes it more durable. The % or rate of melting is therefore lower for man-made snow.

The shape of the snow pile should be such that the ratio between the volume and surface is as large as possible. It has been shown that this creates the lowest total melting. Theoretically, although a half cylinder shape (or sphere) provides the best ratio, this exact shape is difficult to build with snow. The most practical soultion is therefore some kind of cylindar section (see below).

To minimize the surface area it is important to construct and re-work the snow such that the entire surface is smooth and even (using the grooming machine or a backhoe). 

The larger the snow pile is, the less percentage of the pile will melt since a larger pile will have less of its volume exposed to weather and wind.

For Cross-Country venues it is typical to store between 10 000 – 20 000 m³ snow. This may be a suitable size for both reducing the formation of ice (due to the weight of the snow) and optimizing the ratio between volume and surface. 2000 m³ saw dust would cover a 13000 m³ large snow pile. 

Type, thickness and age of the cover material

Since the first snow storage projects (for skiing) around year 2000, venue operators and research projects have been trying to determine the best cover material.  Both natural and manufactured products have been used; materials that enable evaporation (can breathe) and materials that insulate the snow entirely (do not breath). Most common has been to use natural wood based materials that enable evaporation, for example saw dust or wood chips. It is important that the material is breathable and allow evaporation, since the evaporation process uses energy/heat and will therefore cool down the material below.  The cooling effect from the evaporation will have a big influence on the rate of melting.

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As of today there are no manufactured materials (with reasonable costs) that are as effective as old fashioned saw dust, even though several types of “breathable” geotextile fabrics have been tested and compared.  The reason geotextile fabrics are anyhow used are due to their practical advantages. 

In addition to reflecting radiation from the sun, saw dust is able to absorb water, which will cause a cooling effect when evaporated later.  The thickness of the saw dust layer must be approximatelly 40 – 50 cm such that the result of the water absorption, evaporation and cooling process is optimal. 

The thickness of the sawdust influences the rate of melting in Davos (Grünewald et al., 2017)

In Norway it has been observed that new and light colored saw dust works well with approximatelly 20% snow loss. However, after about 3 years of use the sawdust is dirty and will have reduced reflection as well as reduced evaporation, and will then give 30% or more snow loss.

The geotextile fabric will last for at least 3 years, but should then be replaced due to damage from water, wind, dirt and other pollutants.

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Comparing sawdust and geotextile

Detailed measurements from several sites in Scandinavia show that the snow pile’s total and per day change of volume is higher for geotextile fabric than for saw dust. This is mainly due to the lesser insulation and water absorption capability of the geotextile fabric.

The % change of volume is affected by the size of the snow pile, but it is observed an average of 20 – 25% change for saw dust and 30% or more for geotextile fabric.  The % change per day is measured to ca. 0,15 % for saw dust and ca. 0,35 % for geotextile. Using two or more layers of geotextile fabric would reduce the melting and % change of volume by 2 – 4 %.

In Canmore, Canada, at 1400 meter elevation, snow has been stored every summer since 2009 using saw dust. Their average loss has been between 10 – 13%. In 2019 the snow pile was 14 000 m³.

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_{Pictures of the uncovering process shown above (Photos: Mike Norton).}

Although the rate of melting for saw dust is quite a bit less than for geotextile fabric, the work related to covering and uncovering the snow pile is much more labor intensive than for geotextile fabic.    

Saw dust must be covered in a smooth and even layer, and this demands use of a grooming machine and a bulldozer or excavator (see photos above from Canmore).  The saw dust must also be transported to and from the site, uncovered and stored over the winter (if re-used).  

Covering the snow pile with a geotextile fabric is a less demanding process

The geotextile fabric normally comes in rolls of 5 meter width and 50 meter length. These are rolled across and over the snow pile (a special attachment for a grooming machine or tractor can be used). Velcro fastens the rolls/units together, and weights are added on the perimeter edges in case of wind. 

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Geotextile fabric will last 3 – 5 years, but should then be replaced due to damage from water, wind, dirt and other pollutants (the cloth can then be used for new trail surface).

New saw dust works better than old saw dust, and should optimally be dried or partially exchanged every 1 – 2 years. Re-used saw dust will loose much of its capacity after 3 year.

Comparison	Saw dust	Geotextile
Investment/purchase	ca. 20 EUR per m³	ca. 2.30 EUR per sqm
Duration	1 – 3 years	3 – 5 years
Rate of melting	15 – 25%	25 – 35 %
Labour intensive	High	Medium
Sensitive to air temperature and humidity	Low	Medium
Enviromentally friendly	Medium	Good

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Cost

The total cost for storing snow can be divided into the following parts: 

• Snow production 
• Purchasing material and covering with saw dust or geotextile 
• Un-covering the saw dust or geotextile and distributing the snow

Based on studies comparing saw dust (40 cm thick) and geotextile fabric (2 layers), the total expenses are quite equal. It is a bit cheaper to use saw dust for smaller volumes (less that 10 000 m³) while it is cheaper to use geotextile fabric for larger volumes.

The cost of snow production is a bit higher for geotextile fabric since more snow is needed to cover a fixed demand (more melting will occur, so the snow pile must be larger to start with).   Purchasing the material and covering/uncovering the snow pile is more expensive with saw dust (especially if the cover material is replaced every year).

Distributing the snow is the largest cost item (about 50% of the total cost).  For alpine venue this cost is reduced since the snow storage is normally done directly on the slope, and can be pushed out by the grooming machines.

Expenses	Snow production	Cover (incl. purchase of material, labor, depreciation)	Uncover material and distribution of snow
Saw dust	Less	Slightly more	Slightly more
Geotextile	More	Slightly less	Slightly less

A venue (in Norway) should expect that it will in total cost up to 90 000 EURO to produce snow, purchase cover material, cover and uncover the snow pile and distribute snow for a 5 km (6 meter wide and 50 cm deep) Cross-Country course (15 000 m³ snow).  Since the largest expense is the distribution of the snow onto the courses, inexpensive equipment rental adn machine operators can save cost. 

The distribution of expenses are approximately:
Snow production: 15 – 20%
Covering snow pile: 25 – 30%
Uncovering and distribution: 50 – 55%

Transportation of snow

Transportation of snow from a nearby snow storage or a remote production location (for example at a higher elevation in the mountains, or at a “cold” valley) is a costly and energy demanding task. Today this normally takes place by trucks and tractors with trailers. Sometimes it is possible to use grooming machines to push most of the snow out (especially in alpine venues).

A truck or a trailer usually can take about 10 – 15 m³ snow. This means it may take up to 500 trips to transport 5000 m³ snow (which will cover about 2 km of a 5 meter wide Cross-Country course).

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There are ongoing studies and practical trials to see if it is possible to distribute snow from a central location by blowing the snow through plastic pipes. This is however most practical when the snow has low density and is light.

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Examples of snow storage locations 2018

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