It’s been a long time since we discussed just what a snow fence is and how they should be installed. Even though I don’t think they are really going to be needed this year, I figured with winter once again knocking on our doors, it’s time to review how snow fences work and the best places to install them. The information I used for this article came from a number of different sources and I have tried to summarize the information as best I could. Since most snow depths in snow fence studies are still measured and referred to in feet, I’ll use this unit of measure in my examples.
For most of us, the No. 1 reason for putting up a snow fence is to prevent snowdrifts from forming where we don’t want them to – usually roads and driveways. The second major reason for building a snow fence is to try and capture snow in a particular area – to either utilize the snowmelt in the spring or to provide ground cover and insulation in the winter.
In either case, the biggest mistake made when putting up a snow fence is not figuring out the capacity of the fence, or determining how much blowing snow, on average, the fence needs to capture. To estimate the quantity of blowing snow you could expect in a typical winter in agricultural Manitoba, you first need to figure out the fetch, or distance where wind can pick up snow and then deposit it. For most regions in agricultural Manitoba, the prevailing wind direction during blowing snow events is either north or south, so you need to determine how much open space there is in those directions. If you have huge wide-open spaces around you, then you don’t need to worry about distances longer than about four kilometres.
Snow amounts in agricultural Manitoba vary greatly from year to year, but during a typical winter most regions will expect around 100 centimetres of snow. On average, about 70 per cent of this snow will be relocated (the snow will be moved by the wind from where it fell, to a new location), but this does depend greatly on how much natural trapping capacity there is (tall grasses, ditches, et cetera) prior to the snow reaching your snow fence.
If we use an example of a wide-open cultivated field with a fetch length of around 500 metres, you could expect around 25 tonnes of snow to be transported per metre over the winter. This amount increases to around 100 tonnes per metre for fetches over 3,000 metres in length.
So what does this mean? It means that a snow fence will need to be sized and placed so that the fence can capture this amount of snow. If a snow fence is too small, once its capacity is full it will no longer be able to prevent drifting downwind. Also, if the fence is placed too close to the area you are trying to protect, the size of the drift may eventually cover the area, making the situation even worse.
The main feature of a snow fence that determines how much snow will be captured is the height of the fence. To capture 25 tonnes per metre you need a fence that’s around four feet tall. To capture 100 tonnes per metre you’ll need a fence around eight feet tall. Typically pre-made snow fences are four to five feet tall, but what most people don’t realize is that a small gap of around six inches should be left at the bottom of the fence and the ground. This gap helps to optimize the catching ability of the fence and on a four-foot fence, this extra six inches in height can increase the capacity of the fence by 30 per cent. For most places in our region, a 4.5-to 5.5-foot snow fence will do the job. If you find that your situation does need a higher fence, you can either put the effort into increasing the height, or put up a double snow fence, which is two snow fences that run parallel to each other, spaced using the guidelines outlined in the next paragraph.
A rule of thumb for the placement of a snow fence is that a fully grown drift can stretch downwind around 35 times the height of the fence. Therefore, a 4.5-foot fence is capable of producing a drift that is around 150 feet in length – but remember, this is the maximum length, and in some years we never get enough snow or blowing snow to develop a full-size drift.
When installing the fence, you should place it perpendicular to the prevailing wind direction and should extend up to 10 times the fence height in either direction of the area that the fence is trying to protect. This will help take into account variation in wind direction. This means that if the area you are trying to protect is 300 feet wide, your fence should extend around 50 feet past this area in each direction, giving you a 400-foot fence.
While nothing can beat your own personal experience when placing your snow fences, understanding how and why snow fences can work will help you tweak your current setup, allowing you to the get most bang for the buck. With the possibility of this winter being a warm, dry El Nińo winter, it may be important to capture as much snow as possible to ensure a good start next year.