This is my most popular snow-related piece, which first ran in 2016. Today’s very fine snowstorm over the whole Northeast means it’s time for a rerun!
I have a fairly long driveway next to an open field, and at least once every winter, snow drifts across it to the point where things get dicey without all-wheel drive.
For two decades, I have talked about setting up a snow fence to keep out the drifts, but I’ve never gotten around to it. Which is a good thing, because I would have done it exactly wrong.
One consolation: Many people do it exactly wrong.
“I see it a lot. People don’t understand how a snow fence works,” said Robert Haehnel, a research mechanical engineer at the Cold Regions Research and Engineering Laboratory in Hanover, an Army Corps of Engineers facility that studies anything that freezes. (Known as CRREL, it gets my vote for Most Interesting Place in N.H. That You Didn’t Know About.)
So what did they do wrong? At my request, Haehnel walked me through years of research on snow fences, much of it done by a man named Ronald Tabler who wrote the field’s bible: Controlling Blowing and Drifting Snow with Snow Fences and Road Design.
The manual is used by most states, including New Hampshire, to guide placement of snow fences. If you want more details you can easily find it online, all 307 pages of it.
So here’s the most surprising thing: Snow fences don’t stop drifts by blocking the snow that’s blowing right along the ground, as I had envisioned. They stop drifts by disrupting the aerodynamics of the wind passing overhead, slowing it down so it can no longer carry all the flakes it had picked up.
What this means is that snow doesn’t pile up behind a fence, it drops out of the sky in front of the fence – that is, on the downwind side. So if you place the fence next to your driveway, as I planned, you’re guiding more snow to fall right down onto the driveway.
It’s like a snowdrift multiplier!
Tabler even developed a rule of thumb for placing fences. Take the height of the fence and multiply it by 35, and that is the distance you need between the fence and the thing you want to protect, to ensure all the snow will fall out of the air before the wind gets to the target.
For a standard 4-foot snow fence, that equals a distance of 140 feet away. Holy Toledo – that’s a long way.
If you’ve ever seen a snow fence sitting forlornly in the middle of a field, now you know why.
But wait, there’s more. Tabler found that snow fences need a certain “porosity” – holes to keep snow from piling up and knocking it over – and should have a gap at the bottom equal to 10 percent of the height (about 5 inches for that standard 4-foot fence).
Why the gap? It directs some of the wind under the fence and scours away snow that might build up in front of the fence. This matters because the height of the fence above snow determines how well it disrupts wind passing overhead – so if snow builds up in front, your fence becomes effectively shorter and works less well.
“Make sure the bottom doesn’t get buried over time. If it starts out a 4-foot fence and you get a foot of snow, now it’s a 3-feet fence. . . . Every time it gets buried, it’s less effective,” Haehnel said.
Some places, notably Japan, build large snow fences with gaps designed to scour snow off roads when there isn’t room to place the fences at the right distance. This works well, Haehnel said, but has the side effect of increasing the speed of the wind to the point that it can be dangerous for vehicles.
Snow fences can also be used to collect rather than deflect snow, Haehnel said. “An example is setting snow fences upwind of a cattle pond, so you deposit all the snow there. When it melts, you’ve got a water supply for your cattle.”
So there you have it: A boring, overlooked aspect of life in wintertime is actually complicated, counterintuitive and interesting.
I’ll take solace in that the next time my car gets stuck in the driveway.
Interesting. I was under the impression that blowing snow was moving along the ground. The size of particle is relative to the wind strength. I believe it is called salation. My experience is the snow collected behind the fence is not snow flakes but particles that have been migrated along the terrian. I believe moving sno is thought to evaporated if it covers over 2 miles as per studies by R.A. Schmidt.
The snow behind a fence is migrating particles of different size relative to the shear stress of wind. So lite wind erodes small particle that skip along the surface knocking particles of similar mass. So higher winds erode the little particles plus larger particles as per the winds velocity.
So you get this mosaic of different size particles migrating onlong the surface. If a fence collected falling particles the volume of sno would be very small.
The fences in Wyoming are designed relative to fectch (area that blowing migrating sno is upwind of the fence), distance and annual sno fall allow them to design a fence that can catch all the caculated sno that will migrated thru the fence.
The fence cuts down the wind force changing the shear stress and the migrating snow falls out of suspension behind the fence.
And I’ve been accused of overthinking something!!
My wife calls me Rube Goldberg.
Having lived in South Dakota for 10 winters, I can say the folks of North & South Dakota (and Minnesota, Iowa, Wyoming, etc.) know how to do snow fences and “shelter belts”, which use trees to protect their homes and farm buildings.
Here in NH I have no need for either. The wind doesn’t howl every day like it does in those states.
We in New England like to brag/moan about our bad weather, but the Upper Plains has us beat in summer, winter, spring and fall. How people survive there is beyond me!
Very interesting article on snow fencing. While snow is beautiful and wonderful, we’re pretty glad we don’t get much snow in Brazos County, Texas. We do have plenty of fences. Cow and horse fences, all kinds of privacy fences, plenty of barb wire fences, but alas, no snow fences. Thanks for the interesting information.
What if you’re lot is small & you had to build snow fencing 8 feet away from your driveway…
this is Mescalero County, NM-we get drifts 20 feet high in the mountains…would a taller fence help? Like a 10+ foot tall fence? Great article-unfortunately can’t build a snow fence 140 feet away as they are protected lands….
That is a problem for small lots. Sure a taller fence would help, but it also increases the amount of snow that can pile up against it and knock it down – meaning it has to be much stronger. I have no idea what could stand up against 20-foot snow drifts!
I have been saying to my husband for years to get a snow fence because of drifting of very high snow! He doesn’t listen so it takes him a lot longer to snow blow! I dread Wisconsin winters! Not looking forward to it !!! I’m jealous of states that don’t have to go thru this!! Interesting reading about snow fences!
In New England we like to say our winters are tough – but they’re nothing compared to upper Midwest winters.
I have a long driveway that runs three feet from the property line parallel to the direction of the wind. I can’t put a snow fence up on the neighbor’s field, though. I’m stuck paying for plowing, I guess.
Yeah – this finding is more relevant to the Midwest’s monstrous farms.
I enjoyed your article on snow fences. I grew up in northern Maine in the 50’s where there are lots of potato fields, lots of snow, and narrow roads. It was usual to see orange snow fences, the kind with wood slats held together with twisted wire. The snow would build up on the road side of the fence.
We also put orange or yellow tennis balls on our car radio antennae, the kind that telescoped, so that when we came to an intersection the ball would be visible over the snow banks to cars approaching perpendicular to the road we were on.
Thanks for the interesting article.
“car radio antennae” … people under 30 have no idea what that means!
Thank you so much for your post. Had to remove existing treeline for safety. Replanting next spring. I only have 6ft from property line to driveway. My long driveway would be buried. You saved me from wasting time & money.
I plan on putting a temporary fence up with the purpose of keeping small dogs in my yard. It will be up just from December to April. I live in SE Wisconsin where we get about 45 inches of snow in an average season. To achieve my purpose, i don’t have much of a choice where to install it. What I am trying to figure out is what affect it will have on drifting. I will try to describe what I plan. It will be installed roughly in line with the usual wind. For WI standards, there is a bit of a hill the wind comes over from the neighbors property to our yard – probably a drop of 15 feet. There is a 3 foot retaining wall parallel to the neighbors yard, perpendicular to the wind. This is will be the stopping point of the fence. At the house side of the fence, it will be attached to the edge of our deck on one side and at the house on the other. We intend to keep a path from the sliding door of the deck to the deck step and make a path so the dogs can access the yard. I am most concerned that the snow will build up in this area due to the fence. From your article I can’t really figure out if this is going to be a problem. I don’t want to leave much of a gap at the base of the fence in which the dogs would escape when there is no snow. Hope i explained in a way that makes sense. Your comments are appreciated.