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November-December 2012

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Don't Fence Me In: Snow Fences Tame Weather by Redirecting Winter's Winds

It remains one of the legendary stories about driving and weather. The 77-mile stretch of Interstate 80 west of Laramie in Southeast Wyoming had been open for only a few weeks in late 1970 when snow conditions forced its closure; in a dozen places, the highway had practically disappeared beneath mountain-high drifts of windblown snow. Newscasters quickly dubbed the mess “The Snow Chi Min Trail” (this was the Vietnam era). Fighting 100 mph crosswinds, even rotary plows couldn't keep snow off the road. But what makes this weather yarn unforgettable is the way the problem was solved; in effect, highway engineers tricked the blowing snow into plowing itself.

Snow fences, those rambling, slat-wood barriers erected along many of our nation's highways, saved I-80, and continue to perform their seemingly impossible feat every winter. Icons of the American landscape, these “fences to nowhere” channel wind and snow in remarkable ways to clear the road ahead. Think of them as wind-driven snow plows to manage wind-driven snow.

“Yes, the I-80 story is true,” confirms professional engineer Cliff Spoonemore, of the Wyoming Department of Transportation's Winter Research Services. “DOT had promised to keep the road always open, yet the very first snowfalls closed us for months.” This was surprising to officials, who noted that nearby US Route 30 (the road I-80 replaced) usually remained snow-free throughout the winter. What they ultimately realized was that Route 30 ran beside a Union Pacific Railroad right-of-way, which was lined with snow fences to protect the tracks—a long-time practice in railroading. That began the Wyoming Study, which was a pioneering effort to develop snow fences for highway protection.

“For highway safety, snow fences perform two key functions,” Spoonemore observes. “The first is stopping blowing snow from drifting farther, and piling it beside the highway instead of letting the wind pile it on the road. The second is screening blowing snow out of the air to improve driver visibility.” The effects can be dramatic. “Before” photos show a snow-clogged highway. After installing a snow fence, the same stretch appears clear: snow peters out toward the road, and even some of the grass shoulder is exposed. Without a snow fence, drivers face whiteout conditions as ground blizzard winds lift and propel loose snow into the air. After a snow fence is installed, the blizzard has somehow been turned off and the road ahead is visible. “Of course, snow fences can do little against direct falling snow,” Spoonemore muses. “You'll still need to plow that.”

It's all about managing where the snow blows. Yet snow fences don't work by blocking the wind. They're porous; wind and airborne snow both pass through and over them. “The fences act as windbreaks,” explains Spoonemore. “They introduce eddies, slowing the wind, and changing it from laminar to turbulent flow.”

That's the secret: While fast, smooth wind can carry lots of snow, disarrayed wind begins dropping the snow it has gathered—it's kind of like tripping a thief on his getaway. “That's why most of the snow piles on the downwind side of the structure,” Spoonemore notes, “with a much smaller drift upwind.”

The snow begins to accumulate far downstream, as this is where wind is slowed most. The drift rises, curves, and spreads as the airflow shapes it. In time-lapse photos, the result looks like a snow-sculpting contest. Surprisingly, the completed drift can stand considerably higher than the fence that formed it.

The snow mound does reach a critical mass. Once it is “full,” owing to aerodynamics, the fence cannot pile the snow higher. Further airborne snow will pass over undisturbed. “It can take months of wind and numerous snowfalls to reach this point,” Spoonemore notes. “We sometimes call these finished dunes wing drifts,” he adds, “for their curving, airfoil shape.” This seems fitting, as the vocabulary used to describe snow fence vocabulary (air resistance, angle of attack, vortices) sounds more appropriate for designing a fighter jet than for building a wall out of boards.

“We owe all this to the late Dr. Ron Tabler,” recalls Spoonemore. “He was the founder and expert in this field, and a mentor to many of us.” For over 40 years, Tabler studied every aspect of how wind transports snow. His diagrams, formulas, and statistics remain the snow fencing “bible” (see the Sidebar).

Yet surely, most motorists whizz past a stretch of fence-protected highway unaware of how wind and weather have been redirected to clear the road ahead. The finished drifts look uncannily as if they've been plowed'they're so neatly formed, and even rounded gracefully around their edges because of the aerodynamics that formed them.

And amazingly, this is all done by fences that look like something Tom Sawyer might have cobbled together on a lazy, hot summer day: the fences are no more than 1 × 6 slats nailed to 2 × 6 trussing. Yet each feature conceals exacting study. For example, there is an optimal ratio between spaces and boards: “Fifty percent porosity will yield the best results,” Spoonemore notes. Rough-sawn slats are better at catching snow. Even the gap between fence and floor is not rude construction. According to Spoonemore, “This space channels the wind, scouring the ground beneath the fence. An unburied fence works more efficiently, and keeps working.” (Backyard fences sometimes hollow out the snow beneath them by this same rule of physics.) Spoonemore says, “Some of these results come from lab work, including tests in wind tunnels. “But much of it comes from field testing, from Dr. Tabler going out to measure the results of hundreds of drifts after storms.”

Locating a stretch of fence also depends on understanding weather on the front line. Spoonmore reports: “We talk to our Maintenance Foremen and plow drivers. These ‘heroes of the road’ are out in the field, fighting the snow every day with brute force. Without them, traffic would come to a halt. They find the zones and cuts that tend to fill in all the time. We isolate and study those locations. What's the prevailing wind? What's causing the drifts?” Roadside features, such as medians and bridge abutments, often create their own “micro-weather,” which can move and deposit snow in unexpected ways. The effects of terrain far from the road may need to be considered. Field evidence (for instance, wind-sculpted trees) or even aerial photos taken after a major snow event can help explain where and how the snow travels.

Snow fences harvest and then store the snow, almost the way a farmer lays in wheat or hay. This begins with anticipating annual snowfall. The snow transport (the tons of snow the wind will carry) comes from measuring the fetch, or the area of open field near the highway that the wind can scour. “We put all this into formulas derived by Dr. Tabler,” Spoonemore explains, adding, “This tells us the height of fences we'll need.

Fence height is vital; once selected, it serves as a ruler to predict every contour of the finished drift. For example, the drift's maximum depth will occur downwind of the fence, at a distance six times (6H) the fence's height. Height matters: A six-foot fence will trap twice the snow as a four-foot one. Fences might rise to 14 feet.

“Typically, we'll try to set fences perpendicular to the prevailing wind,” Spoonemore advises, “but we can take up to a 45-degree angle.” So when driving, the fences may pace you (like livestock fencing) or run past in a herringboned pattern. A single tall fence is best, but on some terrains, designers may set a series of fences; the snow, in effect, is made to jump hurdles. Most surprising is how far the fence must stand from the road it shelters. “At least ‘35H’ back from the shoulder,” Spoonmore notes. “That's near 300 feet for an eight-foot fence. Any closer might land part of the resulting drift, which is very long, across the road, making the problem worse instead of better.”

Fence sections usually run 16 feet (so standard-length slats can used straight from the lumberyard). They're assembled on-site using jigs. Instead of posts, classic “Wyoming” snow fencing rides on wooden skids, built and tipped into place (think barn raising), then anchored to the soil by five-foot rebars driven through metal cleats. This is done all well in advance of the season; crews raise these winter fences under hot summer sun. The fences lean slightly, and are braced by wooden crossbars to resist winds when they come. Snow fences are eminently practical things; in winter, their horizontal slats may double as ladders in order to help maintenance workers repair them.

Years of study and fieldwork confirm the benefits snow fencing can provide. Over the course of 30 years, fences along the original trial stretch of I-80 have reduced accidents caused by snow-compromised visibility by 70 percent, and reduced road closures by an average of 7.5 days per year. Snow fences prevent road signage from becoming buried, slow the formation of road slush and ice, and even reduce crosswinds, helping commercial drivers of trucks and semis. In many locales, windblown snow can total many times accumulation; this can make snow fencing a cost-effective option to replowing the road.

From the original 11 miles of I-80 that were test-protected in the early 1970s, today more than 500 miles of Wyoming's highways are protected by snow fences. “Wyoming fence,” now the name of a style instead of a place, can be spotted protecting snow-challenged highways throughout the United States and deep into Canada. Properly constructed, these will give 20-plus years of reliable service—unless the cattle get them. “Wind behind the fence is slower,” Spoonemore observes, “so livestock may take refuge there; they'll scrape against the frame and damage it. Or the wind may claim them. … The wind is constantly lifting these fences,” he notes. “Eventually, the rebar may saw right through, or cut the metal cleats loose that are holding the fence down.”

Austere against the winter landscape, snow fences have captured the attention of scenic photographers, presenting to their artistic eye as iconic an image of the American highway as Burma Shave signs or Route 66 markers. More recently, new building technologies have entered the field, including snow fences made from perforated plastic sheet. (These are available by the roll; look for it in your local home improvement center.) Nailed to posts or stretched across wooden frames, these new fences continue the tradition of simple-but-subtle design; the diameter and arrangement of holes, and even the elasticity of the plastic, have all been calculated to best screen snow from the wind. There is even a growing industry around reclaiming the wood from Wyoming fences worn past their use. Repurposed into beautifully weathered furniture and flooring, these boards preserve a patina made by snow-laden winds, tamed as the cars drove by them.

“Snow Man”

“When the ground is completely snow-covered, a motorist's visibility … is inversely proportional to the fifth power of wind speed,” according to the National Transportation Board's “Controlling Blowing and Drifting Snow,” by Dr. Ronald D. Tabler.

Among the many studies on blowing snow issued by the nation's Department of Transportation and State highway agencies, it's rare to find a document that is not written by (or quoting) this same expert. Drawing on experience as a research hydrologist with the United States Forest Service, Tabler (1937-2010) became both the subject's historian (studying stone and lumber snow fences of Victorian days) and its agent of change, refining the traditional railroad fences with dozens of improvements to best suit highway needs.

Through his insights, we can watch windblown snow dunes accumulating, literally one flake at a time. The largest particles, we are told, creep across the snow and crowd into undulating waves. Smaller particles saltate, or bounce over the snowface and release additional snow (like so many colliding fleas) on each landing. Only the very smallest particles, pared down through collision and evaporation, become free-flying (blowing) precipitation. Once the dunes form, they seem almost alive, with wind along their slip faces (an abrupt drop-off along their backs) forming circulation zones of rotating winds.

Replete with tables and complex formulas, Tabler's studies formalize drifting snow into a mathematical subject. Computer programs that he helped to develop are now automating the process. Yet his advice often veers to the admirably practical. His favorite mantra? “Plowing drifts costs a hundred times more than storing snow in fences.”

NICK D'ALTO is a mechanical engineer who writes about technology, society, and adventure.

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