WHAT'S NEW IN FENCING FOR SNOW, WIND AND SAND
Dr. Ron Tabler
Tabler Associates
Introduction
Technological advances in rail transportation have increased the need to control environmental factors affecting
operations. Successful operation of remotely controlled switches, for example, requires that switch points be
free of snow, ice and sand. Thermal scanners used for hot-box detection are also susceptible to dysfunction
when wind-deposited material blocks the optical path and dragging-equipment detectors are disabled by snow
and ice accumulations. The advent of intermodal freight operations has brought an incentive for wind control,
because high-profile double-stacked container and trailer loads have proven vulnerable to wind-caused
derailments.
Although these examples make clear the need for controlling snow, sand and wind, the construction of new
fences on railways has generally declined since the 1950's. Maintenance on existing fences has often been
deferred, and fencing design and guidelines have largely remained unchanged since snow fences were first used
on the Union Pacific Railroad in the 1870's. Although efforts are underway to correct the deficiency, the snow
fence guidelines in the American Railway Engineering Association (AREA) manual are woefully outdated and
superficial. Although this oversight is shared by many state and local road departments, railroads have the most
to gain from improved fencing technology. It seems apparent that expenditures for fence systems could easily
be amortized by eliminating delays caused by inoperable switches and thermal scanners. The prevention of a
single derailment can pay for the most expensive fence system. Finally, as C. B. May of the Burlington
Northern Railroad pointed out back in 1988, there is great potential for using wind fences to reduce fuel costs.
The last 20 years have seen major improvements in fencing for environmental control. The purpose of this
presentation is to bring these improvements to the attention of the rail road industry.
Improvements in Snow Fences
Modern snow fence systems have proven effective in preventing drifts, improving visibility, and reducing road
ice on highways, where benefit/cost ratios as high as 80:1 are typical. As a result, state highway departments in
Wyoming, Arizona, and Montana, and oil companies operating at Prudhoe Bay in Alaska, have invested many
millions of dollars in extensive snow fence systems over the last 15 years. Snow fences have also been used
successfully to protect roads, villages, and airstrips in the Alaskan arctic.
These past applications have shown that properly engineered fence systems can reduce blowing snow by as
much as 95 percent, suggesting that fences can be used by railroads to solve the challenging problems of snow
accumulation in switch points and at thermal scanners, as well as to prevent drift encroachment on tracks and to
improve winter-time trackside access. The few applications of new snow fence technology on railroads consist
of five fence systems comprising over 28,000 feet of 9- to 14-foot tall fencing, constructed in Wyoming by the
Union Pacific Railroad to protect thermal scanners and switches. The comparison of snow conditions at one of
these locations before and after installing a new snow fence system in 1980, shown in Figure 1, provides a
striking example of the effectiveness of properly designed snow fence systems.
The most important breakthrough in snow fences has been the development of a reliable method for estimating
how much blowing snow arrives at a problem location over a typical winter. Because the snow storage capacity
of snow fences in relation to height and other characteristics is also known, it is now possible to size a snow
fence in much the same way as a culvert is sized to accommodate storm runoff. Extensive field measurements
over the last 20 years have proven that the snow storage capacity of fences is proportional to fence height raised
to the 2.2 power. Because construction costs for snow fences are linearly related to fence height, it is clearly
more economical to build a single tall fence rather than a series of shorter fences having the same storage
capacity. Tall fences are also more effective than shorter ones because the snow trapping efficiency increases
with fence height. The 14-foot tall fence shown in Figure 2 can store more than 60 tons of snow per lineal foot
of fence, which is sufficient capacity for total protection at all locations on U.S. railways.
Other improvements include effective guidelines for placement, improved fence designs maximize snow
trapping efficiency and minimize construction and maintenance costs; and new fencing materials reduce
maintenance requirements.
New fence designs make snow fences affordable. Total costs for materials and installation for the 14-foot fence
shown in Figure 2, for example, have averaged under $13.00 per lineal foot for the last five years, and bid prices
of $10.00 per foot have not been unusual for larger projects. These fences are anchored to the surface using
driver rebar anchors and specialty designed steel U-clips. Where pole-supported fences are preferable on
agricultural lands because of their narrower footprint, plastic fencing materials allow vertical supports to be
spaced up to 15 feet apart, and have a useful life of 15 years or more when properly installed (Figure 3). A
polyethylene strap with three, 12.5 gauge embedded steel wires, provides an especially durable material that can
also be used for sand or wind fences (Figure 4). This product, which was initially developed for horse and
livestock fencing, is manufactured by Centaur HTP Fencing Systems in Muscle Shoals, Alabama.
Work is underway to update the AREA Manual's fencing section to incorporate some of the new guidelines for
snow fences. In the interim, specific guidelines and procedures are described in the "Snow Fence Guide"
published by the Strategic Highway Research Program (SHRP), as listed in the selected references at the end of
this paper. A more comprehensive treatment, "Drifting Snow Design Manual," is being prepared by the author
and will be available from SHRP in 1993.
Improvements in Sand Fences
Although the potential for effective sand control is equally as great as that for snow control, there have been no
applications of modem fencing systems to serve as examples. There are examples, however, of the effectiveness
of new guidelines, even when used in conjunction with traditional fencing materials. Figure 5 shows how
properly spaced fences have stabilized a highly erodible embankment of mill tailings. A system of over 50,000
feet of four feet of fencing at this project site provided complete stabilization of over 100 acres.
Historically, dust levees have been used for protecting railways on the Great Plains, and plantings of tamarisk
trees, also known as salt cedar or Athel, have proven effective along railways in southern California and other
locations in the southwest. Past experience showed conventional four or six foot tall fences became buried, and
hence ineffective, after only a year or two. Where earthen embankments are practical and where they can be
stabilized, they can provide effective protection. But with the new guidelines available, fences offer a cost
effective alternative suitable for all locations. Unlike trees, structural fences provide uniform, predictable
protection, require no watering, and are unaffected by insects, disease, and freeze damage as occurred to trees in
the Mojave Desert over the 1990-91 winter.
The wind transport of sand and snow are sufficiently similar that many of the guidelines applicable for snow
fences apply to sand fences as well. But because sand deposits do not melt each year, the storage capacity of the
containment systems must be sized on the basis of long-term sand transport. As with snow fences, the key to
successful control is to be able to predict the quantity of blowing sand at a prospective site, and then size the
containment fences accordingly. Methods are available for estimating the mean annual transport of blowing
sand using historical wind data or geological information, and the sand storage capacity of fences is known.
As with snow fences, particulate trapping efficiency and economy increase with fence height. Sand storage
capacity is proportional to the square of fence height. A 14-foot fence, for example, would hold 12 times as
much sand as a four foot fence, and would have a useful life of at least seven years in the highest sand transport
areas in the Mojave Desert. A sand control fence system could be designed to have a much longer effective life
by planning for staged increases in fence height and by adding rows of fencing upwind.
Other improvements in sand control fences include improved placement criteria maximize effectiveness and
allow for future additions of fences to augment storage capacity; use of tall fences improves cost effectiveness
and sand trapping efficiency; new materials are available for both permanent and temporary fences, including a
new polyethylene/steel wire composite strap; improved structural guidelines reduce costs for construction and
maintenance.
Wind Fences
Over the last ten years, field measurements utilizing computer-controlled data acquisition equipment have
provided maps of wind speed reduction behind fences having a variety of porosities and heights. Knowing how
the vertical distribution of wind speed reduction behind a fence varies with downwind distance, it is possible to
specify the height, porosity, and placement of a fence to affect the desired reduction in wind speed.
The development of strong, durable synthetic fencing materials has also contributed to the feasibility of building
tall fences for wind protection. Plastic fencing materials are relatively inexpensive, easily installed, and relax
constraints on the spacing of vertical supports.
Wind fences, up to 24 feet tall, have been used successfully to eliminate swinging and derailments on ropeways
(Figure 6), demonstrating that fences can also be used to prevent wind-caused derailments on railways. Over the
1991-92 winter, the Burlington Northern constructed a 2,500-foot long wind screen at a location near Browning,
Montana, having a history of derailments occurring in strong winds. The problem track is on a 20- to 60-foot
high fill and a three degree curve, with a six inch superelevation on the upwind rail. As a result of their
exposure and the center of gravity offset on the curve, 26-foot high double-stacked container and trailer loads
are exceptionally vulnerable to the strong winds typical in this area.
The Browning fence (Figure 7) utilizes high-density polyethylene Tensar windbreak fencing over the 2,036-foot
long curved portion of the fence. The 37 percent open area (porosity) of this material reduces wind speed at
track center line by more than 50 percent. The ends of the fence were extended tangentially for a distance of
300 feet to help phase out wind protection and to reduce the accelerated flow that normally occurs around the
ends of barriers. In addition to gradually increasing the distance between track and fence, the transition from
sheltered to exposed conditions was made even more gradual by increasing the porosity to 62 percent for 156
feet, and to 74 percent over the last 144 feet. Variable porosities were achieved by using different spacings
between five inch wide Centaur composite rail.
.
A potential problem with wind fences in the snowbelt is that, unless property designed, they can induce large
snowdrifts on the tracks. The increased porosities used near the ends of the fence reduce this problem, but
additional measures are necessary. At the Browning fence, a four-foot gap was left between the ground and the
bottom of the fencing to accelerate the wind near the ground, which in turn displaces the drift away from the
fence sufficiently to keep the tracks free of snow. Because there is a greater tendency for snow to be deposited
at the ends of a fill, 14-foot snow fences were installed upwind of these locations as an additional precaution.
Conclusions
Although this presentation has distinguished between fences used to control individual elements, the state of the
art makes it possible to design a structure that could serve the multiple functions of controlling access, stopping
blowing snow or sand, and reducing wind loads on trains to prevent derailments as well as to reduce fuel
consumption. The potential for using wind fences to reduce fuel costs would seem to warrant an industry-wide
study encompassing both theoretical modeling and field experimentation.
Effective guidelines and materials are now available for fences to control blowing snow, sand, and wind.
Potential benefits to railroads are clear, but the non-traditional nature of the subject matter makes it difficult for
engineers to become proficient in these areas without special training. This shortage of expertise will slow the
adoption of this technology by railroads until successful demonstration projects have reached critical mass, and
this will not be possible without the support of management. Thank you very much. (Applause)
SELECTED REFERENCES
Tablet, R.D. 1991. Snow fence guide. Strategic Highway Research Program, National Research Council,
Washington, D.C. 16 pp
Tabler, R.D. 1991. Snow transport as a function of wind speed and height. In: Cold Regions Engineering.
Proceedings, Cold Regions Sixth International Specialty Conference TCCP/ASCE (Feb. 26-68, 1991; West
Lebanon, New Hampshire). p. 729-738.
Tablet, R.D. and T. Day. 1992. Innovations in wind protection. In: Proceedings, Fifth Symposium of the
International Organization for Transportation by Rope - North American Continental Section (Steamboat
Springs, Colorado; 30 August-3 September, 1992). In press.
Tablet, R.D. 1993. Drift control design manual. Strategic Highway Research Program, National Research
Council, Washington, D.C. In preparation.
Tabler, R.D. and R.P. Furnish. 1982. Benefits and costs of snow fences on Wyoming Interstate-80.
Transportation Research Record 860, p. 13-20.
Tabler, R.D., JW. Pomeroy, and BW. Santana. 1990. Drifting snow. In: Cold Regions Hydrology and
Hydraulics. American Society of Civil Engineers Technical Council on Cold Regions Engineering Monograph.
p. 95-145.
President Hill: Thank you Don and thank you Dr. Tablet.
At this time I would like to take the opportunity to present plaques to the outgoing directors in recognition of
the job they have done. I would like to call Johnny Johnson and Ken Koff to the podium. The plaque reads,
"Roadmasters and Maintenance of Way Association of America presented for services as director 1987-1992."
1 would also like to call on Keith Nordlund to present him with a plaque in recognition for having served as
treasurer during the past year. I'll now turn the podium over to Mark Walbrun.
President Walbrun: Thank you, Steve. I would like to call our outgoing directors,
Doug DeBerg, Don Sorgenfrei and Joe Lileikis to the podium to receive their plaques that read "American
Railway Bridge and Building Association Director for the years 1989-1992."
President Hill: At this time, I'll ask Ken Johnson to present the Resolution Statement.