Snow Drifting, Snow Fences
Snowpack, by its nature, is affected by wind.
Individual snow crystals can be transported over
very large distances and re-deposited in other
areas. During this redistribution process, the
crystals are also affected by mechanical actions
such as abrasion, reducing their size.
Great Summation by Mark Williams:
Snow crystals are affected by other processes such
as evaporation and sublimation. The amount of
crystal reduction due to these processes is
dependent on the wind speed, relative humidity, and
air temperature primarily. Other factors play a
relatively minor role. The factors involved vary
from storm event to storm event as well as
temporally throughout the accumulation/ablation
season.
The size and shape of individual crystals determine
their transport mechanism. Large, fluffy flakes
are easily airborne and can be lifted to great
heights. Large, angular crystals either roll along
the surface or saltate (bounce) depending on the
wind speed and its weight/density.
Observationally, one can see that in the drifting
process, some areas benefit from greater
accumulations and other areas are “robbed” of the
snow that once was there. Areas of accumulation
are those areas where laminar wind flow patterns
have been broken up to non laminar flow by
obstructions. Conversely, areas that lose snow are
predominately areas where the wind has a long
unobstructed run.
Fig 1. Cornice accumulation area
Figure 2. Cornice calving accumulation area
Fig. 3 Windward side of Farmington peak scrubbed
clean of snow by wind.
There are many applications that can utilize this
principle. There are areas where no snow or very
little snow is desired such as roads. Conversely,
there are areas where greater amounts of snow are
desired for water supply either on a micro or a
macro scale. Ski resorts need snow accumulations in
specific areas, understanding the principles allows
the design of runs that are either not affected by
redistribution or that are favorably affected.
Simple things like the orientation of a cabin door
can be determined (the difference between shoveling
2 feet or 8 feet of snow to get inside during the
winter).
FETCH
Fetch or contributing distance is the length of an
area serving as a source of blowing snow to a
downwind location. The upwind end of the fetch is
any boundary across which there is little or no
snow transport. This boundary can be a forest
margin, deep gully, stream, row of trees, a
shoreline of an unfrozen water body. Basically, it
is any discontinuity that breaks up the wind
patterns such that the snow crystals currently
being transported are dropped out and the wind has
the opportunity to “reload” over the new fetch
distance.
Maximum transport distance
The maximum transport distance is the distance an
“average” sized snow particle travels before
completely evaporating. This parameter cannot be
measured directly, however this conceptual distance
provides a basis for estimating evaporation and
hence, the transport of blowing snow. Although
maximum transport distance varies greatly from
storm to storm depending on relative humidity,
temperature, wind speed and other variables, SEASON
LONG averages are less variable. Wyoming (and
where better than Wyoming to study blowing snow?)
studies indicate a maximum transport distance of
about 10,000 feet. This value may be applicable to
other areas as well. Similar values have been
noted in both Alaska and Siberia. (Basically
Wyoming, but further north with colder
temperatures, lower relative humidities and less
solar radiation.)
Carrying capacity
Carrying capacity is the ability of the wind (or
water, etc) to pick up and transport snow from one
area to another. There is some theoretical upper
limit of carrying capacity that has yet to be
precisely defined. Carrying capacity is dependent
on wind velocity, particle size, and total volume
of particles currently entrained. If the carrying
capacity of a wind is not fully met, it in theory
has the potential of gaining more particles for
transport. Just as with water, there is a
gradation of particle size corresponding to wind
velocity. Heavy particles are found close to the
ground as they settle out first, lighter particles
are entrained much higher in the air currents as
they are more easily transported. the longer a
particle is transported, the smaller it becomes
through the processes of abrasion and sublimation.
Laminar Flow
Wind,(in the aggregate) when viewed over the length
of the fetch, is generally steady laminar flow.
The fence or other obstruction creates pockets of
non laminar flow (eddies) which robs the wind of
its carrying capacity. Since the wind no longer
has the steady velocity nor the steady direction
(in both the horizontal and vertical planes) the
snow particles that it currently carries, fall out.
The height and openings of the fence determine the
geometry of the drift (where it starts and ends,
how high and wide - all are parameters that may be
altered)
Opportunities for drifting
The greatest amount of drifting occurs during and
immediately after storm events. The greatest
potential for trapping snow is within about 10 feet
of the snow surface (biggest particles, most snow).
Above the 10 foot height, the physical logistics of
fencing become a big problem and the expense
becomes prohibitive. After a crust of any kind is
generated on the snow surface, particles available
for transport decrease dramatically - thus
seasonally, snow is more available in the early
season than in the ablation stages.
Relocated precipitation
Relocated precipitation is that portion of water
equivalent precipitation relocated by the wind.
This, therefore excludes snow retained by
vegetation and topographic features, or snow that
melts in place. Most studies show that, at a
minimum, 20 to 25% of local snowpack is not
redistributed by wind (bottom threshold figure),
even in areas of high winds and very low
vegetation. Conversely, up to 75% of snowpack can
be redistributed in certain areas.
Assuming uniform conditions over the fetch
distance, snow transport is give by:
Qt = 0.5PrT(1-0.14F/T)
Where
Qt is snow transport water equivalent (cubic feet
of water per foot of width)
Pr is the relocated precipitation (feet of water
equivalent)
T is the maximum transport distance in feet,
usually assumed to be 10,000.
F is the fetch distance in feet.
The F/T ratio basically give that proportion of
snow that is evaporated prior to the snow being
redeposited on the lee side of an obstruction,
either natural or constructed.
This equation give
snow available for
to know if you are
either clearing or
us a good approximation of the
redistribution, a fact you need
going to construct a fence for
augmenting a drift.
The total water equivalent storage capacity of a
50% porous snow fence on level terrain is
approximately:
Qc = 6.7H2.2
Where
Qc is storage capacity in cubic feet of water
equivalent per foot of fence
H is the height of the fence.
Notes of Porosity and terrain:
Over the years, fences of various porosity levels
have been used and a standard of 50% porosity
determined to be the most cost effective. Solid
fences alter laminar wind patters somewhat, but
most of the particles are retained in the laminar
flow over the top of the compressed flow at the
fence edge. Porous fences increase the boundary
areas of discontinuous flow (simply by creating
more boundary areas) and therefore are much more
effective at trapping blowing snow particles. The
terrain can have a huge affect on the total water
equivalent storage capacity of a fence. If the
fence is immediately adjacent to a cliff (typical
cornice areas in the mountains) then the total
capacity just went through the roof. Conversely,
if it is immediately adjacent to an upward sloping
area, the total water equivalent storage area is
minimized.
As a fence is buried in the seasons snow
accumulation, its effectiveness decreases, because
there is less fence to trap snow.
If we assume T is 10,000 feet and setting Qc = Qt
we can solve for H, the required height of the
fence.
H = 21.38 (Pr (1-0.140.0001F))0.459
NOW: assume that ALL winter precipitation/SWE can
be relocated so that
P = Pr
(Total precipitation is that which is available for
transport relocation)
This assumption overestimates snow transport by at
least 25%, however the over design of a fence
storage capacity assures efficient snow trapping.
Snow trapping declines rapidly after a fence is 75
to 80% full and fences should be designed at least
20 to 25% greater than the snow transport.
If you calculate the height of a fence for a fetch
distance of 10,000 feet and a P of 4 feet, you come
up with a fence height of 37.7 feet. Obviously
some other solution is necessary, because the cost
of building and maintaining a 40 foot high fence is
going to be too stiff as well at the ethereal and
esthetic qualities of such a behemoth being on the
same plane as Smurfberry Cereal.
Enter STANDARD FENCES.
Most fences used for highways are either 4.5 or 9
feet high. You don’t need a 40 foot high fence,
you can use multiple rows of lower fences to get
the job done.
Required to get the same effect as a higher fence:
1) Precipitation falling between the rows of fences
2) Reduced evaporation of blowing snow over that
portion of the fetch occupied by the fences
A simplified empirical equation has been developed
that provides a reasonable approximation.
N ~ (H/Hs)2.18
Where N is the number of rows of fences having a
height of Hs and where H is the required height of
a single fence.
If your objective is to keep a road clear
(relatively) of blowing snow, obviously your worst
nightmare would be to create a drift directly on
top of it. You want to place the fence in a
location such that the end of the maximum drift
location is close to, but not on the road. The
drift will continue to grow throughout the winter
till the ablation season.
The maximum length of the lee drift on level
terrain when the fence is filled to capacity is
taken as 29X to 35X the fence height. Using 35X is
the norm for highway usage as it has greater margin
of error. Thus a 4.5 foot fence should be located
about 160 feet from the highway.
At this distance, the natural scrubbing action of
clean wind will erode snow from the highway instead
of depositing it there. This in turn leads to
lower maintenance costs. (In windy Wyoming, many
times the plows must go out even when it hasn’t
snowed just to plow drifts from windy days.)
Water supply augmentation
In this case, you may want to design a small water
pond such that the drift forms directly within the
boundaries of the pond itself. There are many
studies that have documented the size and shape
distributions of drifts (Tabler and Jairrel having
done a few) which would then allow you to size the
pond to the most cost effective shape. Remember in
these types of analyses to remember natural
topography often provides free augmentation to
drifts and catchments. Thus location, location,
location. If you can find a natural depression,
ridge, etc that will extend your pond, it means far
less construction costs and potentially better snow
trapping efficiency.
Natural barriers:
Fences don’t have to be fences - they can also be
rows of trees or bushes or furrows, even 2 foot
rows of wheat and corn stubble have been used
effectively to trap snow.
When using natural or live barriers, there is an
unpredictable part of snow drift accumulation
proportional to the discontinuities in the
vegetation. Conifers (spruce/fir) tend to be bushy
at the bottom and very sparse at the top:
triangular in shape. Thus as you increase in
height, the density of the barrier decreases. For
most applications (such as water augmentation) you
need to place the live barrier directly on the edge
of where the drift is desired. With live barriers,
the drift (largest portion, deepest segment) will
be immediately after the barrier. With steel,
hypalon or wooden fencing the drift shape and
geometry is significantly different. The deepest
section of the drift may be quite some distance
from the fence itself.
If Cottonwoods, Box Elder and other such deciduous
trees are used, they have the opposite shape of the
Conifers: that is narrow at the bottom and bushy at
the top. They don’t make as good a live barrier as
the conifers do and they require a great deal more
water to survive.
If planning a water augmentation project, pay
particular attention to the natural soils and
geology of the site. Areas with high infiltration
rates (sandy/loamy soils) will see greater snowpack
but likely not much in increased runoff. That is
to say, it most likely will infiltrate the soil and
be lost as far as use is concerned. One way to
deal with this is by selecting sites that have very
low infiltration (clay type soils), very stable
geology (no fissures, cracks, etc). Another,
albeit expensive solution, is to line the area with
some impervious soil or membrane to fascilitate
water storage.
Some logical places for water enhancement fencing
in this photo.
Drifting - avalanches and streamflow hydrograph
alterations.
Avalanches have been used to concentrate snow in
the bottom of canyons, away from exposed areas of
solar radiation. This does several things: 1) by
putting large amounts of snow in the canyon bottom,
it shields the mass from the sun delaying its melt
substantially till later in the season. 2) by
concentrating snow in deep layers, the physical
melt process is substantially delayed. The
combination of these two act to provide streamflow
much later in the runoff season. Depending on the
slide area (accumulation and run-out areas) - as
much as 1 to 2 months later in the season. The keys
are: 1) Provide enough snow to avalanche several
times during the year either by natural of
mechanical means. 2) Have a collection area that is
well protected and deep enough to accommodate
significant snow depth, preferably more than 20
feet deep and as long as the avalanche will run.
This means that the slide path must be relatively
steep, 40 degrees or more and that the slide path
is long and confined. Otherwise, the slide will
spread out instead of concentrating. The best
areas are large open bowls of significant area that
funnel down to very tight, narrow and confined
canyons.
This work by Dr. Mark Williams (INSTARR),
University of Colorado, is very informative.
BLOWING SNOW
Readings
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Williams, M. W., P. D. Brooks, T. Seastedt, and S. Schmidt, Nitrogen and carbon soil dynamics
in response to climate change in a high-elevation ecosystem in the Rocky Mountains, Arctic and
Alpine Research, in review.
Suggested Reading:
Brooks, P. D., M. W. Williams, and S. K. Schmidt, Snowpack controls on soil nitrogen
dynamics in the Colorado alpine, Biogeochemistry of Seasonally Snow Covered Basins, edited
by K.A. Tonnessen, M. W. Williams, and M. Tranter, IAHS-AIHS Publication 228,
Wallingford, UK, pp 283-292, 1995.
Williams, M. W., P. D. Brooks, A. Mosier, and K. A. Tonnessen, Mineral nitrogen
transformations in and under seasonal snow in a high-elevation catchment in the Rocky
Mountains, USA, Water Resources Research, V 32, N 10, pp 3175-3185, 1996.
Brooks, P. D., S. K. Schmidt, and M. W. Williams, Winter production of CO2 and N2O from
alpine tundra; environmental controls and relationship to inter-system C and N fluxes,
Oecologia, in press.
Brooks. P. D., M. W. Williams, D. Walker, and S. K. Schmidt, The Niwot Ridge snowfence
experiment: biogeochemical responses to changes in the seasonal snowpack, Biogeochemistry
of Seasonally Snow Covered Basins, ed by K. A. Tonnessen, M. W. Williams, and M. Tranter,
IAHS-AIHS Publication 228, International Association of Hydrological Sciences, Wallingford,
UK, pp 293-302, 1995.
Williams, M. W., P. D. Brooks, A. Mosier, and K. A. Tonnessen, Mineral nitrogen
transformations in and under seasonal snow in a high-elevation catchment in the Rocky
Mountains, USA, Water Resources Research, V 32, N 10, pp 3175-3185, 1996.
Brooks, P. D., M. W. Williams, and S. K. Schmidt, Microbial activity under alpine snowpacks,
Niwot Ridge, Colorado, Biogeochemistry, V 32, p 93-113, 1996.
Basic Wind Field
Threshold Wind Speed
Modes of Snow Transport
Biogeochemistry
Additional Web Sites
Snowfence and blowing snow, Niwot Ridge, Colorado.
Basic Wind Structure
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Geostropic Winds:
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Winds at altitudes greater than 1 kilometer
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Pressure-gradient forces in equilibrium with Coriolis forces

Winds essentially governed by large-scale weather systems and
independent of surface topography
Earth's surface:
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Friction between the earth's surface and atmosphere
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Results in a "no-slip" condition such that wind speed must be zero.
Boundary layer:

Boundary layer is the section of the earth's atmosphere between the
earth's surface and geostrophic winds.

Wind speed in the boundary layer increases from zero at the earth's
surface to its geostropic value at the top of the boundary layer.
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The thickness of the boundary layer and the distribution of wind
velocity with height above ground level depend on surface roughness.
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Over rough terrain the boundary layer is thicker and the wind speed
increases relatively slowly with height.
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Over flat, open terrain the boundary layer is thinner and the wind
speed increases relatively fast with height.
Measuring wind speed:

Wind speed is usually measured at a height of 10 meters above terrain
that is relatively flat and open.
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Measured wind speed at airports and other meteorological sites will
generally differ from wind speeds at other heights and/or over
different terrain.
Shear stress
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Formal definition: downward transfer of momentum to the surface.
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Descriptive definition: drag of the wind on the earth's surface.
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Units: force per unit area (N m-2).
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Velocity gradients in the boundary layer imply the existence of shear
stresses in the wind flow.
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Shear stress is at a maximum at the earth's surface.
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Shear stress decreases with increasing height above the ground.
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Shear stress becomes zero in the geostrophic wind above the
boundary layer.
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The shear stress exerted by the wind on the snowcover surface causes
the movement of loose snow.
Threshold Wind Speed
Definition
Minimum wind speed at which snow is moved or redistributed by wind.
Complicated function of the physical conditions of the snow surface.
Empirical observations
1. Threshold wind speed increases with increasing temperature and humidity.
2. If the original deposition occurs with wind, the particles will be broken into
small pieces and they will pack to a higher density to subsequently increase
threshold wind speed.
3. Threshold wind speeds increase with time since snow deposition, as a result
of sintering or bonding processes among the deposited snow grains.
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The rate of increase in threshold wind speed decreases with time.
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The increase in threshold wind speed is slower at colder temperatures.
4. Threshold wind speeds are much lower when there is a new source of snow
particles, such as
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new snowfall;
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snow on trees; and
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low snow strength layer.
Modes of Snow Transport
Type of blowing snow, the action or movement of snow grains being
transported by wind, characteristic height above the snow surface,
characteristic wind speeds, and the percent of total snow load
transported by wind for each type of snow transport.
Type
Actio
n
Height
(cm)
Wind speed (m
s-1)
Percent
Load
creep
saltation
turbulent
diffusion
rolling
bouncing
suspensi
on
less than 1
1-100
more than
100
less than 5
5-10
less than 10%
about 80%
more than 15
less than 10%
Biogeochemistry
We have implemented a long-term snow fence experiment at the Niwot Ridge LongTerm Ecological Research (NWT) site in the Colorado Front Range of the Rocky
Mountains in the USA to assess the effects of climate change on alpine ecology and
biogeochemical cycles. The response of nitrogen (N) and carbon (C) dynamics in
high-elevation mountains to changes in climate was investigated by manipulating
the length and duration of snow cover with the 2.6x60\|m snowfence, providing a
proxy for climate change. Results from the first year of operation in 1994 showed
that the period of continuous snow cover was increased by 115 days relative to the
control site and 90 days longer than at the snow fence site the year before
construction (1993) [Brooks et al., 1995a]. We have implemented a long-term snow
fence experiment at the Niwot Ridge Long-Term Ecological Research (NWT) site in
the Colorado Front Range of the Rocky Mountains in the USA to assess the effects
of climate change on alpine ecology and biogeochemical cycles. The response of
nitrogen (N) and carbon (C) dynamics in high-elevation mountains to changes in
climate was investigated by manipulating the length and duration of snow cover
with the 2.6x60\|m snowfence, providing a proxy for climate change. Results from
the first year of operation in 1994 showed that the period of continuous snow cover
was increased by 115 days relative to the control site and 90 days longer than at the
snow fence site the year before construction (1993) [Brooks et al., 1995a].
The deeper and earlier snowpack behind the fence insulated soils from winter air
temperatures, resulting in a 9deC increase in annual minimum temperature at the
soil surface and a 12deC increase at a depth of 15 cm. Warmer soils allowed
microbial activity, measured as @co2@ flux, to continue through much of the
winter. Carbon dioxide production under the deeper, earlier snowpack after
construction of the snowfence was 55% greater than production before construction
of the fence. The loss of @co2@ from snow-covered soils increased from about 20%
of above-ground primary production before fence construction to 31% after fence
construction, with shallow snow sites losing as little as 2% of above-ground primary
production as @co2@. Surface litter decomposition studies were conducted to test if
increased snow depth and duration behind the snowfence increased the rate of
decomposition and N mineralization relative to controls. Initial results show that
there was a significant increase in the litter mass loss under deep and early snow,
with no significant change under medium and little snow conditions.
Snowpack duration and depth also appear to control soil N dynamics [Brooks et al.,
1995b]. Under deeper, earlier accumulating snowpacks, N mineralization was
generally higher (1712-1960\|@mgNm2@) and with smaller spatial variation (CV 426%) compared to shallow and later accumulating snowpacks (5111440\|@mgNm2@, CV 42-83%). In contrast, the lowest nitrification rates were
found under deep/early snowpacks (8-18% of mineralized N) compared to larger
rates under shallow/late snowpacks (16-58% of mineralized N). These results are
consistent with faster rates of decomposition under deeper/earlier snow producing
more mobile N. Nitrogen and C dynamics in high-elevation mountain soils are very
dynamic and appear to be sensitive to small changes in climate.
Microbial activity and other biological processes under snow may be as important to
N and C cycling in high-elevation ecosystems as that occurring during the growing
season [Brooks et al., 1996]. Small changes in climate that affect the timing,
duration, and depth of snow cover may cause large changes in the N and C
dynamics of alpine ecosystems. These biogeochemical changes in soil may in turn
cause changes in plant community structure and composition. Further, N export in
surface waters of high-elevation catchments is generally considered to be from the
storage and release of @no3@ in seasonal snow [eg Williams and Melack, 1991].
These results suggest that microbial activity and N cycling under snow may control
@no3@ export in these surface waters, and that small changes in climate may
produce large changes in the chemical content of surface waters.
Additional Web Sites
Jason Box's Blowing Snow Animation . Jason has several nice animations of
blowing snow, with some suggestive text on the importance of snow surface
roughness caused by blowing snow.
Mark's home page
Department of Geography and
Institute of Arctic and Alpine Research
Comments and inquiries to: markw@snobear.colorado.edu
All contents copyright (C) 1995, INSTAAR and the University of Colorado
All rights reserved.