Analysis of Impact of Proposed Snodgrass Ski Area
Expansion to Avalanche Risk on the Rocky Mountain Biological Laboratory,
Private Structures, and Users of County Road 317
August 2005
Prepared by:
Hal Hartman, applied physicist, Snowmass, Colorado.
Art Mears, P.E., Avalanche-control engineer, Gunnison,
Colorado.
Commissioned
by:
The Rocky
Mountain Biological Laboratory
REPORT SUMMARY
This report
consists of two sections: Section 1
(this section) presents the principal conclusions and recommendations; Section
2 provides the technical details and support for the conclusions.
We have
evaluated the ski area preferred alternatives (shown in the “Preliminary
Concepts”) and determined how this alternative will change the physical characteristics
of avalanches and the associated risk. In accordance with information from the
U. S. Forest Service (the USFS), an “open-gate” policy enabling backcountry
access to avalanche slopes above the Gothic Road and Gothic will be permitted
and is therefore assumed in our analysis. We further assumed, based on ski-area
“preliminary concepts” we have evaluated, that substantial forest will be
removed from the summit area of Snodgrass Mountain (SGM) and various ski trails
and that the upper “Snowbeast” avalanche path will be open to lift-served
skiing. The risk change from
avalanches in and near the Rocky Mountain Biological Laboratory at Gothic (the
RMBL) and the change in exposure to avalanches on private property near Gothic
have been evaluated. Several
avalanche-related issues are summarized in this section and detailed in Section
2: (1) the increased risk to persons
using the Gothic Road resulting from the open-gate policy, (2) increased risk
to the Gothic Road users resulting from use of the “Snowbeast” avalanche, (3)
the change in the amount of snow blown into starting zones which will result if
trees near the summit of SGM and within certain ski trails are removed as
proposed, (4) increase in avalanche frequency resulting from “3,” (5) the change
in exposure to the “Sedmak” cabins, (6) the change in avalanche exposure to the
“Seeman” cabin, (7) the change in the
probability of triggering an avalanche that reaches RMBL property and adjacent
private property near Gothic.
The
following characteristics of the SGM topography, the use of Gothic Road, the
location and access to the RMBL, and the use of the area for recreational
backcountry skiing are unique among ski-area expansions we are familiar with
and have been considered in our analysis:
a)
Terrain
sufficiently steep to keep even the smaller, frequent avalanches moving is
adjacent to over 5,000 feet of the Gothic Road;
b)
The
Gothic Road is the only reasonable access to the RMBL and backcountry areas
leading to the Maroon Bells/Snowmass Wilderness area;
c)
The
Gothic Road has been designated as a non-motorized use area by the USFS (in
response to public request) and receives increasingly heavy use by
cross-country ski and snowshoe use; it is the only travel corridor near Crested
Butte so designated which is outside of Wilderness;
d)
From
the SGM summit area, which is proposed to be served by lifts, approximately
11,000 feet of out-of-bounds ridge crest downhill skiing is accessible without
climbing;
e)
The
11,000 feet of boundary parallels the Gothic Road which is as much as 1,600
feet below the SGM summit; nearly the entire length traverses avalanche paths;
and
f)
The
Gothic Road provides skiers and snowboarders with ready access to ski lifts,
thus setting the stage for repetitive “yo-yo” ascents and descents.
1.1
Change
in risk to Gothic Road users (see Section 2.1 for analytical details)
Skier and
snowboarder use of avalanche terrain above the Gothic Road will increase by a
factor of 10 to 100 because of simplified access resulting from ski lifts and
an open-gate policy. Avalanche frequency is also known to increase as a result
of “artificially-induced” triggers, such as skiers. We interviewed fifteen
acknowledged avalanche experts from Colorado, Utah, and Montana currently
involved in regional or local avalanche forecasting. They estimated that
avalanche frequency would increase by a factor of 2.6[1]
as a result of an open-gate policy. Terrain above substantial portions of the
road is steep; consequently many of the avalanches inadvertently triggered
could reach the road and endanger road users who would not otherwise be exposed
to avalanches. Certain avalanche areas above the road will receive much heavier
use than others, increasing the avalanche frequency substantially in these
areas. We have concluded, based on
the expert opinions mentioned and applied analytical methods, that the risk to
Gothic Road users will be increased by a factor ranging from 2.6 to 14 if
open-gate skiing is permitted. The Gothic Road is used by RMBL personnel to
access Laboratory facilities at Gothic and is used by recreational skiers who
use one of the few backcountry routes currently closed to motorized uses in
Gunnison County. Furthermore, we quantified the encounter probability[2]
that a person or group on the Gothic Road would be hit by an avalanche by
fitting binomial and Poisson distributions, as described more fully in Section
2.1. Our calculations were based on 29 years of avalanche observations at the
RMBL at Gothic, personal observations obtained by traveling the road after
periods of avalanches, an estimated ski season length of approximately 152
days, a travel speed of 1.3 miles per hour, and 5-30 Gothic Road users per day,
on average.
Given the
above assumptions and the premise that open-gate “triggers” and Gothic Road
users are both distributed randomly through time, there would exist a 1:7 (14%)
chance of a person being hit and possibly buried and killed by an avalanche
each year on the road.
1.2 Increased risk
to road users from the Snowbeast avalanche path (see 2.2 for details)
The
“Snowbeast” avalanche path[3]
is discussed separately here because use of steep, avalanche-prone terrain in
this path for lift-served skiing is shown on the CBMR “preliminary
concepts.” However, lift-served skiing
of this path may be withdrawn from the proposal. This avalanche path is as much as 250 feet (80m) wide where
intersects the Gothic Road at approximately mile 1.6 north of the winter
trailhead. Major avalanches can and
have started at 10,800 feet elevation in Snowbeast on the east exposure of the
eastern summit of SGM and have crossed the Gothic Road and East River[4]. Smaller and more frequent avalanches begin
between 9,700 and 10,200 feet; these cross the road more often. Four impacts to the use of the Gothic Road
would be possible: (1) timber removal
from the eastern summit of SGM, as proposed, would substantially increase the
amount of snow transported into the starting zone, increasing frequency and
size of larger avalanches starting on the upper slopes (the effect of forest
removal on snow drifting is discussed further in 1.3 and 1.4), (2) use of
explosives to reduce the risk to lift-served skiers would increase the
frequency of avalanches some of which will reach the road, (3) use of
explosives in upper Snowbeast could propagate slab fractures into adjacent
avalanche paths to the north also increasing risk on the road north of mile
1.6, and (4) an “open-gate” policy will lure skiers onto the steep lower slopes
between 9,700 and 10,200 feet even if the area is not developed for skiing;
this would increase the frequency of inadvertently-triggered avalanches onto
the Gothic Road and the risk to skiers on the road (section 1.1).
1.3 Increased snow
transport into starting zones resulting from forest removal
Removal of
the forest at ridge crest on SGM for purposes of constructing ski runs and ski
lift return stations will create a source area for drifting snow. Consequently,
wind will strip snow from currently forested areas at ridge crest and deposit
substantial amounts of snow in adjacent avalanche starting zones above the
Gothic Road. Drifting snow dramatically increases avalanche danger when
precipitation is combined with wind. Experiments and field observations
indicate that a doubling of wind speed greater than 10 mph results in an
approximate fourteen-fold increase in the rate of drifting. Wind
velocity and direction data have not been obtained at the summit of SGM. Although two winters of wind data recorded
at the Crested Butte Ski Area were obtained, estimates of the annual and maximum
snow transport rates are inconclusive due to the brief period of record. We
then compared wind behavior characteristics found in the limited Crested Butte
Ski Area data with ten years of wind data obtained from a high-elevation site
twenty four kilometers north of SGM.
Approximations of maximum snow transport rates were obtained by
comparing these data. In addition, we
considered clues of prevailing wind direction observed in the field, observed
and measured topographic features, and measured areas dedicated to ski runs and
ski lift return stations on SGM. Wind characteristics associated with the
avalanche cycle of January, 2005[5]
were also used to estimate the maximum six hour cumulative snow transport
rate. We found the wind during this
storm was typical of a two year return period wind event[6].
Moreover, the data suggest that a ten-year return period wind event would have
increased the snow transport rate by a factor of 2-to-3 if trees were removed.
Ski trail grooming operations and/or skiing and snowboarding during episodes of
snowfall and drifting will not decrease snow transport rates significantly.
Therefore we conclude that removing trees at ridge crest will increase
drifting snow and enhance snowfall in adjacent avalanche starting zones
which are located above the Gothic Road.
1.4 Increased
avalanche frequency from effects discussed in 1.3
Trees
located at ridge crest prevent drifting snow from reaching nearby avalanche
starting zones[7]. In steep,
forested areas tree trunks anchor snow slabs, tree braches intercept snowfall
and then gradually drop chunks onto the snow it to form a stable snow cover on
the ground. The forest canopy alters the net energy exchange between the
snowcover and atmosphere. These effects
produce a uniform snow cover resistant to avalanching. Forest fires, tree
cutting and other activities which denude ridge crest areas and avalanche
starting zones are known to increase avalanche frequency and magnitude. Trees at ridge crest on SGM and in starting
zones above the Gothic Road are important inhibiting factors to avalanche
formation. However, creating source
areas for drifting snow by cutting trees at ridge crest will localize thick,
cohesive and highly unstable slabs of snow in the upper reaches of many
starting zones near the summit of SGM. These concentrated deposits of thick,
wind-relocated snow near ridge crest will also increase avalanche potential
energy and maximize snow slab tensile stresses. Therefore, the probability of slab fracture and avalanche
initiation will increase. When
avalanches of dense snow begin they will impact trees and thin the existing
forest cover in the starting zones.
Forest removal at the SGM summit, therefore, is similar to a “positive
feedback” mechanism: drifted snow
creates more avalanches; hard-slab[8]
avalanches thin the forest through impact damage to trees; the reduced tree
density in starting zones further increases avalanche frequency and produces
more forest damage; this leads to more avalanches. As a final point, 29 years of avalanche observations at the RMBL
at Gothic indicate that hard slab avalanche events currently account for less
than 1.5% of all avalanches; this is due entirely to the protective nature of
the existing forest cover. We conclude, therefore, that cutting trees at
ridge crest will alter wind deposition in starting zones and result in more
frequent and larger avalanches.
1.5 Change in risk to the Sedmak Cabins (see 2.5 for details)
These two
privately-owned cabins[9]
are located within an aspen forest on the uphill (west) side of the Gothic Road
about 0.5 miles south of Gothic. The
older aspen trees in this forest are 30-70 years old and the younger trees are
10-30 years old. Small-to-medium sized
avalanches have penetrated the aspen forest in the past and these have advanced
to within approximately 50m (160ft) of the southernmost cabin. The avalanche starting zone on the
unforested slope above the cabins is sufficiently large to produce an avalanche
capable of destroying portions of the forest, reaching both cabins and crossing
the Gothic Road. This is supported by
analytical avalanche-dynamics modeling procedure used in this analysis and by
observations of the effects of avalanches in similar terrain within the
Colorado snow climate. The avalanche terrain below the starting zone and above
the cabins is sufficiently steep to keep moderate-to-large avalanches
moving. We previously estimated that
avalanche frequency will increase as a result of “open-gate” skiing accessed from
the top of SGM (see 1.1, above). Therefore,
we estimate that these cabins will be reached approximately 2.6 times as
often than they would be during the present, natural conditions. This would be due to the increased number of
triggers resulting from the proposed “open-gate” policy.
1.6
Change in
avalanche exposure to Seeman cabin (see 2.6 for details)
The Seeman
cabin is located approximately 260m east of the Gothic Road at the west edge of
an aspen forest and near the end of the natural runout zone[10]
of the “Sedmak South[11]”
avalanche path. The Sedmak South path
starts north of the SGM summit at approximately 10,720 feet (3,270m) elevation
and runs out in the valley at approximately 9,380 feet (2,860m) elevation, a
maximum fall of 410m (1,340 feet). In the
avalanche-dynamics analysis utilized in this study it is classified as a
“medium sized” avalanche path. On
January 12, 2005, an avalanche in Sedmak South stopped within approximately
190m east of the Gothic Road, or 70m short of the Seeman cabin. Based on this 2005 event, a previous avalanche
in February, 1986 which went nearly as far, and the apparent age of the aspen
forest (several decades old, judging from tree sizes), this recent avalanche
had an estimated return period of about 30 years. This 30-year avalanche (along with many other events sampled in
North America) was used to calibrate the Swiss avalanche-dynamics model
“AVAL-1D” to local snow-climate conditions.
To conform to the Gunnison County Land Use Regulations (the “LUR”)
avalanches with 300-year return periods must be considered in land-use planning
and engineering. After calibration, the
Swiss AVAL-1D model was used to simulate a 300-year avalanche. This natural 300-year event (pre-SGM
development) was found to stop roughly 20m (65 feet) short of the Seeman
cabin. However, AVAL-1D is sensitive
to, among other things, the amount of snow released from the starting
zone. Larger amounts of snow will
produce larger, longer-running avalanches, a model result that is well
supported by numerous field observations.
We assumed increased amounts of snow would be blown into the starting
zones as a result of forest removal from near the top of SGM (Section
1.3). This will increase the volume of
the released slab and increase the runout distance, velocity, and impact
pressure potential of the avalanche.
When the released slab volume is increased as a result of wind-deposited
snow, the Sedmak South avalanche path will impact the Seeman cabin and travel
20m (65 feet) past it. This exposure
of the cabin to avalanches, which did not exist under natural conditions, would
result from the forest removal for ski-area development that is shown in the
“Preliminary Concepts.”
1.7
Change in
probability of triggering avalanches into RMBL property (see 2.7 for details)
The steep
terrain, including the northeast exposure of SGM (RMBL path #’s 17 – 11), can
act as a single avalanche path. This
has been documented during five events starting in 1976. The lower starting zones (below the 10,500
to 11,000 foot level) on Gothic Mountain, which are directly above RMBL
property therefore connect with and can be triggered from the northeast slopes
of SGM. Major avalanches have released
from these lower Gothic Mountain slopes and have impacted cabins at the RMBL[12]. These slopes extend continuously for
approximately 4,000 feet and therefore involve a much wider area than that
affected by avalanches starting on the upper slopes of Gothic Mountain. The Snodgrass slopes will become heavily
used (compared to current conditions) when an open-gate policy is in effect and
would certainly be used during periods of higher snowpack instability than at
present. This final conclusion is true
because the proposed open-gate policy will “lure” inexperienced skiers and
snowboarders from the ski area onto steep, avalanche-prone slopes even when the
snowpack is unstable and avalanches are likely. Large avalanches are known to have been triggered inadvertently
by skiers and snowboarders accessing backcountry slopes from other developed
ski areas[13]. Based on our estimate of increased
avalanche frequency resulting from “open-gate” skiing from Snodgrass (see 1.1,
above), we estimate that avalanches will reach the RMBL property, including
some buildings, approximately 2.6 times as often as at present. A similar increased exposure would also
occur on private property in the “Gothic Heights” subdivision south of Gothic
on the lower slopes of Gothic Mountain.
1.8
Recommendations
The
following recommendations are based on the findings and conclusions of this
study as summarized in sections 1.1 through 1.7 and detailed in Section 2. The word “recommendations” is used here with
the understanding that the RMBL does not have direct control in the
implementation of these recommendations.
1.8.1
Artificial release techniques (including all
shock-loading procedures) should not be used on the east-through northeast
slopes of SGM during any conditions. Shock-loading procedures,
including explosives, propane/oxygen exploders and other methods are known to
produce avalanche fracture lines which can extend for long distances through
continental snowpacks such as those found in Colorado as well as snowpacks in
other climates. These methods,
therefore, could produce avalanches that can cover long distances of the Gothic
Road and reach private property (Sections 1.2, 1.5, 1.6, 1.7).
1.8.2
All “open-gate” access from the ski area onto
the east through northeast slopes of SGM should be prohibited. The
number of skiers and snowboarders accessing the avalanche terrain above the
Gothic Road will increase by a factor of 10 to 100 or more over the current
backcountry use. This would
substantially increase the risk to road users, increase the risk to private
property and buildings, and decrease the time when the Gothic Road could be safety
used by RMBL personnel and recreational skiers on the road (Sections 1.1, 1.2,
1.4, 1.5, 1.7).
1.8.3
A 100m-wide buffer of trees must be left
undisturbed and maintained in good condition without disturbance between the
developed ski-area facilities (including any ski runs and approaches to them) and
the top of the east and northeast Snodgrass avalanche slopes. Such a forest buffer could mitigate the
increase in snow transport and avalanche frequency discussed in Sections 1.3
and 1.4 and would eliminate the increased the thickness and density of the
slab. This mitigation would also reduce
the design-magnitude avalanche size so that it would not impact the Seeman
Cabin (Section 1.6).
1.8.4
The proposed Snowbeast ski run and the run on
the north ridge of SGM should be eliminated. These runs, to be located in
the Snowbeast avalanche path and directly on the north ridge of SGM, would also
enable additional snow to be transported into adjacent starting zones (Section
1.3 and 1.4). In addition, they would
provide easy access by skiers to starting zones, increasing avalanche frequency
and size and the risk to Gothic Road users and/or private property (Sections
1.1, 1.4, 1.5, and 1.7).
2.1 A CHANGE IN RISK TO GOTHIC ROAD USERS[14]
2.1.1 ABSTRACT
[1]
Quantifying risk to users of the non-motorized Gothic Road is accomplished by a
step-by-step logically defensible process. Relevant knowledge has been acquired
from three sources: 1) data characteristics 2) domain knowledge and 3) expert
opinion. Simplicity and completeness is the theme of structuring and applying relevant
knowledge. Furthermore, inconsistency and bias have been minimized by combining
estimates from different sources, when possible, and concentrating on only the
most important causal forces.
[2] Key
words: Artificial trigger, avalanche path, binomial probability function,
chance, encounter probability, exposure, natural avalanche frequency, natural
trigger, return period, risk, runout zone, unconfined avalanche path.
2.1.2 INTRODUCTION
[3]
Justification for assessing avalanche risk to Gothic Road users is nontrivial.
For example, over the past 29 years some 7,745 avalanches have been observed
near the town of Gothic by Rocky Mountain Biological Laboratory (RMBL)
personnel (barr, 2005). Furthermore, Mears (1995) estimates that approximately
7,700 feet of Gothic Road can be reached during design avalanche conditions and
on average 1,000 feet of road is affected each winter.
[4] Risk levels are estimated by application
of encounter probability, the sum of all terms except x = 0 in the binomial probability function (McClung, 1999; Triola,
1992). This commonly used equation expresses the chance that a person or a
building will be hit by an avalanche given a finite exposure time and knowledge
of past avalanche occurrences.
2.1.3 METHODS
[5]
Encounter probability, EP,
may be written (LaChapelle, 1966; McClung and Schaerer, 1993)
(1) 
where Δt, in years (yr) is the period of time that a person is
exposed to a potential avalanche; n
is the number of people exposed to a potential avalanche per yr; T is the return period, in yr per
occurrence, of an avalanche reaching or exceeding a specified location.
Furthermore, McClung (1999) assumes that avalanches are rare, independent, and
discrete, with arrival according to a binomial probability function. In
addition, for purposes here, it is assumed that Gothic Road users as well as
skiers and snowboarders descending from the summit of Snodgrass Mountain to the
road are distributed randomly through time.
[6] For conceptual purposes equation (1) is
applied to a specific avalanche path, Snodgrass 15 (see Figure 2.1), which
intersects the Gothic Road approximately 3,200 meters (m) north west of the
trailhead. Selection criteria includes: 1) a well defined avalanche path that
is easily observed from the RMBL 2) an avalanche path that originates at the
summit of Snodgrass Mountain which is preferred by local backcountry skiers and
snowboarders 3) a segment of road that intersects an unconfined avalanche path
(Burrows and Burrows, 1976) and 4) a segment of road that crosses an avalanche
path where vegetation characteristics indicate at least one large avalanche in
an interval of one to two years (McClung and Schaerer, 1993).
2.1.4 DEFINITIONS
[7]
Time, ΔtR, where subscript R associates a variable with the Gothic Road, is defined as the
period of time that a person is exposed to a potential avalanche while traversing
Snodgrass 15.
[8] Time, ΔtS, where subscript S associates a variable with Snodgrass
Mountain, is defined as the period of time that a person is exposed to a
potential avalanche upon reaching the Gothic Road following a ski or a
snowboard descent of Snodgrass 15.
[9] Exposure, nR, is defined as one person traversing the Snodgrass 15
avalanche track on the Gothic Road in a north west direction and then returning
on the road in a south east direction.
[10] Exposure, nS, is defined as the presence of a skier or snowboarder
on the Gothic Road following a descent of Snodgrass 15.
[11] Return period, T, is defined as
(2) 
where
f is the frequency of avalanches
reaching the Snodgrass 15 runout zone.
[12] Quality deep powder skiing and
snowboarding is defined as equal to or less than one skier or snowboarder per
meter of slope width across the fall line for each 0.4 meters of snowfall.
[13] Finally, given definitions above,
equation (1) is rewritten as
(3) 
2.1.5
CURRENT RISK TO GOTHIC ROAD USERS
Assumptions
[14] ΔtR is 10-4.668
yr. Considering all forms of travel observed on the
Gothic Road near the RMBL (b. barr, personal communication, June 2005; A.
Mears, personal interview, February 2005) ski, snowshoe and foot as well as age
and ability plus snow and weather conditions, the average rate of travel is
taken to be 0.6 m·s-1. This value represents approximately 70
percent of the rate of travel determined by timing two groups of skiers on the
road while gliding on a well packed surface in fair weather during March of
2005.
[15] ΔtS
is 10-4.544 yr. Following a descent of Snodgrass 15 it is estimated
that 15 minutes per skier or snowboarder is required to prepare for the return
trip to the Gothic Road trailhead and exit the Snodgrass 15 avalanche path (T.
Schweitzer, personal interview, June 2005).
[16] nR
is 5 exposures. Estimates of people reaching the
RMBL, located approximately 4,800 m north west of the trailhead, on a daily
basis range from zero during periods of extreme avalanche hazard to 40 during
fair weather conditions; the daily average is assumed to be 5 (b. barr,
personal communication, June 2005).
[17] nS
is 1 exposure. barr (personal communication, June
2005) and Schweitzer (personal interview, June 2005) estimate that as many as
10 backcountry skiers and snowboarders descend Snodgrass 15 on a weekend day;
however, each day, on average, they estimate that 1 skier or snowboarder
descends Snodgrass 15.
[18] T
is 1.7 yr per avalanche. 169 avalanches have been
observed on Snodgrass 15 over the past 29 years (barr, 2005), 52 of which have
reached the runout zone. Finally, of the 52 avalanches reaching the runout zone
barr (personal communication, June 2005) estimates that 33 percent have
exceeded the road.
[19] Based on the earliest and latest date of
avalanches reaching the Snodgrass 15 runout zone (barr, 2005), the winter
season spans 157 days.
[20]
Current Risk: Quantifying current risk to Gothic Road users given methods,
definitions and assumptions found in sections 2.1.3, 2.1.4 and 2.1.5 leads to the following conclusion:
There is an approximate 1: 80 yr chance of someone standing on the road being
hit by an avalanche.
2.1.6
A CHANGE IN RISK TO USERS OF THE GOTHIC ROAD
Assumptions
[21] ΔtR is 10-4.668
yr. Considering all forms of travel observed on the
Gothic Road near the RMBL (b. barr, personal communication, June 2005; A.
Mears, personal interview, February 2005) ski, snowshoe and foot as well as age
and ability plus snow and weather conditions the average rate of travel is
taken to be 0.6 m·s-1. This value represents approximately 70
percent of the rate of travel determined by timing two groups of skiers on the
road while gliding on a well packed surface in fair weather during March of
2005.
[22] ΔtS
is 10-4.544 yr. Following a descent of Snodgrass 15 it is estimated
that 15 minutes per individual is required to prepare for the return trip to
the Gothic Road trailhead and exit the Snodgrass 15 avalanche path (T.
Schweitzer, personal communication, June
2005).
[23] nR
is 5 exposures. Estimates of people reaching the
RMBL, located approximately 4,800 m north west of the trailhead, on a daily
basis range from zero during periods of extreme avalanche hazard to 40 during
fair weather conditions; the daily average is assumed to be 5 (b. barr,
personal communication, June 2005).
[24] nS
is 23 exposures. Expert opinion indicates that skier and snowboarder frequency
on public land immediately adjacent to lift served ski areas is approaching the
regularity of skiing and snowboarding found within developed ski area
boundaries (A. Henceroth, D. Maroz, S. Toepher; personal interviews, March
2005). Moreover, the frequency of backcountry use adjacent to operational ski
area boundaries is believed to be approximately proportional to snow quality
(A. Henceroth, D.
Maroz, H. Smith, S.
Toepher; personal interviews, March 2005). Thus,
application of the quality deep powder guidelines mentioned above predicts that
each day, on average, 23 backcountry skiers and snowboarders will descend
Snodgrass 15 provided lift served access to the summit of Snodgrass Mountain.
[25] Quantifying quality deep powder skiing
and snowboarding is based on 172 m of avalanche track width at the 3,096 m
elevation on Snodgrass 15 and average monthly snowfall observed from November
15 to April 15 over the past 29 years at the RMBL (http://www.rmbl.org).
[26] T is
0.6 yr per avalanche. Observations by 15 experts (D.
Atkins, B. Barr, K. Birkeland, J. Brennan, J. Coulter, M. Friedman, A. Gleason,
D. Maroz, M. Rikkers, J. Roberts, H. Smith, S. Toepher, B. Tremper, K.
Williams; personal interviews, 2005) involved in local or regional avalanche
forecasting (McClung and Schaerer, 1993) unanimously attest to an increase in
backcountry avalanches as a result of increased human activity. Furthermore, 13
of 15 experts estimate that artificially triggered avalanches increase the
naturally triggered avalanche frequency by a factor of 1.5 to 6.0 with an
average rate of increase equal to 2.6.
[27] As a consequence of open gate ski area
boundary policies, expert opinion acquired from 6 ski area avalanche
forecasters (J. Brennan, J. Coulter, A. Henceroth, D. Maroz, H. Smith, S.
Toepher; personal interviews, March 2005) indicates that artificially triggered
avalanches increase the naturally triggered avalanche frequency, on average, by
a factor of 2.8. Therefore, the return period of an avalanche on Snodgrass 15
which exceeds the Gothic Road is expected to increase by a factor of 2.8 from
the current value of T = 1.7 yr to T = 0.6 yr.
[28] Nearly 8 of 10 naturally triggered
avalanches that reach the Snodgrass 15 runout zone are soft slab (McClung and
Schaerer, 1993) with an average size ranging between 2 and 3 (Perla and
Martinelli, 1976); both conditions are characteristic of artificially triggered
dry snow slab avalanches (Schweizer, 1999).
[29] Based on a typical ski season in
Colorado, the winter season spans 152 days.
[30] Future
Risk: Quantifying future risk to Gothic Road users given methods, definitions
and assumptions found in sections 2.1.3, 2.1.4 and 2.1.6 leads to the following
conclusion: There is an approximate
1: 6 yr chance of someone standing on the road being hit by an avalanche. This
represents about a 14 fold increase in risk to users of the road provided lift
served access to the summit of Snodgrass Mountain and an open gate ski area
boundary policy.
2.1.7
CONCLUSION
[31] Risk levels will vary according to
specific topographic features in proximity to the Gothic Road. Furthermore,
risk levels will vary with the number of skiers and snowboarders descending
avalanche paths on the north east faces of Snodgrass Mountain and the
coincident number of Gothic Road users. Thus, given findings presented above,
the risk envelope will increase from current levels by a minimum factor of 2.6
and vary to a maximum factor of 14 provided lift served access to the summit of
Snodgrass Mountain and an open gate ski area boundary policy.
2.1.8 REFERENCES
barr, b.,
2005. 29 year data set; personal records of avalanche and weather observations.
Rocky Mountain Biological Laboratory, Gothic. Gothic, CO.
Burrows, C. and Burrows, V., 1976. Procedures for the study
of snow avalanche chronology using growth layers of woody plants. Occasional
Paper No. 23, 1976, Institute of Arctic and Alpine Research. University of
Colorado at Boulder. Boulder, CO.
LaChapelle, E. 1966. Encounter probabilities for avalanche
damage. USDA Forest Service Miscellaneous Report 10. Alta, UT: Alta Avalanche
Study Center, Wasatch National Forest.
McClung, D., 1999. The encounter probability for mountain
slope hazards. Can. Geotech. J., (1999), 36: 1195 – 1196.
McClung D., Schaerer, P., 1993. The Avalanche Handbook. The
Mountaineers. Seattle, WA.
Mears, A., 1995. Snow avalanche technical report - -
Environmental impact statement considerations, Crested Butte Mountain Resort
E.I.S. Prepared for Pioneer Environmental Services, Inc. Gunnison, CO.
Perla, R. and Martinelli, M. Jr., 1976. Avalanche Handbook.
U.S Department of Agriculture, Handbook 489. Ft Collins, CO.
Schweizer, J., 1999. Review of dry snow slab avalanche
release. Cold Regions Science and Technology, (1999), 30. 43 – 57.
Triola, M., 1992. Elementary
Statistics. Addison – Wesley Publishing Company, Inc. Reading, MA.
2.2
GOTHIC
ROAD CLOSURES DUE TO SKI AREA OPERATIONS[15]
2.2.1
ABSTRACT
[1] Five
avalanche paths are found immediately adjacent to the proposed ski area
boundary at ridgeline on Snodgrass Mountain. Consequently grooming operations,
skiers and snowboarders in close proximity to avalanche starting zones will
provide an artificial triggering mechanism for avalanche release during periods
of considerable to extreme avalanche danger. A simple quantitative method is
used to estimate the number of days each winter that ski area operations may
inadvertently trigger avalanches capable of reaching the Gothic Road. Relevant
knowledge has been acquired from three sources: 1) data characteristics 2)
domain knowledge and 3) expert opinion. Finally, to minimize unsystematic
errors and improve reliability, intuitive processes are replaced with
analytical methods.
[2] Key
Words: Absolute instability, artificial trigger, avalanche day, avalanche path,
chance, conditional instability, fracture line, grooming, hard slab, interval
dump board, natural avalanche frequency, natural trigger, remote trigger,
return period, risk, runout zone, starting zone, soft slab, water equivalent
from precipitation.
2.2.2
INTODUCTION
[3]
Avalanche starting zones at ridgeline on Snodgrass Mountain span approximately
1,200 lineal meters (m). Moreover in the past 29 years, 91 naturally triggered
avalanches originating in avalanche starting zones located immediately adjacent
to the proposed ski area boundary have reached their respective runout zones
(barr, 2005). Therefore, due to ski area operations in close proximity to
ridgeline starting zones, there is a chance of inadvertently triggering
avalanches capable of reaching the Gothic Road.
[4] Consequently, it may be necessary to
take preventative measures and regulate the presence of people on the Gothic
Road. Estimating the number of days per winter season where preventative
measures will be required is conditioned on a cause and effect relationship
between precipitation and avalanches (McClung and Schaerer, 1993).
2.2.3 METHODS
[5]
Avalanche occurrence records recovered from the RMBL (barr, 2005) were censored
to include avalanche paths Snodgrass 13 through 17. Censored data were then
filtered to include avalanches originating at or near ridgeline which reached
their respective runout zones. Subsequently, the censoring and filtering
process identified 91 large avalanches observed over the past 29 years among
the 5 avalanche paths.
[6] Weather observation records recovered
from the RMBL (barr, 2005) were censored to include 24 hour water equivalent
from precipitation concurrent with the dates of avalanche activity mentioned
above.
[7] Finally, taking class size equal to 5
millimeters (mm), daily water equivalent from precipitation frequencies on
avalanche days was calculated. Probabilities were then assigned to each
frequency class, plotted and fit with a simple binomial curve (see Figure
2.2.1).
Figure 2.2.1: Probability of at
least one large avalanche originating at or near ridgeline on Snodgrass
Mountain and reaching the runout zone versus 1 day precipitation total.
2.2.4
DEFINITIONS
[8] An
avalanche day is defined as the occurrence of one or more large avalanches
within the 5 avalanche paths mentioned in section 2.2.3. That is, Snodgrass 13
through 17 are taken as an aggregate. This is a reasonable assumption as
continuous fracture lines linked avalanche starting zones Snodgrass 13 through
17 as recently as January 11, 2005. Furthermore, all 5 avalanches exceeded the
Gothic Road (Personal field observations, February 2005).
2.2.5 CALCULATIONS
[9] The
equation of the binomial curve best fit to the probability frequencies found in
Figure 2.2.1 is
(1) 
where x is millimeters of daily water
equivalent from snowfall measured at the RMBL (barr, 2005).
[10] For instance, if 20 mm of daily water
equivalent from precipitation is taken as the single conditioning event that
determines when the presence of people will be regulated on the Gothic Road,
then equation (1) predicts that there is a 1: 13 day chance of a large
naturally triggered avalanche originating near or at ridgeline and descending
to a runout zone. As daily water equivalent from precipitation increases to the
single largest value recorded in 29 years, the probability of at least one large
avalanche is one.
[11] Finally the long running average, based
on 29 years of weather observations at the RMBL (barr, 2005), suggests that 20
mm of water equivalent from precipitation is exceeded 5 days annually; however,
during the winter of 1979/1980 this quantity was exceeded on 12 days.
2.2.6
DISCUSSION
[12] Daily
precipitation is the primary meteorological factor for avalanche formation
(Armstrong and Williams, 1986); moreover, as snowfall increases the larger and
more likely avalanches become (LaChapelle, 1985). For example, Schaerer (1981)
considers 20 mm of water equivalent from precipitation a critical threshold
which affects the stress-strength relationship of the snow cover.
[13] Furthermore, LaChapelle (1985)
acknowledges a distinction between absolute and conditional instability. The
binomial curve best fit to the probabilities found in Figure 2.2.1 is an
expression of absolute instability; that is, it gives the chance that the snow
cover has exceeded a critically weak state given 24 hour water equivalent from
precipitation.
[14] However, during periods of conditional
instability, which may persist for many weeks (Jamieson and Schweizer, 2000)
the magnitude of an artificial triggering force (McClung and Schweizer, 1999)
is difficult to quantify. For example, shear stress must approach shear
strength in the weak layer and the rate of deformation in the weak layer must
be fast enough to provoke fracture (McClung and Schaerer, 1993).
[15] Finally, of the 91 large avalanches found
in the censored and filtered data set, all are soft slab (McClung and Schaerer,
1993) and 76 percent are size 3 (Perla and Martinelli, 1976); both conditions
are characteristic of artificially triggered dry snow slab avalanches
(Schweizer, 1999).
2.2.7
CONCLUSION
(Ski area operations without Snowbeast)
[16] During
periods of considerable to extreme avalanche danger grooming operations, skiers
and snowboarders in close proximity to avalanche starting zones near ridgeline
on Snodgrass Mountain will require regulating the presence of people on the
Gothic Road.
[17] At 20 mm of daily precipitation from water
equivalent, the probability of at least one naturally triggered avalanche
reaching a runout zone is calculated to be 1: 13 days. Said another way, if 20
mm of precipitation from water equivalent is measured on the 24 hour interval
dump at
[18] Consequently, if a minimum risk tolerance
level of 1: 13 days is coupled with periods of conditional instability,
regulating the presence of people on the Gothic Road between November 15 and
April 15 will be required on 5 occasions per season, on average, with each
episode extending for an indefinite period of time; the maximum is taken to be
12 occasions per season with each episode extending for an indefinite period of
time.
2.2.8 CONCLUSION (Ski area operations with
Snowbeast)
[19] Steep
terrain within the Snowbeast avalanche path will require frequent avalanche
control. In addition to ski and snowboard stabilization, periodic application
of explosives will be required to: 1) release soft slab avalanches during and
after periods of snowfall 2) release hard slab avalanches during and after
windy conditions and 3) test snow cover stability.
[20] The goal of avalanche forecasting
according to McClung (2000) “is to minimize the uncertainty about instability
introduced by temporal and spatial variability of the snow cover (including
terrain influences), any incremental changes in snow and weather conditions and
any variations in human perception and estimation.”
[21] Moreover, artificial release of
avalanches is critically dependent on timing in addition to the size of the
explosive and the location of the explosive relative to the snow cover surface
(McClung and Schaerer). Nevertheless, delays in avalanche control work caused
by nightfall and bad weather coupled with the goal of avalanche forecasting,
minimizing rather than eliminating uncertainty, will on occasion result in
unpredictable snow cover behavior.
[22] Thus, closure of the Gothic Road will be
required when explosives are used to prepare the Snowbeast avalanche starting
zone for skiing and snowboarding. This is due to a residual uncertainty in
avalanche control work and a quantifiable chance of inadvertently releasing
avalanches on terrain adjacent to the ski area boundary.
[23] As a first approximation, consistent with
the findings presented in sections 2.2.1 through 2.2.7, the minimum number of
mandatory road closures is 5 per season with each closure extending for an
indefinite period of time; the maximum number of mandatory road closures is 12
per season with each closure extending for an indefinite period of time.
2.2.9 REFERENCES
Armstrong,
B., Williams, K. 1986. The Avalanche Book. Fulcrum, Inc. Golden, CO.
barr, b., 2005. 29 year data set; personal records of
avalanche and weather observations. Rocky Mountain Biological Laboratory,
Gothic. Gothic, CO.
Jamieson, B., Schweizer, J. 2000. Texture and strength
changes of buried surface-hoar layers with implications for dry snow-slab
avalanche release. Journal of Glaciology, Vol. 46, No 152, 2000.
LaChapelle, E. 1966. Encounter probabilities for avalanche
damage. USDA Forest Service Miscellaneous Report 10. Alta, UT: Alta Avalanche
Study Center, Wasatch National Forest.
LaChapelle, E. 1985. The ABC of Avalanche Safety. The
Mountaineers. Seattle, WA.
McClung, D. 2000. Predictions in avalanche forecasting.
Annals of Glaciology. Vol. 31, 2000.
McClung D., Schaerer, P., 1993. The Avalanche Handbook. The
Mountaineers. Seattle, WA.
McClung, D., Schweizer, J. 1999. Skier triggering: snow
temperatures and the stability index for dry slab avalanche initiation. Journal
of Glaciology, Vol. 45, No 149, 1999.
Mears, A., 1995. Snow avalanche technical report - -
environmental impact statement considerations, Crested Butte Mountain Resort
E.I.S. Prepared for Pioneer Environmental Services, Inc. Gunnison, CO.
Perla, R., Martinelli, M. Jr., 1976. Avalanche Handbook. U.S
Department of Agriculture, Handbook 489. Ft Collins, CO.
Schaerer, P., 1981. Handbook of Snow: Principles, Processes,
Management & Use. Pergamon Press. Toronto, Canada.
Schweizer, J., 1999. Review of dry snow slab avalanche
release. Cold Regions Science and Technology, (1999), 30. 43 – 57.
2.3
INCREASED
SNOW TRANSPORT INTO STARTING ZONES RESULTING FROM FOREST REMOVAL[16]
2.3.1
ABSTRACT
[1] A
numerical snow transport model is applied to the loading of avalanche starting
zones immediately adjacent to newly created fetch areas as a result of proposed
ski area expansion on Snodgrass Mountain. Relevant knowledge has been acquired
from two sources: 1) data characteristics and 2) domain knowledge.
Extrapolation is based on the most recent data.
[2] Key
Words: Avalanche starting zone, blowing snow, drifting snow, fetch, load
factor, potential snow transport, snowfall water equivalent, source area.
2.3.2
INTRODUCTION
[3]
According to proposed ski area development option “Snodgrass Light,” removal of
the forest at ridgeline will create a source area for blowing and drifting
snow. Consequently, wind will strip snow from never before exposed areas at
ridgeline and deposit snow in adjacent avalanche starting zones located above
the Gothic Road.
[4] Although two winters of wind
observations recorded at the Crested Butte Ski Area were obtained, preliminary
estimates of maximum snow transport rates were inconclusive due to the brief
period of record. Subsequently, wind behavior characteristics found in the
limited Crested Butte Ski Area data were compared with ten years of wind
observations recovered from a site 24 kilometers north of Snodgrass Mountain.
Comparison of this data along with clues of prevailing wind direction,
topographic features and measurements of proposed areas dedicated to ski runs
and ski lift return stations on Snodgrass Mountain, permitted approximation of
maximum snow transport rates.
2.3.3
METHODS
[5] The
procedure for estimating the quantity of blowing snow contributing to avalanche
loading (Hartman; 1996) is adapted from that proposed by Tabler (1994). To
estimate wind loading over a period of time, potential snow transport is
calculated from the relationship developed by Tabler (1991)
(1) 
where U is the 10 meter (m) wind speed (m/s)
and Qupot is potential snow transport
in time t(s) per meter of width
across the wind.
[6] The total snow available for relocation
over the fetch, Qs is
(2) 
where Qs is in kg per meter of width across
wind, F is fetch length (m) and Swe is snow water equivalent (m) from
precipitation available for relocation.
[7] If potential snow transport is greater
than Qs, then blowing snow deposited
in the starting zone is estimated to be equal to Qs minus the evaporation from blowing snow particles using the
equation developed by Tabler (1975)
(3) 
where Q is snow transport per meter of width
across the wind and X is the maximum
distance that the average size particle can move before completely evaporating.
[8] For a wide range of climates X must be adjusted for temperature and
humidity (Tabler and Schmidt, 1972) using the following equation
(4) 
where RH is relative humidity and T(C) is ambient air temperature.
[9] Wind velocity observations at the summit
of Snodgrass Mountain are nonexistent. Consequently, wind data were recovered
from the Crested Butte Ski Area; however, due to: 1) a brief period of record
2) weather instrumentation in close proximity to the summit of Mt. Crested
Butte resulting in an obstructed field of wind flow to the south and south west
and 3) the effects of small topographic barriers similar to Mt Crested Butte on
wind flow characteristics (Bruintjes et al, 1995; Fohn, 1980), an additional
source of wind data was obtained for purposes of comparison and averaging.
[10] Located 24 kilometers north of Snodgrass
Mountain, data from the Elk Camp weather observation platform were acquired
(Coulter, personal communication, 2005). Selection criteria included: 1)
nearest weather measurement platform 2) weather measurements acquired at an
elevation comparable to the summit of Snodgrass Mountain and 3) length of
record equal to or greater than 10 winter seasons.
[11] Finally, comparison and averaging of the
data along with clues of prevailing wind direction, topographic features and
measurements of proposed areas dedicated to ski runs and ski lift return
stations on Snodgrass Mountain, permitted preliminary approximation of maximum
snow transport rates.
2.3.4
DEFINITIONS
[12] Fetch
length, given proposed forest removal found in the “Snodgrass Light”
development option, is taken to be approximately 100 m. It is recognized that
ski trail width is less than 100 m, for the exception of return station
locations where fetch length is greater. Nevertheless, justification for
assuming fetch length includes: 1) in situ evidence of snow drifting 2) terrain
features located between proposed ski runs and avalanche starting zones and 3)
downwind topographic features (Hartman, 1984).
[13] Two sources of snow water equivalent
contribute to overall snow cover depth in an avalanche starting zone: 1) snow
water equivalent from precipitation and 2) snow water 
Figure 2.3.1: Graphical procedure
for extrapolating wind load over a 6 hour period given a 100 m fetch on
Snodgrass Mountain on January 11, 2005. This period of wind was sufficient to
have transported over 65% of the daily snow water equivalent into nearby
avalanche starting zones.
equivalent
relocated by wind. By proportion to water equivalent from precipitation, the
load factor is defined as a measure of additional snow water equivalent
relocated by wind into an
avalanche
starting zone. Finally, it is believed that most drifting snow will accumulate
in an avalanche starting zone within 100 m downwind of ridgeline (Hartman,
1984).
2.3.5
CALCULATIONS
[14]
Equations 1 through 4, given wind speed recovered from the Crested Butte Ski
Area and Elk Camp weather sites as well as coincident daily precipitation
totals at the RMBL (barr, 2005) over the duration of the January 11, 2005
avalanche cycle (Mears, personal interview, 2005), yield an approximate 1.25
load factor for avalanche starting zones on Snodgrass Mountain.
2.3.6
DISCUSSION
[15] Wind
velocity has never been measured at the summit of Snodgrass Mountain. Moreover,
although two winters of wind observations recorded at the Crested Butte Ski
Area were obtained, estimates of maximum snow transport rates were
inconclusive.
[16] Consequently, wind behavior
characteristics found in the limited Crested Butte Ski Area data were compared
with ten years of wind observations recovered from the Elk Camp site. Thus,
considering wind characteristics associated with the avalanche cycle of January
11, 2005 which exceeded some 1005 m of the Gothic Road (Mears, personal
communication, 2005). the maximum 6 hour cumulative snow transport rate was
typical of a two year return period wind event. Moreover, the data suggest that
a one in ten year wind event could have easily tripled maximum snow transport
rates.
[17] Finally, it is important to recognize
that grooming operations, skiing and snowboarding during episodes of snowfall
combined with drifting will have negligible effects on snow transport rates.
Because snowfall and drifting snow is continuous and grooming operations,
skiing and snowboarding are intermittent, the former condition will dominate
snow transport rates.
2.3.7
CONCLUSION
[18]
Removing trees at ridgeline on Snodgrass Mountain will increase snow transport
rates into nearby avalanche starting zones. Preliminary load factor
calculations predict that drifting snow will augment traditional snow cover
depths by approximately 50 percent. Finally, 6 hour cumulative wind transport
rates associated with the January 11, 2005 avalanche cycle could be easily
tripled given a ten year return period wind event.
2.3.8 REFERENCES
barr, b.,
2005. 29 year data set; personal records of avalanche and weather observations.
Rocky Mountain Biological Laboratory, Gothic. Gothic, CO.
Bruintjes, R., Clark, T., Hall, W.
1994. Interaction between topographic airflow and cloud / precipitation
development during the passage of a winter storm in Arizona, Journal of
Atmospheric Sciences, Vol. 51, No. 1.
Fohn, P. 1980. Snow transport over mountain crests, Journal
of Glaciology, Vol. 26, No. 94.
Hartman, H. 1984. Snow redistribution from fetch to starting
zone, International Snow Science Workshop (Aspen, CO; 24-27 October, 1984)
Proceedings, 196 -197.
Hartman, H. 1996. Forecasting high continental avalanches: A
field workers perspective, International Symposium on Snow & Related
Manifestations (Manali, H.P., India, 26-28 September, 1994) Proceedings,
425-429.
Tabler, R. 1975. Estimating the transport and evaporation of
blowing snow, Symposium on Snow Management on the Great Plains (Bismarck, N.
Dakota; July, 1975) Proceedings, Great Plains Agricultural Council Publication
73, 85-104.
Tabler, R. 1991. Snow transport as a function of wind speed
and height, Cold Regions Engineering, Cold Regions Sixth International
Specialty Conference (West Lebanon, New Hampshire, 26-28 February,1991)
Proceedings, 729-738.
Tabler, R. 1994. Design guidelines for the control of
blowing and drifting snow, Strategic Highway Research Program, Report No.
SHRP-H-381, National Research Council, Washington, D.C.
Tabler, R. and Schmidt, R.A. 1972. Weather conditions that
determine snow transport distances at a site in Wyoming, UNESCO/WMO. Symposia
on the Role of Snow and Ice in Hydrology (Banff, Alberta; 6-13 September, 1972)
Proceedings, 118-127.
2.4
INCREASED
AVALANCHE FREQUENCY FROM EFFECTS DISCUSSED IN 2.3[17]
2.4.1
ABSTRACT
[1]
Removing trees at ridgeline on Snodgrass Mountain will increase snow transport
rates into nearby avalanche starting zones. Preliminary load factor
calculations found in section 2.3 predict that during windy conditions,
drifting snow will augment snow cover depths by approximately 50 percent. As a
result, avalanche characteristics established over the past century will be
altered. A change in avalanche characteristics is assessed given relevant
knowledge acquired from three sources: 1) data characteristics 2) domain
knowledge and 3) expert opinion.
[2] Key
words: Avalanche starting zone, cohesive, core, deformation, drifting,
engelmann spruce, fracture line height, fracture propagation, hard slab,
increment borer, lodgepole pine, radiation flux, snow transport rate, soft
slab, source area, stress field, subalpine fir, wet, wet loose.
2.4.2
INTRODUCTION
[3] Forests
affect snow distribution, snow cover strength and radiation flux. For example,
forests on the windward side of ridges prevent wind relocated snow from
reaching adjacent avalanche starting zones (McClung and Schaerer, 1993). In
avalanche starting zones forest features such as tree stems, tree branches
reaching into the snow cover and compacted snow below tree crowns alter snow
cover deformation and stress fields (Gubler and Rychetnik, 1990). Also, the
forest canopy moderates heat exchange between the snow cover and the atmosphere
(Schaerer, 1981). Thus, trees at ridgeline and in avalanche starting zones
serve as an effective avalanche prevention system.
2.4.3
METHODS
[4]
Avalanche occurrence records recovered from the RMBL (barr, 2005) were censored
to include avalanche paths Snodgrass 13 through 17. Censored data were then
filtered to include avalanches originating at or near ridgeline which reached
their respective runout zones. Subsequently, the censoring and filtering
process identified 91 large avalanches observed over the past 29 years among
the 5 avalanche paths. Also, data were binned by type and size (Perla and
Martinelli, 1976).
[5] Weather observation records recovered
from the RMBL (barr, 2005) were censored to include 3 day water equivalent leading
up to the dates of avalanche activity mentioned above. Three day water
equivalent concurrent to avalanche activity was then correlated with avalanche
size. Finally the median, a common robust and resistant measure of central
tendency (Wilks, 1995), water equivalent value in each avalanche size class was
selected and plotted.
[6] Field observations on Snodgrass Mountain
were conducted June 20 through June 23, 2005. Surveys included: 1) avalanche
path characteristics 2) ridgeline topographic features 3) estimates of forest
age, density and diversity at ridgeline as well as in avalanche paths and 4)
inspection of forest and topographic features separating avalanche paths.
Figure 2.4.1: Avalanche size as a function
of 3 day precipitation. On Snodgrass Mountain, avalanche size is dependent on
snow supply.
2.4.4
FINDINGS
[7]
Censoring avalanche occurrence records specific to Snodgrass 13 through 17
identified 513 avalanches observed over the past 29 years. Of the
aforementioned avalanches, 416 are identified as soft slab with an average
fracture line height of 0.7 m whereas 5 are identified as hard slab with an
average fracture line height of 0.6 m (McClung and Schaerer, 1993). The
remaining 92 are characterized as loose, wet loose or wet slab (McClung and Schaerer,
1993).
[8] Avalanche size as a function of 3 day
precipitation is presented graphically in Figure 2.4.1. Furthermore, on
Snodgrass Mountain, Figures 2.2.1 and 2.4.1 imply that the probability of
avalanche occurrence and avalanche size is strongly dependent on snow supply.
[9] Ridgeline topographic features,
beginning 100 m upwind of avalanche starting zones Snodgrass 13 through 16, are
characterized as convex with initial slope angles ranging from + 6° to + 24°,
positive slope taken as ascending, while slope angles approaching avalanche
starting zones range between - 2° and - 12° respectively.
[10] Trees found at the summit of Snodgrass
Mountain and at ridgeline are engelmann spruce, lodgepole pine and subalpine
fir. Developmental classes are seedling, sapling, pole, mature and senile
(Burrows and Burrows, 1976). Individual tree heights reach approximately 17 m
and forest density exceeds 1000 trees per hectare. Cores recovered with an
incremental borer indicate that the age of mature trees exceeds 200 years.
[11] Near the summit of Snodgrass Mountain,
avalanche starting zone angles range from 30° to 45°. Topographic features are
concave across as well as down the fall line.
[12] Trees located in avalanche starting zones
near the summit of Snodgrass Mountain are engelmann spruce, lodgepole pine and
subalpine fir. Near ridgeline where slope angle exceeds 35° and tree density
reaches approximately 1000 trees per hectare, diffuse avalanche indicators are
present (Burrows and Burrows, 1976). However, openings in the forest with
minimum lengths of 30 m and widths of 15 m are numerous. Farther downslope,
response of trees to tilting, breakage and scarring is extensive (Burrows and
Burrows, 1976).
[13] Finally mid slope, between the summit of
Snodgrass Mountain and the Gothic Road, tree species separating avalanche paths
Snodgrass 13 through 17 include those mentioned above. Dominant developmental
classes are seedling, sapling, and pole. The presence of mature trees is
limited and senile trees are rare where forest density is approximately 500
trees per hectare. Where the forest canopy trends from open to closed (Burrows
and Burrows, 1974), diffuse avalanche indicators are present.
2.4.5
DISCUSSION
[14]
Transects recovered from areas slated for tree removal at various points along
the Snodgrass Mountain ridgeline illustrate a uniform convex or “hump-shaped”
terrain profile upwind of avalanche starting zones. Combining the
aforementioned profiles with downwind terrain features, Hartman (1984)
demonstrates that similar terrain relationships maximize source area drifting
and avalanche starting zone deposition. Consequently the supply, distribution,
and hardness of snow reaching avalanche starting zones characteristic of the
past 100 years will be altered.
[15] Snow supply determines avalanche size and
frequency. Consistent with findings presented in section 2.3, Increased Snow
Transport into Starting Zones Resulting from Forest Removal, preliminary load
factor calculations predict that drifting snow will augment traditional snow
cover depths by approximately 50 percent. Furthermore, 6 hour cumulative wind
transport rates associated with the January 11, 2005 avalanche cycle could
easily be tripled given a ten year return period wind event. Thus, there is
solid evidence that avalanche frequency and size will increase due to a boost
in snow supply as a result of wind loading.
[16] Distribution patterns of snow deposited
in the upper reaches of avalanche paths will be redefined as a result of
removing trees at ridgeline. For example, potential avalanche energy levels,
atypical of the past 100 years, will increase due to concentrated masses of
snow deposited near ridgeline.
[17] By dimensional analysis, the impact
pressure of flowing snow is estimated by
(1) 
where I is impact pressure, ρ is snow density and v is velocity. It is well known that
wind effects reduce snow crystal size; consequently, once deposited in an
avalanche starting zone, fragmented snow particles increase slab density.
Finally, as equation 1 points out, impact pressure increases proportional to
snow density.
[18] Jamieson and Johnston (1988), and Rosso
(1986) show in situ a dependence of tensile strength on density and
microstructure. For example, Rosso demonstrates that as slab density increases
from 140 kg·m-3 to 260 kg·m-3, tensile strength increases
by an approximate factor of 16. Consequently, high tensile strength values
maximize the initial length scale of fracture zones (Gubler and Rychetnik,
1990).
[19] Greater slab avalanche fracture heights
as well as fracture lengths will occur as a result of thick wind relocated
deposits of snow located near ridgeline. Gubler and Rychetnik (1990) say that
slab avalanches with fracture heights between 0.8 m and 1.5 m may start in
forest openings with minimum lengths of 30 m and widths of 15 m. Moreover,
McClung and Schaerer (1993) claim that a minimum forest density of 1000 trees
per hectare is required to prevent avalanche formation. Therefore, considering
findings mentioned above, decay of the forest between ridgeline on Snodgrass
Mountain and the Gothic Road will occur.
2.4.6 CONCLUSION
[20] Creating source areas for drifting snow
by cutting trees at ridgeline will alter the supply, distribution, and hardness
of snow reaching avalanche starting zones characteristic of the past 100 years.
As a consequence, once set in motion downslope, thick deposits of dense wind
relocated snow will dramatically thin the existing forest cover. As the forest
decays and loses its protective qualities, long-term avalanche characteristics
will be redefined.
[21] As a final point, 29 years of avalanche
observations at the RMBL indicate avalanches involving dense wind relocated
snow account for less than 1.5 percent of all occurrences. On Snodgrass
Mountain, less than 1 percent of all avalanches consist of dense wind relocated
snow; this is due entirely to the protective nature of the existing forest
cover.
2.4.7 REFERENCES
barr, b.,
2005. 29 year data set; personal records of avalanche and weather observations.
Rocky Mountain Biological Laboratory, Gothic. Gothic, CO.
Burrows, C. and Burrows, V., 1976. Procedures for the study
of snow avalanche chronology using growth layers of woody plants. Occasional
Paper No. 23, 1976, Institute of Arctic and Alpine Research. University of
Colorado at Boulder. Boulder, CO.
Gubler, H. and Rychetnik, J. 1990. Effects of forests near
timberline on avalanche formation. International Snow Science Workshop
(Bigfork, Montana. 9-13 October, 1990) Proceedings, 73 -93.
Hartman, H. 1984. Snow redistribution from fetch to starting
zone, International Snow Science Workshop (Aspen, CO. 24-27 October, 1984)
Proceedings, 196 -197.
Jamieson, J. and Johnston, C. 1988. In situ tensile strength
measurements of alpine snow. International Snow Science Workshop (Whistler,
B.C. 12 -15 October, 1988) Proceedings, 103 -112.
LaChapelle, E. 1985. The ABC of Avalanche Safety. The
Mountaineers. Seattle, WA.
McClung D., Schaerer, P., 1993. The Avalanche Handbook. The
Mountaineers. Seattle, WA.
Perla, R., Martinelli, M. Jr., 1976. Avalanche Handbook. U.S
Department of Agriculture, Handbook 489. Ft Collins, CO.
Rosso, R. 1986. In situ measurements of the snowpack.
International Snow Science Workshop (Tahoe, California. 22-25 October, 1986)
Proceedings, 210 – 215.
Schaerer, P., 1981. Handbook of Snow: Principles, Processes,
Management & Use. Pergamon Press. Toronto, Canada.
Wilks, D. 1995. Statistical Methods in the Atmospheric
Sciences. Academic Press. San Diego, CA.
2.5
INCREASED AVALANCHE SIZE RESULTING FROM DRIFTING SNOW AND
SEDMAK CABIN EXPOSURE[18]
2.5.1
Data Used.
The average depths of new snow slabs are required data in some
avalanche-dynamics models, including AVAL-1D which was used in this report.
Slab depths during 30, 100, and 300 year return period avalanche-release
conditions were calculated from detailed daily winter weather records taken at
Gothic since 1975. To determine slab
depths of new snow that could be released during single large avalanches the
largest 10 storms during the period of record were studied. Storms were defined as unbroken periods of
snowfall in which the snow depth at the Gothic study plot continuously
increased during the storm (i.e. where settlement, which increases the strength
and stability of the snowslab was exceeded by new snow accumulation, which
increases the stress). The 10 largest
storms ranged in duration from 6 to 15 days.
Data taken for each storm included:
dates, maximum and minimum temperatures, daily new snow, total new snow
(S daily
snowfall), total water equivalent of new snow, and total snowpack depth. Average slab densities, r, were
computed r = S(H2O)/(D HS). Data are summarized in the following table.
Table
1. Periods of thickest new-snow slab
formation, Gothic (1975 – 2005)
|
Dates of persistent snow depth increase (storm) |
S (storm
H2O equiv., m) |
D HS
(depth increase, m) |
r
(average density, kg/m3) |
Duration (days) |
|
Feb 15-26, 1993 |
0.189 |
0.87 |
217 |
11 |
|
Feb 3 – 11, 1994 |
0.102 |
0.90 |
113 |
8 |
|
Feb 27-Mar 6, 1995 |
0.142 |
1.00 |
142 |
7 |
|
Jan 3-11, 2005 |
0.162 |
1.08 |
150 |
8 |
|
Feb 7-14, 1995 |
0.158 |
1.11 |
142 |
7 |
|
Dec 29-Jan 5, 1981/82 |
0.121 |
1.20 |
101 |
7 |
|
Feb 26-Mar 3, 1978 |
0.122 |
1.20 |
102 |
6 |
|
Jan 16-31, 1996 |
0.182 |
1.27 |
143 |
15 |
|
Feb 10-21, 1986 |
0.251 |
1.29 |
195 |
11 |
|
Feb 13-23, 1980 |
0.200 |
1.36 |
147 |
10 |
2.5.2
Release slab thickness. Slab thickness, D, (slope normalized
component of depth, H), is used in avalanche-dynamics computations. To compute velocity, impact pressure, and
runout distance, slab depths of 30 and 300 year return period conditions are
usually considered in calculations, mapping and avalanche zoning. The 300-year return period is also used to
define the design avalanche for land-use planning and engineering in Gunnison
County. We extrapolated the slab depth
increases in Table 1 to the longer return period used in County land-use
regulations by assigning each data entry in column 3 a “return period”, based
on the 30 seasons of historical record[19]
then plotting and fitting the return period, T, vs. slab depth, H, to a)
log-normal and b) double log (“extreme- value” or Gumble) distributions. The log-normal distribution predicted DHS = 1.92m for a 300-year event; the extreme-value
distribution predicted DHS = 1.64m
for a 300-year event. The Gumble
extreme-value distribution is preferred because the fitted line provides a
better fit to the data (R2 = 0.93 verses 0.83 for the log-normal
distribution) and extreme-value distributions have been previously shown to
provide a good fit to hydrologic and avalanche data (Linsley, et. al, 1958;
McClung and Mears, 1991; McClung and Schaerer, 1993). The extreme-value distribution is shown below where the y-axis
represents the measured (or predicted) slab height, and the x-axis is the
“reduced variant,” RV of the return period T, where RV = Ln(Ln[T]). Finally, these slab depths were increased by
5% to 10%, depending on location, for the higher elevations of the starting
zones. Slab thicknesses were calculated by multiplying starting zones depths by
cos q, where q is the
starting-zone slope angle.
2.5.3
Avalanche-dynamics analysis. The AVAL-1D dynamics model (Christen,
Bartlet and Gruber, 2002) simulates avalanche runout distances, flow depths,
velocities and impact pressures. It was
used to compute avalanche runout distance and define the red and blue hazard
zones in conformance with the definitions in the Gunnison County Land Use
Resolution (the “LUR”). This program was calibrated to local snow climate,
snowpack and observed avalanche conditions that have been documented in the
East River valley near Gothic and other local areas. The following steps were used in application of AVAL-1D:
(a)
Potential slab-release areas were identified in the field,
from historical observations, and through analysis of topographic maps,
Gunnison County orthophotos, and U.S. Forest Service stereo photographs. The hazard zones were based on Gunnison
County criteria[20].
(b)
Avalanche path steepness was measured on the ground and
scaled from detailed topographic maps and/or U.S. Geological Survey quadrangle
maps.
(c)
Slab depths released from starting zones were calculated for
various return periods by the methods outlined in section 2.5.2.
(d)
Released slab volumes were calculated (by AVAL-1D) by
multiplying slab thickness times slab area.
(e)
The width of avalanche starting zones, tracks and runout
zones during large avalanches was measured on the ground, on topographic maps
and from direct observations.
(f)
Avalanche runout distances, flow heights, velocities, and
impact pressures were computed by AVAL-1D.
(g)
The predicted runout distances were compared with large
avalanches with known, observed runout distances, thus calibrating the
conditions to those that have occurred in this central Colorado snow and
avalanche climate.
(h)
The red and blue hazard zones (as defined in Gunnison
County) were computed by AVAL-1D and modified, where appropriate, with
avalanche frequency data obtained in the Gothic area.
2.5.4
Avalanche frequency and size increase at the Sedmak cabins
FIG 2.5.1 2.51
The so-called Sedmak[21]
cabins are located below a small aspen forest. Directly above the forest a
distinct avalanche starting zone exists.
The existing cabins lower left center) and the starting zone above the
aspen forest (center) are shown in photo 2.5.1. Small-to-medium sized avalanches have penetrated the forest and
approached to within approximately 50m (160 feet) of the southern cabin, as
evidenced by forest growth patterns and damage to the forest. An avalanche dynamics and runout analysis
was completed as described above in Sections 2.5.1 through 2.5.3. Additional wind-relocated snow blown from
forest removed from Snodgrass Mountain was
not assumed in this case. The
analysis indicated that major avalanches with return periods in the 100-300
year range will impact the cabins, run through the forest and cross the Gothic
Road. This is a natural consequence of
the current location of these buildings.
However, the starting-zone slopes above the forest will, under the
preferred CBMR alternative, be reached far more often by skiers and
snowboarders due to the open-gate policy.
This increased use will cause avalanches of all sizes to be triggered approximately
2.6 times as often based on the expert opinions obtained and reported in
Section 2.1. Consequently, these cabins
will be reached and damaged by design avalanches and smaller avalanches more
often than at present.
2.5.5
References
Christen, M, P. Bartelt and U. Gruber, 2002, AVAL-1D,
Numerical Calculation of dense flow and powder snow avalanches, Swiss Federal
Institute for Snow and Avalanche Research, Davos, Switzerland, 132 pps.
Linsley, R., M. Kohler and J. L. H. Paulhus, 1958, Hydrology
for Engineers, McGraw-Hill Book Company, Inc., New York, 340 pps.
McClung, D.M. and A. I. Mears, 1991, Extreme-value
prediction of snow avalanche runout:
Cold Regions Science and Technology, v. 19, p. 163-175.
McClung, D.M. and P. Schaerer, 1993, The Avalanche Handbook,
The Mountaineers, Seattle, 271 pps.
2.6
CHANGE IN AVALANCHE EXPOSURE TO THE SEEMAN CABIN DUE TO
WIND-RELOCATED SNOW[22]
2.6.1 General
Fig 2.6.1
The results of the analyses described in sections
2.5.1 through 2.5.3 were applied to avalanche paths above existing or proposed
buildings, bridges, or other fixed facilities, including the Seeman cabin. The photograph (2.6.1) is a view of the this
cabin taken from the lower track of the Sedmak South avalanche path, approximately
1000 ft (300m) east and 50m (160 feet) above the building. Photograph 2.6.2 is an overview of the
avalanche terrain taken from the south side of Gothic Mountain showing the
cabin (with the red roof) at the left side of the photo. The edge of the aspen forest may result from
the runout of major avalanches. The
large avalanches of 1986 and 2005 both stopped within approximately 20m – 30m
of the western (right) edge of the forest and 70m from the cabin. Judging from
the forest age and apparent frequency these avalanches probably had return
periods of 30 years or less, an estimate correct to the nearest half order of
magnitude. These observed avalanche runouts were used to calibrate the
coefficients used in AVAL-1D. The
runout distance of the 300-year event in this path has not been observed
therefore it was simulated through use of the model. The procedure for using
AVAL-1D for computing the increased runout distance resulting from wind
redistributed snow and slab thickness increase is described in 2.6.2.
2.6.2
Slab thickness increase
Snow slabs in starting zones will increase in thickness as a
result of wind-transported snow (Section 2.3) resulting from increase timber
removal from the summit and ridges of Snodgrass Mountain. We computed the
average increase in slab thickness utilizing the following relationship:
(1)
Dadj = [1 + (EF – 1)(100/296)(1.1)]H Gothic(300)
cos θ, where
Dadj is the adjusted slab depth, EF is the
wind-drift enhancement factor over the upper 100m slope length of the starting
zone (Hartman, pers. comm., March, 2005), the factor (100/296) distributes this
wind-loaded snow over the 296m starting zone slope length in Sedmak South, the
factor (1.1) adjusts slab depth at Gothic to the starting zone approximately
350-400m above Gothic, H Gothic(300) is the 300-year slab depth at Gothic
(Sec. 2.5.2) and θ is the mean starting-zone angle. From (1) we calculate an
Fig. 2.6.2
adjusted slab thickness, Dadj of 1.69m in the
starting zone of Sedmak South. Dadj
was then used simulate the avalanche dynamics using AVAL-1D. Internal and external friction terms in the
AVAL-1D model were determined by calibration from observed avalanches in the
immediate area. The terrain was measured on the ground and on topographic maps.
The 300-year avalanche, assuming current conditions, will
flow 60m (200 feet) farther than the 30-year event in an easterly direction
toward the cabin, stopping 20m (65 feet) short of it. However, the 300-year avalanche will flow 40m farther when
additional snow is blown into the starting zone and the slab thickness is
increased as a result of forest removal from the summit of Snodgrass. The Sedmak South avalanche will therefore
impact the cabin and travel 20m (65 feet) past it after the slab has been
increased in thickness by the wind-transported snow. This exposure to the design avalanche at the Seeman cabin does
not exist currently.
2.7
Fig 2-3
CHANGE IN PROBABILITY OF AVALANCHES REACHING RMBL PROPERTY[23]
Fig 2.6.2
Private property within the RMBL and adjacent private
property (e.g. within the Gothic Heights Subdivision on the lower Gothic Mt.
slopes) will be reached by avalanches more often than at present as a result of
the open-gate policy that enables access to avalanche-prone slopes. This is true because the entire avalanche
area beginning at Snodgrass Mountain and extending onto Gothic Mountain is one
avalanche path, given the proper snowpack conditions. Photos 2.6.2 and 2.7.1 provide overviews showing the terrain that
can be connected by a single fracture.
Continuous and connecting slab fractures up to 1,450m (4,750
feet) long have been observed on five occasions[24]
in avalanche paths on the northeast exposures of Snodgrass Mountain during the
past 29 winters, an average return period of only 6 years. This occurred most
recently in January 2005 when the entire northeast exposure of Snodgrass
released simultaneously. Long,
continuous slab fractures have also been observed on the lower starting zones
of Gothic Mountain (below the 10,500 to 11,000-foot elevation level), directly
above RMBL facilities. These observed
fractures on Gothic Mountain[25]
have been up to 1,200m (4,000 feet) long.
The lower slopes on Gothic Mountain do not avalanche as often as the
steep slopes on the upper portion of the mountain, however, because of the
gentler gradient these lower-slope avalanches can release deeper slabs, be
larger and will affect a much wider area.
The avalanches from the lower slopes, therefore, have the potential to
affect more of the RMBL facilities than avalanches from the upper slopes of the
mountain. Two cabins were reached and
one damaged in 1978 from avalanches beginning on the lower slopes of Gothic
Mountain, below 11,000 feet elevation.
The long, connecting slab avalanches discussed above are
themselves connected by an avalanche path located where the lower east-facing
slopes of Gothic Mountain intersect the northeast-facing slopes of
Snodgrass Mountain. A continuous
fracture approximately 200m (650 feet) long at the 10,800-foot level was
observed within this intersection area in February 1996. This avalanche fractured on slope exposures
ranging from northeast through southeast.[26]
Fig 2.7.1
Direct observations, therefore, indicate that the
northeast-facing terrain on Snodgrass Mountain is connected to the east-facing
terrain on Gothic Mountain. During
unstable snowpack conditions, long, continuous slab fractures may initiate at
either the northern or southern end of this “Snodgrass-Gothic avalanche
complex” and propagate for as much as 3,000m (2 miles) in either
direction. Such conditions are
inevitable considering the 300-year return period that must be considered in
land-use planning and engineering in Gunnison County.
The Snodgrass Mountain slopes would become heavily used
(compared with current backcountry use) if an open-gate policy[27]
were in effect. The slopes would certainly be used during periods of higher snowpack
instability than at present. This is
true because an open-gate policy will lure skiers and snowboarders who may have
little knowledge of avalanche conditions onto steep, avalanche-prone slopes. This may occur even when the snowpack is
unstable and avalanches are likely.
This increased use of avalanche terrain during potentially dangerous
conditions would lead to a greater frequency of avalanches of all sizes
(discussed in Section 2.1). Some of
these avalanches would propagate for long distances above private property and
some of these will produce avalanches that will reach private property and
buildings.
We conclude, based on the of the findings of Section 2.1 and
this section, that avalanches will reach the RMBL property, including some
buildings, 2.6 times as often as at present.
This increased exposure will occur primarily within the “red,” or
“high-hazard” avalanche area as delineated on the avalanche map “Gothic
Mountain Avalanches & RMBL.”
Avalanche frequency would probably also increase within the “blue,” or
“moderate-hazard” zone.
Report prepared by:
Hal Hartman, applied physicist, Snowmass, Colorado.
Art Mears, P.E., Avalanche-control engineer, Gunnison,
Colorado.