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. 



[1] This is a simple average of the expert opinions; this factor would be larger (approximately 2.8) when only the opinions of experts working at ski areas are averaged.

[2] Encounter probability expresses the chance that a person or building will be hit by an avalanche given a finite exposure time and knowledge of avalanche probabilities or return periods.

[3] “Snowbeast” is an unofficial name.

[4] The latest major avalanche was on Feb 14, 1995.

[5] The Gothic Road was covered over 3,300’ of length during this period of avalanching.

[6] However, precipitation during this storm was characteristic of a 10-year return period event.

[7] Starting zone – Slope generally > 30° inclination.  Avalanches release, increase in mass, and accelerate in this zone.

[8] Hard slab refers to dense layers of snow which are usually deposited by wind.

[9] One cabin has recently been purchased and is no longer owned by “Sedmak.”

[10] Runout zone – The lower, or distal zone of the avalanche path where avalanches decelerate, stop, and form a deposit of high density snow and entrained debris.

[11] The “Sedmak South” path is path #13 in the RMBL data set.

[12] Avalanches starting from the 10,500 to 11,000 foot level were observed in March, 1978.  Two cabins were impacted and damaged by avalanches starting from these elevations on Gothic Mountain.

[13] The “Dutch Draw” avalanche fatality adjacent to the Canyons Ski Area in Utah is the latest example.  This occurred in January, 2005.  It was a large, long-running avalanche triggered in uncontrolled backcountry terrain and accessed from the top of the Canyons “9990” lift.  Another example is a “100-200 year return-period” avalanche triggered from the north face of Mt. Alyeska Ski Area, Alaska in 1981. 

[14] Prepared by Hal Hartman, Applied Physicist, Snowmass, CO. July 13, 2005.

[15] Prepared by Hal Hartman, Applied Physicist, Snowmass, CO. July 13, 2005

[16] Prepared by Hal Hartman, Applied Physicist, Snowmass, CO. July 13, 2005

[17] Prepared by Hal Hartman, Applied Physicist, Snowmass, CO. July 13, 2005.

[18] Prepared by Art Mears, P.E., avalanche-control engineer, Gunnison, CO, July 2005.

[19] For example, the largest value has a return period of 30 years, the 2nd largest 15 years, the 3rd largest 10 years, and so on.

[20] Avalanche areas are subdivided into “red” and “blue”  hazard zones defined as follows:  Red (high hazard) zone – affected by avalanches with return periods of 30 years or less and/or by avalanches producing impact pressures on flat surfaces normal to the flow direction of 600 lbs/ft2 or more.  Residential construction shall be prohibited in Red Zones; driveways and subdivision roads shall avoid areas where avalanches have return periods of fewer than 10 years; utilities shall be buried or otherwise designed to minimize avalanche exposure.  Blue (special engineering) zone – affected by avalanches with return periods of more than 30 years but less than 300 years and by avalanches capable of producing impact pressures on flat surfaces normal to the flow direction of less than 600 lbs/ft2.  Residential building construction shall be permitted in the Blue zone only if that construction has been certified by a qualified professional engineer licensed in the State of Colorado to withstand avalanche impact and static loads and that the structure has been otherwise protected by external avalanche-defense structures that have been similarly certified.  (Gunnison County LUR, January 8, 2001).

[21] One cabin has recently been purchased and is no longer owned by Sedmak.

[22] Prepared by Art Meears, P.E., Avalanche-control engineer, Gunnison, CO, July 2005.

[23] Prepared by Art Mears, P.E., Avalanche-control engineer, Gunnison, CO, July 2005.

[24] In 1976, 1984, 1986, 1995, and 2005.

[25] The latest observation was in March, 1978.

[26] Fractures across slopes of varying exposures is not unusual.  For example, fractures in a cirque north of Gothic Mountain in February, 1995 were continuous from north through south (270° of exposure variation).  This has been observed in other mountain ranges as well.  The necessary ingredients appear to be (a) a continuous slab, (b) a relatively weak layer, and (c) a high-quality shear fracture.

[27] An open gate policy enables ski area users to enter the backcountry from developed ski areas “at their own risk”, even if these areas are within avalanche terrain.