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US HIGHWAY 550 AVALANCHE REDUCTION PROJECT
SAN JUAN MOUNTAINS OF COLORADO
Don Bachman
Denny Hogan
Lead and Associate Forecaster
CAIC/CDOT Silverton Avalanche Forecast Office
Post Office Box 418
Silverton, Colorado USA 81433
INTRODUCTION
In September 1992, the Colorado Department of Transportation (CDOT) contracted the
Colorado Department of Natural Resources, Avalanche Information Center (CAIC) to provide
avalanche condition evaluations, forecasts and recommendations for avalanche control and
road closure, and other related services for U.S. Highway #550 in the San Juan Mountains
of southwestern Colorado. Two CAIC forecasters are based midway along the 37mi/54Km
avalanche-prone route, in the town of Silverton (Pop. 400; 9320'/2830m asl), from November
1 through May 1. This paper will describe project elements and forecasting methodology and
will summarize weather, avalanche events and forecasting results for two winter seasons.
Setting
The discovery of gold and silver in the late l9th century dictated the
need for access into the highly active avalanche terrain of the San Juans. US Highway
#550, which follows an historic wagon road, is now an important transportation corridor,
linking Farmington, New Mexico and Durango, Colorado with the Montrose/Delta/Grand
Junction region to the north. Three Passes (Coal Bank, 10580'/3216m; Molas, 10910'/3316m
and Red Mountain, 11090'/3370m) are traversed by about 400 vehicles/day during the winter,
passing under or across 101 identified avalanche paths.
The area of greatest concern on the highway is the East Riverside
Slide, scene of 5 fatalities through 1987. In 1985, a 185' wide avalanche shed, costing
$2.7 million was constructed at the main gully intersection of this path. In March, 1992,
a sixth fatality resulted from an avalanche on an adjacent path, just north of the shed.
This incident prompted the development of the avalanche reduction program. The East
Riverside area accounts for no more than 20% of the highway hazard, and dozens of other
incidents involving the public and CDOT personnel have occurred over the length of the
highway, some with injuries.
The town of Silverton is totally dependent upon US t550 for access
beyond the mountain valley. No other populated community in the conterminous United States
has such a potential for isolation due to avalanche related road closure, which disrupts
daily delivery of mail, goods and services and access to medical services.
Snow climate
The project area is characterized by relatively low snowfall, cold temperatures, and
location considerably inland from coastal areas, factors which define a continental snow
climate. Weather systems most commonly produce orographic precipitation, which is
dependent upon the position of terrain features in relation to the regional setting. Thus
precipitation events will generally favor one portion of the highway over others during
much of a storm period. Snow cover is often shallow (60" - 120" in the starting
zones) and develops in a pattern of frequent storms having < 12" per 24 hr.
period. This condition leads to persistent structural weakness in the snow pack from large
temperature gradients, and development of faceted grains at basal and mid-pack layers.
(Higher intensity snowfall, > 12"/24 hr. produce most natural avalanche events.)
Average winter water equivalent at Red Mountain Pass is around 25", while extremes
can range from 12" to 40". Temperatures generally range from -25 degrees C to +5
degrees C. These elements, coupled with frequent ridge top winds > 15 mph, will result
in both hard and soft slab avalanches, during the winter season. Spring temperature
warm-up periods will often produce wet avalanches across the road.
Avalanche Inventory
Avalanche paths which cross the highway range in vertical fall
dimension from 60' to 3200' with starting zones of < 1 acre to 150 acres or greater in
a few cases. Based upon frequency estimates of return period, the project lists 25 paths
that run one or more times annually, 35 paths at 1-5 years and 16 paths at 5-10 years. The
remaining paths release erratically, but most have reached the highway in the past 25
years. Eighty-three avalanche paths can produce damaging events, while 19 of these would
destroy a heavy vehicle. Frequency estimates are based upon 40 years of record (of varying
quality) from CDOT, the US Forest Service and the Institute of Arctic and Alpine Research
(INSTAAR).
AVALANCHE FORECASTING
Methods
Avalanche forecasting methods utilize conventional observations and subjective analysis by
experienced forecasters. Each forecaster has more than 20 years avalanche related
experience in the continental snow climate, including 12 years of specific forecasting
responsibilities for the lead forecaster. Snowpack measurements are made at four sites
along the highway corridor. Total depth and snowfall totals are recorded. Frequent
ramsonde and snowpit investigations were made. Daily observations of highway avalanche
paths and backcountry indicator samples helped forecasters develop pattern recognition
skills. Each observed avalanche occurrence was recorded in detail and entered, along with
weather and snowpack observations, onto a data base. Continuous monitoring of storm
periods by traveling the highway verified forecasted potential. Radio communication and
contact between forecasters and CDOT maintenance personnel helped develop immediate
response to changing conditions.
During the 1993-94 season two remote instrument packages providing wind
speed and direction, and temperature information were installed on a ridge above the north
segment of the highway at 12000'/3660m and at 12900'/3,930m adjacent to Red Mountain Pass.
Five Soil Conservation Service SNOTEL sites, located along the highway corridor for water
yield and management purposes, provided daily precipitation and temperature information.
Advisories
The core of the project is the development and dissemination of daily
avalanche potential evaluation and forecast advisories for three highway segments.
Evaluations are made for the time of the advisory, and forecasts are routinely issued for
a 25 hour period. Updates are issued in response to changing conditions. Avalanche
forecasts are based upon quantitative weather forecasts provided by the CAIC Forecast
Office in Denver and real time stability evaluations.
Avalanche release potential is characterized as low, moderate, high and
extreme which are defined in terms ranging from very unlikely to certain. Travel and
maintenance restrictions vary from none, to total road closure. The advisories are
distributed to CDOT personnel, law enforcement officials and other interests through
personal contact and FAX line.
Control & closure recommendations
Contracted obligations with CDOT extend to recommendations for control
and closure of the highway when necessary. These recommendations are included in the
advisory and specify which highway segments and groups of paths require control work and
by what methods. Closure recommendations may be made prior to control efforts when
potential for natural release renders highway unsafe for travel. The highway is then swept
and the gates shut until safe travel can resume. CDOT then responds with explosive control
methods which may include helicopter bombing and avalauncher delivery of high explosives.
The avalanche forecasters accompany control teams to help direct the efforts and record
results.
OUTCOME
Weather summary
The 1992-93 winter was characterized by a series of strong storm systems which entered the
San Juans on a southern fetch with comparatively mild temperatures and abundant moisture.
Snowfall water equivalent on Red Mountain Pass was 36.50", well above the mean of
25.50" established over a 12 yr. period of accurate data gathered by INSTAAR from
1971-1983.
The 1993-94 winter was much lighter with a snowfall water equivalent of
17.00. on Red Mountain Pass at the 3rd week in April.
Avalanche summary
A total of 142 avalanches crossed the highway during the 1992-93 winter. Explosive control
accounted for 65, while 87 released naturally Of the 87, 25 small events crossed the road
while it was open and the remainder after road closure. Debris dimensions at centerline
ranged from about 2' x 30' to 4' x 1300' to 20' x 250'. During an extraordinary
storm period from 2/19 through 2/24, 42 avalanches crossed the road, covering the
centerline for a total distance of 3.9mi/6.3Km, with average dimensions of 6.8' x 500'.
All but a few of these events were natural while the road was closed. Total road closure
days south of Silverton were 7 days and north of Silverton (Red Mountain Pass) 17 days.
Numerous other short closures were made for control work. Two incidents were noted. An
avalanche struck the mail truck, with no damage, and a fully loaded semi tractor-trailer
plowed into debris in another avalanche and had to be abandoned for 3 days.
A total of 94 avalanches crossed the highway during the 1993-94 winter.
Average dimensions were about 4' x 220'. Total road closures were 2 days for the entire
highway. A few closure periods were for control work. No avalanche incidents involved the
public, though two plows inadvertently got stuck in avalanche debris.
Forecasting summary
During the 1992-93 season 172 advisories provided forecasts and recommendations for the
three separate highway segments. A total of 517 forecast decisions were made: 420
"LOW", 69 "MODERATE", 20 "HIGH" and 8 "EXTREME".
Forecast decisions are scored against the next advisory's evaluation, based upon rating
definitions. Results showed that rating periods of "moderate" or above, were
under forecast in 21% of the cases, while 34%of those rating decisions were over forecast.
Over forecasting reflects a conservative bias for the operational project. Forecast
accuracy for the entire decision sample was 91.2X%
Forecast decisions for 1993-94 were 422, of which 390 were for low, 24
for moderate and 8 for high. Forecasts were under estimated for 7 low periods and 2
moderate. There were 3 over forecasts at moderate. Overall forecast accuracy for the
season was 97%.
DISCUSSION
This project marks the first systematic on-site forecasting effort undertaken for Colorado
mountain highways. Institutional constraints resulted from the newness of interagency
advisory and decision-making roles, however, cooperation between forecasters and local
maintenance personnel was instrumental in the success of the project. Frequent discussion
of weather forecasts, avalanche conditions and logistics at the maintenance barn level
resulted in increasing trust and appropriate mutual response to potentially dangerous
situations. Public interest was high and frequent media coverage and personal contact with
community leaders, law enforcement personnel and citizens, helped interpret this somewhat
controversial program.
Some factors leading to forecasting error were:
- 1. Uncertain meso-scale weather forecasts;
- 2. Lack of real-time weather data at starting zone elevations;
- 3. Difficulty in accurately assessing contributory factors.
- 4. Inability to expeditiously perform explosive testing.
Future Program
The Avalanche Reduction Project is now a state-wide line item program. Full time
forecasters went on duty for the 1993-94 season on the I-70 and mountain pass corridors
west of Denver and on Wolf Creek Pass. The Silverton Forecast Office added Lizard Head
Pass, south of Telluride, and CO #110 to the Gladstone mine to their forecast
responsibilities. A comprehensive CDOT statewide avalanche atlas is nearing completion. An
Avalanche Hazard Index is in preparation as an inventory and decision-making tool.
Future sophistication of the operational forecasting programs will include additional
remote and direct instrumentation, a blending of analytical modeling into the conventional
forecasting techniques, and improved communication with Colorado Department of
Transportation personnel.
ACKNOWLEDGEMENTS
Knox Williams, Director - Colorado Avalanche Information Center, Denver, Colorado
Ed Fink, Highway Maintenance Superintendent, CDOT Avalanche Coordinator, Aurora, Colorado
REFERENCES
Armstrong & Armstrong, 1983, "San Juan Mountain's Avalanche
Study...", USDI Bureau of Reclamation Report 1-07-81-V0230, Division of Atmospheric
Resources Branch, Denver, CO USA 80225
Mears, A.I., 1992, "The East Riverside Avalanche
Accident of 1992: Engineering and Snow Safety Considerations", 1992 ISSW Proceedings
Pg. 246 257. c/o CAIC 10230 Smith Road Denver, CO USA 80239
McClung, D. & Schaerer, P., 1993, The Avalanche
Handbook, The Mountaineers, 1011 SW Kickitat Way, Seattle, WA USA 98134
Colorado Avalanche Information Center Annual Reports. 1992-93 & 1993-94
Colorado Department of Natural Resources, CAIC 10230 Smith Road, Denver, Colorado 80239
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