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Key Words:Airblast,Crown face,Dry slabavalanche,High-
way, Slidepath
ABSTRACT
BridalVeilFallsavalanchepathreleasedJanuary31,
1996.The resulting avalancheran down to and across the
floorof Provo Canyon.It continued up the opposite side
ofthecanyon,crossedUSHighway189and destroyed
thepowerlinesbeyondthehighway.Italsoblewtwo
railroad cars about fifty meters and severely damaged the
base terminal of the four passenger tram. This was part of
a much largeravalanche that included two more drain-
ages south of Bridal Veil.
Located in the WasatchMountains of Utah, Provo Can-
yon isat 40º21' north latitude,111º36'west longitude.
Provo Canyon hasthe three largestslidepathsthat threaten
a highway in the state of Utah. They are Bridal VeilFalls,
Lost Creek, and SlideCanyon. The starting zoneandtrack
of Bridal Veilhave an area of approximately 160 hectares
and a vertical drop of 1626 meters. An interesting feature
ofthetrack isa cliffjust abovethe runout zone.About
120metersinheight,the cliffhas a perennial waterfall
that the slidepath is named after.
The storm cycle that producedtheBridal VeilFalls ava-
lanche began January16,and bythe31sthaddropped
239 centimeters of snow (H2O equivalent 181 millimeters)
on the Utah Department of Transportation (UDOT) snow
study plot at Aspen Grove (see precipitation table).Total
snow depth at Aspen Grove (elevation 2100 meters) was
18 centimeters on January 16 and 122 centimeters on the
31st.The ten days before the storm cycle was clear with
no precipitation. Beforethe storm cycle thesnowpackwas
76 centimeters deep at the Sundance study plot at 2438
meters, and was weakrecrystallized snow. On January 31
thetotaldepthattheSundancestudyplotwas254
centimeters.
UDOT did avalanche control (with positive results) on
the north side of Provo Canyon on the 18th and the 28th.
Avalanchecontrol work by UDOT on February 1 triggered
anavalanche in SlideCanyonthatrantowithinabout
ninety meters of US 189.Slide Canyon drains into Provo
canyon from the north, about two kilometers northeast of
Bridal VeilFalls (see map).UDOT does not do avalanche
control in any of the avalanche paths on the south side of
Provo Canyon with the exception of the Deer Creek Dam
Chute.
During the helicopter bombing missionon the 28th I
could seeavalanchedebris in the track of Bridal VeilFalls.
A post control flyby of Bridal Veil resulted in an estimate
by the chiefguide of Powderbirds helicopters that only
about 20% of the path had released. This information was
relayed to me late on the 29th.By early morning on the
30th twenty five centimeters of snow (H2O equivalent 27
millimeters) had fallen at the UDOT study plot overnight.
Snow continued falling at the rate of 2 to 3 cm/hour, with
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moresnowforecast.Average windspeeds atthesummit
(elevation2515 meters) of SundanceSki Areafor this period
were in the range of 9 to 16 meters/ second.
Some facts about the wind data are in order here.The
onlylocalwinddata available isprovided bySundance
Ski Area.The Sundance summitweatherstation has the
highest anemometerin the area,but it isratherfaraway
from the starting zones of Bridal Veil,Lost Creek, andSlide
Canyon.Bridal Veil is the most distant,sitting on the op-
posite side ofProvo Canyon 5.5kilometers tothe south.
Theotherdisadvantage (whenattempting toassessava-
lanche hazard)is that the summit of Sundance is over 650
meters lower than the starting zones of the slidepaths that
threaten the highway.
By mid morning of the 30th I had decided that it was
time for action. I requestedanotherhelicopterbombing mis-
sionfortheslidepathsthat UDOTbombs(thiswasde-
layed untilFebruary first whenthe weatherfinally allowed
flight).AtthesametimeIwarned mysupervisorofthe
extremeand increasing avalanche danger nowposed by
Bridal VeilFalls. He directedme to notify the Utah County
Sheriff which I did. During my phone call to the sheriff,I
suggested that the sheriffinstall avalanchewarning signs
at Bridal Veil and make a press release to warn the public.
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