|
gram isn't channeled to exclude unimportant data. This
channelingof the programto stay focused on anindividual
problemisthehardpar tanddependsontheskill,
knowledgeand experience oftheprogramer,and thisis
what determines if it's a valuable program ornot.
Sointhisregard,when dealing withnumerousvari-
ables which have a compounding effect on each other,a
human brain can still outperform a computer.But the key
hereis experience.Justas a nearest neighbor program isn't
worthmuchwithoutmanyyearsofdata,eitherisour
intuition worth much without a lot of data, or experiences
loggedinourmemorybanks.Eventhough acomputer
may soon beat the world chess champion,the avalanche
game, whichis infinitelymore complexthanthefinitegame
of chess,will still be relying on humans.
SNOW PITS
There is always the question of whetherit's betterto dig
extensive"datapits"ortodighastypits.Foragiven
amount of time it's more valuable to dig numerous hasty
pits because you get a better perspectiveof the conditions
which sometimes vary greatly overshort distances.It is
important to dig enough data pits so you feel comfortable
knowing how the different layers are changing.Also it's
important to record data pits forfuture reference.
At Big Sky, with a continental snowpack, the main con-
cern during mostofthe season is the temperature gradi-
ent, facettedgrain layers.The main contributing factor, as
to varying TG development in a given area, is the depth of
the snowpack.With this in mind it's a good idea to probe
around to find a spot with a representative snow depth to
dig a pit in .Also,to save time on a large path, a few pits
can bedug in different locationsatvarying depths fora
reference. Then by probing across the slopeandat different
levels you can getan idea about the varying TG conditions
by noting the depth of the snowpack at each probe site.
Snowpitsaren'tveryreliable informulating stability
evaluations.Explosives and ski cutting are more reliable.
The problem with getting stability information from snow
pits is the inexact science of shear tests.
The authorhas observed that shear tests conducted on
the crown face of deep slab avalanches many times didn't
reveal the sliding layer as the easiest shear.Shear tests all
share common problemsthat make them unreliable:
*They only test a small area.
*The load or stress is applied unnaturally fast.
*Whena columnisisolatedthestresscausedbythe
uneven creep and settlement of different layers in dif-
ferent areas of the path is removed.
Thenaturalstressthatisimpartedbythesurrounding
snowpack is removed when a column is isolated.Youstill
have the stress which is caused by the weight of the over-
lying snow. But this may notbe representativeof theoverall
interrelated stress of the whole slope.Explosives and ski
cutting are more accurate because the inclining snowpack
is in it's natural state when thesetests are performed.With
explosives and ski cutting you eliminate one (#3) and part
of another (#1) of the three problems listed above.
218
|
EXPLOSIVES AND SKI CUTTING
Skiareashave theopportunitytousethemorereliable
methods ofexplosives and skicutting along with the in-
puts of snow and weather data to evaluate snow stability.
These more reliable methods are necessary because of the
desire to have slopes open on a daily basis.Ski areas are
abletoaccomplishthisbecausetheyhavetherequired
manpowertocontinuouslymonitorarelativelysmall
amountofavalancheterrain.Theworkersaren'tsome-
whereelse trying to figure out what's going on in the start-
ing zones.They can just go there and find out what's hap-
peningwith thesnowandweather ataparticularslide
path.During a storm,orif it's getting wet during the day,
the avalancheworkers can keep ski cutting suspected ava-
lancheslopes.Many timesthiswillreduce thehazard
enough that the slope can remain open.If the hazard be-
comestoogreatthe area isclosed,eithertemporarilyto
allowcontrolwork(avalanchehazardreduction),then
reopened, orleftcloseduntil conditionsimprove.This
would usuallybe thefollowing day,whenfull routeswould
be done using explosives and ski cutting to both evaluate,
and mitigate the hazard as needed.
The basic procedure of bombing or shooting a slope and
then ski cutting and opening it are pretty straightforward.
It doesn't matterwhich is the weak layerorwhat type of
crystals failed,oreven whether it avalanched ornot.As
longasthehazard isdeemedlowenough,theslopeis
opened and everybody goes skiing.
But you can learn more about stability from explosives
and ski cuttingthanwhetherit slid or not.If a slope doesn't
slideafterbeing testedwithan explosiveitisgenerally
thoughtto be safe for skiers.It is assumedthat theexplosive
imparted more stress into the snowpack than skiers will.
There is the consideration of post control releases, which
won't beaddressedin this paper. If anavalancheis released
with an explosive orby ski cutting, stability can be evalu-
ated by observing the crown line propagation characteris-
tics.
Ifthefractureline propagates horizontallyacrossthe
slopefora longways theslabwould beless stablethan
one where the fracture line propagatedacross the slope at
a downwardangle. If only a circularareaaroundthe charge
was released, andit just took out the area directlybelow, it
would indicate more stability than one that propagatedto
thesides.Ifonlya circular area aroundthe charge was
released and itrode up onthe snowbelow,itwouldbe
more stable yet.
If the explosive didn't causean avalanche,it can still be
observed whether it caused tension release cracks ornot.
Tensionrelease crackswithout an avalancheindicate a co-
hesive slab without a weak enough sliding layer to cause
an avalanche.By examining the depth of the crack it can
be determined which is the potential sliding layer.By ob-
servingthenumberand locationofthetensionrelease
cracks it can be determined where the tension zone is.
CONCLUSION
Ski areas employsome differenttechniquesthanbackcoun-
try, heli-ski and highway forecasters, forevaluating snow
stability.Explosives and ski cutting are the primary tools
used for evaluating newsnow stability at ski areas.Where
as explosives and snow pits are the primary tools used for
evaluating old snow stabilityat ski areas.
|
|
1
2
