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PreliminaryInvestigationsofGlide/CreepMotionSensorsinAlta,Utah BobRice1,DanHowlett2,RandDecker1 1 Department of CivilEngineering,3220 MEB University of Utah,SaltLake City, Utah 84112 2 Snow Safety, Alta Ski Lift Co. And Chairman, Center for Snow Science at Alta, P.O. Box 8007, Alta, Utah 84092

Key Words:Creep,Glide,Slab Avalanche,Forecasting

ABSTRACT

Creepandglideinanalpinesnowpackcanleadto
snowpackfailureincludingsnowavalanching.With a
rapid rate of change in creep or glide with respect to time
without an increase in snow strength, avalanchesmay be
expected.Deformation in the snowpackby creepandglide
are necessary measurements in determining stress in snow
slabs.In order to detect in real time the creep and glide in
analpinesnowpack,theuseofcreep/glidesensorsare
utilized.These sensors are capable of detecting deforma-
tion rates andavalancheevents.The sensors areconnected
by hardwire to a multiplexerand a datalogger where real
time data are collected on creep and glide motion.This
motion deflects the sensors which are fixed within a one
meterpoly vinyl chloride pipe which is attached to a co-
axial spring.The deflections are monitored on two axis.
Voltagescorrespondingto angleof tilts are recorded. These
voltages are producedby a solid state accelerometer.Pre-
liminary testing during the winterof 1995-1996was car-
ried out on Mt. Baldy in Alta, Utah.Valuablelessons were
learned during the winterthat will be implemented next
winter.A numberof significant events were captured by
thesensors.Theseeventsincluded boththeelasticre-
sponse of the snowpack whenexplosives resulted in wide-
spread avalanching and the rapid increase in creep rates
whichprecededavalanching. The causeandeffectbetween
successfulavalanchecontrolwithexplosivesand
correspondinghigh creeprate events warrant further field
investigation.Thegoaloftheprojectistoimproveor
optimizeavalanche forecasting and controlusing creep/
glide motion sensors.

1. INTRODUCTION
   
   Measuring instabilities in an alpine snowpack have been
   ongoingformanyyearsbyresearchers.Evaluating and
   predicting these changesin slopes have been approached
   by examining acoustic emissions and various methods of
   measuring creep and glide.Researchers have concluded
   that these phenomena indicate the development of insta-
   bilities.However,by examining these phenomena in real
   time,this information could prove invaluable in assisting
   avalanche forecasters.
   DuringtheWinterof1995/1996threeglideand
   creep(C&G) sensors where installed on Mt.Baldy in Alta,
   Utah.These sensors are designed to measure differential
   creepandglide motion in real time in an alpinesnowpack.
   By monitoring creep and glide in real time, rapid changes
   in the deformation of the snowpack can signal approach-
   ing instabilities and avalanching.The preliminary testing
   focusedonsiteselection,laboratorycalibrationand
   repeatability of the A/D sensorsignal, installation,initial
   snowpackmeasurementsand climaticeffectsonthein-
   strumentation.

2. METHODS AND PROCEDURES
   
   2.1Objective of the Creep and Glide Sensor
   
   The sensoris designed to measure snow creep and glide
   characteristics in avalanchestarting zones.The sensor is
   a poly vinyl chloride(PVC)pipeone meter inlength.Within
   the PVC pipe anaccelerometer and temperature sensor are
   fixedtomeasuretheangle ofdeformationofthesensor
   and thetemperature ofthe surrounding snowpack.The
   signalsaremeasuredinmilliamps.Thecurr entis
   converted to a voltage which is then calibrated over -90 to
   0 to +90 degrees angle from vertical in both longitudinal
   and traversedirections.The deflections are monitored on
   two axes to detect skew and lateral motions.The sensor is
   attachedto acoaxialsteel spring thatactsas areturn mecha-
   nism.Thesensor is thenthreadedonto an1.27 cm diameter
   ready rod (all thread) that is fixed to the ground.

Deflections are calculatedby:

D=hsinQEq. (1) where: D=deflection h=hypotenuse length of 1m Q=angle of tilt Snow movement accelerations are calculated:

A=

Eq.(2)

acceleration

2.2 Location of Creep and Glide Sensors in Alta

The C&G sensors were installed on Mt. Baldy in Harold's
slide path.The starting zone of Harold's is located at an
elevationof3,110meterswithaslopeaspect of40de-
grees, a slope steepness of 37 degrees and a length of 122
meters.This slope was chosen because the starting zone
isrelativelyundisturbedbyareaskiersandcouldbe
accessedand monitored frequently.Furthermore, the sen-
sors are in close proximity to high resolution weather data
collection sites.
Three C&G sensors were installed in Harold's starting
zone.In order to measure the differential creep and glide
movements the threesensors wereplacedat varying heights
from the surface of the ground.Sensor#1 was placed at
the ground, sensor #2 was placed1 meter above the ground
and sensor#3was placed twometers above the ground.
The readyrods were fixed to thegroundandplumbto slope
angle.Rod lengths were increased with couplers.

2.3 Calibration of Creep and Glide Sensors

PriortoinstallingtheC&Gsensors,thesensorswere

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