A v a l a n c h e

D y n a m i c s

a n d

Densities within the avalanche weremeasured by two dif-
ferent methods.First with the use of a capacitanceprobe
(Louge et al 1996) which measures the dialectric constant
of snow passing in front of its sensors.Each sensorstrip
on the probe consists of a sensor, a ground and a guard.A
buffer amplifier maintains the guard at precisely the same
sinusoidal voltage as the sensor to protect it from any stray
capacitances.Inadditionthesensorisconnectedto
processing circuits using a guarded coaxial cable sothat
cablecapacitancedoes notparticipatein themeasurements.
Through proper calibration this dialectric constant can be
related to the density of the snow. The second method for
measuring the density was with optical sensors (Dent et al
1994).Each photoelectric sensorin thearraycontained
both an infraredlight emitting diode (LED) andan infrared
sensitivephototransistormountedsidebyside.The
phototransistor was mounted so as to measure the amount
of infrared light from the LED that is reflected from objects
passing in front of the sensor. This reflectance is not only a
function of thesnow's density, but also thesnow's structure
andwater content. By calibratingthesensors with thesnow
press(figure2)beforehand,thesignalscanberelated
directly to the snow's density.Through cross correlation
between adjacentoptical sensors measurements of velocity
at different points in the avalanche can also be obtained
(Dent et al 1994).
The depth (height) of the flow is measured using a light
swinging arm attached to a rotational potentiometer.As
the slide passes the shed, a small skid on the arm rides on
thesurfaceofthesliderotatingthearmandhence the
potentiometer.Voltages proportional to the angle of rota-
tion are then recorded.This depth gauge is calibrated by
raising the skid to known heights above the running sur-
face.
Shear and normal stresses were measuredusing a shear
plate.The shear plate is a roughenflat 23 x 28 cm alumi-
num plate.This plate is rigidly mounted in a sturdy box
bytwocantileveredarmsfittedwithstraingaugesina
wheatstone full bridge configuration (Figure 3).This box
is mounted to the side of the shed, below the slide running
surface so that the plate is flush with the surface.As the
slideflowsoverthe plate,voltagesacrossthebridge are
recorded. The recordedvoltages are assumed proportional

Figure 2: Snow press

EXPERIMENT

Revolving door,a small east facing avalanche path in the
Bridger mountain rangeof SouthwesternMontana hascliff
bands that feed into a narrow 100m chute with an uniform
slope of approximately 30 ^obefore runningout into a stand
of small pines.In the center of revolving doorisa large
boulder,behind which a small instrument shed has been
built.By contouring snow in the early winter, it has been
possibletoreleaseavalanchesthatpassnexttothis
instrumented shed.
Preparation of the site priorto triggering of a slide first
involves removing any new snow from the side of the shed
exposing the top 30-40 cm of the observation window.The
slopeisthensmoothedand contouredsothattheslide
willrunstraight and uninterrupted past theshed.Care
must be taken to insure that there are no changesin slope
orbumpsthatwilldisrupttheflow,causingeddiesor
deposition on the running surface.Afterthe slide path is
prepared, it is then instrumented (figure 1).

Figure 3:

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