Geology Reference
In-Depth Information
136
II. Introduction to Geologic Hazards
conditions are probably dangerous. If the snow cracks
and the snow cracks run, this indicates slab avalanche
danger is high. If you must ascend or descend a dan-
gerous slope, go straight up or down along the side; do
not make traverses back and forth across the slope.
Take advantage of areas of dense timber, ridges, or
rocky outcrops as islands of safety. Use them for lunch
and rest stops. Spend as little time as possible on open
slopes."
QUESTIONS (8, PART E)
Aspen, Colorado
There are two zones of avalanche hazard presented on the
Aspen map (Figure 8.29 in the colored maps section).
Darker zones are known avalanche areas, and lighter zones
are areas that may sometimes have small avalanches.
Lighter areas also represent extensions of the known areas.
Bryant (1972) notes, "The most obvious avalanche paths are
in gullies or on steep treeless [or sparsely vegetated] slopes
below treeline."
1.
Calculate slopes in degrees or percent for the following
paths or potential slide areas:
a.
the path marked A, just south of Tourtellotte Peak
QUESTIONS (8, PART D)
1.
What are the most common slopes for avalanches in
degrees and in percent?
degrees
percent
b. the path marked B, for its full length from the highest
elevation in the shaded zone above the B to its lowest
point near the word "Fork."
2.
Why are avalanches not as common on slopes that are less
steep or that are more steep?
c.
the lighter-colored zone immediately south of Aspen,
beneath the ski lift east of Pioneer Gulch.
3.
Where on a slope profile is an avalanche most likely to
begin?
d.
Describe the relationship between the results you cal-
culated above and your answer to question 1, part A.
Based on your calculations, are the avalanche areas
depicted on the map justifiable on the basis of their slope
(as the only data)?
4.
If the winds from a storm are blowing out of the west, on
which side of a mountain will most of the snow accumu-
late? Why?
5.
Describe three possibilities for a safe route through aval-
anche terrain,
a.
2.
Now inspect other slopes that have mapped avalanche
paths depicted on Figure 8.29. Are these slopes likely to have
avalanches based on the answer to question 1, part D?
b.
3.
In addition to slope, what other kinds of data might the
author have used in depicting avalanche areas on this map?
c.
4.
Mears (1976) gives the following formula for calculating
the runout distance (defined as the lower boundary of the
track to the outer limit of impact) of an avalanche with a con-
fined path:
PART E. AVALANCHE PATH IDENTIFICATION
Avalanche hazards in the Aspen, Colorado, area have
been simplified for this exercise; actual hazards from
avalanches depend on the local snow and terrain con-
ditions, and may be more or less than those used for
illustration in this exercise. There is still much to learn
about avalanches, and caution in areas or times of
increased hazard is imperative.
Avalanche paths are separated into three zones:
the starting zone, the track, and the runout zone.
Figures 8.25, 8.26, and 8.27 illustrate these and their
captions discuss important aspects of avalanche travel
and hazards. Use these figures as you investigate the
maps discussed below.
S = 214 + 11.4A
where S = runout distance in meters, and A = area of
the starting zone in hectares.
This formula is based on snow and terrain conditions found
in Colorado, and may not be as applicable in other loca-
tions. A hectare is an area of 10,000 square meters, or 100 m
on a side. It compares with an acre, which has 43,560 square
feet, and has about 209 ft on a side. One hectare equals 2.47
acres and 1 meter is 39.4 in. See Appendix I for additional
conversions. Use the formula given by Mears to calculate
the runout distance of the avalanches from McFarlane
Creek. Use Figures 8.26b and 8.27 to help identify the points
of the McFarlane Creek path. Figure 8.28 is a topographic
