HomeMy WebLinkAboutPRJ14-0031 Bighorn TH Submittal 1-30-14 DRAFT w AttachmentsJanuary
30,
2014
Vail
Town
Council
C/O
Jonathan
Spence
75
S.
Frontage
Road
Vail,
Colorado
81657
Re:
Avalanche
Hazard
Map
Amendment
Application
Dear
Vail
Town
Council:
Mauriello
Planning
Group
has
been
retained
by
the
Bighorn
Townhouse
Association
to
assist
with
an
amendment
to
the
Town
of
Vail
Avalanche
Hazard
Map.
Pursuant
to
the
Town
Code,
this
letter
shall
serve
as
the
formal
request
for
this
map
amendment.
Introduction:
The
Association
is
requesting
the
amendment
for
the
properties
located
at
4708
and
4718
Meadow
Drive,
known
as
the
Bighorn
Townhouses,
located
in
East
Vail
across
from
Bighorn
Park.
The
Bighorn
Townhouses
are
identified
by
the
Avalanche
Hazard
Map
as
located
within
both
the
Blue
(Moderate)
and
Red
(High)
Avalanche
Hazard
Area.
Included
with
this
application
is
a
detailed
report
prepared
by
Arthur
I.
Mears,
P.E.
Inc.,
one
for
the
foremost
experts
in
the
world
with
regard
to
snow
avalanches,
which
concludes
that
the
Town’s
hazard
maps
should
be
updated
to
reflect
the
conclusions
of
his
report.
The
conclusions
are
based
upon
decades
of
new
studies
and
modeling
techniques/technology
that
were
previously
not
available
when
the
Town’s
hazard
maps
were
originally
studied
and
adopted
in
1975
and
1977.
The
revised
mapping
would
place
the
Bighorn
Townhouse
properties
outside
of
any
hazard
area.
The
amendment
would
also
benefit
other
neighboring
properties
and
Town
owned
lands.
Background
and
Regulations:
The
Bighorn
Townhouse
properties
are
located
within
the
Blue
Hazard
Avalanche
Area
and
the
Red
Hazard
Avalanche
Area
as
mapped
by
the
Town.
These
areas
are
defined
by
Chapter
21:
Hazard
Regulations
as
follows:
BLUE
HAZARD
AVALANCHE
AREA:
An
area
impacted
by
a
snow
producing
a
total
static
and
dynamic
pressure
less
than
six
hundred
(600)
pounds
per
square
foot
on
a
flat
surface
normal
to
the
flow
and/or
a
return
interval
in
excess
of
twenty
five
(25)
years.
PO Box 4777 Eagle, Colorado 81631 970.376.3318 www.mpgvail.com
By Shelley Bellm at 11:10 am, Feb 06, 2014
PRJ14-0031
RED
HAZARD
AVALANCHE
AREA:
Any
area
impacted
by
a
snow
avalanche
producing
a
total
static
and
dynamic
pressure
in
excess
of
six
hundred
(600)
pounds
per
square
foot
on
a
flat
surface
normal
to
the
flow
and/or
a
return
interval
of
less
than
twenty
five
(25)
years.
Section
12-‐21-‐10:
Development
Restricted,
of
the
Town
Code
prohibits
the
construction
of
any
structure
within
a
Red
Avalanche
Hazard
Area.
Structures
may
be
built
in
Blue
Avalanche
Hazard
Areas
with
proper
mitigation
measures.
Furthermore,
for
existing
structures
within
Hazard
Areas,
no
building
permit
shall
be
issued
for
the
exterior
expansion,
alteration
or
addition
to
existing
structures
except
for
windows,
skylights,
and
minor
alterations.
The
following
map
is
the
Town
of
Vail’s
GIS
map
of
the
Avalanche
Hazard
Map
for
the
Bighorn
Townhouse
area.
The
map
clearly
shows
how
the
current
hazard
mapping
impacts
the
Bighorn
properties.
As
currently
mapped,
these
properties
are
severely
restricted
from
redevelopment
and
remodel
activities,
are
subject
to
extraordinary
insurance
costs,
and
are
impacted
in
their
ability
to
be
marketed
and
sold
due
to
this
outdated
and
now
inaccurate
40
year-‐old
mapping.
Chapter
21:
Hazard
Regulations
allows
for
amendments
to
the
Geologic
Hazard
Maps
when
more
detailed
site
specific
information
is
available.
A
request
is
to
be
submitted
to
the
Community
Development
Department
followed
by
a
hearing
before
the
Town
Council
(no
Planning
and
Environmental
Commission
Review
is
required).
The
Town
Council
then
makes
the
following
finding:
Town
of
Vail
Existing
Snow
Avalanche
Hazard
Mapping
Bighorn
Townhouses
If
the
site
specific
geologic
investigation
establishes
by
clear
and
convincing
evidence
that
the
property
should
not
be
designated
as
a
geologically
sensitive
area,
the
town
council
shall
direct
the
department
of
community
development
to
amend
the
map
appropriately.
This
avalanche
area
was
originally
studied
by
Ron
Halley,
P.E.
of
Hydro-‐Triad
engineering
in
1975.
This
avalanche
path
was
named
the
KAC
Avalanche
Path.
This
1975
mapping
was
used
as
a
basis
for
the
report
on
snow
avalanches
produced
in
1977
which
was
ultimately
adopted
by
the
Town
of
Vail.
The
image
below
is
the
Hydro-‐Triad
mapping
that
was
interpreted
onto
the
Town’s
snow
avalanche
hazard
map.
In
1990,
Arthur
I.
Mears,
P.E.,
Inc.
completed
an
analysis
and
re-‐mapping
of
the
KAC
avalanche
which
reduced
the
blue
avalanche
hazard
to
the
Bighorn
Townhouses.
The
last
KAC
avalanche
probably
occurred
between
1950
and
1962.
Avalanche
Hazard
Expert:
The
applicant
has
engaged
the
services
of
Arthur
I.
Mears,
P.E.,
Inc.,
Natural
Hazards
Consultants,
to
review
the
previously
map
snow
avalanche
hazard
and
its
relationship
to
the
Bighorn
Townhouses.
Mr.
Mears
also
performed
all
of
the
analysis
and
mapping
for
the
debris
flow
and
debris
avalanche
for
the
Town
of
Vail,
which
mapping
is
still
used
today
as
the
basis
of
the
Town’s
hazard
maps.
Mr.
Mears
has
a
B.S.
in
Civil
Engineering
and
an
M.S.
in
Geology
from
the
University
of
Colorado,
Boulder.
Based
in
Gunnison,
he
formed
Arthur
I.
Mears,
P.E.,
Inc.
in
1981.
Mr.
Mears
has
been
an
avalanche
consultant
on
over
1000
projects
in
9
states
and
8
countries.
He
has
published
over
35
technical
and
research
papers
and
works
with
international
colleagues
from
Canada,
Switzerland,
Norway
and
Austria.
Mr.
Mears
provides
hazard
K.A.C.
Avalanche
Study
from
Sept.
1975
mapping,
risk
analysis,
mitigation
strategies
and
design
parameters
to
protect
people
and
infrastructure
from
avalanches,
debris
flows,
and
rockfall.
His
clients
include
utilities,
transportation,
mining,
municipalities,
engineers,
planners,
land
developers
and
homeowners.
More
information
about
Mr.
Mears
credentials
and
experience
can
be
found
at
his
website
www.mearsandwilbur.com.
Conclusions
of
Mears
Study:
Based
on
the
investigation
(included
with
this
submittal),
Mr.
Mears
has
concluded
that
the
property
is
beyond
the
range
of
design
avalanches
(100-‐year
avalanche)
and
that
a
map
amendment
should
revise
the
Town’s
Avalanche
Hazard
Map
to
reflect
this
analysis.
The
following
map
is
a
revised
Avalanche
Map,
using
the
Town’s
current
GIS
mapping
(solid
blue
and
red
colors)
of
the
Avalanche
Hazard
for
the
KAC
Avalanche
Path,
with
the
proposed
revision
to
the
map
based
on
the
current
analysis
of
the
Avalanche
Hazard
of
the
KAC
Avalanche
Path
(red
and
blue
outline).
Analytical
methods
for
estimating
the
magnitude,
extend,
and
destructive
energy
of
design
avalanches
have
changed
since
the
1975
and
1977
studies.
These
methods,
advanced
mostly
in
Europe
through
research,
direct
observation,
and
computer
modeling
have
allowed
for
more
accurate
estimations
in
recent
decades.
In
addition,
substantial
regrowth
of
the
forest
in
the
area
has
diminished
the
probability
of
large
avalanches.
A
significant
forest
fire
affected
the
upper
reaches
of
the
avalanche
area
prior
to
the
1970s.
Town’s
Current
Avalanche
Hazard
Mapping
Proposed
Avalanche
Hazard
Mapping
based
on
Mears’
Investigation
The
conclusion
from
the
analysis
indicates
the
following:
1.Design-‐avalanche
extent
and
the
high
(red)
and
moderate
(blue)
hazard
zones
should
be
modified
as
shown
on
Figure
3.
The
outer
limit
of
the
design
avalanche
will
stop
approximately
200
ft.
south
of
the
condo
units.
The
Town
of
Vail
avalanche-‐zone
map
should
be
updated
to
reflect
this
change.
2.Destructive
powder
avalanches
will
not
affect
the
condo
units.
A
very
low-‐density
dust
cloud
may
reach
the
units
and
extend
to
the
south
of
Meadow
Drive
but
it
will
not
produce
damaging
pressures
or
endanger
people.
3.Because
the
design
avalanche
will
not
reach
the
condo
units,
mitigation
to
protect
from
avalanches
will
not
be
needed
and
is
not
recommended.
4.Debris
flow
will
not
affect
the
units.
Muddy
floods
may
reach
the
condos
however,
structural
damage
will
not
occur
and
danger
or
injury
to
residents
will
not
be
a
problem.
Mitigation
from
debris
flow
is
not
recommended.
We
have
included
the
following
attachments
in
support
of
this
application
to
amend
the
Town’s
Avalanche
Hazard
Map:
1.Investigation
of
Arthur
I.
Mears,
P.E.,
Inc.
of
the
Avalanche
Hazard
for
Bighorn
Townhouses
(January
29,
2014)
,
located
at
4708
and
4718
Meadow
Drive,
in
accordance
with
the
requirements
of
Chapter
21:
Hazard
Regulations,
of
the
Vail
Town
Code.
2.K.A.C.
Avalanche
Study,
Vail
Colorado
(1975)
by
Ronald
Halley,
Avalanche
Consultant.
3.Evaluation
of
the
Snow
Avalanche
Hazard
in
the
Valley
of
Gore
Creek,
Eagle
County,
Colorado
(1977)
by
the
Institute
of
Arctiv
and
Alpine
Research,
University
of
Colorado.
4.Shape
Files
of
the
proposed
revisions
for
use
by
the
Town
of
Vail
for
the
amendment
to
the
GIS
Map
for
the
Avalanche
Hazard
for
the
Bighorn
Townhouses.
Thank
you
for
your
time
and
consideration
on
this
matter.
Should
you
have
any
questions,
please
do
not
hesitate
to
contact
me
at
970.376.3318
or
dominic@mpgvail.com.
Sincerely,
Dominic
Mauriello,
AICP
Principal
1
Arthur I. Mears, P.E., Inc.
Natural Hazards Consultants
555 County Road 16
Gunnison, CO 81230
Tel/Fax: (970) 641-3236 January 29, 2014
Bighorn Townhomes c/o Dominic Mauriello Via email
RE: Avalanche hazard at condo buildings
Dear Ms. Bernardo:
I completed a site visit on January 27, 2011 and analysis on January 28 and
February 4, 2011. This report was reviewed and updated slightly on January 28,
2014. As a result of this work I conclude the following: The residential units
are beyond the range of design avalanches and will not require mitigation.
Details of my methodology and support for my conclusions are contained in this
report, as follows. Important limitations to my conclusions are discussed under
the “Limitations” section of this report.
Location, previous work and avalanche exposure
The area studied is located at 4708 & 4718 East Meadow Drive in Vail, Colorado
(e.g. “Condos”) below and to the west of the “KAC Avalanche Path1.” This was
first studied and named by Ron Halley. P.E. of Hydro-Triad engineering in 1975, the first site-specific study of this path. His mapping has been used as the basis
for the Town of Vail avalanche-zone maps. According to the 1975 study, unit
4718 is in the high-hazard or “Red” zone and unit 4708 is located at the boundary
of the intermediate-hazard or “Blue” zone. In December, 1990, A.I. Mears, P.E.,
Inc. completed an analysis and re-mapping of the KAC avalanche. This study modified the avalanche boundaries reducing the blue zone and provided
avalanche loads on the Bernardo residence. Both the 1975 and 1990 studies
have been modified in this current analysis. Modifications are justified because
of the current availability of updated avalanche-dynamics analytical techniques
that have become available in the last decade.
The KAC avalanche starts on steep, open north-east facing slopes between
10,500 and 9,500 feet elevations, approximately. Roughly 20-25 acres of
1 The KAC Avalanche was named by Ronald L. Halley, P.E., the president of Hydro-Triad, Ltd. In a study commissioned by Mr. Donald J. Tomas and completed in September. 1975.
2
avalanche starting zones2 exist within this steep, open forest. Inspections of old U.S. Forest Service aerial photos indicate that the last major avalanche in this
path occurred between 1950 and 1962. It widened the avalanche track below
9,000 feet elevation and destroyed portions of the forest (Figure 1). This
avalanche probably reached the meadow immediately below the steep terrain but it is unclear from physical evidence if it reached the current locations of the Condos or Meadow Drive. Compare Figure 1 with Figure 3.
Figure 1. 1962 aerial photo
showing damage from the KAC avalanche in Vail that probably
occurred between 1950 and
1962. Forest fire damage is also visible on adjacent slopes.
The magnitude, extent and destructive energy of design
avalanches3 can be estimated
through analytical methods
widely applied mostly in
central Europe, especially in Switzerland. Older versions
of such methods were applied
in the 1975 study by Hydro-
Triad and were used as the basis for the avalanche map of the KAC avalanche
used in Vail zoning. However substantial research, direct observations of avalanches worldwide, and computer modeling have taken place in Europe in the
subsequent decades. Furthermore, re-growth of the forest throughout upper and
intermediate parts of the avalanche path4 has diminished the probability of large
avalanches. Figure 2 is a photo of the lower part of the path taken on January
27, 2011 near the site labeled “Tree Damage” in Figure 1.
Avalanche-Dynamics Analysis
We applied an updated Swiss avalanche-dynamics modeling technique (the program AVAL-1D) to compute avalanche speeds and runout distances (stopping positions) of dense flowing avalanches. Avalanche speeds computed
were less than 30m/s (65mph) in the avalanche above 10,000 feet elevation.
Speed decreased quickly to less than 15m/s (33 mph) at the point where
avalanches discharge from the main gully above the meadow. Avalanche runout
2 Starting zones are areas generally steeper than 30 degrees and are areas where avalanches
begin, accelerate and increase in mass. 3 In Vail, the design avalanche is the largest or most destructive event expected in a 100-year
period, a “100-year avalanche.” This event has a constant annual probability of 1%. 4 Avalanche path: the entire area where an avalanche moves, including the starting zone, the
track (where greatest speed is reached) and the runout zone where the avalanche stops (the open meadow in the KAC path).
3
in the meadow south of the condo units was computed; design avalanches will stop south of the condo units as shown in Figure 3 where avalanche red and blue
zones have been delineated.
Figure 2. Lower part of KAC avalanche path (labeled “Tree Damage” in Figure 1). Re-
growth of forest is apparent.
Powder avalanches can also occur during periods of widespread dry-slab releases from the primary starting zones between 9,500 and 10,500 feet
elevation. However these will be confined to the steeper terrain above 9,000 feet
within and adjacent to the central gully of the KAC path. Powder avalanches will
not produce destructive forces at the base of the KAC avalanche path or at the condo units.
Conclusions and recommendations
The following conclusions and recommendations are based on 1) the site
inspection conducted on January 27, 2011, 2) our previous studies of avalanche potential in the Vail area and elsewhere in North America and Europe, 3)
previous work in the KAC avalanche path by Hydro-Triad and myself, 4) analysis
of aerial photographs dated 1939, 1950, 1962, 1974, 1984 and current imagery
available on the internet, and 5) the avalanche-dynamics analysis referenced.
4
1. Design-avalanche extent and the high (red) and moderate (blue) hazard zones should be modified as shown on Figure 3. The outer limit of the
design avalanche will stop approximately 200 feet south of the condo
units. The Town of Vail avalanche-zone map should be updated to reflect
this change. 2. Destructive powder avalanches will not affect the condo units. A very low-density dust cloud may reach the units and extend to the south of Meadow
Drive but it will not produce damaging pressures or endanger people.
3. Because the design avalanche will not reach the condo units, mitigation to
protect from avalanches will not be needed and is not recommended. 4. Debris flows will not affect the units or proposed new structures. Muddy floods may reach the condos however structural damage will not occur
and danger or injury to residents will not be a problem. Mitigation from
debris flows is not recommended.
Limitations to this work
General
1) You as my client should know that while our company can and does
attempt to uphold high professional standards, the state of scientific and
engineering knowledge is incomplete, and does not always permit certainty. The complex phenomena involved in avalanches cannot be
perfectly evaluated and predicted, and methods used to predict avalanche
behavior change as new research becomes available. While we can and
will offer our best professional judgment, we cannot and do not offer any
warranty or guarantee of results.
Site-Specific
1) Avalanches larger than the Town of Vail 100-year design avalanche are
possible. Such events may travel farther than the blue-zone limit mapped
on Figure 3. 2) Furthermore, the design avalanche mapped assumes the current forest
cover and vegetation. Widespread destruction of the forest in the starting
zone by fire or other causes could increase the areas of avalanche
release and the size of avalanches.
Report prepared by,
Arthur I. Mears, P.E. Avalanche-control engineer
5
Figure 3. Updated mapping of the KAC avalanche path, East Vail, Colorado. Red and
Blue zones were defined in accordance with Vail regulations.
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February 6, 2011
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1
K. A. C.AVALANCHE
STUDY
VA I L, COLORADO
Prepared For
Donald J. Thomas
King Arthur's Court
Development
ay
Ronald L.Halley
Avalanche Consultant
1
Sept., 1975
I
f
HYi`~R0°TWA0 ti''t;
September 19, 1975
1
Mr. Donald J. Thomas
13555 Coliseum Drive
Chesterfield, Missouri 63107
Dear Mr. Thomas:
Enclosed is our report on the Avalanche Study for the so-called
KAC Avalanche Path. The study was performed in accordance with our
proposal letter of February 28, 1975.
The proba6le avalanche runout zones have been defined, and a
discussion of the impact of the avalanche on potential development
within the King Arthur's Court has been included in the report.
If there are any questions, please contact us.
Sincerely,
HYDRO-TRIAD, LTD.
Ronald L. Halley, P.E.
President
RLH/mh
Encl: Report
1
bN, MISSiv;ilPri .AVE. - SUITE 10 LAKEWOOD, CCILORADO 80226 PHONE 303•934-2477
K A C AVALANCHE STUDY
TABLE OF CONTENTS
Pa9e
Introduction . 1
Location 1
Terrain . . 2
Geology . . . . . . . 2
Vegetation 3
Land Use Zoning 4
Historical Avalanche Occurrences . 5
General Gore Valley : 5
KAC Avalanche 6
Aval anche Zoni ng . . 8
KAC Avalanche Analysis . 12
Character of the Avalanche Path 12
Snow Depth vs. Recurrence Interval 13
Runout Zone Analysis 16
Conclusions and Recommendations 21
LIST OF FIGURES
Figure I General Location Map
Figure II Avalanche Path
Figure III Avalanche Path Profile
Figure IV Zoning Map
1
K A C AVALANCHE STUDY
INTRODUCTION
1
This study of the avalanche path located in the Bighorn area of
the Gore Valley was authorized by Mr. Donald Thomas, one of the owners
of property sited in the valley floor below the avalanche path. This
particular avalanche path has generated a considerable amount of furor
within the county and the town of Vail over the past two to three
years. The various public discussions concerning the KAC avalanche
path will not be reviewed herein, as most are a matter of record.
This study is based upon the physical facts of the slide path,
the mountain meteorology producing the snowpack conditions found in
and adjacent to the Gore Creek Valley and an avalanche analysis
including definition of modes of the avalanche and dynamic consider-
ations. The runout area is defined by a red zone (relatively high prob-
able occurrence with high impact pressures) and a blue zone (r^elatively
remote occurrence probability and lower probable impact pressures).
t These zones are generally in accordance with the Swiss Avalanche
Zoning Planl, although modified somewhat to fit the Gore Valley.
Location
The KAC avalanche path is located on the south valley wall near
the Bighorn area in the eastern end of the Gore Valley. The Town of
Vail has recently annexed the valley section including the Bighorn
area and the runout zone is within the Vail Town limits. The location
of the KAC avalanche is shown on the general location map, Figure I.
i
2
1
Terrain
The general terrain of the Gore Creek Valley is highly conducive
to major climax type avalanches, especially along thesouth valley
wall. The ridge line extending from Vail Pass, between Black Gore
and Main Gore Creek on the north and Turkey Creek, Two Elk Creek and
Mill Creek on the south, ranges in elevation from 10,800 feet (3300
meters) at Vail Pass to 11,800 feet (3600 meters) at the point between
the headwaters of Two Elk and Mill Creeks. The valley floor at the
Bighorn area is approximately 8500 feet (2600 meters). The valley
show5 the U-shaped characteristics of the glacier action during the
Pleistocene epoch and the upper portions of many of the major avalanche
paths have nivation or small "hanging" glacier hollows.
The terrain forms steep slopes down into the valley with slopes
often exceeding 38 degrees (78 percent) and frequent cliff bands of the
more resistant rock members (limestone and sandstone) of the Minturn
Formation.
Geology
The Gore Range is one of the major massive uplift sections of
Precambrian crystalline rock typical of the mountainous regions of
Colorado. The uplift fault block forming the main element of the
range is flanked by metamorphic gneisses and by the sedimentary for-
mations extending westward.
The two major faults flanking the range are the Gore Fault which
bears roughly north-northwest and the Frontal Fault which trends
approximately parallel.
3
1
The Gore Fault can be easily identified in the field, as it is
highly visible traversing through the area east of the top of Vail
Pass then northwestward cutting through the Main Gore Creek just upstream
of the confluence with Black Gore immediately upstream of the Bighorn
area. Westward of the fault, the sedimentary rocks of the Minturn and
Maroon Formations are very evident.
The Maroon Formation conformably overlies the Minturn Formation.
t
The Maroon Formation consists of red mudstone or shale, siltstone and
fine-grained sandstone. The Minturn Formation can be identified by
the distinctive beds of coarse grained gray to reddish sandstones,
conglomeritic sandstone, sandy and silty shales as well as the pinkish
gray to gray limestone beds.
The cliff bands that are evident along both sides of the Main
Gare Valley between Bighorn and the Vail Village center are the more
resistant sandstone and limestone members of the Minturn Formation.
The cliff and diagonally cut arroyo within the lower end of the KAC
avalanche path are exposed portions of a limestone member of the
formation.
Vegetation
The KAC avalanche path extends from the valley floor, 8500 feet
elevation (2600 meters) to near timberline at 11,100 feet (3400 meters)
and traverses basically three ecosystems. The valley floor
can be described as a meadow vegetal type system that up until the early
1960's was intermittently used for meadow hay production with some
irrigation and livstock grazing. The ditches are still evident in
the field.
4
1
Above the valley floor to an approximate elevatian of 9000 feet
2750 meters) extends a mixed stand of open aspen graves, small meadow
pockets (derived primarily from geological factors) and lodgepole pine
or pouglas Fir. There are a few scattered Ponderosa Pine in this zone.
From 9000 foot upward, the primary vegetal types consist of relatively
dense forest of Engelmann Spruce, subalpine fir and lodgepole pine.
The grassy or shrub-covered meadow area along the bottom of the
ephemeral KAC stream and on the northeast aspect sloping into the
stream is the relatively frequent avalanche release zone.
Land Use Zoning
The valley walls and adjacent mountains throughout the 6ore Valley
area are part of the White River National Forest and therefore are
under the jurisdiction of the U.S. Forest Service. The current zoning
of the valley floor within the privately owned land is shown on Figure
IV. This indicates the bulk of the valley floor immediately adjacent
to the runout area of the avalanche is currently zoned "Low Density
Multiple Family". The Bighorn Subdivision, Fifth Addition, is
apparently two-family residential as is Tract C and D of the Bighorn
Townhome Subdivision.
1
1
NISTORICAL AVALANCHE OCCURRENCES
General Gore Valley
The Gore Valley has seven major climax type avalanche paths along
the south valley wall that have been identified between the village area
and the confluence of Black Gore and Main Gore. These avalanche paths
have been named primarily by Forest Service personnel and from west to
east are as follows:
Clubhouse
Frontage Road
Waterfall
Old Muddy
Timberfalls
King Arthur (KAC)
Vail Meadows
Another large avalanche path is located east of the Black Gore -
Gore Creek confluence on Black Gore that has been designated as the
Siberia Slide. This avalanche path may impact the new Interstate I-70
alignment.
In between these major avalanche paths are numerous small slides
and wet spring type slide paths that wi11 require close examination in
case of any potential development in or near the runout areas.
During a heavy wet spring snowfall of May 6-7, 1973, many of the wet,
spring-type slides along the valley wall slid. Fortunately, these wet
spring type avalanches have minimal runout distances and no existing
structures were jeopardized.
6
1
Discussions with old-time Gore Valley or Minturn residents, as
related by Jim 6regg, U.S.F.S. and Whit Borland, indicate the last major
climax avalanche cycle in the valley was during the 1940's, although some
comments would indicate major avalanches may have occurred in the early
1950's. Examination of snow course records would indicate 1943, 1947
and 1952 as being the most likely years for major avalanche occurrences.
Exact determination of these reported occurrences would require detailed
interviews with older local residents and examination of the meager
climatic data available for that period. While detailed newspaper
research and personal interviews have produced good information on the
history of various major avalanche paths in the San ,luans Silverton
area2, it is doubtful that local newspapers in Eagle County would have
included the same detail, since the Vail Pass Highway, U.S. 6, is relatively
new (1930's) and few of the local ranchers "wintered" in the 6ore Valley.
K A C Avalanche
Detailed examination of the tree growth in and adjacent to the KAC
slide path indicates major avalanche activity within the past 30-35 years.
Other major avalanches have occurred within the past 100 years.
The old burn area that is clearly visible, see Figure II, at the
head of Mill Creek has been a factor in avalanche activity in the 1930's
and 1940's. The forest fire apparently burned just to the upper edge
of the KAC gulley drainage. The resultant open area with the prevailing
west wind up Mill Creek would aggravate snow deposition loading on the
steep upper slopes of the KAC avalanche path. The exact date of the
Mill Creek fire is not known, but due to the rather cormnon practice of
7
setting forest fires to obtain jobs fighting the same fire during the
depression years, the early 1930's is a likely period. Whatever the
source, natural or man-caused, the burn area has definitely been a
factor in avalanche activity on the KAC avalanche. The burn area is
slowly revegetating, slowly, mainly due to the elevation and is holding
more snow at this time (personal observation, winters 1974, 1975).
1
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1
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AVALANCHE ZONING
Avalanche zoning is a facet of mapping of natural hazard zones
such as geological hazards, floodplain mapping and snow and ice avalanches.
The Swiss have progressed further in physical and legal definition of
avalanche zoning 1'4, but various mountainous areas within the United
States have either passed or are contemplating avalanche zoning (vis,
Resolution of the San Juan County Regional Planning CorrQnission, adopted
November 5, 1975; Avalanche Zoning for the City and Borough of Juneau,
Alaska and Avalanche Zoning Ordinance for the City of Ketchum, Idaho).
An interesting aspect of the Avalanche Zoning Ordinance of the City of
Ketchum (Sun Valley), Idaho is allowing owner-occupied single-family
homes within a so-called "Extreme Avalanche Hazard Sub-Zone" with the
stipulation that the owner may reside in the dwelling year-round, but
the dwelling may not be rented between November 1 and April 30.
The Swiss avalanche zoning standards may be generalized by the
following three categories:
White Zone:Terrain is free of avalanche hazard. It might be
affected by the air blastof dust avalanches the
pressure of which does not exceed 100 kilograms (kg)
per square meter (20.5 pounds per square foot).
Red Zone:Terrain which is exposed to frequent and powerful
avalanches. This means avalanches with:
a pressure of l to 3 metric tons per square
meter (200 to 600 pounds per square foot) and a
return period of 30 years or less;
a pressure of more than 3 tons per square meter
and a return period of 90 years or less.
Blue Zone:The blue zone is a transition zone between white and
red. This area is affected only seldom or slightly
by avalanche5. This means avalanches have:
a pressure of more than 3 tons per square meter
over 600 pounds per square foot) and a return period
of more than 90 years;
1
a pressure of 1 to 3 tons per square meter and a
return period of more than 30 years;
a pressure of 0.1 to 1.0 tons per square meter (20
to 200 pounds per square foot).
These zone definitions set forth the relative avalanche activity
and probable impact loads, the zones do not set forth what development
activities are allowed within these zones. Allowed or disallowed
activities within the zones must be established by the local or
regional public agencies. The following excerpts from "The Avalanche
Zoning Plan" by Hans Frutiger are pertinent:
Now it will be possible to determine building specifi-
cations for the different zones. These will prohibit construction
in zones unfit for development. Definitions, however, are necessary.
For example: building restrictions could not be applied to the
construction of underground structures such as water reservoirs,
or to temporary buildings used only in sumner. It might also be
permissible to allow farm buildings such as haylofts and sumxner
stables, which are protected by avalanche control structures.
On the other hand, buildings connected with big traffic or
gathering of people such as hotels and schools might be excluded
from the blue zone.
The zone of transition, especially if it is broad, can be
divided into smaller sections. This can be done by distinguishing
between more and less endangered areas. It would be illogical to
require building reinforcements to have the same load capacity
on the edge of the "red zone" as on the edge of the "white zone".
Therefore, avalanche pressure bands with ranges from 3.0 to 2.0
t/mz, 2.0 to 1.0 t/mz and 1.0 to 0.0 t/mz can be provided.
Eventually, a plan for evacuation must be drawn for the transition
zone. The right to effect evacuation must also be contained in
the regulations of the avalanche zoning plan. In practice the
community authorities can only achieve an evacuation with the
help of an Avalanche Warning Service. It has to give the
technica7 advices. Therefore, such a communal service has to
be provided necessarily for every settlement having "blue zones".
As can be seen, the transition zone'(blue zone) occupies a special
position. A correnent must be made now, which could have already
been mentioned in the chapter dealing with legal aspects. Because
1
10
property rights should be protected as much as possible, the
federal judicial practice requires an unquestionable legal
foundation in arder to effect general building restrictions or
building limi'tations which must be evoked as a result of
avalanche danger. It is the transition zone that leads to
problems and which makes it idfficult to arrive at decisions
that satisfy public and private interests. It is inherent in
the peculiarities of avalanches, especially the ones that
occur at greater time intervals, that their extent and there-
fore their potential to do damage, can only be estimated.
Therefore, small errors can be introduced even by qualified
plan researchers, since no one is in the position to state
objectively what exactly will happen. Such a possibility
should not be a reason to decide in case of doubt in favor
of the landowners since this could result in a decision which
is neglecting the appropriate safety requirement. We should
be aware that a wrong decision can have catastrophic conse-
quences."
The definition of the Extreme Hazard or Red Zone, as used in this
analysis, is basically the same as recomnended by the Swiss and can
be stated as follows:
Red Zone:Terrain which is exposed to frequent and powerful
avalanches. This means avalanches with:
1) a pressure of 200 to 600 pounds per square foot
i to 3 metric tons per square meter) and a return
period of 50 years or less;
2) a pressure of more than 600 pounds per square
foot (3 metric tons per square meter) and a return
period of 100 years or less.
Blue Zone:Terrain that is a transition between the red and white
zones. This area is affected relatively infrequently
100 years or slightly by avalanches. This means
avalanches have:
1) a pressure of more than 600 pounds per square
foot (3 metric tons per square meter) and a return
period of more than 100 years;
2) a pressure of 200 to 600 pounds per square foot (1
to 3 metric tons per square meter) and a return
period of more than 30 years;
3) a pressure of 20 to 200 pounds per square foot (0.1
to 1.0 metric tons per square meter) and a return
period of less than 50 years.
11
The 30 year recurrence interval used in the Swiss Avalanche
Zoning Plan is probably due to climatic cycles that have been established
in the Alps. The modification to 30 year, 50 year and 100 year utilized
herein is to place the avalanche zoning on a more comparable basis to
the existing U.S. Federal Flood Insurance Act which uses the 50 and
100 year recurrence intervals as benchmark hazard levels.
1
1
1
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1
K A C AVALANCHE ANALYSIS
t Character of the Avalanche Path
The KAC avalanche path has a northeast aspect and a maximum
vertical drop of approximately 2350 feet (720 meters). The length
of the avalanche path is approximately 5400 feet (1650 meters) from
the start of the runout zone or edge of valley floor to the upper
end of the path.
The release zone consists of two areas; the lower area relatively
free of significant tree growth due to geological conditions, slope
and frequency of avalanches and the upper area with a significant
stand of timber but showing evidence of avalanche activity both as
string" slides and more general releases.. These two areas are
delineated on Figure III and are of almost equal areal extent, 26.6
acres and 26.8 acres (10.15 and 10.85 hectares).
The lower area has an almost due east aspect and has local
steep sections of 40-42 degrees (84-89 percent), although the nominal
slope for the majority of the area is 36 degrees (73 percent). The
bedrock in this area is exposed in many locations as narrow cliff
bands and dipping rock surfaces. Soil cover is shallow and shows
evidence of relatively rapid soil creep due to the steep slope and
relatively weak rock-soil interface. With the predominate grass-
shrub ground cover, the winter snow cover has very limited frictional
strength at the ground surface.
t
13
The upper release area is not as steep as the lower area and
possesses a deeper soil mantle and a significant stand of timber.
Average slopes in this upper release area range from 32 to 36
degrees (62 to 73 percent) although the upper endhas local slopes
of 38 degrees (77 percent). The snow deposition from the Upper Mill
Creek basin has been significant in the past due to the burn area,
but with the re-vegetation continuing, this factor is becoming less
of a problem for the upper release area.
Snow Depth vs. Recurrence Interval
The west side of the Gore Range has a strong orographic uplift
condition that generates the relative abundance of snow that makes
the Vail ski area known internationally for good dependable powder
snow. Snow measurement courses for this area consist of the Department
of Agriculture Soil Conservation Service stations at Shrine Pass
and Vail Pass plus the snow courses maintained by Vail Associates near
the Patrol Building at the top of the ski area and near Mid-Vail.
The S.C.S. Shrine Pass snow course measurements were initiated in
1942; the Vail Pass course in 1952. The Vail ski area measurements
were initiated in 1963, although some miscellaneous measurements were
conducted as early as 1960.
The S.C.S. Snow Depth Frequency Analysis published in 1974
data through 1971) indicates a one percent probability (100 year
recurrence interval) snow depth of 77 inches (1.95 meters) for the
Shrine Pass course and 86 inches (2.3 meters) for the Vail Pass
course for April 1. The ten percent probability (10 year recurrence
1
14
interval) snow depth is 76 inches (1.78 meters) far the Shrine
Pass course and 73 inches (1.85 meters) for the Vail Pass course.
In an analysis performed by Whit Borland5 in 1972 on the Clubhouse
Gulch Avalanche and using the nine years of record from the Vail
Associates station, the 100 year recurrence interval snow depth was
indicated to be approximately 140 inches (3.6 meters). This record
is extremely short for a reliable frequency analysis, and the course
character such as wind deposition and aspect are not fully defined
at this time.
These snow depth frequency relationships cannot be used directly
in avalanche analysis for a variety of reasons, but primarily due to
the fact that the maximum snow depth usually occurs in April which
is after the peak period of slab avalanche activity in the Central
Rockies, whieh is normally late December to mid March, and that the
avalanche activity is the resultof cumulative factors within the
snowpack, such as snow genesis, resulting largely from temperature
history of the snowpack.
As an example, the following is a list and date of occurrence
of the avalanche accidents or incidences within Colorado as listed
in "The Snowy Torrents", both the January 1967 and the March 1975
editions. These two editions cover the period 1910 through 1971.
Black 6ear Mine April 2, 1926
Arapahoe Basin November 18, 1951
Arapahoe Basin January 18, 1957
St. Mary's Lake February 24, 1957
Dam Slide (Berthoud)April 8, 1957
Loveland Basin February 12, 1958
Berthoud Pass April 29, 1958
La Plata Peak March 19, 1960
15
Aspen February 23, 1961
Arapahoe Basin November 24, 1961
Loveland Pass January 7, 1962
Twin Lakes January 21, 1962
Dotsero March 4, 1962
Red Mouritain Pass March 3, 1963
Homestake Lake January 31, 1965
Geneva Basin December 20, 1965
Loveland Pass January 7, 1967
Arapahoe Basin November 26, 1967
Aspen Highlands February 15, 1968
Leadville February 24, 1968
Niwot Ridge January 26, 1969
Loveland Basin January 27, 1969
Redcliff January 29, 1969
Breckenridge December 25, 1969
Red Mountain Pass March 2, 1970
Breckenridge January 10, 1971
Snowmass February 27, 1971
Aspen Mountain March 6, 1971
Aspen March 16, 1971
Pole Creek, San Juans October 17, 1971
Vail November 28, 1971
This list does not represent a full assessment of the critical ava-
lanche periods during the various winters but only when people or property
were involved in snow avalanches. It does give an indication of when
during the winter season the majority of avalanche incidences occurred.
By month, this would be:
MONTH NUMBER OF INCIDENCES
October 1
November 4
December 2
January 9
February 6
March 6
April 3
The October incident involved elk hunters crossing a snow filled
gulley. The Black Bear Mine incidences on April 2 occurred as the re-
sult of a 14-day storm cycle starting on March 22, 1975. The Dam Slide
on April 8, 1957 was released by artillery fire after a very intense
spring storm. It was, however, a hard slab, size 5(very large) thet
evolved into a mixed powder avalanche during passage down the avalanche
slide path.
16
The point of this discussion is to indicate the probable higher
risk during the late December to mid March period and that not just
total snowpack depth but snow genesis, temperature and storm history
are major factors in developing high hazard and recurrence interval
data for avalanche activity.
For this analysis of the KAC avalanche, two different snowpack
depths were examined as to maximum velocity and runout considerations.
These snowpack depths were 59 inches (1.5 meters) and 78.5 inches
2.0 meters) and these depths have been taken as equivalent to the
average release area snow depth for the two recurrence intervals
used in the Red and Blue runout zone definitions.
Runout Zone Analysis
The detailed analysis of the avalanche dynamics and runout zone
definition involved: 1) assessment of the types of avalanches that can
occur at this location, i.e. loose snow, slab, powder, wet snow
avalanches or mixed type avalanches; 2) definition of the release
zone, slide path dynamics and runout distance and impact pressures
for the more critical types of avalanches. A detailed discussion of
snowpack genesis, avalanche types and dynamics will not be included
herein, as this information is discussed at depth in the various refer-
ence books.
The loose snow avalanche can occur on this avalanche path, but
would involve only the top layers of the snowpack resulting from a
particular storm cycle and would be 9imited in areal extent and runout
distance. In all probability, the loose snow avalanches would be
17
associated with the local cliff bands in the steeper sections of the
release zone(s) and the cliff area near the bottom of the slide path.
The wet-spring type avalanches have occurred relatively frequent
on this slide path, but those originating in the release zone(s)
do not carry through the relatively flat section, Sections B-B and
C-C on Figure III. midway down the slide path. Debris from wet spring
avalanches in this section were observed by the author in the spring of
1973and 1975. An extremely large wet snow avalanche could theoretically
carry through this section, but the bulk of the snow would lodge in the
ravine or arroyo cutting diagonally across the slide path below the
lower cliff band.
The critical type of avalanche for this major slide path in
determining runout distance and impact pressures in the valley floor
are the slab avalanche and powder avalanche or a mixed slab-powder
avalanche. Typically in the Central Rockies, the powder avalanches
originate as soft slab avalanches and through slide path geometry or
velocity considerations evolve rapidly into powder or mixed state
avalanches.
For the KAC avalanche path with the gully section near Section
A-A and the lower cliff band, the mixed state or the powder avalanche
will be the condition of the snow mass as it enters the valley floor.
A full-blown powder avalanche as would be defined by the equations and
criteria of Voellmy would tend'to be confined due to the restrictions
1 within the mid avalanche track and due to the approximately 30 degree
velocity vector change.from the release zone into the slide path.
18
This analysis is based upon the method utilized by Frutiger on
the Juneau, Alaska.and Twin Lakes, Colorado avalanches which utilizes
the equations of Voellmy and Salm. The primary equations are:
V0
2 = to R (sin 0 - le cos YY'0) Gulley F1ow
40 Aoyo
Powder Avalanche
V2 2gho U o
fL
Runout Distance 2
V1
S ~2
2g cos ~U - Tan ~U) + ~1
21~ hl~
Froude Number
2
F 9h Where if F4.1 streaming flow
if F 7 1 shooting flow
P 9 V2(1+
P
P1_P21
Where:
V velocity (avg.) of the snowmass at the point
indicated by the subscript
ts = coefficient of ground friction (m/s2)
400 to 600 m/s2)
4( friction coefficient (0.1 to 0.3)
angle of ground surface from horizontal
density of snow, subscripts indicate whether state
is snowpack, flowing snow or deposited
h depth of snow
19
g= acceleration of gravity, 9.82 m/sec2
A= cross sectional area of flowing snow
S = runout distance, meters
P= pressure on perpendicular obstruction
The analysis indicates the initial slope velocities within the
release zone are in the lower part of the range that could, under
proper conditions, evolve into a powder avalanche. An analysis of
flow regime indicates for flowing snow mass, the Froude Number is
greater than 1 and thusly "shooting flow" would exist. Calculations
were made for bath a flowing mass and a mixed condition flowing and
powder avalanche. The fiield evidence was utilized to establish the
cross sectional area at the various points and to establish the
historic range of velocities at the cliff and rock nose area.
The trajectory for the snow mass coming off the cliff was ca1=
culated for various potential velocities and it was found that all
velocities below approximately 42 meters per second (95 miles per
hour) would allow the snow mass to impact uphill of the rock nose
in the arroyo) and this area would have to fill before the remainder
of the sliding mass would carry on through to the valley floor.
Sections across the ciiff-rock nose area were measured in the
field and the volume of snow to fill this area is approximately 38,000
cubic meters. With an initial density of 0.20 gm/cm3, a deposited
density of 0.35 gm/cm3 and an average release area depth of 1.5
meters, this arroyo would absorb snow from eTeven acres (4.4 hectares)
of the release area. This volume and the attendant turbulence and
energy loss for the remaining snow crossing the arroyo are definite
factors in any evaluation of this avalanche path.
20
Consideration was given to the possibility of a powder avalanche
with velocities in,excess of 42 meters per second at the cliff.
Velocities in excess of 42 meters per second under conditions of a
large powder avalanche on a relatively open slope is very possible as
velocities as high as 90-100 meters per second have been reported .
Due to the geometry of this slide path and the evidence from the trees
below the rock nose, it is the opinion of the author that the probability
of a full-blown powder avalanche with velocities in excess of 42 meters
per second is sufficiently remote as not to be a design case for this
particular avalanche path, given the adopted runout zone definitions.
The calculations for the runout distance and impact pressures
were made based upon equations of Voellmy and Salm and the evidence
obtained in the field from the avalanche path. A velocity vector
change of the sliding snow due to the geometry of the cliff, arroyo
and rock nose is evident from the debris on the rock nose. This was
considered in the runout zone definition.
1
CONCLUSIONS AND RECOMMENDATIONS
The conclusions derived from this study are shown on Figure II,
i.e. a High Hazard or Red Zone of avalanche runout that has a maximum
length of approximateiy 600 feet and an Intermediate Nazard or Blue
Zone width of 60 feet. The definition of the zones is given under
the chapter "Avalanche Zoning" of this report.
A case might be made to define the limits of the runout zone
on the basis of conditions that would occur on a more infrequent, i.e.
considerably greater than 100 year recurrence interval, since the
potential for loss of life exists. A parallel could be drawn between
the conceptualdesign philosophy currently utilized in the design of
dams and spillways in which the design floods are based upon Probable
Maximum Precipitation (PMP) and PM Flood type events and definition
of avalanche runout zones.
The Swiss have considered this problem in the evolution of their
avalanche zoning criteria and the co?ranents of 6audenz Bavier of Chur
contained in Chapter XU, page 157, of "Avalanche Protection in Switzer-
land", General Technical Report RM-9, March 1972, are pertinent.
De Quervain has expressed himself with regard to the
avalanche catastrophe of 1968 in Davos as follows relative
t
to this problem: If one wishes to exclude all possible
risk by taking as a basis of zoning not only the regular
avalanche activity but also all isolated historical events,
then various well-known localities wou7d have to place
entire regions under the ban.' And again he writes: Thus,
probably in the future there will remain a residual risk,
whether because in the space of centuries one must accept
one enormous catastrophe or because more frequently one
must reckon with less intense damage.'"
22
1
The Swiss, while living with avalanche problems of far greater
magnitude than we experience, to date, in the United States, have
adopted a rational, practical approach to the risks involved and
the zoning limitations.
Since two of the Bighorn Townhouse and some of the uncompleted
KAC buildings lie partially in the Red Zone and partially in the Blue
Zone, some consideration by the Town of Vail will be necessary to
establish a policy for these buildings and the potential inhabitants.
Various alternatives are available which range from allowing continued
construction and occupancy to restrictive zoning and not allowing
habitation within the avalanche runout zones. Another possibility
is avalanche defense structures, either retarding structures in the
release zone or protective structures in the runout zone.
Retarding structures in the release zone would be on National
Forest land and would require considerable cooperation and coordination
with the U.S. Forest Service. The cost of these structures would be
relatively high and environmental impact considerations will be
significant.
Defense structures in the runout zone would have to be massive
t to afford protection to the buildings and would require careful design
to insure a reasonable degree of protection. It would be premature at
this time to define, in detail, the structure that would be required,
but the construction costs for full protection would be at least
200,000.
23
If there were no buildings within either the red or blue zones,
the most propitious action would be to exclude any construction within
either zone. The KAC development, however, has several buildings under
construction; and the Bighorn Townhomes have been completed and occupied
for several years. The buildings on the KAC development that lie within
the red zone should not be completed for habitation during the winter
months. Any building that is completed within the red or blue zone
should be assessed for potential damage to adjacent structures in case
of a major avalanche. If additional buildings are constructed on the
KAC property, the new buildings should be sited out of both the red
and blue zones.
The Bighorn Townhomes present a somewhat different problem in that
these buildings have been completed and occupied for several years. There
is definitely a hazard involved for these buildings and any winter
inhabitants. Based upon "Encounter Probabilities for Avalanche Damage"
by Ed LaChapelle, Alta Avalanche Study Center, March 1966, and considering
a fifty year life for the building and a fifty year recurrence interval
for a major avalanche, the chances for avalanche damage to these.buildings
is approximately six out of ten during the expected life period.
REFERENCES
1. The Avalanche Zoning Plan", by Hans Frutiger, USDA Forest
Service, Translation No. 11, July 1970.
2. Development of Methodology for Evaluation and Prediction of
Avalanche Hazard in the San Juan Mountain Area of South-
western Colorado, INSTAAR, December 1974.
3. Cold Region Science and Engineering, Part III, Section A-3
Avalanches", by Malcolm Mellon, May 1968.
4. Avalanche Protection in Switzerland", 6eneral Technical Report
RM-9, March 1975.
5. Evaluation of the Snow Avalanche Hazard in the Valley of Gore
Creek, Eagle County, Colorado", INSTAAR.
6. Clubhouse Avalanche, Vail, Colorado", Whit Borland, July 1972.
7. Racquet Club Avalanche", Whit Borland, December 1972.
8. Avalanche Forces and the Protection of Objects", by Sommerhalder,
translation by E. LaChapelle, Alta Avalanche Study Center,
November 1967.
9. The Snowy Torrents, Avalanche Accidents in the United States
1910-1966", Dale Gallagher, Ed., January 1967.
10. The Snowy Torrents, Avalanche Accidents in the United States
1967-1971", Knox Williams, March 1975.
11. On the Destructive Forces of Avalanches", Voellmy, Trans. No. 2,
Alta Avalanche Study Center, March 1964.
12. Snow Frequency Analysis for Colorado and New Mexico Snow Courses,
1974", April 1974.
13. An Example of Damage From a Powder Avalanche", M. Martinelli, Jr.
and K.D. Davidson, 1966.
14. Guidelines and Criteria for ldentification and Land Use Controls
of 6eologic Hazard and Mineral Resource Areas", Colorado
Geological Survey 1974.
15. The Weather and Climate of a High Mountain Pass in the Colorado
Rockies", Art Judson, November 1965.
1
16. Avalanche Zoning for the City and Borough of Juneau, Alaska",
Hang Frutiger, January 1972.
17- San Juan County, Colorado Avalanche Zoning Resoiutions"
18. Avalanche Zoning Ordinance for-the City of Ketchum, Idaho"
19. Encounter Probabilities for Avalanche Damage", Ed LaChapelle,
March 1966.
20. Snow Avalanches, A Handbook of Forecasting and Control Measures",
U.S.D.A. Forest Service No. 194, Rev. October 1968.
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Acres 8~Red Runout Zcne Suile 0/
DBuilding A
10 _75 ha Hectores ~Lakewood Colo, 80226S~epf. 1975 ,lab. ew.r_ N da;..# _I if.tt,~~
i
EVALUATZCN OP THF, SNOW AVAI.A:YC:Ir, iaZARD IN TfiE
VALLiY 0F GOriE CREEK, EAGLE CQTlPZTY~ COLOKADO
Final Report
Prepared for thE tot,m of Vail, Go:oraclo
by
In3titute of Arctic and A2pine Recearc:
Univers3ty of Colorado
culdc;r, Colorado
Research Team:
Richard F. Madole, Project Director
Paul Carrara
Mike Glenn
Art Mears
Paula Krebs
Funded by
hational Aeronautica and Space Administration
NASA-PY Project Grant Number NGL-06-003-200
I~IIIIIIII!IIUIII~IIi~IIIflIIIIIIIIIIIIINII~IIII~IIII~IIIIC
ACKIQOWLEDGLCIENTS
INSTt'1A.R personnel on this project gratefullytacknowledge the asEtistar.ce
of the towm of Vail in providir.g living quarters and office support for the
duration of the field season. w'e wish to thank especially Terry Aiinger for
his cooperation and active support of this research effort, and to thank
Kent Rose and Jim Lamont for their aid. We also thank Ed Browning, and
Jim Gregg of the U.S. Forest Service, and ArC ,7udson, and Fete Martinelli
of the Rocky i4cuntain Porest and Range lxperiment Station (U.S.F.S.) for
helpful advice and assistance. Sincere appreciation ie expressed to the
field assistants, Marks Anders, i•.ichael Eurnham, Dave Groeneveld, and
Sandy Zicus, for their valuable help.
TABLE OF COIdTE:VTS
SECTION PAGE
I.INTRODUCTION I
II.CONDITIONS IN SNO[d PACK LEADING TO AVALANCtIE RELEASE 2
Paul Carrara)
Avalanche Types 2
Genesis in the 5now Pack 2
Equitemperature Metamorphism 4
Temperature Gradient Metamorphism 4
Physical Characteristics of Avalanche Hazard Areas 4
Some Factors Conducive to Avalanche Formation 6
Meteorological
Topographical 7
III.METHODOLOGY
The Problem
Art tiears)
7wo Types cf Avalanchea: Wet and Dry 8
Art Mears)
Calculation of the Velocity and Impact of "Climax"
Powder Avalanches 9
Art Mears)
Caleulation of Starting Zone Volumes 12
Art Mears)
Vegetative Analysis of Maximum Avalanche Width 13
Paula Krebs and Mike Glenn)
Sizes of the Avalanche Runout Areas 15
Art Piears)
Impact Pressures Within the Runout Area 16
Art Mears)
The Avalanche "Windblast"16
Art Mears)
1
SECTION PAGE
IV.WET SD',OW AVALA2dC?;ES pir' UNC0:7F7.Nr.D 5'L0Pi;S 17
Paul Carrara)
Velocity, Impacts and Runout Distances of the
Unconfined Wet Avalanche 17
V.SiJDi~77ARY 19
Large Major Avalanches 14
Small Slope-Avalanchee 20
Clubhouse 21
Waterfall 22
Timberfalls 23
Ol:d Muddy (Racquet Club)24
King Arthur 25
Vail Meadows 26
SELECTED REiERI;iiCES 27
GLO5SARY 29
iv
LIST OF FZGURES
FIGURE PAGE
lA.Slab avalanche 3
1B.Loose snow avalanche 3
2A.Gully avalanche S
2B.Unconfined slope avalanche 5
3.Aftereffecta of powder avaldnche impact 10
GA,Wet enow avalanche confined in an avalanche track 11
GB.Powder enow avalAnche in the acme avalancha track
as 4A.
SEcrTON i
IN1'RODUC'LION 4 R
The snow avalanche, a mass fiow cf snow and/or ice down a slope, is
one of the most widespread and serious natvral hazards in montane Colorado.
The high, steep, and unbro}:en slopes that extend ircn above timberline
down to habitable altitudes, such as 8,200 feet (2,500 m), represent the
greatest danger. Velocities attained by some 1a:ger 2valanches on sucn
slopes may exceed 224 mph (100 m/s) and impact forces may be on the order
of several hundred lb/ft2 (t/n2) Ie11or, 1968).
Ever since man has gone into the mountains he has been plagued by
avalanches. The recsrrence of winters of excep*ior.ally heavy snows result-
ing in avalanches and disasters are well docuaeated in Switzerland where
villages have been located in the hign mountzin valleys for more than a
thousand years (Fraser, 1967). The largest avalar.che disaster recorded in
the United States claimed 96 lives at Wellington, tYashington in 1910
Gallagher, 1967). _
The probleiv of avalanches is not new to Colorado. During the height
of mining in the late 1800's and early 1900's virtually hundreds of niners
were killed by avalanches (U.S. Dept. Agric., 1968). Since 1950 avalanches
have claimed a total of 41 lives in Colorado, and during this 23 year peri-
od Colorado leads all other states in avalanche fatalities (A. Judson, per.
comm.) 1473). The number of avalanche fatalities will only increase w:.th
increasing recreational use of the mountains and development of mountain
properGy.
The widespread occurrence of natural hazards and the rapid expansionofpopulationcentersinmountainousColozadoposeamajorproblenfor
government, planners, developers, ar.d the public in general. 11,1ountainous
terrain forces population cer.ters and the nighways linxing Chem to locate
along valley floors that are prone to multiple hazards including floods,
debris flows, and landslides as well as avalanches. The Vail area, partic-
ularly Vail East, exemplifies the problem. Of ccnsiderable concern is the
fact that much of the public is unaware of the kinds and 2bundance of these
natural hazards.
The town of Vail has been aware of the natural hazard problem ar.d in
late 1972 appealed to the INSTAAR NASA-PY project team for support tnrough
the then relatively new program funded by the NASA (.dational Aeronautics
and Space Adminlstration) Office of University Affairs. This program
whieh is devoted to applying space technolo~y and the latest remote sensing
technology to real world problems requires, ameng other things, that user
groups requesting support be willing to act upcn the study results. The
need to delinit natural hazards in the Vail area ar.d the willir.gness of
municipal officiale to act upon study results clear2y complied with the
NASA-PY program guidelinea. Because of this and the suiCabi'_iCy of NASA
high altitude aircraft imagery for delimiting natural hazards in th:s
area, the work described in the following pages became possible,
I
V
9
i
E
SECTION II
CONDITIONS IN SNOW PACK LEADING TO AVALANCHE RELEASE
The defin±tion of an avalanche, as used in this report, refers to a
mass of snow and/or ice falling, flowing, or sliding rapidly down a slope
under the force of graviCy. Wherever snow occurs on steep slopes the
possibility of an avalanche is present. Avalanches occur by the thousands
every winter in mountainous Colorado, and the probability af loss of life
and property is certain to increase with inereased winter recreational use
of the taountains.
Avalanche Types
There are two basic avalanche types, the loose snow avalanche and
the slab avalanche, which are recognized by differences in the mechanical
properties of sr.ow in the starting zones. The distinction is based on the
amount of internal cohesion existing between the individual snow particles
LaChapelle, 1970). Lcose snow has litCle internal cohesion and slides as
a formless mass. In contrast, a slab avalanche has more internal cohesion
and can transmit static stresses over a great distance at high speeds.
This results in a large volume of snow releasing from the starting zone.
Slab avalanches (Figure 1.A) involve a much larger area than the loose snow
avalanche, and the starting zone of a slab avalanche contains a fracture
line which may be several hundred or thousand feet long. Slab avalanches
are classified as soft or hard, depending on the condition of the snow
coming to rest in the runout zone. Ttie difference is meteorologically
induced. The hard slab is thought to be the result of cold temperatures
and strong winds (LaChapelle, 1966).
Loose snow avalanches (Figure 1B) tend to occur when a fluffy snow
accumulates on steep slopes during times of little or no wind. Changes in
the snow pack, ei=her induced by temperature or meltwater, reduced what
littZe internal cohesion may be present and the snow gives way to acquire
atubility at a lnwer angle oT repose. Wet, loose snow avalanches, although
not as large as slab avalanches, are very common in the spring nnd early
summer and are a hazard to mountaineers in the high country.
Slab avalanches constitute a major hazard because in many instances
they flo;a over great distances and can inflict extensive damage because of
high velocity and large size. Wind-blown snow commonly develops slab con-
ditions, and wind is interpreted to be one of the dominant factors 3n slab
formation (LnChapelle, 1970). Slcib avalanches begin sliding ae a single
large slab, uaually on an incompetent layer such aa depth hoar or a buried
sun crust.
Genesis in the 5now Pack
Basically, avalanches occvr in an unstable snow pack when the strength
of the snow can no longer support its oG-n weight. Instability i.s produced
by changes vrithin the snow pack usually associated with temperature.
f
d
f
r
s yI
NN
t
U
1
w----
4
Equitemperature rietamorphism
If the temperatures are the same or at -Zeas''t Fsicailar throughouz 'the
depth of the snow pack, a process known as equiteraperature metamorphism can
occur (LaChapelle, 1967). This process is most ragid at 32°F (0°C) but
slows with decreasing temperature. At temperatures below -40°F (-40°C) the
metamorphism takes place very slowly. The snow crystals char.ge shape sa
that the ratio of surtace area to volume approaches a rinimum. Total surface
area is reduced by the transfer of water vapor Co tne paints of contact
between particles. The it2dividual ice particles tend to weld together in
a process called siatering (P,amsier and Kisler, 1966), The snow nack is
strengthened and stability is increased. However, there are other proceases
whichdecrease snow strength and etability.
Temperature Gradient P,etanorphism
Cold a3r temperatures can create a situation whereby snow at the top
ot the pack is colder than near the bottom. When the temperature oradient
is large enough and the snow pack has a high permeability to air, a process
referred to as temperature gradienC metamorphism occurs (LaCtiapelle, 1964).
The equilibriun o£ k,ater vapor pressure in the interstitial spaces is
temperature dependent. The equiliUrium point varies with the temperature
gradient and water vapor flows £rom regions of higher pressure (warmer snow)
to areas with lower pressure'(cold.er snow). This causes a grain to grain
transfer of waCer vapor. Vapor is condensed a-- ice on the coldsurface of
an adjacent grain. Opposite this deposition ooint, vapor is removed to be
deposited as ice on the cold surface of the nexC grair.. Tenperature ;radi-
ent metamorphism results in the upper part of the snow pack Saining raterial
at the,expense of the lower part of the pack. In addition, this process
tends to produce cup shaped crystals wfiich possess little cohesion and
which undergo almost no sintering. This usually produces larger crystals
with fecrer ice bonds whose tensile strength, resistance to shearing, and
lodd bearing capacity are greatly reduced. If the process is compieted,
the snow involved develops into a mass of crystaZs with little or no interztal
eohesion. This is callea depth hoar. Tlie occurrenee of depth hoar in a
snow pack on steep slopes can lead to dangerous avalanche conditions, ana
ia responsible for some of the Zarge "climax" avalar.ches which have cccurred
in Colorado.
Physical Characteristics of Avalanche Hazard Areas
There are two main types of avalanche hazard areas: the well-defined
avalanche chute or gully, and the steep planar barren slope which may "rur_"
over a considerab]_e area. A well-channeled avalanche track usually leads
from a bowl-shaped catchment basin at its head, the starting zone. When
snow in the catchrsent basin is released, it is funnelled into the gully
where it is generally confined until reaching tfie valley floor below. Forces
fzom these avalanches ulay be very high. These avalanches are capable of
cuuaing extensive damaee Co buildinEs und loss o£ life. Fiuure 2A illuatratea
the component parta of thie type oC avnlanche.
The other tyne of av3lanche hazard area occurs on asteep hillside with
little or no vegetation (Figure 2Il). In theae areas, it is difficult to
r~
y {t\\~
4,.4
41 1~1
slk-~J 1r
rK~~Y}
A
lit~t4 Y:1
y 1 r l< lc~ `c~/~-,y
r `
i f `
t~ . ~
j~j 1
w i. l
r 4,,,-„4 AVALANCHEUPJCONFINEDSLOPE
GULI.Y AVALANCHE
Fisure 2B.
Figure. 2A.
delimit the cat'chnent basin Uecause the entire hillside may release if
slab conditions develop. Avalanches can occur on essentially any slope
of 15° to 60°, but ure most corcmon slopes of.30°% 40 45°. SZopes less~
than 25° are not steep enough to be hazardous except under rare and pecul-
iar conditions, and tnose sreater than 60° are so steep that snow tends
not to accumulate on them (LaChapelle, 1970). Wet slides are an excention
to this. There are reports of wet slides starting from a slope as gentZe
as 12° (CotCman, 1966).
Areas prone to avalanches may be associated with indicator plant
eomzr,unities and/or structure of vegetation. Tracks that avalanche annu-
ally will be nearly devoid of trees. Those thaL- run relatively frequently
are outlined by aspen wEiile the hillsides around them may be covered by a
coniferous forest. Trimlines separating small trees fren larger ones are
oiten evident on hillsides. In some areas avalanc}iing is restricted to
small "stringer" slides wiiose locations are revealed by narrow lanes devoid
of trees. Also some smaZl nvalanches can take place on forested slopes and
flow through the forest without destroying it. Two inportant points to
remember are (1) any steep slop.2 can "run" under proper conditions,and
2) even a smaZl slide of anly a hundred feet or so is enouoh to trap und
kill a human.
The high frequency avalanche paths are only a part of the avalanchz
hazard problem, ar.d perhaps the least worrisome part at that. Of r.:ore
concern are the not so obvious areas ot infrequent avalanching. These
avalanches are analogous to the 50- or 100-year flood. They do not occur
often, but when they do, the damage tends to be severe. As point2d out
bJ Gallabher (1967), the climax avalanche may "run" only ence per one or
two centuries, and iC is these that hold the records for deaths and property
damage. The inirequency of these large, deadly avalanches is such that
Lhere tend to be few clues to the outer limits. The areas involved are
generally covered with forests w•hich, to the uninfo:med observer, appear
to be safe. However, clues to detect the boundaries do exist in the liviag,
damaged, and dead vegetation.
Somo Fnctore Conducive to Avalanche Formntion
Meteorological
Wind will redeposit snow on the lee side of ridges, thus loading r
these slopes with more snow than if there had been no wind. Winds greater
than 15 mph will usually move suow. Wind can also compact snow to form a
slab, which when fractured begins sliding as a unit.
Rapid accumulatian wi.ll not allow the snow to settle. Generally snow-
fall rates of 1 inch per hour produce hazardous conditions.
Wnter contrnt mny build tap to nuch a point thnt the intcrnal cohcelon
of the tmuw puck iH reQuced. Thie ie tho enuea of wot enow nvnluncheet in
the spring.
Cold temperatures may lead to fornation of dep[h hoar. If a tempera-
ture gradient exists ir, the snow pack, the sintering process and its stabil-
izing influence will be greatly sloked.
7
Snow cr'ysEal structure is important because small needles and pellets
result in more dangerous conditions than dendritic or star-shaped cryytals
LaChagelle, 1969).t 3
Storms iniluence potential avalanches. A large number of avalanches,
usually loose snow avalanches, occur during or shortly after atorma.
Tapogranhical
Slope asnect is considered important because snow on north-facing
slopes is nore susceptible Co depth hoar conditions and, therefore, is nore
liicely to slide in mid-winter. SouCh-facing slopes are dangerous in the
spring season.
Slope anvle, as mentior.ed before, 3s important in that most avalanches
occur on slopes of 30° to 45°. However, avalanches can occur on slopes
ranging from 15° to 60°.
Slope Drofile is to be considered because slab avalanches are more
likely to occur on conve.t slopes where tensional forces are present.
Snow depth is an important factor. If the snow depth exceeds the
average height of ihe ground surface roughness the avalanche hazard is
inCreased.
For those interested in furthe'r reading canceraing aval2nches refer
to the selected bibliography in the back of this report.
t-
1
srcTioH zzi
2IETHODOLOGY t
The Problem
Several moderately large avalanche pachs exist on the north and north-
east facing walls of the valley of Gore Creek (Plates 1 and 2), lhe ava-
lanches c:hich run in these paths pose destructive potentials which'are
difficult to evaluate because they rarcly run to their full capacity. Be-
cause oi their hig}i velocitins and "flutd" properties, thesc avalanches do
ttot stop on the mountainsides, but travel unknown distances out onto the
valley floor. Several of these "ruuout" areas are pr?vata].y owned, and a.re
presently being developed. The probability that these avalanches will
reach buildings wnich are occupied during the ski seasor. is dangerously
high in several cases.
The iollowing questiona must be answered to evaluate the degree of
thie hazurd:
1. Wliat area will Che runouC of the largest expecCecl avalanches
cover?
r
2. Whatrwill be the impacL• forces upon cbstacles within these
areas?
3. What is the probAbility that the large or "cilmax" avalanche
will occur?
The methodology used to answer these questiona ia preeented in this
chapter.
Two Types of Avalanches: Wet and Dry
Wet snow avalanches and dry snow or "powder" avalanches both occur in
the major avalanche paths along the valley of Gore Creek. It is necessary
to distinguish between them because they differ greatly in density, velocity,
runout area covered, and the nature of impact with obstacles.
Powder avalanches typically consist of a fluidized, 2ow density mixture
of snow and air. They often reach velocities 4-n excess of lOG mph but dc
not travel faster than 200 mph except on rare occasions (:fellor, 1968).
The flow of these low density powder avalanches is probably turbulen., and
upon impact toith obstacles behaves like a true fluid. Consequently, the
results of the powder avalanche impacts observed in the field havebeen
analyzed using the relationships of aerodynamics.
Wet snow avalanches usually travel at much lower velocities, flow
closer to the ground, and follow irregularitiea ir. the topography nore
closely than powder avaZaaches. The density of the moving nnow may ba as
high as the density of the snow deposit before it released. Wet snow
9
avalanches, because of the±r hioh density, nay behave as a compressible
solid upon impact with a rig3d body (:fellor, 146,8~, causing very higN
inpact pressures in spite ot their low velocities. Tne high kater cor.tent
of these avalanches has a iuoricating eftect, ar.d they may trsvel ZOIIgdistancesintherunoutarea, especially when they are confined to a gu11yA. Judson, per. comm,, 1973).
Of these two avalanche types, the powder avalanche has the potential
of covering the largest area in the ruaout z.ene. Therefore, the possible
runout areas of these powcier avalanches should be determir:ed bec2use they
are capable of covering larger areas than wet snow avalaaches.
Calculation of the Velocity and Impact af "Climax" PaWCIer Avalanehes
Tite impact pressure of a poc.der avalanche is a funetion of flow densiCyandvelocity. Although density is unknown, reasonab2e upper and lower
limits may be esticaated. A very lightly loaded powder avalanche has a bulk
density of ice particles in turbulent suspension as low as 10-4- grs/cm3.(1)Since the density of air at 10,000 feet elevation is about I0-3 gm/cm3 Chis
is also the approxj.mate lower limiC for avalanche density (i•lellor, 1968).Mellor (1968) suggests an upper linit for powder avalanche density of i0-2
gm/cm3, which is the approximate minimum density for snow in which all the
particles are in mutual contact. ilowever, powder avalanches may be some-
what denser close to ttle ground, so for purposes of analyzing ivipacta a
higher value of 2 X 10-2 gn/cm3 has been used in calculatier:s.
Observations in the field showed that powZer atalaach° impac*_.s near
the lateral bour.daries of the track generally caused breaking of the trunks
of Englemann Spruce (Picea en~el:,iannii), Lodgepole Pine (Pir.us contorta) ,
and Aspen {populus tr.cmu].oide,) at heights of 5 to 15 feet above L'ne ground,and there was often widespread damage to limbs or nearby trees up to 35 feet
above the ground (rigure 3). The Vail Meadows avalanche reached flcw
heights of 120 feet above the central gul-ley about halfway down the Crack.
I[ was not uncommon to £1nd the broken stu:nps of Spruce trees three feet
in diameter which had failed through the beading stress applied to L-hem
by an avalanche.
Where wet snow avalanche damage was observed, it was much more extensive.
Large trees were u;ually uprooted as ttight be e:cpected frcm a dense slurryofsnowmovir.g close to the ground. Consequently, it was not difficult to
distinguish between wet and powder snow avalanche damage. Tne central gullies
of the large avalanche paths showed wet snow avalanche danage. Figure 4
depicts the relative sizes of powder and wet snow avalanches flowing in the
same track.
Fie1d data collected to determine avalanche impact pressures included
measurement of the cross sections of tree trur.ks where bendin,; failure had
taken plnce. In Vail Mcadowa, King Arthur's and Timberfalls gullies, trees
broken or damnged by pok-dez avalanche impnct wera observed along the entize
l)lgm/cm3 Q 62.4 lbs/'r"T~.
J/~~1f/~y'r
tt~`
4*^t
I{1.~II! IJ~} . f; 1 a l
y~X_~~~~i~~~~~y~., ~'I~,.F s t ui. r l ~O. ,f•s'','~1,•r,i~r',~'%';` Y%'i t~i;:_.'y'{, 1t,~Y 1 f f I f} t ~
I~/ j'4i ~vr i~VA~'l~. y~,~,~l' j(ftJ~ ~ y~
y r / i s rY~
rk'',i~~~5r~.»
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fir~ r J,r
5 r~,~r c c
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f ~ 'v s ~ ~ l. ~ ivr S c'",i ?5r'
i~`/~Irf~~ _ i~+"-r~` r,Sl j~€1~'~ r{
i i Ik:..t ~ 1~..~w F~ f I-V w f 1~ L 1 f j/.~ t
1 t ) l 1 t S , r.. •.y jroti1ti\a,E•r"~ 1
ol( ti!
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1-J~1..t J t~ t ~ ~..~ill1l~Yir.~.~7 t l~, 1.~ .
i q j 1.' y l y ~ - 7 t `E!i45y~p t f Zti;!.~/~1 t~ :1.~,~',f`I~r G rn„ / i~~ ~ K(4 Y~ui{~ K 1~ Y•
1~~~~f `i~ ii • ~y Z~YY4 fJ~l~r~~l#~''I`.r~l S',h.
f~~~
j' i 1"1 i e~'' :yll, ~ 1~J ~ r.~.:/ /fi M `7~Y'r 1" y"r.'~~'~ w+`.~u//~i/! +1?..V•'~.:
1 I~~~~i~~~~A~1S ~ ~e Y 1t ~ i
i~~TJ Y+.
fi SamL
v,.~v,v t , • he e 1atc
aYo~.?~ I•-~
1i.1 7"'•:.I' : f p~z,,deX e
Fisure 4a• track asr4A' B~ 1 featxes
r_ .~s-.-G'•1 e~1 b~r~p~i f fecte~.
13
purpose because they are almost scale consistent from point to point. Tne
portio;i of thepiioto cove°ing t.ze Gore Creek avalanche area was enlarged
to a scale of approxir:ately 1:12,000 (1" = i.,G00'). Resolution at this
scale was about 5 feer.. The starting areas•wer@ outlined, as ahoun on
Plates 1 and 2, assuming the following:
1. All of the open, unforested areas could release eimultaneously,
and,
2. In forested areas adjacent to the open areas where destruction
in the timUcr was observed, 1/2 of thia surface area was used
to calculate avalanche relcase volumes.
During the summer of 1973 the starting areas of Clubhouse, Frontage
Road, Waterfall, Old I~fuddp, Timberfalle, N.ing Arthur's, and Vail Meadows
avalnnche areay were visited, The following observations are genera2, and
apply to all these areas: ,
Z, Extensive areas existed wh3ch were almost coapletely devoid
of trees.
2. Gradients on the upper north and northeast facing slopea
typically measured 60% to 80%.
3. The slopes were smooth and grass covered.
4. In some cases dowmdipping rcck strata leud to downslope move-
ment of rocks and soil and appeared to linit extensive tree
growtih.
5. At some locations extensive destruction from powder avalanching
was evident wi,*_hin the mature forest, especially adjacent to
the open starting areas. This inc3icated thaC some of the snow
within the Porestcould contribute to a maximum volume avalanche
release.
6. Avalanche Taovement down the central gullies can cause a szmul-
taneous release of sr.ow from areas lower on the mountain,
adding to the avalanche volume.
7. Localized avalanche activity probably takes place more fzequentlp
in the starting areas, and tracks limiting revegetation of these
areas.
Vegetati.ve Analysis of Maximua Avalanche k'idth
The mhinum width of the clir.!ax snow avalanche is a requzred parameter
in the analysis of impact forces and runout distances. With changes in the
terra3n the avalanche track is altered. The extent of these alterations
are dependent on the nature of the terrain characteristics, and indicate
the volume and the velocity of the snow. An analysis or the vegetation
14
across the track and into the undisturbed forest on either side is a way
to quantify the volume and velocity of the snow involved in the inzreouent
ard seldoM observed cllmax avalanche. An assunFftion of the developed,
methodology is that the ma}:in:um width of the clinaa avalanche ccincidea
with the u;aximura lateral exter.t oL disturbed vegetation along the track.
The undisturbed vegetation i5 characteristic of the sature forest and
departures fron th7s are detectable as "disturbed vegetation" in the ava-
lanche track.
The samplir.fi technique does not require description and analysis of
the vesetation Lor the entire avalanche area. Ir.stead, detailed informa-
tion is collected f.or smaller areas whicil are theoretically represeneative
of the total area. Vegetation is sampled along lines, called transects,
and yield a cross-sectional description of the vegetation from within the
forest on one side of the avalanche track into the forest on the other side.
The transects, or lines for sampiing, are laid out perpendicular to the
avalaache traclc at selected points where the channel characterist=cs change.
Examples of transect placement are at the top of the track markir.g the
loiver edge of the starting zone, at'some intermediate "normal" points along
the traclc, at points where the slope gradienC signiLicantly changes, at
point.9 wlicra Lli(i dirrctlo<i or ch:irnctcrlvticn of the channcl chnnp,c, attci aC
t:hc boCCum uf. tho nv:llnncho Crticic miirlclnE; ne Lop of the runouC •r,ana, n
topop,rnphic or Ineal re7.1eL crosh-accti.on way surveyed tor ench trunrleci.
The correct placement of the transects and the cross-sectioas of each were
ttecessary to determine the variations of the margins of the avalanche as a
response to changes in topography. Lach transect consisted of two =egmients,
one going to the right of the Crack and one going to the left. Beth seg-
ments began at the draiii•age channel in the avalanche track and ended in the
mature forest on opposite sides of the gully.
A transect is an elongated sample plot in which the vegetational data
are recorded as a list-count of plants as they are encountered. This
method is useful to indieate transitions in vegeCation, as in this instance
betweett the mature forest ar.d the avalancP:e track. Within contiguous sample
units along the transect a list of species, i.e. the kind of plants, and the
number of indivi.duals of each species were recorded. Two sizes of samnle
units wcre employed, one for woocly vegetation and oae for herbaceous (non-
woody) vegetation. For the tzees and snrubs the sarple units were five oy
five meter quadrats sucessively placed along the transects. Besides
recording the species and number of indi•aiduals within each sample uni*_,
the diameters of the trees were noted as we11 as aay scars or branch trimming.
Within these larger quadrats for woody vegetation one meter by one-half
meter quadrats were selected as the sample units for herbaceous vegetation.
The detailed information frota field data collected in this nanr.er poztrays
the distribution,and the apparent changes in density of speciesalong the
transects.
The data fron each segment were analyzed separately because differing
environmental factore, such aa slope aspect, influenced each segnent. The
procedure used was a coaputer program of factor aaalysis described by BrayandCurtie (1957). This analysis groups the data into clustera of saiapie
15
units having similar vegetative characteristics. Where there is a change
in the composition of the vegetation a diffwrer.C Fluster ;ts designated.
For exa7ple, data from one segmienC extending from Che drainage into the
mature forest mignt be grouped into a creei: co:nmunity, a n:2adow-like grase
area, shrubs ar.d tree seedlir.gs, and mature forest. 1'he break between the
cluster of sample uniCS representing shrubs and tree seedlings and the
cluster of sample units representing the mature forest indicates the max-
imum lateral extent of disturbed vegetation. This is interpreted to be
the maxinum width oL the avalanche. Ay plotting the sample units of the
two segments along the cross-section of any particular transect tne lateral
boundary of the avalanche can be located, 5ome indication of the volune af
snow filling the channel during the climax avalanctie can be calculated from
this in£orLZatian.
Branch trinning and scars on the lower portions of tree trunks near
this lateral boundary give additional evidence of the flow depth and max-
imum width of the avalanche track. Frequently found were the snapped off
trunks of trees nea: these lateral Uoundaries. t3y measuring the size of
these stumps and by noting what species of tree they were, the force
required to cause f<:ilure can be calculated. Data from various U.S. Forest
Service research laborato.ties are available which evaluates the internal
stren;th of different *.aoods. Combining this informaCion with the known
location of the stumps miniaum values of the forces generated within the
snow avalanche can be derived.
In summary, the maximum la.teral extent of the climax snow avalanche is
depicted by thc point where the characteristics of the maturn forest 3isap-
pear. Within the mature fcrest there is random deadfall, little ground
vegetation, and few young trees. Within the avalanche track near the lateral
boundary is found unifornly oriented downed timber, meadow-like vegetation,
and shrubs with seedling conifers. In some instances the maximum width of
the avalanche caunot be detected as precisely as described above. Here,
field notes, personal observation and e:zperience, and computer analysis are
combined *_o determine the naximum lateral extent of the climax snow ava-
lanche.
Sizes of the Avalanche Runout Areas
The measured and derived data used to calculate the size of the climax
avalanche runout areas were the following:
1. Probable range of avalanche velocities.
2. Probable range of avalanche flow densities.
3. The width of the avalanche at the top of Che runout.
4. The heighK of the avalanche at the top of the runout.
5. The topography of the runout area.
i
16
Voellmy"(2955) developed a method for calculating avalanche runout
distance. For this study, Voellmy's equations have been modified to include
the effects associa*ed witn lateral sp:eading oe the avalanche in the runuut
area. The results obtained througn this modification seem reasonable when
compare3 u-ith observed runout areas in other parCs of Colorado. The runout
areas for the six large avalanche gullys in the valley of Gore Creek are
shown on Plates 3 and 5.
Impact Pressures Within the Runout Area
As the avalanche decelerates in the runout area, its density increasea
probably causing impact pressures to increase also. Obstacles near the
outer limits of the rur.out area nay actually be exposed to greaCer impact
pressures than those near the bottom of the avalanche track. For example,
at Timberfalls, im;act pressures at the boCtom of the track would be ubout
1,000 pounds per square foot, but could increase to more than 2,OC0 pounds
per square foot after the avalanche has traveled severai hundred feet into
the runout area. From a planning standpoint to try to compromise the ava-
lanche hazazd by building withiri the'outer limits of the runout area is not
logically sound.
The Avalanche "Windblast"
Powder avalanches sometipes have a high velocity gust of air associated
with their descent. The maxinum velocity of this gust is probably limited
by the terminal velocity of the avalar.che during its descent, bnt possibly
the parcel of air comprising this gust naintains a high velocit}• for a
longer period nf time than the avalar.che itself-. If both the avalanche and
the wind gust reach the runout 2rea at the same time, the gust will depart
fron the avalanche front as the snow mass decelerates. The air blast may
traveZ unexpectedly long distances across the valley as long as it is unim-
peded by majo: obstacles, thereby extending the destructive zone of the
avalanche.
As discussed previously the ma:cir„um avalanche velocities possible
along the valley of Gore Creek are about 170 npn. The inpact pressure
associated with a 170 rsph gust of clear air at this altitude is 65 pounds
per square foot. However, after the air parcel has traveled 1,500 feet
tha velocity could still be 140 mph, producing impact pressures af 45
pounds per square foot. This is still a potentially destructive pressure
to buildinSs.
Although the boundaries oi the "windblast" effect could not be mapped
in this report it should be considered when pl.anning structures in the line
of avalanctie descent, even when they are beyond the rur.out liraits. This is
particularly important wnen buildings are dcsigned to have large areas of
glass facing the mountainaide.
SECTION IV
FIET SnOW AVALANCHES Oh UNCONFINED~AOPES
Aerial photo inspection along with later f3eld observations clearly
indicated that the large gullys descending the southern wall of the valley
of Gore Creek are not the only avalanche hazard present in the valley.
The steep hillside in areas between the gullys where the slope ranges from
25° to 40° also "runs" with small wet snow avalanches.
These wet snow ava]anches, similar to the wet slides in the gullys,
usually travel at los;er velocities and flcw closer to the ground than
corresponding powder avalanches. The flow densities of these wet ava-
lanches may be as great as the density before release. Destructive poten-
tial by these wet snoG? avalanches may be higti because of this high density.
One small avalanche last spring broke an aspen tree which was 40 cm in
diameter, and carried it almost to Gore Creek.
Velocity, Impacts and,Runout Distances
of the Unconfined Wet Avalanches
Although not possessing the tremendous runout potential of avalanches
occurring in the major gullys, the wet avalanches on the steep valley side-
walls are a definite hazard. Impact forces associated with these avalanches
may be high due to the high density of snow. To delimit the runout distances
of these avalanches on the avalanche hazard Maps was considered important.
Field observaL•ions turned up evidence of runout distances and destruc-
tive forces. Furthermore, fi.eld data delimited those areas subject to
wet-spring avalanches. The equations of Voellpy (1955) and Sommerhalder
1964) were used to delimit the extent of the wet'slides in areas where
nvalanche debris Iias been removed.
Assumptions in the calculations included 3epth of released snow, the
ground friction coefficients, and coe£ficient of internal friction. How-
ever, the values whi.ch were used in the equations must approxiMate the
actual ones because field observa*_ions tend to support the ealculated
results. For avalanches ariginating below the linestone outcrops the
depth oi released snow was considered to be approximately one rceter, a
value thought to be a realist:ic depth. Values for the coefficient of
ground friction used were 400La /52 for grassy areas (5orr,merhalder, 1964).
Internal friction may approach zero in the case of extremely wet avalanches
but for the majority the coefficient value will be around 0.15 (Sommerhalder,
1964).
Runout distances ior these wet avalanches are much less tnan those of
the large gu11y avaianches but due to the high densities, impact forces
from these wet avalanches are surprisingZy high. Runout distances for wet
avalanches in various areas of the valley of Gore Creek range from 15 to
150 meters. These distances are dependent on the starting slope angle, the
nature of the vegetation on the starting slope and in the runout zone, the
ie
depth of the"released snow, the slope of the runout zone and the water
cantent of the moving snow.
4 y
VelociLies of wet snow avalanches may approar_h 68,4 nph (30 m/s) for
a large release. This is still much slower than those velocities obtained
by the large climax poiader avalanches.
The following table gives an indication of veloci*_ies, impacts and
runout distances on various slope angles assuming a grassy surface in the
release zone, 1 m of released snow with a density of 300 kt,/n3 and a grassy
runout surface sloping at 10°. These conditions are common in the valley
of Gore Creek.
Starting Impact Runout
Slope Anp,le Velocity mph (m/s)pounds/ft2 (+/m2)Distances feet(m
25°29.4 (13.1)1,904 (9.5)245 (75) .
30°33.4 (14.9)29660 (13)233 (71)
35°36.8 (16.4)3,070 (15)266 (69)
40°40 17.8)3,480 (17)220 (67)
SECTIOY V
SUhAfARY
Gore Creek flows in a westernly directionfor approxinately 12 miles
before it joins the Eagle River at Dowds Junction. Gore Creek is joined
from the south by one of its major tributaries, 31ack Gore Creek a few
miles from its headward margin. At this point the valTey becomes broader
and the valley gradient lessens. fiowever, larse gullys can be seen descend-
ing the southcrn vZlley wall which are bare of vcgr_taCion in many places.The uvalanche tiazard in the valley of Gore Creek is associated with theae
gullys nnd Cne valley sidewall. The vaSley oi Gore Creek ia cuC into the
Aiinturn Formation which consists oY 5,800 zt. 1,800 m) oi shales, sand-
stones, and conglomerates of Pennsylvanian age (320 to 280 ruillicn years
ago) interbedded with several beds of limestone and dolomite in its upper
twa-thirds. For a detailed anaiysis of tiie locaZ bedrock the reader may
refer to several sources on this sub,ject (hreto, 1949; Hanshaw, 1958;
Berger, 1961; and Poelchau, 1963).
Large Major Avalanches
The major gullys which constitute the avalanchc hazard in this valley
are located from the Junction of the Gore and Black Gure creeks, and extend
down to the vicinity of the Clubhouse Slide. All of these avalanches, with
the exception of the Vail Meadows Slide, run unobstructed to the valley
floor, wilich lies at elevations.of 8,200 ft. 2,500 m) to 8,400 ft. 2,550 m),and which is endangered by avalanches. The vertical drop from the hea3 of
these gullys to the valley floor is 1,970 to 2,800 ft. 600 to 850 m). 1ne
bowl-shaped depressions high up on the h311side are the caCChment basins for
the associated avalanche tracks. These bowls probably were nivation hollows
during the Pleistoeene when the climate was considerably cooler.
Evidence that these slides have "run big"'in the past comes from the
obvious trimlines, bzolcen tree stumps, and in sone cases by the alzgneddebrisontherunoutfansatthebottomofthetracks. However, lack of
this evidence does not imply the avalanche track never ran. Z:uch of the
debris brought down by these avalanches were removed Uy ranchers using tne
fans as hay neadows (Jim Gregg, pers. cocm., 1973). Present day construc-
tion crews have also reraoved debris as the area o;as bulldozed to 1ay
foundations for condominiums and homes.
Wet slides in the spring are a com.mon occurrence in these gullys. The
sides of these gullys aze ratner steep (30°-40°) ar.d with the warm weather
in April and 24ay snow in these gullys rnay slide. Although these caet slides
do not usually flow for great distances out onto the runout ian, the poten-tial avalanche hazard still re,..ains high. The wet sl4des keep the track
c2ear of obstructiona and vegetation so that A r~3jor powder avalanc:~e Ylow-
ing down the track in winter will encounter only a mminiBUm of surface
friction. In short, the wet spring slides occurring in the gullps prepare
the avalanche track for the climax powder avalanche.
zo
SmaZ1 SZope-Avalanches
In other areas wtiere r.o large avalanche gul2ys are present slope
avalanches are very ccm.mon. Unlike the large gully type of avalanchee,
these avalanches do not posses crell-defined catcnment basins, tracks, and
runout fans. Tne slcpe avalanctles occur on the steep valley sidewall
25° to 40°) and usually originate immediately below the proncnnced lime-
stone outcrop along the valley wa11, alttiough some large wet avalanche
trac3cs originate above tnis unit: The slope avalanches are Co the west
of Timberfalls gu11y, where the slopes are covered by as,nen. In sosce areaQ,
the existence of slope avalanches is marlced by a doiN-ns7_ope absence of treea.
However, soze wet avalanches can eviden*_ly run out c?f forested areas as
well without destroying much of the vegetation cover.
These bankslides appear to be more common zn the aspen forested areas
as opposed to the coniferous forested areas. One reason for this may be
that the non-foliated aspen trees aiford little protection to the underlping
snow pack during the spring warm-up allowir.g lubricution of the snow pack by
meltwater. This contrasts sharply to snow pack in areas of conifer forest
that are in shade (C. Whelin, per. comm,, 1973).
While not possessing the extensive runout potentia2 of large gully
avalanches, unconfined wet ava2anches certainly are capable of inflicting
daaage to buildings and causing loss of human ?ife. Many lives lost to
avalanches have been from smail.ova2anchee that run less than se-.eral
hundred feeE (Fraser, 1967).
r
q.
ClURHOUSE
E
T. Description of Terrain
A. Startins Zone
1. Area: 30 acres
2. Average inclirLation: 50%
3. Maximu.-n inclination: 80`/<
4. Elevation: 10,200 to 9,400 £eet
5. Orientation: N to NtJ
B. Avalanche Track
l. Average width: 145 feet
2. Maximum width: 150 feet
3. Maximum flow height:' 70 feet
4. Elevation aC bottom: 8,400 feet
C. Runout Zone
1. Average inclination: 14%
2. Type of surface: mixed grass, shrub, and sspen forest
II, Ayalaache Destructive Potential
A., Zn the Track
1. Maximum velocity: 100 to 130 mph
2. Maximum impact pressure: 1,000 to 1,300 lbs. per sq. ft.
B. In the Runout Zone
1. Maximwn runout distance: 650 to 800 feet
2. Maximam impact pressure: 1,000 to 2,000 lbs. per sq. ft.
3. Maximum distaTice of observed debris: 750 feet
22
WATERF6LL
R {h
1. Deacription of Terrain
A. Starting Zone
1. Area: 20 acres
2. Average inclination: 53%
3. rfaximum inclination: 70%
4. Elevation: 10,700 Co 10,100 feet
5. Orientation: NE
S. levalanche Track
1. Average width: 170 feet
2. Maximum width: 180 £eet
3. Maximum flow height: 80 feet
4. Elevation at bottom: 8,500 feet
C. Funout Zone
1. Average inclination: 20%
2, Type of surface: grassy meadow
11. Avalanchc Destructive Potential
A.. In the Track
1. Maximum velocity: 100 to 130 mph
2, Maximum impact pressure: 1,000 to 1,300 lbs. per sq. ft.
S. In the Zunout Zone
1. Maximum runout distance: 750 to 910 feet
2. Maximum impact pressure: 1,000 to 2,000 lbs. pe~ sq. ft.
3, 2•'.aximum distance of observed debris: 820 feet
23
TI218ERFALLS
1. Description af Terrain S
A. Starting Zone
1. Area: 47 acres
2: Average inclination: 57%
3. Maximum inclination: 80%
4. Elevation: 10,900 to 9,600 feet
5. Orientation: NE
B. Avalanche Track
1. Average width: 300 feet
2. Maximum width: 340 feet
3. Maximum flow height: approximately 100 feet
4. Elevation at bottoa: 8,650 £eet
C. Runout Zone
la Average inclination: 1.0'0
2. Type of surface: grassy meadow
II. Avalanche Destructive Potential
A. In the Track
l. Maximum velocity: 120 to 150 mph
2. Maximum impact pressure: 1,000 to 1,500 lbs. per sq. ft.
B. In the R.unout Zone
1. Maximum runout distance: 1,040 to 1,260 feet
2. Maximum inpacL pressure: 1,500 to 3,000 lbs. ger sq. ft.
3. faximum distance of observed cebris: 780 feet
24
OLD MG'DDY (RACQUET CLUB)
I. Description of Terrain
A. Starting Zone
l. Area: 30 acres
2. Average inclination: 54%
3. Mar.imum inclination: 80%
4. Elevation: 10,600 to 9,400 feet
5. Orientation: NE
B. Avalanche Track
1, Average width: 150 feet
2. Maximum width: 250 feet
3. Maximum fiow helght: 70 feet
4. Elevution at bottom; 8,600 feet
C. Runout Zor.e
1, Average inclination: 5%
2. Type of surface: grassy meadow with some mudflow debria
II. Avalanche Destructive Potential
A. In the Track
1. Maximum velocity: lOQ to 130 mph
2. Maxinium impact preaeure: 800 to 1,100 lbe. per aq. fC.
B. In tlic Runout Zone
1. Maximum runout diatAnce: 770 to 950 feet
2. Ma.ximum impact pressure: 1,000 to 2,000 lbs. per sq. ft,
3. Maximum distance of observed debris: none could be found
V .
2J
K11YG l'illyj]VR
x {3
I, Description of Terrain
A. Starting Zone
1, Area: 45 acres
2. Average inclination: 57%
3. Maximum incli.nation: 85%
4. Elevation: 11,000 ta 9,600 feet
5. Qrientation: NE
B. Avalanche Track
1. Average width; 300 feet
2. Maximum width: 380 feet
3. Maximum flow height: 110 feet
4. Elevation at bottom: 8,700 feet
C. Runout Zone
1, Average inclination: 7%
2. Type of surface: grassy meadow
II. Avalancne Destructive Potential
A. In the Track
1. Maximum velocity: 120 to 150 mph
s~~avS 2. Maxinum 3mpact pressure: 1,000 to 1,500 lbs, per sq. ft.
B. In the Runout Zone
L 1. Maximum runout distance: 1,080 to 1,320 feet
2. Maximum impact pressure: 2,000 to 4,000 lba. per aq. ft,
3. Maximum distance of observed debris: 730 feet
t.s~-
26
VAIL MEt1DOWS
a 4
I. Description of Terrain
A. Starting Zone
1. Area: 55 acres
2. Average inclination: 50%
3, Maximum inclination: 80%
4. Elevation: 11,300 to 10,000 feet
5. Orientation: NE
B. Avalanche Track
1. Average width: 210 feet
2. Max3mum width: 280 feet
3. Maximum flow height.: 120 feet
4. Elevation at bottom: 9,000 feet
C. Runout Zone
1, Avalanche hits a 100-foot high hill directly at bottom of
runout zone
II. Avalanche Destructive Potential
A. In the TracK
1. Maximum velocity: 150 to 170 mph
2. Maximum impact pressure: 1,300 to 1,700 lbs. per sq. ft.
B. In the Runout Zone
The avalanche hits a 100-foot high hill at the bottom of the
track where much of the energy is lost. Part of the avalanche
overtopa the hill, and part is deflected to eiChez si3e.
27
w
SELECTED REFERE:tiCES
Bray, J.R. and J.T. Curtis. 1957. An ordination lof the upland forest
communities of southern Wisconsin. Ecol. Monogr., V. 22, p. 217-234.
Berger, W.H. 1961. Areal geology of the central part of the Minturn
Quadrangle, Colo. Unpubliehed M.Sc. thesis, i7niv. of Colo., 105 pp.
Betts, Ii.S. 1919. Timber, its etrength, seasoning and grading. McGruw-
Hill, N.Y., 234 pp.
Daugherty, R,, and J. Franzini. 1965. Fluid mechanica with engineering
applications. McGraw-Hi11, N.Y., 574 pp.
Fraser, C. 1966. The avalanche enigma. John Murray, London, 301 pp.
Gallagher, D. ed.), 1967. The snowy torrents--avalanche accidents in
the United States, 3910-1966. U.S. Forest 5ervice, Alta Avalanche
Study Canter, 144 pp.
Hoerner, S.T. 1965. Fluid-dynamic drag. S.F. Hoerner, Midland Park,
New Jersey, 452 pp.
Hanshaw, B.B. 1958. Struc*_ural geology of tne tvest side of Y.he Gore
Range, Eagle County, Colorado. Unpublished :i.Sc. thesis, Univ. of
Colorado, 138 pp.
LaChapelle, E.R. 1966. Avalanche forecasting - a modern syntnesis. In:
International symposium on scientific aspects of snow and ice
avalanches, Int. Assc. Sci. Hydrol., pub. 69, p 350-356.
LaChapelle, E.R. 1969. Field guide Co snow crystals. Univ. of Washington
Press, Seattle and London, 101 pp.
LaChapelle, E.R. 1970. The ASC of avalanche safeCy. Colorado Outdoor
Sports Co., Denver, Colorado.
Mellor, M. 1968. Avalanches. Cold Regions Science and Engineering,
Part III; Engineering, 5ection A3, Snow Technology, 215 pp.
Poelchau, H.S. 1963. Geology of the Gore Creek area, Eagle County,
Colorado. Unpublished M.Sc. thesis, University of Colorado, 76 pp.
Shoda, M. 1965. An experinental study on dynamics oi avalanchirg snow.
In: Symposi-um of Davos, International Association of Scientific
Hydrology, Publication no. 69.
Somnerhalder, E. 1964. Avalanche Forces and the Protection of D'ojects.
Translation no. 6, Alta Avalanche 5tudy Center, U.S. Fc-est 5ervice.
Tweto, 0. 1949. Stratigraphy of the Pando area, Eagle County, Colorado.
Colorado Sci. Soc ProceedinSs, v. 15, no. 4, p 149-235.
zs
U.S. Dept. of Agric. 1968. Snow avalanches--a har.dbook of forecasting
and control measures, U.S. Forest Service, Agric. Handbook 194.
k
Voellmy, A. 1964. On the destructive force o£ avalanches. Translatioa
no. 2, Alta Avalanche Study Center, U.S. Forest Service, 64 pp.
l
30
an?le of renose: The maximum angle of slope at which loose, cohesionless
material will come to rest.*
avalanche. A large nass of snow and/or ice falling, sliding, or flowing
very rapidly under the force of gravity.*
avalanche track. That part of the hillside leading from the catchment
basin to the runout zone. In many cases it is a gully and can be
distinguished by its lack of vegetation.
branch triri. Loss of branches on the lower portion of a tree trunk. Caused
by material moving past tree snow, air).
catchnent basin. The main collection region for the snow at T.he head of
the avalanche track (syn. starting zone).
centigrade. Unit of temperature in metric system, water freezes at 0°C
and boils at 100°C. To convert to °F = 9/5°C + 32.)
clastic. Pertaining to or being a rock or sediment composed principally
of broken frasments that are derived from preexisting rocks or
minerals and that have been transported individually for some disCance
from their places of origin.*
climax avalanche. A large or major avalanche which is a result oL cumulative
factors working over a longer interval of time than those avalancl:es
associated with a single sCorn.t
cohesion. Shnar strength in a snow pack not related to interparticle
friction.*
conglomerate. A coarse-grained, clastic sedimentary rock composed of
rounded fragments larger than 2 nm in diameter (granules, pebbles,
cobbles, boulders) set in a fine-grained matrix o° sand, silt, or
any natural cementing agent.*
conifer. An evergreen; e•g. pine, spruce, fir.
dead-fall. Downed, dead trees.
density, vepetative. Number of individuals in relation to the space in
which they occur.
Denotes definitiops taken from Glossary of Geology, Araerican Geol.ogical
Institute, 1972.
t Denotes definitions taken from Gallagher, D. ed.). 1967. The snowy
torrents--avalanche accidents in the UniCed States, 1910 1966. U.S.
Forest Service, Alta Avalanche Study Center, 144 pp.
I
mDenotes definition taken from Ramsier, R.O. and C.M. Keeler. 1966. The
sintering procesa in anow. Journal of Glaciology, V. 6, No. 45, 421-
424.
31
deDth hoar. New centers of crystallizatfon caused by vertical diffusion
of water vapor. These crystals are of a di€ferent character tha~
the original snow, and often are cup shaped and layered. Cohesion
is very poor between the crystals. A steep tercperatnre gradient
within the snow cover usually will induce such fornations.t
dolomite. A carbonate sedimentary rock consisting chiefly of the mineral
dolomite in composition, or a variety of linestone or aarble rich in
magnesium carbonate.*
equitemperature r.:etas:orPhisr.i. A process of modification of ice crystals
in a snow pacic, characterized by vapor transfer fron regions of high
surface energy to regions of low surface energy in a relatively
constant-temperature, below-freezing environment and leading to the
formation of uniform, well-rounded grains.*
factor analysis. A statistical method of evaluating data for homogeneous
or similar characteristics.•
fluid. A substance that is permanently deformed with the slightest stresa.
formation. The basic or fundamental rock-stratigraphic unit in the local
classification of rocks, consisting of a body of rock generally
characterized by some degree of internal lithologic honogeneity or
distinctive lithologic features.*
fracture line. The well defined line across the top oi the avalanche
path where the s1aU breaks away fromthe stable snow. The fnce of
the fracture ia perpendicular Co the slope t
gradient. A degree of inclination, or a rate of ascent or descent of an
inclined part of the Earth's surface wirh respect to the horizontal.
IC ia expressed as a ratio, a fraction, a percentage or an ar.gle.*
I
kilometer. A measurement of length in the metric system, equal to 1,000
meters (1 kilometer - 0.6214 miles).
lee. The part or side of a hill or ridge sheltered or turned away from
the wind.*
limestone. A sedicaentary rock consisting chiefly of calcium carbonate,
primarily in the forn of the mineral calcite. Limestones are formed
by either organic or inorganic processes, and may be detrital,
chemical, oolitic, earthy crystalline or recrystallized.*
list-count. A method of vegetation data collection in which species are
listed and the number of individuals of each species is recorded.
loose snow avalanche. A snow avalanche that starts at a poi,:t and widens
downhill, in snow lacking cohesion.*
meter. The basic unit of length in the metric system, equal to 3.281 feet.
32
metric ton. A thousand kilugrams, equal to 2,205 pounds. *
nivation. Erosion o£ rock or soil beneatli a•sno*bhnk or snow patch afld
around its fluctuar.ing margin, caused mainly by frost action but
also involvin; c,iemical weuthering, soliflucCion, and meltwater
transport oi wcathcring products.*
nivation hollow. A small, shallow recess, depression or clrque-like
basin iorred 'oy a snow patch or snowbank.
Pennsylvanian. A period. of the Paleozoic era thought to have covered
the span of time between 320 and 280 million years ago.*
permeabilitv. The property or capacity or a porous rock, sedimenC or
soil for transmitting a fluid without impairment oi the structure
of the nedium; it is a neasure of the relative ease of fluid flow
under unequal pressure.*
Pleistocene. An epoch of the Quaternary period, after the Pliocene of
the Tertiary and betore the Holocene (syn. Ice Age; Great Zce Age;
glacial epoch).*
powder avalanche. An avalanche conposed of dry, loose snow. As used in
this report, a high velocity cloud of ice particles maintained in
suspension by turbulence, with an effective density between 10'3
gm/cm3 and 2 x 10'2 gmjcs,3•
quadrat. A unit for vegetation sampling. Depending on the Cype of
vegetation, a quadrat may be the entire sample area or a subunit
within a larger sample plot. The size varies with the type of
vegetation being sampled (syn, sample unit).
remote sensi.ng. The measurement or acquisition of information of some
ti property of an object or phenomenon, Uy a recording device that is
not in physical or irtimate contact with the object or phenomenon
under study.*
runout zona. That area in the avalanche path where the debris comes to
rest t
sample plot. An area delineated for vegetation sampling.
sample unit. An organizational subdivision of a sample plot used in
vegetation analysis (syn. quadrat).
sandstone. A medium-grained, clastic sedir.ientary rock conposed of abundant
and rounded or angular fragments of sand size set in a fine-grained
matrix and more or less firmly.united by a cementing material.*
sedimentarv rock. A rock resulting from the cor.solidation of loose sediment.
that has accumulated in layers consisting of inechanically formed
fragments of older rock transported from its source and deposited in
water or from air or icc, or a chemical rock forned by precipitation
from solution, or an organic rock consisting of the renains or secretions
of plants and anlmals.*
33
ahale. A fine-grained, detrital sedir„entary rock formed by the consolida-
tion of clay, silt or mud, and c2iaracterized by finely stratified
structure and/or fissility that is approximqtely parallel to the,
bedding.*
sintering. The process by which ice and snow particles bond together at
temperatures 'Delow the melting point. Evaporation-condensation is
Lhe cajor mechanism by which sintering proceeds under normal atmos-
pheric conditions.p
slab. A layer in, or the whole thickness of, a snow pack whose internal
cohesion is larde compared to its external adhe-,ion to other snow
layers or the ground. The characteristic identifying property of a
snow slab is Che ability to sustain clastic deformation under streas
and hence the propagation of fractures.*
slab zvalanche. A snoW avalanche that starts from a fzacture line, in
snow possessing a certain amount of cohesion.*
slope aspect. The direction toward which a slope faces with respect to
the compass or to the rays of the sun.*
species. A kind of organism. As used in this report, a kind cf plant.
The plural of soecies is also species.
sun crust. A type of snow surface forned by refreezj.ng of sur£ace snow
that had been selted by the sun.*
emperature rradient. The rate of change in temperature with vertjcal
height in a snow pack,expressed as °C/cm.
temperaturc r,radient metamorphism. A nrocess of modification of ice
crystals in deposited snow, characterized by vaper transfer under
strong vapor pressure and temperature gradients, and reeulting in
the growth of complex shaped crystals usually with stepped or layered
surfaces.*
tensile strength. The maximum applied tensile stress that a body can
withstand before failure occurs.*
tension. A state of stress in which tensile stresses predominate; stress
that tends to pull a body apart.*
topography. The general configuration of a land surfare or any part cf
the Earth's surface, including its relief and the position o: its
natural and man-made features.*
transect. A line along which vegetation is sampled or data collecte3
syn. eample plot).
tree sapling. A young tree with a height between sig and fifteen feet.
34
tree seedlina. A young tree with a height less than six feet.
trimline. A sharp break or boundary in vege'tation. Specifically in the
case oi avalanches, a downslope absence of trees in the active ava-
lanche zone in contrast ro nearby bordering mature forest in these
areas not afiected by avalanches.
trunk scar. An area on a tree trunk where the bark has been removed by
the abrasive action of material noving past the tree.
turbulent flow. Flow in which the flow lines are confused and hetero-
geneously mixed, as opposed to laminar flow.*
water vapor. Water in the gaseous state.
wet avalanche. An avalanche composed of damp snow (high water content)
usually caused by a eudden thaw. Common in the apring season.
windblast. A hfgh velocity gust of sir sometimes aasociated with large
avalanches.
J
j.
Clubhouse
i
Gore
f~~~-----
Frontaye
I.
II I~ryr"'^i- .
Sa
Y I
J
I,~ I~~~1~~i 1 ~ i,~,i'J ~t~
i'j~~ f`_~. t
il~ i
Gore Creek S?g~ '0
v ? _i ~ i
Golf Course Ci
1 f'~i;~
C~ 8 280 Interstate 70
PLATE 5
CLIMAX AVALANCHE RUNOUT
Golf course area
O 500 1000 1500 FEET
i rI r
0 100 200 300 400 METERS
N Contour Interval = 40 feet
5idewindar 1
Terroy
j'
I
1~--•-.,-•-J\
e.aoo:_:---
s
Gore Crcek 1. I,1~:
pLATpELANCI~E RuNOUTS
GI.IMAX T
East of galf course FE
looo t~oo
0 50-~Q 400 MEYERS
o 1~9 200
Intervoi = 40 feet
Contour
vc:il Mladows
wa~o~ran
ranU niuiur
Timbarfa!!s r- -
O'd Muddy 7~r
1:
y i i
i i I /
1 i
1 l._ I t f f
j00OIO.00d
w,ooo ro,ooo-t"`
r_
a+'SOD f
9,000
l
g,000--1---~f~
t y.
f.
r..,,,i••~Ca~re;r~',CrE~K,./~k
PLAv E S
GCRc CREEK AVALAP.CHE AREfi
Upper Go:e Volley
0 IO:iO 2000 r'EcT
1 0 300 6~00 METEF6
Contow In'erve! = 200 feet
See PLATE''3 ior Rur.out Detai;s t
n
w~sa:'r~»,d.Paat¢:..s~su4a.uv.u......._. a,1
Clubhause
r-
frontcge 10,000
i
9,000
00~~l ~
OfO/C.(80K
rc--~"in?erstote 70 ATE 2
GORE CREEK AVALANCHE
A;eo: wesf part
0 IG00 2000 FEET
N
0 200 400 600 A7ETERS
Conlcur Intervai - 200 feet
Saa PLATE 5 for Runou: Detoile
u..~~
g
ClnGhouse
S Fron!age
10,000
y
i\oOO 4
2
f ~
l i~ J~^~~~~~~~.~9,000
re Creek
f
lntarstate 70 ATE3
GORE CREEK AVALANCHE
Araa: west parf
a4 0 1000 2000 FEET
f------r==`r'
y IV 0200 406 ECO ML7ERS
Ccntour Interva7 = 200 feet
See PLA7E 5 tor Runout Dataiis
fi
i
Me
r
King P.-thur
8,800 .I O:d Muddy
L.
8.6~0--~' ti\t F
Y
y
r
J
f c A `os ~
151z-F \ r"`~11
a
f
q
r~•.';:<iii=~_/Gor Creeke
o
U.S.6
a
c•
l
r_..~....~..,_m,~.~,,.....~..-r~~r.m~..,..r..... _ _ .u._
warerfaii
Timberfa3s
7 r,
f
M' J52O r'~-~~
tt fl.r--_~~~- .!8 W,_i- ti i~:~ ry,~
i-• .13..~,Gora
i~J- i•4Y,~~~• ~y;~Ela.,;• `s
P~Caa 'J`
f,
IL.A7E 3
CLIMAX, AVALANCNE RUtr'OUTS
Upoer Gcre Creek Uclley
a SCO 1000 I500 FEE`
Q 10 200 3pp 4'p0 ME'ERS
Contour intcrval = 40 feet
Area ef we! snow ovalanches
l
Runout o` Icrge powCcr avcla.nchos
V..r....~........e~.s+..+w.~. .A_n.. r . v.... . . .v 1
f•
c
l
Wotertalt
TimberTalls r
CCC" "1:~.1-'\_. i
V:~`.~V;1 F'./y` .
i j~P----8,400-
Gore
C---w..;e
S I%------'
U.S. g
e I •i
PLATE 3
CLIMAX AVALANCHE RUNOUTS
Upper Gore Creek Velley
0~--~- 5~00 1000 1500 FEET
0 160 266 300 400 METERS
Contaur Interval = 40 feet
Area of wef snow avalanches
y
Runout of lorge powder avalonches
l
e
1
s
r-.-----f iaio7 \w•. :-f"~-- "-%'`-.,'--'"r 4'aa»r`f.
r l
J `r /
98--.r'
i FI V
j/ .
J hi'~IVT~~ r
l „l i1~
r'^
1~"y;r ~009`8'-
5~~oy~a4a!1 tPPnW Plp nyUV burN s*opoaµi II ~
LOCAL GUYERtiN1EtiT MA";ACE^.5ENT COtiSULTANTS PRESS R'cLETSI
1740 Wi:liama54z¢t ( Lcr.uec Coiurado 80216 / T1:one (30J 399-7059
June 23, 1979 A recent study has verified t}~at the lacation of t},c
j Core Valle}'IVate; Dist.ict water storage tank in the Va:2 .
A1 E ht 0 R A N DU Af Mea3ows avalanche path has ne notential detrimental efEect
ott re,sidences below :t.
This is the conc;usion of a Avalanche Scudy cocipletedT0: IIoard of Directors, Gore V~lley iYater Uistrict at tlie request o£ the Gore tialley l'+ater Dist.rict Ly 1{}•drn-
fi'iad Ltd. This scudy was commissioned in respoi»e to tUe
i~RCAL' JII[U65 1'. Coilins - fears a.;id cwicarns raiscd last year as tv the eiEect> a£ [iie.
RP.; AvalancFe Stud ~Present~tion to Vail Toian Council SOO,1100 gallon water storage tank loc~ited ii~ the a~raLanche
Y path.
Ron Ha11ey is schedulcd to make a verUal presentation of T}~e most recent analysis uses thc generally ?eceptcd
the Avalauiche Study Co the Vail Town Counci7. ut their study 100-year avalanche und dssesses iCS impoct on thc tauk. 2
session SnJ thcir formal meeting, "iuesday, July 5.founJ that when the wate; stcragc turk is Ri)% ful;, it o-~ill
It is expected [hnt the presentation to the Town Council coinpletely resist the avalanche. If the tauk is iess C~ui.l,
it is suhject to sliding ofi i.ts hase and rupturing. Hti~.ever,
Study Session will occur at ahcut h:60 p.m. (the study sessi_n oven iE a.X shuuld rupture, the water rcleused would consti-
s'tarts at 2:09 an:l ends at S:OG and iae'vz renuested to 6e late tute le5s than 1$ uf thc volwne of siww already ;uavipg down
on the agendxl; anL it is expected tnat presentaYion bcfore Lhe Ltie avalanche path, T'be tank itsclf xoitiid movu veTy little,
Town Council i.ill t•e early in the inezting, wIlic}; starxs at 7:30
I believe it would be helpful to haye as many lloai'd mentbeis pra-Tl+c Gore Valley N'ater [listrict indicates that the tank
senC at ihese meeti.ngs as possibl4 to re-emphasize the tact that hns scldom 6een Icss than BOa ful] and the cl;ar.ces of tnis
Gere Galley Water District has fulfil:ed its co~.mltmenL ta assess occui'iing at the sase cime an.avalunche would occur are ex-
the xvalanche hazard. I do not L4lieva that iX 1s pecesSai'y foT ti'emely snall. Regardicss, the 14ater Pistrict i3oard has ra-
Tom GrimshatiJ or myse;f to be present.solved to maintain at least 80% oI the volunie in the t2nk,
Enc.osed is a copy of a news release which w:ll be sent to excepT in a cssz of extreTe eiiergency. The cost oi buildi>>g
The Vail ?iail and to the Vail Villager next blunday. Please let
a diveTSion stiuctt:re to prqtect the t»nk ir the eF'ent an aca-
me know if you have any objections or suggesfed chan8es to it.lanche st:ould occur u'nen it i.as less x'..sq 30'~ full xou11 be
pve; $25,000, as coiapare3 wich a replacemcnt ccst on the tank
JPC:ajh of aUout $100,000,
Enc,
cc; I'honas T. Grimshaw Mos2 imrtortant, the study reyeals Ghere is no adde.i dangar
Stunle± Bernstein ~To d~nhill structures, bux does recommerd that the Town o: Vail
Craig Rooillard allou na structuTes to be built in the ii:g}t-Haza7d Zcn9 As al-
Rop Flaliey Yeady defined in p;evious 5tudies,
q preser,tat,alt oF the Tepart wili be madF be:qre the L'ai:
Town Council on Tuesdwy eyening, July 5 bY Ronkld I., Hailey, p,E „
of Hydro-Triad Ltd. A copy of thc report is pn fzle iR the Vail
Planning office.
JPG;ajha
a