FLOOD06042

• County: Denver
• Community: All
• Stream Name: All
• Basin: South Platte
• Title: Colorado Landslide Hazard Mitigation Plan
• Date: 1/1/1988
• Prepared For: State of Colorado
• Prepared By: CGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

u.s. 24 bas apparently been active since before construc-
tion of that highway in 1930. Repeated patching and
overlays on U.S. 24 have resulted in 8 to 10 feet of asphalt
. in the roadbed.
Formal design studies fur 1-70 began in the DO'M!s Junc.
tion area in 1963. The old landslides were recognized and
methods to allow construction were incorporated into the
highway design. However, .....ra1 problems relating to soils
and geology have been experienced since completion of
1-'10. These include landslides, fill failure, bin-wall distress,
and flows of artesian water from the pavement. After the
ColoIado Department of Highways (CDOII) experienced
the partiaI loss of an approximately 300-foot section of U.S.
24 between Dowds Junction and Minturn in 1985, renewed
investigation of the Iandsliding problem was initiated.

Descriptions of the Four Landslides

Meadow Mountain landslide
The Meadow Mountain landslide is a compound slope
failure consisting of shallow to deep (up to 40 feet) earth.
flows occurring on the surface of at least three much
deeper (90 to 160 feet) translational landslides inYoIving
bedrock. The basal surface bas numerous bedrock shear
zones present above the basal shear. The landslide area
covers more than one-half square mile.

Whiskey Creek
The Whiskey Creek landslide probably furmed initialJy as a
massive eartbfIow or a series of earthflows, This landslide
is the IaIgest in the area. However, it is also probably the
oldest and 1east active with the exception of the toe of its
east lobe, adjacent to 1-'10.

Dowds No. 1
The lowest part of the Dowds No. 1 landslide is composed
of ancient river gravels resting on Minturn Formation
bedrock. The gxavels represent a furmer course of the
Eagle River and probably are evidence that _ment of
the upper part of the landslide altered the course of the
river, furcing it to the north. This part of the landslide bas
a long, documented history of slow movement and bas
damaged 1-'10, the nearby highway bridge, and its west
approach.
The upper half of the landslide shows no evidence of
modem movement. It is composed of very large blocky
material consisting of arkosic sandstone of the Minturn
Formation with individual blocks rnnging up to 30 feet in
diameter.

Dowds No. 2
This is the smallest landslide in the area. It consists
predominantly of IaIge blocky material from the Minturn
Formation with a clay and sand matrix. The landslide bas
been active recently and damaged 1-'10 in 1983,

Potential Impacts of Landsliding at
Dowds Jundion
Catastrophic landslide failure in the Dowds Junction area
could result in the fullowing impacts: 1) loss of life, 2)
significant economic loss, 3) backwater fiooding of the
towns of Minturn and West Vail if a very large landslide

46

fonn, and 4) down-valley flooding of portions of
d unincorporated communities of Eagle-\'3i1,
, W>lcott, Eagle. and Gypsum if there is
dam formed by the landslide.
40 options for mitigation were presented by
Earthflows Task Fon:e in response to the
ntial threats posed by the landslides. The
re based on poSSIble geologic and bydrologic
s if major earthflows occur and river damming
results. 'ons included: taking no action, requiring flood
insurance monitoring, regulating development, relocating
people structures, erecting physical structures,
establis' drainage control, using anchors, attempting
weather odification, and many others (see Appendix 5).

Glen Springs

CASE STUDY NUMBER 2

DEBRIS-FLOW HAZARD

(Glenwood Springs.

arfield County. Colorado)

Springs is a small mountain city located in a oar-
at the junction of the Roaring Fork and Colo-
(Figure 18). It is one 01 Colorado's older com-
munities d bas been affected by debris flows throughout
its histo . Significant parts of existing development and
othe ' attractive potential growth areas are subject to
the deb ' -flow hazard. Glenwood Springs was chosen as a
case stu because: 1) it typifies the debris-llow hazard
that dozens of Colorado's mountain communities, 2)
the area been the subject of successively more detail-
ed studies by the Colorado Geological Survey in
coopexa' n with the city and Garfield County over the
past 14 , 3) the excellent information base and relative
predicta 'ty of hazard zones make a variety of mitigation
efforts sible, and 4) there bas bee~ recent activity, with
debris occurring in JSTl7, 1981, 1984, and 1985.
Altho the debris-flow and flooding hazard in Glen-
wood S was recognized early in the city's history,
id . 'on and analysis of the source areas were not
begun fu until the late I97Os. Most of the city is
located debris fans which have historically been very
active 20). At least 18 major damaging debris-flow
episodes have occurred since 1900.

'ption of the Debris-Flow Hazard

are the most serious type of landslide hazard
to affect lenwood Springs. Debris flows are slurries of
rock, so , or:ganic matter, water, and air that flow rapidly
down p -existing drainage channels until they are
deposite in fim-shaped cones where the channels enter
the 'valley floor. Damage results from impact, burial,
and fI ' . Debris flows are extremely common in Glen-
wood Sp , and appear to be the predominant fonn of
flood t for most of the smaller watersheds of the

mbination of local geology, geogxapby, and climate
Ie lor the frequent debris flows at Glenwood