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

Springs. The mountain slopes surrounding the city consist
of sedimentary rocks whose weathering products are
highly suscepbble to debris-flow activity. Sudden failure of
the accumulations of soil and debris can be triggered by
saturation due to snowmeh and torrential rain. Debris fans
and aprons are very prone to bydrocompaction and require
speciaJ dIainage and foundations even where not subject to
debris flooding.

Impacts of Debris Flows at Glenwood Springs

Direct damages incurred in the debris-flow events of the
I970s and 19805 totaled millions of doilaIs. Although in-
direct losses are undocumented, they included loss of
tourist tIade and fish kills in the Roaring Fork River and
other streams.
In 1982 the City of Glenwood Springs contracted for the
prepaI3lion of an engineering study and control plan for
debris fiows. Debris-fiow basins were ranked in terms of
hazard severity, and the hyd!lluJic properties of debris
fiows and flash fioods in selected basins were detennined.
Conceptual designs of workable alternative control systems
for combined debris fiow and stonn runoff were also
fonnuJated. The plan included nonstructural mitigation
methods such as zoning and land-use restrictions, fiood
warning systems, and maintenance programs for channels
and debris souree areas; and structural methods such as
fioodproofing of buildings and construction of debris basins
and dams, eneIgY dissipators, drop structures, channels,
and storm sewers.

Lamplite Park

CASE STUDY NUMBER 3

RIVER BLUFF RETREAT REACTIVATED

BY HUMAN ACTIVITY

(Lamplite Park subdivision, Grand

Junction, Mesa County, Colorado)

Lamplite Park is a subdivision in the City of Grand Junction
(FIgUre 18). LandsJiding occurred there after construction
activity reactivated an old landslide which had its origin in a
river bluff OYersteepened by erosion of the Colorado River.
Although quite sman, this landslide caused severe damage
and fo=d the abandonment of severs! homes that were
only a few years old. This case is of interest because the
geologic process of oversteepening and failure of uncon-
solidated or weak rocks by localized riverine or coastal
erosion occurs widely throughout the United States and
because full utilization of available geotechnical data would
have prevented the ....re losses experienced at this site.
The effects of the Lamplite Park landslide were first
noticed in the early 198Os, shortly after the housing
development was completed. However, aerial photographs
indicated that a landslide had existed on the site as early as
1954. Photo analysis also showed that the bead scarp had
apparently receded about 50 feet between 1954 and 1913.
Investigation of the land-use history revealed that the site

48

had been d as a gravel pit. junk yanI, and a clean-fj]]
dump prior to development.
When th subdivision was proposed in 1976, the
geote . report identified the landslide and made
severs! mmendations regarding construction on the
site, inclu .' avoidance. The main part of the site was
developed 1982. At. that time, the property passed to a
second oper who built a row of houses on the north
side of S Clara Avenue in the immediate area of the
head scarp 21). At. this time clean-fj]] dirt was
trucked to site, dumped, and graded to level and ex-
tend the b s of these lots.
Within a ar or so of construction, at least two houses
began . problems associated with differential
movement. y NOYember 1984, these two houses were
condenm and the residents had filed lawsuits against the
county,' and individuals involved with the construction.
By 1988,' structures (nine units) had been moved off
the site, an one other structure remained condenmed.

scription of the Landslide at

Lamplite Park

The Lamp . landslide is a complex rotational failure. The
slide mass is a relatively thin (10 to 20 feet thick) sec-
tion of the IdJard Mesa temlCe gravel deposn. The slip
surface is turated, soft, extremely weathered shale;
bedrock is ered Mancos Shale. The slip surface is
saturated dischaIge from a perched water table in the
terrace Is. This causes loss of strength and an in-
pressure.
This was old metastable landslide that was reactivated
in the head scarp area as a direct result of residential
developme ,which caused loading of the top of the land-
slide and a in the ground-water regime.
A two-p geologic investigation determined that the
main cause f damage to the houses was failure of the fj]]
material had been placed out OYer the bead of the old
landslide. e fj]] itself increased the weight on the top of
the lan . ,while the development on the fj]] resulted in
increased il moisture. The ultimate'resuIt was a reactiva-
tion en n along the original landslide failure surface.

cts of the Landslide on the

Community.

impact on the co1)llDunity has been the loss
of 10 home (9 structures) located on the landslide.
Originally e city-owned and private utilities beneath Santa
Clara Aven e were also thought to be at risk. When the
houses we occupied, there was a risk of personal injury
associated 'th the possibility of a sudden foundation col-
lapse or osion of a ruptured gas line caused by the
slow deteri ration of the bearing support for the buildings
and the p e of laJ:ge open tension cracks,
Stab . n of the upper portion of the landslide was
considered d discounted due to the poor benefit/cost
ratio be n construction cost and the present or
anticipated e of the property and improvements. It was
concluded ere was no cost-effective way to allow long-
term con' ed residential use of the lots north of Santa
Clara Aven e east of address 1154.