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

Chapter 2
THE LANDSLIDE HAZARD

In order for local officials and emergency response per-
sonnel to correctly formulate and implement landslide
hazard emergency response plans, they need to be in-
formed about landslide processes and impacts. To at.
tempt a solution without understanding the problem is
inefficient, if not dangerous.
The term "landslide" is used to describe a wide
variety of processes that result in the downward and
outward movement of slope-forming materials composed
of rocks, soils, artificial fill, or a combination of these.
The materials may move by: falling, toppling, sliding,
spreading, or flowing (U.S. Geological Survey, 1981).
The various types of landslides can be differentiated by
the kinds of material involved and the mode of move-
ment. A classification system based on these
parameters is shown in FIgUre 3. Other classification
systems incorporate additional variables such as the rate
of movement and water, air, or ice content of the slide
material.
Although landslides are primarily associated with
mountainous regions, they can also occur in areas of
generally low relief. In these areas landslides occur as
cut-and-fill failures (highway and building excavations).
river bluff failures, lateral spreading landslides, collapse
of mine-waste piles (especially coal), and a wide variety
of slope failures associated with quarries and open-pit

mines. The most common types of landslides are
described below.

TYPES OF LANDSLIDES

Falls

Falls (Figure 4) are abrupt movements of masses of
geologic materials that become detached from steep
slopes or cliffs. Separation occurs along surfaces such as
fractures, joints, and bedding surfaces, and movement
occurs by free-full, bouncing, and rolling. Falls are
strongly influenced by gravity, mechanical weathering,
and the presence of interstitial water.
Depending on the type of earth materials involved,
the result is a rockfall, soilfall, debris full, earth fall,
boulder full, and so on. All types of falls are promoted
by undercutting, differential weathering, excavation, or
stream erosion.

Topples
Toppling failures (FIgUre 5) are distinguished by "the
forward rotation of a unit or units about some pivotal
point, below or low in the unit, under the action of
gravity and forces exerted by adjacent units or by fluids
in cracks" (Varnes, 1978, p. 12).

TYPE OF MATERIAL
TYPE OF MOVEMENT ENGINEERING SOILS
BEDROCK Predominantly coarse I Predominantly fine
FALLS Rockfall Debris fall ! Earth fall
lOPPLES Rock topple Debris topple I Earth topple
ROTATIONAL FEW Rock slump Debris slump : Earth slump
SLIDES UNITS Rock block slide Debris block slide I Earth block slide
TRANSLATIONAL - 'MANY - I
UNiTs Rock slide Debris slide I Earth slide
LATERAL SPREADS Rock spread Debris spread I Earth spread
Rock flow Debris flow I Earth flow
FLOWS I
(deep creep) (soil creep)
COMPLEX Combination of two of more principle types of movement

Figure 3. Abbreviated version of Varnes' classification of slope movements (Varnes, 1978).

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