FLOOD00566

• County: Statewide
• Title: Nature's Building Codes - Geology and Construction in Colorado
• Date: 8/1/1979
• Prepared For: State of Colorado
• Prepared By: DNR
• Floodplain - Doc Type: Educational/Technical/Reference Information

Characteristics

In many stream valleys, the debris fans, bui It up over the
centuries at the mouth of the small tributaries, offer
attractive places for development. Frequently, in an effort
to avoid mainstream flooding, the debris fans are bui It upon
without the real ization that they, too, are subject to
periodic debris flows and flooding. Oepending on the cl imate
and geology, the fans may recover quickly from the
destructive effects of a debris flow event, offering little
visual evidence of the active processes. In general, the
existing channel on a debris fan cannot accamodate the next
large debris flow. It cannot be assumed to be the only
hazardous location on a debris fan. The land form Is built
up over the years as debr I s flows per i ad i ca I I Y depos i t
materials across the entire debris fan or portions of it.
During a large flow a new channel may result from plugging
of the existing channel with debris. Usually, the
destructive forces of the debris flow decrease as one moves
from the narro'N', steep apex of the fan to the broader,
gentler slopes down gradient. Correspondingly, the size of
the mater i a I depos i ted decreases as the debr I s f low moves
across the fan. Debr i s fans often are vegetated with
cottonwood or aspen trees, grasses and shrubs In a distinct
contrast to adjacent plant growth. Some debris fans In the
high mountains also are subject to avalanches.

Consequences

Structures and improvements on the apex of the fan may be
destroyed or bad I y damaged wh I lei mprovements farther down
on the fan may only experience water and mud damage. Erosion
and depos i t i on on an act i ve fan by success i ve debr I s flows Is
to be expected.

Aggravating Circumstances

Man's activities which could increase the natural hazards are
similar to those cited under debris flow and mud flow. In
addition, however, significant, short-term alterations to the
debr i s fan I and form can increase the hazard on part i cu I ar
areas on the fan. l'4ass i ve earth mov i ng on the fan cou I d
create an artificial diversion or channel which would cause
temporary preferential flow directions during a debris flow.

Mitigation

The best form of mitigation is based upon an understanding of
the natural processes of a debris fan and locating and
constructing Improvements accordingly. Given the condition

of a dave loped dabr I s fan, measures that can be taken to
decrease the hazard I nc I ude bu i I ding mass i va earth structures
on the uph i I I s I de of houses or other 1 mprovements to d I vert
the flow to one side or the other, planting a dense row of
trees, erecting retaining walls, and channell ing the stream.
These measures shou I d be cons I dered on I y after a camp I ete
understand i ng of the process Is obta i ned because in many
j nstances they cou I d be of Ii tt I e benef J t and cou I d even
increase the hazard to other developed areas.

Land Use

Land uses on debris fans range from open space to relatively
intensive use. Intensive use may be appropriate after a
thorough geologic study and understanding of the debris fan
is obtalned. Some fans have very deeply entrenched channels,
indicating that during recent geologic time, the dominant
process has been erosion on the fan rather than deposition.
Such a debr i s fan may be safe for deve I opment. LInt j I
determined otherwise, however, bui Idlng on a debris fan
should be considered hazardous.

A debr i s f low smashed Into th i s G I en wood Spr I ngs home in
1977. The house I s a part of a res i dent i a I deve I opment
placed on a debris fan which had been bui It up over the
years. Note the height of the flow as indicated by the
water Imud mark at second f loor I eve I.

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