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<br />
<br />Characteristics
<br />
<br />In many stream valleys, the debris fans, bui It up over the
<br />centuries at the mouth of the small tributaries, offer
<br />attractive places for development. Frequently. In an effort
<br />to avoid mainstream flooding, the debris tans are bui It upon
<br />without the realization that they, too, are subject to
<br />periodic debris flows and flooding. Depending on the cl imate
<br />and geology, the tans may recover quickly from the
<br />destructive effects of a debris flow event, offering I ittls
<br />V i sua I ev I dance of the act j va processes. I n genera I, the
<br />existing channel on a debris fan cannot accomodate the next
<br />large debris flow. It cannot be assumad to be the only
<br />hazardous location on a debris fan. The land form Is bui It
<br />up over the years as debris flows periodically deposit
<br />materials across the entire debris tan or portions of It.
<br />During a large flow a new channel may result from plugging
<br />of the existing channel with debris. Usually, the
<br />destructive forces of the debris flow decrease as one moves
<br />from the narrow, steep apex of the fan to the broader,
<br />gentler slopes down gradient. Correspondingly, the size of
<br />the mater i a I depos I ted decreases as the debr i s flow moves
<br />across the fan. Debris fans often are vegetated with
<br />cottonwood or aspen trees, grasses and shrubs In a distinct
<br />contrast to adjacent plant growth. Some debris fans in the
<br />high mountains also are subject to avalanches.
<br />
<br />Consequences
<br />
<br />Structures and improvements on the apex of the fan may be
<br />destroyed or bad I y damaged wh i I e I mprovements farther down
<br />on the fan may only experience water and mud damage. Erosion
<br />and deposl t I on on an act i ve fan by success i ve debr i s flows is
<br />to be expected.
<br />
<br />Aggravating Circumstances
<br />
<br />Man's activities which could increase the natural hazards are
<br />similar to those cited under debris flow and mud flow. In
<br />addition, however, significant, short-term alterations to the
<br />debr I s fan I and form can increase the hazard on part i cu I ar
<br />areas on the fan. l'o1ass i ve earth mov I 09 on the fan cou I d
<br />create an artificial diversion or channel which would cause
<br />temporary preferential flow directions during a debris flow.
<br />
<br />Mitigation
<br />
<br />The best form of mitigation is based upon an understanding of
<br />the natura I processes of a debr i s fan and I ocat I ng and
<br />constructing improvements accordingly. Given the condition
<br />
<br />of a developed debris fan, measures that can be taken to
<br />decrease the hazard i nc I ude bu II ding mass i ve earth structures
<br />on the uphi II side of houses or other improvements to divert
<br />the flow to one side or the other, planting a dense row of
<br />trees, erecting retainIng walls, and channelling the stream.
<br />These measures shou I d be cons I dered on I y after a camp I ete
<br />~nderstand i ng of the process is obta i ned because I n many
<br />I nstances they cou 1 d be of Ii tt I e benef I t and cou I d even
<br />increase the hazard to other developed areas.
<br />
<br />Land Use
<br />
<br />Land uses on debris fans range from open space to relatively
<br />intensive use. Intensive use may be appropriate after a
<br />thorough geologic study and understanding of the debris faa
<br />is obtained. Some fans have very deeply entrenched channels,
<br />Indicating that during recent geologic time, the dominant
<br />process has been erosion on the fan rather than deposition.
<br />Such a debris fan may be safe for development. Until
<br />determined otherwise, however, building on a debris fan
<br />Should be considered hazardous.
<br />
<br />
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<br />
<br />A debris flow smashed into this Glenwood Springs home In
<br />. 1977. The house is a part of a resl dent i a I deve I opment
<br />p I aced on a debr i s fan wh I ch had been bu i I t up over the
<br />years. Note the he I ght of the f I ow as I nd i cated by the
<br />water/mud mark at second floor level.
<br />
<br />17
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