FLOOD02285

• County: Statewide
• Community: State
• Stream Name: All
• Basin: South Platte
• Title: Alluvial Fan Flooding
• Date: 1/1/1996
• Prepared For: State of Colorado
• Prepared By: National Research Council
• Floodplain - Doc Type: Educational/Technical/Reference Information

INDICATORS FOR CHARACTERIZING ALLUVIAL FANS

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check Natural Resources Conservation Service (NRCS) soil maps or drilling and logging records
from water wells, lf none of these sources is available, field reconnaissance can be done to
determine whether or not the landform consists of alluvial sediments.

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Morphology

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Alluvial fans are landforms that have the shape of a fan, either partly or fi"ly extended.

To meet the cnteria in the committee's definition of an alluvial fan, the landform of
interest must have the shape of a fan, either partly or fully extended. Flow paths radiate outward
to the perimeter of the fan. This criterion can be assessed with topographic maps. For example, in
Figure 3-2a the landform downstream from the Lawton Ranch, Montana, has the shape of a fan
that is nearly fully extended. This landform is known as the Cedar Creek alluvial fan and is a
classic example of a fan with nearly ideal morphology.

Location

Alluvial fan landforms are located at a topographic break,

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To meet the criteria in the committee's definition of an alluvial fan, the landform of
interest must be located at a topographic break where long-term channel migration and sediment
accumulation become markedly less confined than upstream of the break. This locus of increased
channel migration and sedimentation is referred to as the alluvial fan topographic apex, Figure 3-2
shows that the Cedar Creek alluvial fan begins at a topographic break, which in this case is a
slightly embayed mountain front. As Cedar Creek exits its narrow bedrock canyon, it becomes
less confined and is able to migrate more freely. Less confinement can lead to greater channel
widths and smaller channel depths. As a result, the occurrence of deposition increases, and flow
paths become more unstable.

Defining the Boundaries of an Alluvial Fan

Where are the toe and lateral boundaries of the alluvialfan?

Toe

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The distal terminus, or toe, of an alluvial fan commonly is defined by

. a stream that intersects the fan and transports deposits away from the fan,
. a playa lake,
. an alluvial plain, or
. smoother, gentler slopes of the piedmont plain.