FLOOD01621

• County: Statewide
• Community: State of Colorado
• Title: Flood Hazard Delineation on Alluvial Fans and Urban Floodplains
• Date: 1/1/2001
• Prepared For: State of Colorado
• Prepared By: J.S. O'Brien
• Floodplain - Doc Type: Educational/Technical/Reference Information

Table 3. Hud.t'low sample Viac::olli ty and. yield stress
as a Function of BE~d.:L.ent concentration.
= B,c,. ~, = B,c,.
n a,e a?e
Source Points a, B, r' Points a, B, r'
1- Aspen, Colorado
Pit 1 10 3.60 X 10-2 22.1 0.99 10 1. 81 x 10" 25.7 0.92
2. Aspen, Colordo
pit 4 10 5.38 X 10-l 14.5 0.95 10 2.72 x 10-0 10.4 0.93
3. Aspen, Colorado
Natural soil 9 1. 36 X 10-3 2B.4 0.96 10 1. 52 X 10-1 18.7 0.83
4. Aspen, Colorado
Mine Fill 9 1. 28 X 10-1 12.0 0.87 9 4.73 x 10-2 21.1 0.91
5. Aspen, Colorado
Natural Soil 16 4.95 x 10-~ 27.1 0.83 14 J.8J x 10-l 19.6 0.92
6. Aspen, Colorado
Mine Fill 10 2.01 x 10-- 33.1 0.85 10 2.91 X 10-l. 14. J 0.84
7. Glenwood Springs,
Colorado #1 14 2.SJ X 10-3 23.0 0.93 14 3.45 x 10-l 20.1 0.96
8. Glenwood Springs,
Colorado #2 8 6.48 x 10-1 6.2 0.94 8 7.65 x 10-2 16.9 0.97
9. Glenwood Springs,
Colorado #J 10 6.32 x 10'3 19.9 0.95 8 7.07 x 10-~ 29.8 0.91
10. Glenwood Springs,
Colorado #4 9 6.02 x 10.' 33.1 0.96 9 1. 72 X 10-3 29. ~5 0.75
11. Iida, China (1938) S 3.73 x 10-~ 36.6 0.99
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(From O'Brien, 19S6)

Cv = sediment concentration by volume
ry = viscosity in poises
1 = yield stress in dynes/cm2
r2 = correlation coefficient
al and I3l are empirical exponent and coefficient.~:..
Points refer to the number of different sediment concentration samples.

3.3 Flow Paths and Avulsion

Flood flows have a tendency to stray from well-defined
channels on alluvial fans. Chann",>l abandonment associated with
sediment deposition and scour is refe,rred to as avulsion. Damages
from channel avulsion and migration are often neglected in
engineering analyses. During a flood event, debris and sediment
may plug the channel causing it to be overtopped. The flow,
seeking a new path, may then begin t,o incise a new channel. Once
the flow has left the channel, it becomes a two-dimensional flow
problem on an unconfined surface" Flow obstructions such as
buildings, walls and levees will guide the new flow path. History
has shown that channel training on an alluvial fan can be a risky
venture. Levees and channelization are an attempt to control the
very processes which have created t:he fan"

3.4 Return Period Frequency for Alluvial Fan Even.ts

The paucity of field data an alluvial fan discharges is
attributable to several factors including the lack of gages on
ephemeral fan channels, difficulty in est.imating peak discharge
after e', ::mts occur, and unavailablE~ rainfall data. Exceedance
probabilities for discharges in un9aged alluvial fan channels are
difficult to estimate.

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