FLOOD02366

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Basin: Statewide
• Title: The Big Thompson Flood of 1976 Field Trip Guidebook
• Date: 1/1/1976
• Prepared For: Annual meeting of Geological Society
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

the substrate on which structures rest, erosion of roadways and other

earth-fill structures, and partial to complete burial of manmade works by

detritus. These processes occur at rates ranging from imperceptibly slow

to rapid and are often episodic, as for example in the case of landslides

and soil creep. Landslides are most common when ground moisture conditions

approach saturation or when slopes are naturally or artificially cut. The

majority of rockfalls, however, seem to be related to springtime freeze-

thaw cycles or occurrence of moderate to heavy rainstorms. Debris avalanch-

ing and sliding, as well as spectacular fluvial events accompanying storms

such as the Big Thompson event, appear to be aperiodic in that precise

short term temporal and aerial prediction is very difficult if not im-

possible. Determination of location, rate and frequency of hazardous geo-

morphic processes is a desirable research goal in many geologic hazard

studies.

Geomorphic Criteria For Identification Of Geologic Hazard Areas

Recognition of geologic hazard sites in the Big Thompson Canyon area

can be accomplished in two ways. Distinctive landforms and deposits pro-

vide nearly unequivocal evidence when definitely related to active hazardous

processes. Slope, thickness and composition of surficial materials are

frequently good indicators of hazard areas as well. In both cases, the

degree of hazard is also a function of recurrence and severity of the

associated processes and specific site planning for structures and human

activities. Therefore, identification of geologic hazard sites is straight-

forward in the presence of observable field evidence whereas quantification

of risk is usually difficult and may be subjective.

Soil creep. Soil creep is the relatively slow downslope migration of

regolith, In the Big Thompson Canyon soil creep is most commonly associated

with fine-grained, unconsolidated colluvial deposits on moderate to steep

27