WSP07943

• File Number: 5700
• Description: Flood Protection Section - Flood Control Projects
• State: CO
• Basin: Statewide
• Date: 10/25/1982
• Author: CWCB
• Title: Debris and Flood Control Plan for Portland and Cascade Creeks at Ouray - Colorado - Final Report
• Water Supply Pro - Doc Type: Report/Study

000930

4.6

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assumed normal depth. Considering the severe steepness of the flumes, this is

an accurate assumption. Analysis of the debris transporting capacity was more

difficult. In this case gravity flow was assumed with a flood viscosity of

100 poise. This viscosity is approximately 10,000 times that of water alone

and is based on viscometer tests of typical debris flow samples from western

Colorado currently being studied by Mr. J. S. O'Brien at Colorado State

University.

Results of these calculations are presented in Tables 4.1 through 4.4.

These tables include capacities for the flumes flowing full, and with one or

two feet of freeboard. The calculations were not performed for two feet of

freeboard on Cascade Creek, since its channel is shallower and the capacities

at two feet of freeboard are small in comparison to the discharges of interest

in this study.

According to the estimated capacities, both the Portland and Cascade

flumes have sufficient capacity to pass water discharges of the lOO-year

return period level or greater, except for the most downstream reach of the

Cascade flume. (This is not entirely true, since the short section of the

Portland flume where the walls have been destroyed would have very little

capacity.) With one foot of freeboard, all reaches of the ,flumes, except the

downstream section on Cascade, can pass the SO-year or greater flood.

However, when debris is considered, the Portland Flume can pass discharges

varying from the 10-year to the 25-year flood. The upstream reaches of the

Cascade Flume have the capacity to handle approximately ~he 10-year flood.

Unfortunately the downstream reach of the Cascade Flume below Righway SSG caw

not pass a significant flood with debris because of the flat slope in this

area..

Observations of the flumes during and after the 1982 floods showed that

both flumes had flowed to capacity in the sections that had not plugged.

Based on the return periods of the estimated flume capacities, the largest of

the 1982 floods on Portland creek had a return period of approximately 25

years. Similarly, the return period of the flood on Cascade Creek was at

least 10 years. A portion of the Cascade flow left the channel and flowed

down Eighth Avenue, never entering the flume. Therefore, the actual discharge

on Cascade Creek was larger than indicated by the highwater marks.

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