FLOOD06925

• County: Larimer
• Community: Greeley
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Master Floodway Planning
• Date: 3/11/1983
• Prepared For: Greeley
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Debris and Flood Control Plan for Portland Creek and Cascade Creek at OUray,

Colorado

The City of OUray is located at the base of the San Juan Mountains of

southwestern Colorado. Due to its location, OUray frequently experiences

flooding and debris problems from the two main drainageways through town,

Portland and Cascade Creeks. These tributaries of the Uncompahgre River drain

steep watersheds above the town. Because of their geologic origins and incli-

nation, these upstream watersheds experience mud and debris flows when high-

intensity summer thunderstorms occur over their areas. OUray is built on the

debris fans left by Portland and Cascade Creeks between the base of the moun-

tains and the Uncompahgre River. The only means of transporting the flow of

water and debris through the city are two concrete flumes, one located on each

creek. These flumes often plug, causing water and debris to spread across

streets and through houses and businesses. Although structural damage is

usually small, the resulting cleanup effort to remove rocks and mud is quite

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extensi,.ve.

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The purpose of the ,SLA study was to develop solutions to the problems

associated with the flooding and debris flows. Two types of solutions were

devised. The first considered relatively inexpensive mitigation measures that

the city can afford without large sums of outside money. The second solution

was to rework the flume system to prevent damages from all but the rarest

events. It would be a costly solution, and would require outside funding.

In devising the solutions, SLA had to review historical accounts of past

events, their characteristics, and the extent of damages. The frequencies of

debris flows were estimated. The upstream watershed was studied to determine

the availability of debris supply, and the potential volume of debris flows

was estimated for various storms. In devising structural measures to contain

the flows, computations of velocities, impact forces, and depths were

required. This information was then used in recommending mitigation measures,

including resiting bridges, enlarging and reshaping the flumes, locations and

details of debris deflecting walls and debris basins, and maintenance

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requirements.

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