FLOOD02337

• County: Statewide
• Community: State
• Stream Name: South Platte River
• Basin: South Platte
• Title: Geomorphic Assessment at Surveyed Cross-sections South Platte River
• Date: 6/1/1996
• Prepared For: UDFCD
• Prepared By: Michael Stevens
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Channel
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inadvertent and intentional grade-control structures that affect the bed profile is given in
Appendix C, Table L The weirs and dams predate 1983. Most of the District's grade-control
structures have been built since 1986.

There are two situations which motivate the placing of a new grade-control structure in the river.
When the thalweg degrades to the limiting elevation determined in the 1985 Master Plan, Urban
Drainage arranges for a grade-control structure to be built to prevent damage that could result
from further lowering of the bed. At other times, the limiting elevation has not yet been reached
but a good opportunity arises to place the grade-control structure immediately. This occurs in
conjunction with other activities along the river; for example, infrastructure construction.

Since 1986, the grade-control structures have been a large factor in the cessation or mitigation
of degradation in the Rural and Suburban reaches of the South Platte River in the Denver
metropolitan area. In fact, some beds that were degrading before construction of grade control
have come back to their former higher levels. Such is the case at the riprapped grade-control
structure between Cross Sections 27 and 28.

From March 1986, the river bed at Cross Section 28 (downstream from Interstate 270) was
degrading at a rate of about one foot a year (Fig. 7). In April 1990, a riprapped grade-control
structure with a boat chute was constructed approximately one-half mile downstream.
Immediately degradation was arrested and the river bed began to rise. Since that time, the bed
has stabilized at nearly the same level it was in 1986.

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Structure
Built

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Thalweg

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Mar-86 Aug-87 Dec-88 May-90 Sep-91 Jan-93 Jun-94 Oct-95 Mar-97

Figure 7.

History of bed and thalweg level changes at Cross Section 28. The downstream
grade-control structure was completed in April 1990.

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