FLOOD08209

• County: Adams
• County: Arapahoe
• County: Douglas, Jefferson
• Community: Denver Metroploitan Area
• Stream Name: South Platte River, Chatfield Dam to Baseline Road
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 8/1/1984
• Prepared For: Denver Metropolitan Area
• Prepared By: Wright Water Engineers, Inc.
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

VIII-l

SECTION VIII
GEOMORPHOLOGY AND SEDIMENTATION

The South Platte River regime, channel and floodplain characteristics were
controlled by alternative periods of drought and wet cycles. Its width,
depth, bends, bank height and slope were constantly readjusting to natural
phenomena. The current regime began to develop about the time of the Civil
War with diversions of water for irrigation.

DESCRIPTION OF RIVER
For purposes of this Master Plan, the South Platte River has been categor-
ized by Stevens (1983) as either Rural, Suburban, or Urban as follows:

o Rural River. The river meanders and is sinuous with one high bank and
one low bank; it is wide and shallow; the channel has a gravel and sand
bed with bars.

Later, through Denver, the river was fixed between permanent banks and chan-
nelized. The U.S. Army Corps of Engineers channelization project will fix
the bed and banks of the river from Columbine Valley to Oxford Street. The
river will, to an extent, be controlled from Columbine Valley upstream to
Chatfield Reservoir to control bank erosion through the Littleton Floodplain
Park.

o Suburban River. In these reaches the river has less sinuosity; banks
have been stabilized with riprap in some places; there is less gravel
and sand bars and less vegetation; and floodplain filling has occur-
red.

o Urban River. The river has been channelized as exemplified by the
river through the City of Denver.

The Adams County portion of the South Platte River, depending upon the
alternate selected may, to some extent, be fixed between protected banks
with the bottom elevation of the stream controlled via drops.

It is necessary to understand the natural forces and regime of the South
Platte River so that the improvements to the river are compatible with the
forces and river regime; otherwise, a constant effort would be needed to
control the river channel.

From Chatfield Reservoir to Brighton, the South Platte River is 41.2 miles
long with a sinuosity of 1.19. Sinuosity is a measure of the bends or
curvature. To the south of Clear Creek the river drops 13 feet per mile.
From Clear Creek to Brighton the river drops 9 feet per mile.

All tributaries to the South Platte River are steeper than the mainstem.

Previous detailed work by Stevens (1983), Chen (1982), Hunt (1954), Hurr
(1975), and others have provided data and opinions for use in this Master
Plan. Past studies by the Wright Water Engineers staff on channelization of
the Middle Rio Grande (USBR, 1958), degradation below Flaming Gorge Dam
(USBR, 1957) on the Green River, and recent Clear Creek channelization
design for Coors (WWE, 1984), provide a perspective on the sedimentation
problems which must be resolved to meet the needs of the South Platte River
if the modifications and plans are to be reasonable ones.

River Characteristics
The river cannot be channelized, straightened, deepened, narrowed, or
widened without adverse long-term consequences requiring significant and
continued work and maintenance.

For purposes of morphological description of the South Platte River, the
study area has been divided into five reaches. Table VIII-l provides a
tabulation of river characteristics for each of the five reaches. These