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

11.2

GEOLOGIC SETTING
The South Platte has been numerously described as "too thick to drink, too
thin to farm, too shallow to sail on, too broad to shoot a rifle across."
In its pristine state, the South Platte River adjusted its width, depth and
sinuosity in response to the amount and type of sediment and amount of water
supplied to it by its tributaries, In the 1880's agriculture began on the
South Platte River and the character of the river was forever altered.

recreation studies. The only area in this Phase A report for which alterna,
tives were not developed is from Columbine Valley to Oxford Avenue. This
reach will be incorporated into the U.S. Army Corps of Engineers channel
improvement project.

Prior to the advent of irrigation, the channel width ranged from 1,500 to
2,500 feet with multiple thalwegs, The raised water table and continuous
stream flow resulting from irrigation caused the river to change to a single
thalweg channel and reduced channel width.

In addition to the Phase A report, this Master Plan study included the pre,
paration of a flood hazard area delineation report for the reach from Oxford
Avenue to Sand Creek, a distance of approximately 15 miles. It is expected
that this flood hazard area study will provide the basis for floodplain
regulation along the river by those communities through which it flows.
This report has been provided to the Urban Drainage and Flood Control Dis-
trict under separate cover.

The South Platte River flows in a valley cut into the landscape during
Pleistocene or recent times. The valley floor is protohistoric and historic
alluvium, mostly reworked gravel overlain by a few feet of dark humus.

From Chatfield Dam to Baseline Road the low flow channel of the South Platte
River is 41.2 miles long. The valley length is 34.5 miles.

An important part of this Master Plan study is a gravel mlnlng study per-
formed for the reach fran Basel ine Road to Interstate 270, near the con.
fluence of Sand Creek. The purpose of this study is to develop gravel min,
ing standards, criteria and guidelines which can be used by Adams County in
the administration of gravel mining regulations.

The profile of the river from Chatfield to Brighton is generally concave
downward as expected with alluvial rivers, The average river bed slope from
Chatfield Dam to the mouth of Clear Creek is approximately 13 feet per mile.
Thereafter the average slope decreases to approximately 9 feet per mile to
Brighton. Where there are no major tributaries, the general river bed slope
of the South Platte River remains constant. Bed slopes of all major tribu-
taries are much greater than that at the mainstem.

STUDY AREA
This Phase A report covers the South Platte River from Chatfield r.eservoir
to Baseline Road in Brighton. This reach is approximately 40 miles long.
The Phase A study involves problem area identification, and alternative
development, along with associated hydraulic, hydrologic, water quality and

Previous Studies
There have been no previous master pl an studies on the South Pl aUe River
through Denver. The U.S. Army Corps of Engineers completed studies of the
South Pl atte River and its tributaries through Mctropol itan Denver in 1974
and 1977. These studies were floodplain information reports which delin.
eated the floodpl ain and identified flood problems. The main objective of
these studies was to promote non.structural floodplain management. In 1977,
Gingery Associates, Inc. prepared a flood hazard delineation for the South
Platte River through Adams County. This study used hydrology prepared by
the U.S. Army Corps of Engineers. In 1983, the Federal Insurance Mminis.

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map also was based on previous Corps of Engineers studies. Numerous other
flood insurance studies have been prepared for short reaches of the river
for various municipal ities through which the river flows.

Throughout the years individual communities bordering on the river have done
studies of various aspects of the river including flooding, recreation,