FLOOD01635

• Designation Number: 558
• County: Douglas
• Community: Douglas County Unincorporated
• Title: Flood Hazard Area Delineation - Plum Creek Watershed
• Date: 11/1/2001
• Designation Date: 1/2/2002
• Prepared By: UDFCD, WRC Engineering
• Floodplain - Doc Type: Floodplain Report/Masterplan

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II. STUDY AREA DESCRIPTION

The Plum Creek Watershed lies in the northern part of Douglas County, which is located southwest ofthe Denver
metropolitan area. The Town of Louviers is located in the northern part of the Plum CreekWatershed, and the Town of
Sedalia lies in the southeast part ofthc Plum Creek Watershed_ In addition, US Highway 85 divides the watershed into
two parts from north to south_ The confluence of Plum Creek with Jarre Creek and West Plum Creek is located southwest
of the Town of Sed alia and northwest of U.S. Highway 67. The growth of the Denver metropolitan area is affecting the
northern part of Douglas County, and residential development is occurring at a rapid growth trend north and east of the
study area (Reference-! 0)_

The study area for this project is about 49_6 square miles. Several large creeks (West Plum Creek, Indian Creek,
and Jarre Creek) and several small drainage basins enter the study area from the south_ These off-site areas have a
combined drainage area of about 146 square miles. This study does not include East Plum Creek (or its drainage area),
which combines with West Plum Creek to fonn Plum Creek In general, the Outfall System Planning and Flood Hazard
Area Delineation of the Plum Creek Watershed are to be used to assist in planning of safe and efficient developments
to accommodate population and economic growth, while maintaining the rural atmosphere and riparian corridors along
the Plum Creek mainstream and its tributaries_

The Plum Creek Watershed is considered by Colorado Division of Wildlife and Colorado Natural Heritage
Program to be one of the ten most important areas for conserving diversity in the state_ It is home for several rare and
potentially endangered species_ This area has been focus of attention since early 1997, when the Preble's meadow
jumping mouse was proposed for federal listing as threatened or endangered. The Plum Creek Watershed consists of
grasslands and shrubby and forested riparian character and is a prime and ideal habitat for small mammals, and
endangered species such as the Ute ladies'-tresses orchid_ Some reaches of Plum Creek, which remain inundated year-
round, provide habitat for several endangered fishes, including the common shiner, brassy minnow, Iowa darter, and
northern red belly dace (Reference-I 1)_

Roadways which crosS the streams and obstruct the stream flows are the most significant factor causing flooding
in the study area_ In addition, several man-made structures such as buildings and fences are also categorized as flow
obstructions_ Records indicate that a high-intensity rain event occurred in the Plum Creek Watershed on the afternoon
ofJune 16, 1965_ This heavy rain caused a massive flood that severely damaged and washed out road embankments and
bridges on County, State and Interstate highways_ Seven homes, a church, the Grange hall, and the lower part of the main
street in Sedalia vanished during the flood. Five bridges, many culverts, and about four miles of the Denver and Rio
Grande Western Railroad's track were damaged_ The recurrence interval of this flood was estimated as greater than 500

years (Reference-12). Subsequentto this flood event, the Plum Creek mainstream channel has been moving both laterally
and vertically as it attempts to refonn to a stable channel alignment and grade_ Over the past 35 years, this movement,
and associated aggradation and degradation, has affected the I OO-year floodplain elevations and area of inundation. In
1978, the Federal Emergency Management Agency (FEMA) conducted a Flood Insurance Study (FIS) for Douglas
County (Reference-l 1)_ This study defined the I DO-year floodplain for a majority of Plum Creek_ A revised study was
conducted in 1985 to include additional reaches to be studied by approximate methods. No base flood elevations were
provided. Additional studies were prepared from which the FlS was updated on March 15, 1993, none of which affected
the reaches included in the current analysis_ Thus, the floodplains for which the current study will be compared to were
either developed by approximate methods or were based upon channel conditions prior to 1978_

The current Flood Insurance Rate Maps cover the following creeks and tributaries within the current study area:
1_ Detailed Studied Areas -
Plum Creek-

East Plum Creek -

One mile north of the Denver Highline Canal crossing upstream to the
confluence with West Plum Creek_
From confluence with West Plum Creek upstream to Castle Rock
(outside of study area)_
Through the Indian Creek Subdivision_

Indian Creek -

2. Approximate Studied Areas -
West Plum Creek
Sterling Gulch
Highlands Gulch/Oxide Draw (unlabelled on FIRM)
Louviers Gulch (unlabelled on FIRM)
Indian Creek (outside oflndian Creek Subdivision)
Lehigh Gulch
Daniels Park Drain (labeled Indian Creek on FIRM)
Tributary 6500 (un labelled on FIRM)
Jarre Creek
Cohen's Drainage (un labelled on FIRM)

The FHAD study includes detailed studies of all the areas in the study area which were previously studied by

FEMA using approximated methods (except for Cohen's Drainage, which is not restudied as part of this FHAD)_ In

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