FLOOD00479

• Designation Number: 575
• County: Douglas
• Community: Douglas County
• Stream Name: Plum Creek and East Plum Creek
• Title: FHAD - Plum Creek and East Plum Creek
• Date: 9/3/2004
• Prepared For: Doulgas County
• Floodplain - Doc Type: Floodplain Report/Masterplan

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II. STUDY AREA DESCRIPTION, FLOODING HISTORY, AND PREVIOUS STUDIES

A.

STUDY AREAS I>ESCRIPTION

II, FLOODING HISTORY

The Plum Creek and East Plum Creek watersheds are located within northern Douglas County. southwest of
the Denver metropolitan area. The study area is within the South Platte Riycr Basin. Plum Creek
contlucnces with the South Platte River at Chattidd Reservoir and continues upstream to its confluence
with East Plum Creck and West Plum Creek at State Highway 67 in Sedalia. The drainage area for Plum
Creek at Chatfield Reservoir is approximately 319 square miles (Figure 1). 'Ibe drainage area is
approximately 166 square miles at Scdalia. East Plum Creek extends from its confluence \\ith Plum Creek
upstream. well into the Pike National Forest. The drainage area approaches 129 square miles at the
conHuence \,,'ith Plum Creck.

Large flood events have been documented along Plum Creek and East Plum Creek in 1933 and 1965.
On September 9 and 10, 1933. nooding occurred as a result of heavy rain on the divide separating
Cherry Creek from Plum Creek. Big Dry Creek, and Little Dry Creek. It \....as estimated that the flow
along Plum Creek reached 5,500 cubic feet per second (cis) during the peak of the storm (Reference 1).

The Plum Creek Watershed is considered by the 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 the focus of attention since 1997,
when the Preble's meadov..' jumping mouse was proposed for federal listing as threatened or endangered.
The Plum Creek Watershed consists of grasslands with shmbby and forested riparian character and is prime
and ideal habitat for small mammals. and endangered species such as the Ute ladies'~tresses orchid. Some
reaches of Plum Creek. which are perennial. provide habitat for several endangered fishes. including the
common shiner. brassv minnow, Iowa darter. and northern red belly dance (References I and 3),

On June 16, 1965, very intense. heavy rainfall occurred at three locations in the Plum Creek Basin.
Over 12 inches of rain was documented near Castle Rock and over 14 inches ofrain fell near Larkspur
within four hours. This intense rainfall contributed to over 126,000 cfs and 36.800 efs peak flows
along East Plum Creek and West Plum Creek respectively. Discharges along Plum Creek were
estimated to be as much as 154,000 cfs near the Town of Louviers. It was estimated that the unit
runoff rate. just below Castle Rock approached 1,170 cfs per square mile for a drainage area of
approximately 108 square miles. The damage along East Plum Creek and Plum Creek was extensive.
Various debris. including sand, boulders. and trees were deposited. Several roadway embankments
were severely eroded or washed out. Bridges were lost at Wilcox Street, Interstate 25, and
WoJtsburger Road near Castle Rock, in addition to State Highway 67 and 105 in Sedalia. tv1uch of the
Towns of Castle Rock a.nd Sedalia were inundated by the !looding (Reference I).

The study area for Plum Creek extends from approximately 4,000 feet upstream of Airport Road upstn:am
to the contluence with East Plum Creek and West Plum Creek. located just downstream of State Highway
67 in Scdalia. This reach of Plum Creek consists of sparse residential and commercial development. with the
majority of development located around the To\',:n of Sedalia. The land is predominately semi~arid prairie.
Short to medium length grasses, along with scattered brush and trees cover the majority of the overbank
areas. The channel consists of sand and gravel in most areas. Ilcavier tree and woody vegetation grO\\lh
can be found immediately adjacent to the channel at various locations.

C. PREVIOUS STUDIES

The stud\' area for East Plum Creek extends from its cont1uence with Plum Creek upstream to the limits of
the Urb;n Drainage and Flood Control District (UDFeD) boundary. located approximately 1.000 feet
upstream of the Burlington Northern and Santa Fe (BNSF) Railroad. Similar to Plum Creek. the study reach
of East Plum Creek consists of sparse residential and commercial development. with much of development
located around the Town of Sed alia. The land consists predominately of semi-arid prairie \'egetation. Short
to Illt'dium leng.th grasses. along with scattered brush and trees cover the majority of the overbank areas.
The channel consists of sand and gravel in most areas. Heavier trees and woody vegetation can be found
immediately adjacent to the channel area').

Plum Creek and East Plum Creek were originally studied as part oCthe Federal Emergency Management
Agency (FE~1A) Flood Insurance Study (FtS) for Douglas County and Unincorporated Areas
(Reference I). Originally published in 1978. the FIS \...as updated in 1993 and 1996. The updates did
not incorporate any changes to the Plum Creek and East Plum Creek floodplains. As part of the FIS,
flood hazard areas were detined for the 100~ and SOO-year floodplains; IO~. 50-, 100-. and 500-year
Hood pro tiles were established; base flood (IOO-year) elevations (BFEs) were determined. and a
regulatory flood way \vas developed.

In November 2001, a Flood Hazard Area Delineation (FHAD) was published by the UDFCD for the
Plum Creek Watershed (Reference 3). This study reanalyzed the portion of Plum Creek from Chatfield
Reservoir upstream to approximately 4,000 feet upstream of Airport Road. TIle 2001 FHAD was used
to establish the downstream hydraulic boundaries on Plum Creek l'Or this study as well as tic-in limits
and locations with Jarre Creek and West Plum Creek.

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