FLOOD05802

• County: Douglas
• Community: Douglas County, Castle Rock
• Stream Name: East Plum Creek
• Basin: South Platte
• Title: Floodplain Information Report
• Date: 9/17/1986
• Prepared For: Park Funding Corporation
• Prepared By: Greenhorne & O'Mara, Inc.
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hydraulics: The water surface profiles for the various recurrence intervals
on East Plum Creek were computed using the U.S. Army Corps of Engineers HEC-
2 backwater program. The starting water surface elevation at Wilcox used in
the HEC-2 run was obtained from the profiles at the upstream study limit of
East Plum Creek in the published Douglas County frS. This elevation is in
agreement with the starting water surface used in the new draft FIS.
According to the Douglas County FrS, the existing bridge at Wilcox will pass
the IOO-year peak discharge with approximately 5 ft. of freeboard. The
roughness coefficients used for channel and overbank areas" are .035 and
0.45, respectively. These values were used in the draft FrS and were
verified for this analysis by field inspection. Locations of selected cross
sections used in the hydraulic analysis are shown on the Flood Profiles and
on Sheet 1 of 1. Table 2 presents a summary of the computed water surface
elevations at the selected cross sectibns. Copies of the HEC-2 runs are in
the Appendix.

Floodplain Delineation: The floodplain shown on Sheet 1 of 1 was determined
USing more detailed topographic information than was used in the draft FrS.
The channel cross sections, channel profile, and the floodplain delineation
are based on new topographic mapping (photo date: 2/26/86) at an original
scale of 1" = 50' and having a l' contour interval. The resulting water
surface profile varies somewhat from the draft FIS which was based on
surveyed channel cross sections. The floodplain delineation also varies
somewhat due, in part, to the base mapping used in the draft FrS which was a
U.S.G.S quadrangle with a contour interval of 20' and a scale of 1" = 1000'.

III. FlOOOWAY

Encroachment on floodplains, such as structures and fill, reduces the
floodcarrying capacity, increases the flood heights and velocities, and
increases flood hazards. Therefore, the area of the IOO-year floodplain is
divided into a floodway and a floodway fringe. The floodway is the channel
of a stream plus any adjacent floodplain areas that must be kept free of
encroachment so that the IOO-year flood can be carried without substantial
increases in flood heights. Minimum standards for the Town of Castle Rock
limit such increases to 1.0 foot.

The floodways presented in this study were computed for stream segments on
the basis of equal conveyance reduction from each side of the floodplain.
Floodway widths were computed at cross sections. Between cross sections,
the floodway boundaries were interpolated. The results of the floodway
computa ti ons are tabul a ted for sel ected cross secti ons and a re shOwn in
Table 3. The computed floodways are shown on Sheet 1 of 1.

The area between the floodway and the IOO-year floodplain is termed the
floodway fringe. The floodway fringe encompasses the portion of the
floodplain that could be completely obstructed without increasing the water-
surface elevation of the IOO-year flood by more than 1 foot at any point.
Typical relationships between the floodway and the floodway fringe and their
significance to floodplain development are shown in Figure 3.