FLOOD04017

• County: Jefferson
• County: Denver
• Community: Columbine Valley, Denver
• Stream Name: Dutch Creek, Lilley Gulch, Coon Creek,
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 5/1/1978
• Prepared For: Jefferson County / Arapahoe County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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This study assumed that all irrigation ditches were full of water and

that no interception of runoff occurred at any open ditch. It was also

assumed that no change would occur to the operation, maintenance and

topographic appearance of any ditch in the future totally developed

Basin.

occur at the Dutch Creek Crossing, than was assumed in the other studies.

This report also shows a reduced area of sheet flow or flood plain over-

flow on the north side of Dutch Creek in Columbine Valley. This may be

attributable to the larger flow diversion estimated at the Fairway Lane

crossing in this report, or to the use of more detailed mapping and digit-

ized cross sections in this area resulting in different ground elevations

at various locations. In all other areas of the Basin, the flood plain

delineations were found to be consistant between the various studies.

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The results of this Flood Hazard Area Delineation study were compared

with two other previous flood plain information studies:

- Flood Plain Information Report, Goldsmith Gulch, Dutch Creek, Lilley

Gulch and Coon Creek, Volume VI, Denver Metropolitan Region, Colorado,

October 1971, by the U.S. Army, Corps. of Engineers.

- Proposed flood elevation determinations for the Town of Columbine Valley,

Colorado, National Flood Insurance Program, March 14, 1977, by the

Department of Housing and Urban Development, Federal Insurance Adminis-

tration.

Flood Plain and Floodway Data

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Differences in flood plain delineations between this report and the other
studies primarily are attributable to the following:
1. This report used more recent aerial photography.
2. Mapping used in this report is more detailed with
a 1"-100' horizontal scale and a 2-foot contour interval.
3. Different procedures were used in the hydrological investigation
for each study which resulted in a variation in streamflow dis-
charges.
4. Different assumptions were used pertaining to land area develop-
ment which also contributed to the variation
in streamflow discharges.

The limits of future flooding are outlined on the attached drawings and

tabulated in Table C-l in Appendix C, Flood Plain and Floodway Reference

Data. The table lists the potential flood elevations, flooding widths

and Reference Line elevation at each cross section for the drainage

reaches studied. The flood profile Thalweg (reference) line and flood

plain width at any point on the streams is graphically shown on the

drawings.

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Floodways have also been computed for each stream. A floodway repre-

sents that part of the flood plain which is required to pass a 100-year

flood event without raising the flood profile more than an acceptable

amount and which represents hazards to personal safety and welfare.

In this study, the floodway represents that portion of the flood plain

where the 100-year flood velocities exceed 2 to 3 feet per second or

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where the water surface profile would not increase more than 0.5 feet
after encroachment. Floodway widths, representing the I imj ts of en-
croachment into the flood plain at each point in the stream, are tabu-

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The largest variation of the flood plain delineation between this study
and the other two seemed to occur in the Town of Columbine Valley. This
report shows a more definite flood plain extending along Fairway Lane
from the crossing of Dutch Creek eastward to the Platte River, which
indicates a larger flow split diversion estimated to

lated in Table C-l of Appendix C. The water surface elevations to be
expected after encroachment are also tabulated.

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Flood Velocities

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Average flow velocities during a 100-year flood for each creek are pro-

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