FLOOD04012

• Designation Number: 184
• County: Jefferson
• Community: Unincorporated Jefferson County
• Stream Name: Bear Cree, Mt. Vernon Creek, Sawmill Gulch, etc.
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Southern Jefferson County
• Date: 12/1/1981
• Designation Date: 2/5/1982
• Floodplain - Doc Type: Floodplain Report/Masterplan

I

I

I

- Parmalee Gulch. (Sheets 22 through 27)

I

This stream drains the residential area of Indian Hills and is

crossed by numerous undersized road bridges and private driveway

culverts. Nevertheless, the backwater effect created by these struc-

tures is mostly very local and few buildings are within the flood

boundaries. Most of the reach studied is located in a deeply incised

valley and potential encroachment seemS insignificant. At the up-

stream end of the study the valley widens; nevertheless, very little

encroachment is possible.

SUMMARY

I

This report has identified the probable flooding limits of poten-

tial lOO-year flood events on several stream reaches in southern

Jefferson County.

I

The floodplain limits delineated in this study are based on: (1)

present development conditions in the basin; (2) existing, mapped

floodplain conditions; and (3) no blockage at existing culvert and

bridge sections. Even if these assumptions are realized, floods of

greater magnitude can and will occur in which the limits of flooding

would exceed those shown in the report,

I

- Deer Creek. (Sheets 28 through 34)

I

At the upstream end of the study reach the channel is deep with

very steep walls so flows of major floods are not likely to spill out

of the channel. Further downstream the valley widens and Deer Creek

Canyon Road will be overtopped in a few locations where low areas

north of the road will be subject to shallow flooding. At the down-
.

stream end of the study the floodplain widens considerably onto an

alluvial fan where the flooding will be shallow. The floodway pro-

posed in this area has been delineated to follow the general path of

the stream and its width computed to maintain the rise in water

surface elevation approximately below one half of a foot. The exist-

ing development is limited to a few farm structures.

Presently, Some of the study reaches are considered to have a high

flood damage potential. The preservation of the floodplain will

serve, at least, to maintain the present level of damage potential

while any improvements would reduce, to some degree, the flood damage

potential. Reducing the flood hazard potential can be accomplished

through structural improvements to bridges and channels, flood proof-

ing, and by introducing an operational and effective floodplain

management program. A major portion of the reaches studied are

presently undeveloped. For these reaches, the enactment and enforce-

ment of sound floodplain regulations will bes t serve future develop-

I

I
I

I

I

Flood Velocities

ments.

I

I

Average flow velocities computed for the reaches studied are high

and range from 6 to 18 feet per second in the channel and from 2 to 10

feet per second on the overbank. The velocities have been computed

assuming a minimum specific energy for most of the reaches, but in

reality the flow will be unstable. Consequently, subcritical and

supercritical flows will be encountered, the latter causing even

higher velocities and hydraulic jumps. The high velocities can cause

severe erosion, dislodge boulders and uproot trees. The water surface

elevations have been computed assuming a flow free of debris, but

substantial variations in water surface elevations might occur due to

blockage at structures and changes of channel section.

I

I

I

I

-9-