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

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HYDROLOGIC AND HYDRAULIC DETERMINATIONS

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Fac~ors Which Reduce the Flood Hazard

Hydrologic Analysis
General

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Flood control improvements, such as channel improvements, reser-

voirs and bridge improvements, as well as good floodplain management

practices, such as flood proofing and controlling development in

floodplains, will reduce the potential hazards in floodprone areas. A

major portion of the reaches studied is presently undeveloped. In

these areas the enforcement of sound floodplain regulations will best

serve future developments. In the developed areas, very little

improvement has been done to the natural channel and numerous bridges

and houses are creating obstructions to the natural flow of a lOO-year

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RELATED FLOOD STUDIES

For the study reaches covered in this report, the only gage site

with a record suitable for a flow frequency analysis is on Bear Creek

at Morrison. In order to determine flood discharges for the ungaged

reaches two different methodologies were employed. A regional fre-

quency analysis was used for the study reaches with a catchment area

larger than 15 square miles, and the Colorado Urban Hydrograph Pro-

cedure (CURP) was used for the study reaches with smaller catchment

areas. Therefore, the flood discharges for Bear Creek (except in the

vicinity of the Morrison gage), Turkey Creek, and Deer Creek were

determined using the regional frequency analysis approach while the

flood discharges for Mt. Vernon Creek, Sawmill Gulch, and Parmalee

Gulch were determined using the CURP approa.ch. Flood discharges of

lOO-year return periods were used for all reaches analyzed in this

study.

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flood.

In these areas a combination of structural flood control

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measures and good floodplain management practices would probably be
most advantageous.

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~~~ional Frequency Analysis

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A Flood Plain Information report (Reference 4) was prepared by the
U.S. Army Corps of Engineers, Omaha District, in October 1971 for the
reach of Bear Creek through Morrison and a portion of Mount Vernon
Creek. Their original topographical mapping, being the only avail-
able, is used for this study (Sheets 4 through 8) as well as most of
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their information on cross-sections, structures and roughness

ficients. The same information was used for the Flood Insurance

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Report for the Town of Morrison
the Federal Emergency Management

(Reference 5) prepared
Agency (FEMA).

Study
by PRC/ECI for

In the regional frequency analysis, 15 gages in the vicinity of

the study area, including the Morrison gage on Bear Creek, were

selected for analysis on the basis of record length and hydrologic

similarity. Since the flood events in the study reaches comprise two

distinct and generally independent events, either rainfall or snowmelt

floods, each type of event was analyzed separately. However, each

type of event was assumed to follow a log-Pearson Type III distribu-

tion.

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For each gaging station, an annual peak flood series was deter-

mined for rainfall events and for snowmelt events. The three sta-

tistical parameters which define the log-Pearson Type III distribu-

tion, the mean, standard deviation, and skew, were then computed for

the common logarithms of the annual flood series for each type of

event.

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