FLOOD10152

• County: Boulder
• County: Adams
• County: Jefferson
• Community: Broomfield, Westminster
• Stream Name: Big Dry Creek Tributaries
• Basin: South Platte
• Title: FHAD - Broomfield Area
• Date: 6/1/1979
• Prepared For: Broomfield, Westminster
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

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STUDY PROCEDURE

The first step toward producing this report was to identify the goals and

objectives, The consultants interviewed parties affected by the drainage

and flood control program and found their goals and objectives to be con-

sistent with the goals of the UD&FCD,

Once the hydrological studies were completed, flood peak water surface

profiles were computed and plotted to define the flood elevations along

the present channel. Discharges for the 10-year and 100-year events under

future development conditions were used to compute the profiles. The

basic procedure in determining the floodplain was the HEC-2 backwater pro-

gram developed by the U.S. Army Corps of Engineers, Manual hydraulic

calculations were performed to define the water surface profiles in areas

not readily adaptable to computer analysis,

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Field surveys were conducted to gather information relevant to the physical

characteristics of the existing drainage basin. The consultants visited

the drainage basin and inspected the hydrologic and surface characteristics

in terms of topography, vegetation, soil, and geology, as well as the char-

acteristics. Land use and future development forecasts were provided by the

cities of Broomfield and Westminster. The results of this data gathering

process are detailed in this report.

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Precipitation information was obtained from Volume I I I of the NOAA Atlas

(Ref. 20) to determine the design rainfall events used for runoff analysis.

Soils data was provided by the Soil Conservation Service.

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The next step was to analyze and evaluate the data that had been gathered.

The evaluation included development of a hydrologic computer model for flood

hydrograph generation (Ref. 2).

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Flows based on 2-year, 5-year, ID-year, and 100-year frequency rainfall

were determined for future basin development using the existing drainage

system in order to asses the effect of future basin development, As a

result of this determination, together with other information that was

gathered, existing and potential problem areas were identified and evaluated,

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Computer modeling indicated that additional future urban development, with-

out upstream storage, would cause an increase in storm runoff, both in rate

and volume, On that basis, it is evident that there would be further dete-

rioration in the natural character of the stream channel,

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