FLOOD04918

• County: Adams
• County: Boulder
• County: Jefferson
• Community: Broomfield
• Stream Name: South Platte River
• Basin: South Platte
• Title: Flood Insurance Study - City of Broomfield
• Date: 6/1/1973
• Prepared For: Broomfield
• Prepared By: FIA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

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DESCRIPTION OF WORK
Streamflow records are not available for the area surrounding

Broomfield, and discharge data for small drainage areas is very limited;

therefore, the conventional methods of flood analysis do not apply and

other methods were used to determine T-year peak discharges. Much work

has been done in the area of urban rainfall-runoff analysis by local

consultants, the U.S. Geological Survey, and other groups. The Denver

Regional Council of Governments with assistance from the Federal

Department of Housing and Urban Development funded the publication of

an Urban Storm Drainage Criteria Manual for use by local engineers and

designers in design and analysis of drainage structures. Some of the

data in the manual was used in the Broomfield drainage analysis. Peak

discharges were determined utilizing a synthetic unit hydrograph and

local rainfall records. These peaks were adjusted and (or) checked

by other rational methods before final acceptance and a graph of

drainage area versus peak discharge was plotted for each area.

Twenty-three cross sections were surveyed within the study area.

Water-surface profiles were determined using the U.S. Geological Survey

step-backwater program along with hand computations for flow-over-the-road

and culvert analysis. Although velocities were calculated to be at or

near supercritical in a few of the cross sections, profiles are

representative of the channel and are believed to be satisfactory for

flood insurance purposes. However, due to the steep slopes and high

velocities that would be encountered in a floodway analysis, results

would be unreliable and therefore that analysis is not practical.

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