FLOOD01759

• County: Broomfield
• Community: Broomfield
• Title: Flood Insurance Study - City and County of Broomfield, CO
• Date: 11/25/2003
• Prepared For: Broomfield
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

Starting WSELs for Gay Reservoir Channel and City Park Channel were
taken from stage-disch~rge relationships computed for Big Dry Creek
for the FrS for. Adams County (Reference 7).. The starting WSELs for
Airport and Brandywine Creeks were computed using the elevations at
their confluences with Big Dry Creek. The starting WSEL for North
Branch Airport Creek was determined at peak flow at its confluence
with Airport Creek. For all other streams, the starting WSELs were
based on ~and calculations at control sections.

The 100-year shallow flooding for City Park Channel along
120th Avenue, West Lake overflow near Grove Way, and Nissen
Reservoir Channel near Tennyson Street was evaluated using manual
hydraulic computations. Various sheetflow areas are present on many
of the streams. The 100-year floodplain has been delineated to
encompass all of these additional areas. The complete filling of
these sheetflow areas will increase elevations downstream, thereby
increasing the IOO-year flood hazard. For any channel modification
or fill in the sheet flow areas, a detailed review of the improvement
should be undertaken. to determine its effects downstream.

The hydraulic analyses for this study were based on unobstructed
flow. The flood elevations shown on the profiles are thus
considered valid only if hydraulic structures remain unobstructed,
operate properly, and do not fail.

All elevations are referenced to the North American Vertical Datum
of 1988 (NAVD 88).

4.0 FLOODPLAIN MANAGEMENT APPLICATIONS

The NFIP encourages State and local governments-to adopt sound floodplain
management programs. Therefore, each FIS. produces maps designed to assist
communities in developing floodplain management measures.

4.1 Floodplain Boundaries

To provide a national standard without regional discrimination, the
I-percent-annual-chance (lOO-year) flood has been adopted by FEMA as
the base flood for floodplain management purposes. The
0.2-percent-annual-chance (500-year) flood is employed to indicate
additional areas of flood risk in the community. For each stream
studied by detailed methods, the 100- and 500-year floodplain
boundaries have been delineated using the flood elevations
determined at each cross section. Between cross sections, the
boundaries were interpolated using topographic maps at a scale of
1:2,400, with a contour interval of 2 feet (Reference 6).

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