FLOOD08164

• Designation Number: 309
• County: Weld
• Community: Erie
• Basin: South Platte
• Title: Flood Insurance Study - Erie, Colorado, Boulder and Weld Counties
• Date: 9/1/1990
• Designation Date: 1/1/1991
• Floodplain - Doc Type: Floodplain Report/Masterplan

Cross section data for Coal Creek in the area north of the Union
Pacific Railroad were obtained from topographic maps at a scale of
1:4800, with contour intervals of 2 and 4 feet (Reference 6).
Field measurements were taken by the SCS to obtain elevation data
and structural geometry of all bridges and culverts.

Roughness coefficients (Manning's "n") for Coal Creek north of the
railroad were determined by the SCS through field inspections. The
"n" values range from 0.085 to 0.120 for the existing channel and
overbanks areas.

Roughness coefficients for Coal Creek south of the railroad were
determined by the WRC (Reference 1). These values were reviewed
and approved by the CWCB. The "n" values range from 0.015 in the
channel to 0.055 in the overbank.

Locations of selected cross sections used in the hydraulic analyses
are shown on the Flood Profiles (Exhibit 1). For stream segments
for which a floodway was computed (Sect ion 4.2), selected cross
section locations are also shown on the Flood Insurance Rate Map
(Exhibit 2).

Water-surface elevations for floods of the selected recurrence
intervals on Coal Creek and the Coal Creek West Line Overflow were
computed through use of the COE HEC-2 step-backwater computer
program (Reference 8).

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 National Geodetic Vertical
Datum of 1929 (NGVD). Elevation reference marks used in this study
are shown on the maps; the descriptions of the marks are presented
in Elevation Reference Marks (Exhibit 3).

4.0 FLOODPLAIN MANAGEMENT APPLICATIONS

The NFIP encourages State and local governments to adopt sound
floodplain management programs. Therefore, each Flood Insurance Study
provides 100-year flood elevations and delineations of the 100- and
500-year floodplain boundaries and 100-year floodway 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

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