FLOOD10617 ‎(2)‎

• County: Boulder
• Community: Boulder County and Incorporated Areas
• Title: FIS - Boulder County and Incorporated Areas - Vol 1
• Date: 10/4/2002
• Prepared For: Boulder County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

Channel roughness factors (Manning's "n") for these computations
were assigned on the basis of field inspection of the floodplain
areas and engineering judgement. Bridge geometry and elevation
info~tion was obtained fram the Colorado State Highway Department
and the City of Longment, when available, and measured in the
field.

The hydraulic analyses for this study were revised to include
information presented in floodplain information reports for
Lefthand Creek, Dry Creek No.1, and St. Vrain Creek (References 1
through 4).

For St Vrain Creek detailed analyses through the Lyons area, the
analyses used field conditions represented by bridge and valley
cross sections surveyed in 1971. Water-surface profiles were
determdned fram backwater computations employing the Standard Step
Method (Reference 46). Starting water-surface elevations were
taken fram the USACE report concerning the St. Vrain Creek
(References 8 and 44).

The roughness coefficients (Manning's "n") used in the study were
determdned by field survey and ranged from 0.045 to 0.055 for the
main channels, and from 0.060 to 0.100 for the overbank. Head
losses at bridges were computed using data in Hydraulics of Bridge
Waterways, published by the u.S. Department of Transportation
(Reference 50).

Water-surface profiles for the 10-, 50-, 100-, and 500-year floods
for Middle Boulder Creek and North Beaver Creek through the Town of
Nederland were developed using the SCS WSP2 computer program
(Reference 35). Starting water-surface elevations were determdned
using stream slope at the starting valley cross sections.

Cross section data were obtained by field measurements. All
bridges, culverts, and other structures were surveyed to obtain
elevation data and structural geometry.

Roughness coefficients (Manning'S "n") were estimated by field
investigation and from pictures of each stream and its respective
floodplain using USGS Water-Supply Paper 1849 (Reference 51), Open
Channel Hydraulics, by Ven Te Chow (Reference 52), and Handbook of
Applied Hydraulics, by Davis and Sorenson (Reference 53). The
roughness values for the main channels ranged fram 0.040 to 0.075,
depending on the locations. The roughness coefficients for the
floodplain ranged from 0.016 to 0.160.

Cross section data for Coal Creek through the Town of Superior were
taken from 1974 photography and mapping of the study area
(Reference 19). All bridges and culverts were surveyed to obtain
their elevation and size. The no~l openings of the bridges and
culverts were used to determine flood data at road crossings.
Typical Manning'S "n" values for the channel and overbanks ranged
from 0.085 to 0.120 (Reference 19).

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