FLOOD10684 ‎(2)‎

• County: Clear Creek
• Community: Clear Creek County and Unincorporated Areas
• Title: FIS - Clear Creek County and Incorporated Areas
• Date: 3/19/2007
• Prepared For: Clear Creek County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

on the Same Stream" (Reference 6). This method was used because the drainage
area of Clear Creek within the corporate limits of Idaho Springs varies from 192
to 256 square miles, and the ratio of these areas to the Lawson gage area is 2.00
or less.

In Georgetown, hydrology for South Clear Creek and for Clear Creek upstream
from its confluence with South Clear Creek was based on the Mountain Region
regression equations. Parameters used in establishing peak flows were: drainage
area of Clear Creek upstream from its confluence with South Clear Creek; mean
annual precipitation upstream from this point; drainage area of South Clear
Creek; and mean annual precipitation from the mouth of the creek (Reference 1).

For the other streams studied in this report, values of the 10-, 2-, 1- and 0.2-
percent-annual-chance peak discharges were determined using a series of
regression equations from which flood characteristics can be estimated from
measurable physical and climatic parameters of drainage basins. Drainage area
and average annual precipitation over that area are the two factors required to
estimate flood flows.

Results of the regional hydrologic analysis were compared with statistical
analyses of historical discharge records at two gaging stations in Lawson and
Golden, Clear Creek County, operated by the USGS (References 7, 8, 9, 10 and
11). The Lawson gage, which provided the major portion of the data, had a 25-
year length of record. Analyses of these two stations followed the standard log-
Pearson Type III method outlined by the u.S. Water Resources Council
(Reference 12). Comparison of records on Clear Creek at Lawson with results of
the regional hydrologic analysis showed a maximum difference in flood flows of
29 percent and an average difference of only 10 percent. This error is acceptable,
considering the uncertainties inherent in the analysis and the short period of
record. The regional analysis was, therefore, adopted for use in Clear Creek
County for both gaged and ungaged areas.

A summary of drainage area-peak discharge relationships for each stream studied
in detail is shown in Table 3.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding from the sources studied
were carried out to provide estimates of the elevations of floods of the selected
recurrence intervals. Users should be aware that flood elevations shown on the
FIRM represent rounded whole foot elevations and may not exactly reflect the
elevations shown on the Flood Profiles or in the Floodway Data tables in the FIS
report. Flood elevations shown on the FIRM are primarily intended for flood
insurance rating purposes. For construction and/or floodplain management
purposes, users are cautioned to use the flood elevation data presented in this FIS
in conjunction with the data shown on the FIRM.

Cross Sections

In the Town of Silver Plume and the two re-studied reaches of Clear Creek, cross
sections for the hydraulic analyses were developed from topographic mapping at

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