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FLOOD10684 (2)
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Last modified
11/23/2009 12:39:50 PM
Creation date
5/21/2007 2:50:12 PM
Metadata
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Template:
Floodplain Documents
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
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<br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied by detailed methods in the community, standard <br />hydrologic and hydraulic study methods were used to determine the flood-hazard data <br />required for this study. Flood events of a magnitude that is expected to be equaled or <br />exceeded once on the average during any 10-, 50-, 100-, or 500-year period (recurrence <br />interval) have been selected as having special significance for floodplain management <br />and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and <br />SOO-year floods, have a 10-, 2-, 1-, and 0.2 percent chance, respectively, of being equaled <br />or exceeded during any year. Although the recurrence interval represents the long-term, <br />average period between floods of a specific magnitude, rare floods could occur at short <br />intervals or even within the same year. The risk of experiencing a rare flood increases <br />when periods greater than 1 year are considered. For example, the risk of having a flood <br />that equals or exceeds the I-percent-annual-chance flood in any 50-year period is <br />approximately 40 percent (4 in 10); for any 90-year period, the risk increases to <br />approximately 60 percent (6 in 10). The analyses reported herein reflect flooding <br />potentials based on conditions existing in the community at the time of completion of this <br />study. Maps and flood elevations will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish peak discharge-frequency <br />relationships for each flooding source studied by detailed methods affecting each <br />community. <br /> <br />There are only two stream gaging stations within Clear Creek County which are <br />operated by the United States Geological Survey (USGS). One, located on the <br />Fall River, was not used in the hydrologic analysis for this study due to shortness <br />of record (9 years, 1930-1938). The other gage is located on Clear Creek near <br />Lawson, Colorado (Station No. 06716500), upstream of Idaho Springs. It has a <br />drainage area of 147 square miles and 29 years of record. <br /> <br />For all streams, flow-flow frequency data were based on a regional hydrologic <br />analysis set forth in Manual for Estimating Flood Characteristics of Natural-Flow <br />Streams in Colorado (Reference 5). <br /> <br />Hydrology for Clear Creek at the northern corporate limit of Georgetown <br />(downstream from its confluence with South Clear Creek) was derived through a <br />statistical analysis of annual peak flows at USGS gage 06716500, Clear Creek <br />near Lawson (period of record: March 1946 to present). This information was <br />then transferred to the subject location using a weighting of the computed flood <br />peaks at the gage with flood peaks calculated with regional regression equations <br />(Mountain Region equations). Drainage area ratios are also employed in this <br />method. Data used in establishing peak flows were as follows: 38 years of <br />annual peak flows from the Lawson gage data; drainage area at the gage; <br />drainage area at Georgetown's northern corporate limit; and mean annual <br />precipitation above the gage. A statistical analysis of 38 years of gage data using <br />log-Pearson Type III fitting of data was conducted (Reference 1). <br /> <br />The method used to obtain the 10-, 2-, 1-, and 0.2-percent-annual-chance flood <br />flows for Clear Creek in Idaho Springs was taken from "Sites Near Gaged Sites <br /> <br />6 <br />
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