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FLOOD10404
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Last modified
1/26/2010 10:13:33 AM
Creation date
11/6/2007 2:58:50 PM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
128
County
Larimer
Community
Larimer County and Unincorporated Areas
Basin
South Platte
Title
FIS - Larimer County and Unincorporated Areas
Date
9/1/1978
Designation Date
1/31/1979
Floodplain - Doc Type
Historic FEMA Regulatory Floodplain Information
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<br />Dry Creek has several lakes and storage reservoirs w ich reduce the <br />contributing drainage area by approximately 13 perce t, although <br />they are not really flood control structures. Dougl ss Lake, also <br />an irrigation reservoir, does reduce flood peak disc arges signifi- <br />cantly. <br /> <br />. <br /> <br />There are no flood control structures on the Cache L Poudre River <br />in the study area although there are two irrigation ams on the <br />North Fork Cache La poudre River. The effect of the two dams is <br />considered negligible in the study area. There are 0 flood control <br />structures which reduce discharges on Buckhorn Creek, Redstone <br />Creek, Fish Creek,or Fall River. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />- <br /> <br />For the flooding sources studied in detail in the communi y, standard <br />hydrologic and hydraulic study methods were used to deter ine the flood <br />hazard data required for this study. Flood events of a m gnitude which <br />are expected to be equalled or exceeded ~ on the avera e during any <br />10-, 50-, 100-, or SOO-year period (recurrence interval) ave been <br />selected as having special significance for flood plain m nagement and <br />for flood insurance premium rates. These events, commonl termed the <br />10-, 50-, 100-, and SOO-year floods, have a 10, 2, 1 and .2 percent <br />chance, respectively, of being equalled or exceeded durin any year. <br />Although the recurrence interval represents'the long term average period <br />between floods of a specific magnitude, rare floods could occur at short <br />intervals or even within the same year. The risk of expe iencing a rare <br />flood increases when periods greater than 1 year are cons'dered. For <br />example, the risk of having a flood which equals or excee s the 100-year <br />flood (1 percent chance of annual occurrence) in any 50-y ar period is <br />approximately 40 percent (4 in 10), and, for any 90-year eriod, the risk <br />increases to approximately 60 percent (6 in 10). The ana yses reported <br />here reflect flooding potentials based on conditions ex is ing in the <br />community at the time of completion of this study. Maps d flood <br />elevations will be amended periodically to reflect future changes. <br /> <br />3.1 Hydrologic Analyses <br /> <br />~ <br /> <br />Hydrologic analyses were carried out to establish th peak discharge- <br />frequency relationships for floods of the selected r currence inter- <br />vals for each stream studied in detail in the commun'ty. <br /> <br />The discharges on the Big Thompson River, the North ork Big Thompson <br />River, and their major tributaries were developed ba ed upon a <br />combination of gage records and regression equations contained in <br />the Colorado Water Conservation Board's Technical Ma al No.1, <br />prepared by the u.s. Geological Survey (Reference 8). The locations <br />of the stream gages analyzed and their respective ye rs of record <br />are as follows: Drake gage (47 years) located on th Big Thompson <br /> <br />10 <br /> <br />. <br /> <br />K <br /> <br />~ <br /> <br />. <br />
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