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FLOOD08620
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Last modified
1/25/2010 7:15:05 PM
Creation date
10/5/2006 3:45:58 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
463
County
Boulder
Community
Longmont
Basin
South Platte
Title
FIS - Longmont, CO, Boulder County
Date
9/1/1987
Designation Date
5/1/1998
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />Various structural improvements such as grass- or concrete-lined <br />channels, detention ponds, and outfall culverts have been imple- <br />mented on Spring Gulch in the :segment from 9th Avenue to 3rd Ave- <br />nue, These improvements are inadequate to contain even the 10- <br />year flood discharge, <br /> <br />No flood protection measures ijave been taken on Dry Creek No. 1 or <br />in Loomiller Basin, except for using Loomiller Pond as a flood- <br />storage facility. Various prQperties along the streets in Loomiller <br />Basin have low retaining wall$ along the street frontage to prevent <br />yard flooding, <br /> <br />In addition to the physical f+ood prevention measures mentioned, <br />the City of Longmont initiated an ordinance in May of 1973 for <br />flood plain regulation and ha$ followed with flood plain manage- <br />ment policies outlined in theist. vrain Comprehensive plan (Refer- <br />ence 5). <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in,detail in the community, standard <br />hydrologic and hydraulic study meth9ds were used to determine the flood <br />hazard data required for this study~ Flood events of a magnitude which <br />are expected to be equaled or exceeded once on the average during any <br />10-, 50-, 100-, or 500-year period (recurrence interval) have been selec- <br />ted as having special significance tor flood plain management and for <br />flood insurance rates. These event$, commonly termed the 10-, 50-, 100-, <br />and 500-year floods, have a 10, 2, 1, and 0,2 percent chance, re-spec- <br />tively, of being equaled or exceeded during any year. Although the re- <br />currence interval represents the lo~g-term average period between floods <br />of a specific magnitude, rare flood~ could occur at short intervals or <br />even within the same year. The risk of experiencing a rare flood increa- <br />ses when periods greater than 1 yeat are considered. For example, the <br />risk of having a flood which equals!or exceeds the 100-year flood (1 <br />percent chance of annual exceedencel in any 50-year period is approxi- <br />mately 40 percent (4 in 10), and, f9r any 90-year period, the risk in- <br />creases to approximately 60 percent' (6 in 10), The analyses reported <br />herein reflect flooding potentials based on conditions existing in the <br />community at the time of completion,of this study. Maps and flood eleva- <br />tions will be amended periodically ~o reflect future changes, <br /> <br />3,1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak dis- <br />charge-frequency relationships for floods of the selected recurr- <br />ence intervals for each stream studied in detail in the community. <br /> <br />Frequency-discharge data for ~wo of the streams studied in detail <br />in the City of Longmont are based on information published in COE <br />Flood Plain Information repor~s for Lefthand and St. Vrain Creeks <br /> <br />12 <br />
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