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Last modified
11/23/2009 1:21:16 PM
Creation date
10/4/2006 9:40:58 PM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
186
County
Boulder
Community
Longmont
Stream Name
Lefthand Creek
Title
Floodplain Information Report - Lefthand Creek, Volume I: Mouth to Foothills Highway, Boulder County and Longmont
Date
12/1/1981
Designation Date
4/1/1982
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />STUDY AREA DESCRIPTION <br /> <br />Table 1 <br /> <br />Study Reach Description <br />Lefthand Creek was studied in detail from its mouth near Longmont, <br />Colorado to a point approximately 0.5 miles upstream of the Foothills <br />Highway. The length of study was about 13.7 stream miles. The <br />contributing drainage area varies from 72 square miles at the mouth to <br />56.4 square miles below Geer Canyon which is immediately upstream of the <br />limits of this study, Stream bed elevations vary from 4916.3 feet mean <br />sea level (m.s, 1.) at the mouth to 5614.8 feet m.s.1. at the upstream <br />limit of study. Stream bed slopes range from 8.5 ft/mi1e near the mouth <br />to 135.7 ft/mi1e near the upstream limit of the study. <br />Development along Lefthand Creek consists mainly of single-family <br />houses and their re 1 ated structures in both rural and urban sett ings. <br />Development in the floodplain occurs throughout the study reach, but is <br /> <br />Drainage Basin Characteristics <br /> <br />Lefthand Creek is located in Bou 1 der County approx imate 1y 30 mil es <br />northwest of Denver. Its headwaters originate on the eastern slope of <br />the Continental Divide at elevations in excess of 13,000 feet. The creek <br />then flows in an easterly direction draining into St. Vrain Creek near <br />Longmont, <br />Approximate drainage areas are given in Table 1. <br /> <br />Drainage Areas <br /> <br />Location <br /> <br />Lefthand Creek at mouth <br />Lefthand Creek at u/s study limits <br /> <br />Area (sq,mi.) <br /> <br />72.0 <br />56,4 <br /> <br />concentrated in an area about 1.5 miles upstream from the mouth. <br /> <br />Obstructions to flood flows within the study reach include natural <br /> <br />Normal annual precipitation over the study area is. approximately 16 <br /> <br /> <br />inches. Temperatures range from a high of about 95 degrees Fahrenheit to <br /> <br /> <br />a low of -20 degrees (Reference 6). The topography of the Lefthand Creek <br /> <br />obstructions such as large rocks, brush, and other vegetation growing <br />along the streambank, and man-made obstructions such as bridges, <br />buildings, and culverts. During floods, these obstructions cause <br />backwater conditions that may increase the flood depths or they may cause <br />constrictions that increase the flow velocities. Brush washed out during <br />floods is carried downstream and may collect at bridges and in culverts, <br />thus creating a damming effect and overbank flow. As flows increase, <br />masses of debris may break loose and cause a wall of water and debris to <br />surge downstream until another obstruction is encountered. In some <br />instances, debris may collect to the point where structural capability is <br />exceeded and a bridge is destroyed or abutments and approaches eroded and <br />roadbeds damaged. <br /> <br />basin varies from rugged and heavily forested canyon in the western part <br /> <br />of the basin, to slightly rolling prairie land east of the canyon mouth <br />where pasture and crop lands border stream banks lined with willow brush <br />and cottonwood. <br /> <br />3 <br />
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