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<br />- <br /> <br />I <br />I <br />I <br />- <br />- <br />- <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />- <br />- <br />I <br />I <br />II <br /> <br />Lower Big Dry Creek Hydrologic Study <br /> <br />The drainage plan discusses the Big Dry Creek channel, describing it as relatively similar in <br />nature from below Standley Lake to the Weld County line. The channel includes a normal flow <br />channel between 15 and 40 feet wide and between 3 and 10 feet deep. The banks of the normal <br />flow channel are steep and sometimes vertical. Erosion of the banks is common and may be seen <br />along the entire reach of Big Dry Creek. The normal flow channel meanders through a wide <br />floodplain which is as much as a half-mile wide at the Weld County line. The floodplain is <br />generally uninhabited, except for a few homes and farm buildings. The channel appears to be in <br />a relatively active me~mdering state, exhibiting numerous horse-shoe bends which appear to be <br />migrating in typical stream fashion. Flood flows exceeding the normal flow channel can spread <br />over pasture and crop1.mds in the wide floodplain to depths varying from a few feet to as much as <br />20 feet, depending upon constrictions at the various road and railroad crossings (Muller 1989). <br /> <br />Muller (1989) summarized three general problems identified in the Greiner (1986 and 1988) <br />flood hazard studies: <br /> <br />1. Channel bank e:rosion and migration of channel meanders, <br /> <br />2. Several homes or businesses located in the 100-year floodplain, and <br /> <br />3. A large number of street and highway crossing structures which have inadequate capacity <br />to carry the fully developed lOO-year flood. <br /> <br />To address these problems, Muller (1989) recommended a floodplain regulation approach that <br />relied on floodplain ;and floodway ordinances to control the location of development. In <br />addition, improvements to crossing structures and erosion control structures within the channel <br />and a levee around thl~ Broomfield Wastewater Treatment Plant were also recommended. The <br />study recommended that the channel of Big Dry Creek be maintained in its present and natural <br />state, including maintenance of the meandering channel pattern. However, as development <br />increases upstream, greater and more frequent flood flows and greater sustained low flows will <br />be experienced, increasing the rate of channel bank erosion. The study indicated the general <br />degradation of the channel bed will be relatively minor, because the channel is already at a very <br />flat slope (Muller 1989). <br /> <br />971-179.092 <br />June 2005 <br /> <br />Wright Water Engineers, Inc. <br /> <br />Page 35 <br />