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WSP11881
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Last modified
1/26/2010 3:19:11 PM
Creation date
10/12/2006 5:15:18 AM
Metadata
Fields
Template:
Water Supply Protection
File Number
8443.400
Description
Narrows Project - Reports
State
CO
Basin
South Platte
Water Division
1
Date
8/1/1966
Author
US DoI BoR
Title
Narrow Unit Colorado South Platte Division Missouri River Basin Project Appendix I - Designs and Estimates
Water Supply Pro - Doc Type
Report/Study
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<br />.- ,-. C. Q <br />." uu <br />{... '-I' <br /> <br />. <br /> <br />Seepage Effects <br /> <br />The seepage problems at the Narrows damsite result from the <br />presence of permeable, alluvial sand which fills the ancestral South <br />Platte River channel. <br /> <br />The Geological Survey has prepared a groundwater contour map <br />which indicates that Bijou Creek is fed by groundwater for a dis- <br />tance of 6 to 8 miles above its confluence with the South Platte <br />River. <br /> <br />The entrenchment of the river in the shale has developed a <br />subsurface bedrock ridge, the crest of which generally follows <br />the south bank of the present South Platte River. This ridge <br />apparently has been breached by Bijou Creek where the latter empties <br />into the South Platte River. Except for the latter area, the shale <br />ridge acts as a groundwater dam which causes a higher water table <br />in the area south of the river. The possibility of Bijou Creek <br />intercepting at least part of the seepage from beneath and around <br />the right abutment of the dam and diverting it to the South Platte <br />River appears probable. <br /> <br />. <br /> <br />Electrical analogy studies have indicated that the additional <br />hydraulic head on the groundwater body, imposed by a full reservoir, <br />in the area downstream from the Narrows axis and south of the South <br />Platte River would be entirely dissipated in the vicinity of the <br />town of Fort Morgan. It should be understood that this condition <br />is indicated without the beneficial effect of Bijou Creek acting <br />as a drain. Had an allowance been made for Bijou Creek, the analy- <br />sis would undoubtedly have shown that the creek would have assisted <br />in the dissipation of hydraulic head in the area east of Bijou Creek. <br /> <br />A board of consultants was convened in August 1948 to consider <br />whether a safe structure could be built at the Narrows axis. These <br />consultants visited the Narrows Dam site; examined the drill logs, <br />geologic maps and sections, design study drawings, reports and <br />memoranda; and discussed the different phases of the investigations <br />with Bureau employees, They concluded that a safe dam could be <br />built even though there would be large seepage losses around and <br />under the right end of the dam. <br /> <br />Reconnnendations <br /> <br />The high seepage losses should be accepted and dam design <br />studies should allow for the consequent uplift pressures. Where <br />the dam sould be founded on Pierre shale, excavation should be made <br />to unweathered rock which should then be coated with an asphalt <br />compound. <br /> <br />. <br /> <br />3 <br />
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