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Lower South Platte River Study <br /> Colorado Water Conservation Board <br /> December 2001 <br /> Costs presented above were used to develop a preliminary estimate for a 15,000 acre-foot <br /> enlargement ($22.2 million). <br /> ' New Sites on North Side of River <br /> ' As shown on Figure 4.1, the study team identified twelve storage sites on the north <br /> side of the South Platte River from Julesburg Reservoir extending 15 miles west into <br /> eastern Logan County. These sites are on draws with intermittent and perennial <br /> ' streams generally flowing from the northwest to southeast and then into the South <br /> Platte River. <br /> ' The geologic setting paralleling the South Platte River's north side is characterized <br /> by sloping uplands dissected by the tributaries flowing southeast to the river. The <br /> ' drainages are deeply incised, cutting through an ancient gravel deposit that covers <br /> vast areas of the region. Many of the ridges and plateaus are capped by a thin veneer <br /> of this gravel, which probably lies in contact with the underlying weathered Brule <br /> ' Formation in some locations. The valleys are partially filled with stratified clay and <br /> silt alluvium mantled with up to 3 feet of eolian materials. Occasional deposits of <br /> sand and gravel occur below the surficial layers and above the soft,weathered silty- <br /> ' clayey bedrock. The thicknesses of these materials are unknown within the valleys <br /> except for areas with scattered occurrences of bedrock outcrops. Several prominent <br /> topographic high spots consisting of well cemented, very resistant sandstone and <br /> conglomerate are remnants of the previously overlying Ogallala Formation. <br /> ' The Brule Formation should be a suitable dam foundation because of its relatively <br /> low permeability. The few outcrops exposed in the general vicinity are too <br /> weathered to indicate bedding thickness or jointing characteristics. Pressure <br /> ' grouting in the Brule bedrock foundation should be effective in reducing any zones <br /> of high secondary permeability. The soils mapped in this area had laboratory tested <br /> permeabilities between approximately 0.6 to 2.0 inches/hour(4.2E-04 to 1.4E-03 <br /> ' centimeters/second) (Logan County Soil Survey, 1977). <br /> On-site materials containing low permeability clay(embankment core),free draining <br /> sands (filter blankets and drains) and silty, clayey sand (soil cement)may be <br /> available at the identified sites for use in constructing embankment dams. Drilling <br /> ' or trenching would be necessary to confirm the occurrence and volume of these <br /> materials and to firmly establish technical feasibility of dam and reservoir <br /> construction. <br /> GEI Consultants, Inc. 4-11 01002 Dec 2001 Lower South Platte River Study <br />