C153379 Feasibility Study

• Contract/PO #: C153379
• Contractor Name: Brook Forest Water District
• Contract Type: Loan
• Water District: 9
• County: Jefferson
• Bill Number: SB 80-67
• Loan Projects - Doc Type: Feasibility Study

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the Louviers from the top of its Pleistocene (Brady) soil to the base of the

deepest part of the buried channel is about 150 ft.

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Due to the thickness of the Louviers Alluvium, the composition varies

significantly. The samples analyzed by Gardner averaged 45 percent subrounded

gravel, 30 percent sand and 25 percent silt. Some of these samples, however,

contained less than 5 percent silt and clay (Gardner, 1967). This change in

percentage of fines is also indicated in the permeabilities measured by

Woodward-Clyde-Sherard (1964) in the Louviers. These ranged from 280,000

ft/year (2.7 x 10-1 cm/sec) to 232 ft/year (2.2 x 10-4 cm/sec) with an

average of 2500 ft/year (2.4 x 10-3 cm/sec). The highest permeabilities

generally occur, however, only in the deepest portion of the channel at Hardin

and at Narrows, where the deepest Louviers Alluvium is a coarse-grained sandy

gravel (Woodward-Clyde, 1978). Due to these high permeabilities, the deep

Louviers channel will provide a "conduit" for seepage beneath the proposed

Hardin Dam that will require design of positive cutoff or an upstream blanket

to limit potential seepage.

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Overlying the Louviers Alluvium is the Broadway Alluvium (Table III-I)

which is much thinner than the Louviers but is exposed on the surface over a

much larger area. The Kersey Terrace west of Box Elder Creek and the Green

City Flats at Masters are underlain by the "pink facies" of the Broadway

(Gardner, 1967). This unit at Masters consists of not more than 30 to 40 ft

of clean, feldspar-rich, pebbly coarse sand derived from the Precambrian

granite and gneiss located in the headwaters area of the South Platte and

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Cache la Poudre Rivers.

The upper surface of this terrace is now located

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approximately 30 ft above the present South Platte. Due to the recency of

deposition of the Broadway (latest Pleistocene), no well-developed soil or

calcareous cement occurs at the surface of these terraces.

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The Broadway Alluvium is locally overlain by or intertongued with a

finer-grained, but time-equivalent, unit which has been called the Bijou Flats

Tongue of the Broadway (Gardner, 1967). This tongue represents the deposition

by Piedmont tributaries of the South Platte of sand, silt and clay eroded

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