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

from fine-grained sedimentary rocks such as the Laramie and Denver-Dawsoc

Formations. These fine-grained materials (75 percent sand, 20 percent silt

and clay) were deposited as alluvial fans in the South Platte channel durin~;

major floods in Bijou, Kiowa and Box Elder Creeks (Gardner, 1967). The

upstream surface of the Box Elder Creek fan presently lies about 60 to 70 ft

above the South Platte channel, but the downstream surface is only 25 to 30

ft above the channel. Gardner (1967) estimated that the thickness of the

Bijou Flats in the Weldona Quadrangle could be as much as 60 ft. Well logs in

the Kersey area, however, indicate that the Bijou Flats Tongue is only 15 to

30 ft thick along Box Elder Creek (Schwochow et al., 1974).

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The Sand Hills Formation is the most widespread surficial unit found

within the proposed dam and reservoir area at Hardin (Figure III-I). Although

shown in Table 111-1 as the oldest Holocene unit in the area, the Sand Hillll

actually includes several periods of depo,;ition, including late Pleistocene.

The Sand Hills consis ts of yellowish-bro,,'Il, well-sorted, fine-grained, non--

cohesive aeolian sand with some silt. This sand is generally formed into

dunes whose long axes trend approximately N35W and may be as much as 10 milell

long. Although the average thickness of this formation is 50 ft, some dunes

may reach a height of 170 ft (Gardner, 1967). At the proposed Hardin axis,

the Sand Hills Formation covers both abutments to a maximum depth of 50

ft (Figure 111-2). Due to its low density, the Sand Hills may be subject

to relatively high settlement, and may even be subject to hydrocompaction

when wetted in areas which have not been irrigated. Due to its fine-grained,

well-sorted and non-cohesive properties, the unit will also be very sus-

ceptible to erosion by wave action in a reservoir. Rapid reservoir drawdown

in areas covered by the Sand Hills Formation may also cause slope failures

due to the lack of cohesion.

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The Piney Creek Alluvium (Table III-I) forms a terrace that varies from
15 to 25 ft above the present channel of the South Platte River. The Piney
Creek, consisting of dark gray to yellowish-brown sandy silt and organic sand,
is generally derived from local bedrock and older surficial units (Gardner,
1967). Although originally widespread, particularly in the creeks and draws

1II-8

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