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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tributary to the South Platte, the Piney Creek has been extensively eroded and
covered by aeolian sand. Present exposures in the Hardin area are apparently
limited to a terrace just below the Bijou Flats Tongue near the mouth of Box
Elder Creek and to small intermittent or ephemeral draws such as Lost Creek
(Figure III-I). The Piney Creek Alluvium will, therefore, have little effect
on the proposed dam or reservoir.

The Post-Piney Creek Alluvium includes the present channel and flood-

plain deposits of the South Platte River and its tributaries, as well as the

small alluvial fans and bog deposits formed in some of the tributaries. The

9- to 1S-ft thick channel and floodplain deposits of the South Platte were

tested for grain size by the Bureau of Reclamation and averaged 33 percent

gravel and 67 percent sand. These coarse-grained deposits, mapped as Qpp in

Figure III-I, have been concentrated by removal of the fines from reworked

Louviers and Piney Creek Alluvium (Gardner, 1967). The Hurley Terrace (mapped

as QpP1) and the alluvial fans (QpP2) are finer-grained and less extensive

than the channel deposits. The Hurley Terrace is located 8 to 12 ft above the

present South Platte channel and has a lithology similar to the Piney Creek

Alluvium (Gardner. 1967). The alluvial fans also contain sandy and silty

alluvium derived from the Piney Creek Alluvium and the Sand Hills Formation.

Due to the unconsolidated character and the potential for organics, partic-

ularly in the alluvial fans, the Post-Piney Creek Alluvium should be stripped

from any dam foundation or abutment areas.

S. Seismicity

Historically, the eastern Weld County area has been little affected

by seismic activity. The largest historical event occurred on November

7. 1882. with an estimated Modified Mercalli (MM) Intensity of III at Greeley

(Dames & Moore, 1981, Fig. 8-1). A series of man-induced earthquakes began in

the Commerce City area. 48 miles southwest of the proposed Hardin Dam site,

during the mid 1960s. The largest of these events occurred on August 9, 1967,

and measured S. 3 on the Richter scale (Hadsell, 1968). The most recent of

this series occurred on April 2, 1981. but measured only 4.3 on the Richter

scale. This 1981 event has, however, been linked by the State Engineer's

III-9