WSP05705

• File Number: 8407
• Description: Platte River Basin - River Basin General Publications
• State: CO
• Basin: Statewide
• Date: 3/11/1964
• Author: CWCB
• Title: Engineering and Geologic Investigation and Analysis of Alternate Proposed Dam Sites on the South Platte River - Morgan and Weld Counties-Colorado
• Water Supply Pro - Doc Type: Report/Study

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-20-

seepage analysis of the lower Weld County site.

Lower Weld County A.xis

The dam embankment, which is to include an interior zone of

well compacted borrow clays, is not expected to allow appreciable

seepage through it, particularly by comparison to the potential

seepage through the foundation materials beneath the dam. We

have assumed for our analyses that there is 0 seepage through

the embankment, and through foundation cutoffs or blanlwts placed

for the purpose of cutting off seepage.

Investigations were con-

fined to those necessary to determine the in-place permeability

of the foundation layers, so that estimates of seepage could be

made under various cutoff alternatives.

It was assumed that the

bedrock forms an impermeable lower boundary.

The foundation materials for the dam appear to fall into

three major categories each having substantially different pel' me-

abilities. These are, from the bottom up: (1) a zone of coarse

sand-gravel-cobble alluvium in an inl1cl', buried channel appraxi-

mately 60 feet deep and 1,800 feet wide, having a horizol1tal

permeability of 280,000 feet per year as indicated by a single

pumping test (see Figure 4); (2) flood plain alluvium which con-

sists of finer grained sands and gravels with silt and clay

lenses. Two permeability tests in this material indicated an

average permeability (hor izontal) of 2,500 feet per year; and

(3) Aeolian deposits which cover most of the area consisting of

dune sands, silts and clays, and having an average permeability

(two tests) af 700 feet p~ year. The lattcr materials form a

relatively impervious blanket over most of the reservoir area,

and in particular the old "flood plain alluvium" which must be

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