7886

• UCREFRP Catalog Number: 7886
• Author: Hydrosphere Resource Consultants.
• Title: Yampa River Basin Recommended Alternative, Detailed Feasibility Study
• USFW Year: 1995.
• USFW - Doc Type: Final Report.
• Copyright Material: NO

Reservoir Site Characterization I
random fill zones for the dam enlargement. They also concluded that the local commercial
borrow areas for sand and riprap were sufficient in size to provide the adequate quantities of
quality material needed for the embankment construction.
Seismic Risk
The seismotectonic investigation characterized the Elkhead Reservoir area as having low
seismic risk. A Magnitude 5.5 to 6.5 event represents the maximum credible earthquake for
this area (Uinta-Elkhead seismotectonic province). The Magnitude 6.5 event at a distance of 70
km at a focal depth of 20 km would yield a ground acceleration of 0.2g. It is recommended
that the dam and the structures be designed for a peak bedrock acceleration of 0.2g.
Evaluation of the geotechnical testing results indicate that the foundation will not liquefy during
the design ground acceleration.
MAPPING
During the. Phase I Study one of the critical needs identified was for improved Elkhead
Dam and Reservoir site mapping, particularly for elevations above the top of the existing dam
to the maximum logical dam height. Since aerial mapping had been accomplished for the City
of Craig during November, 1990, (at a time when the reservoir was significantly drawn down)
it was felt that supplemental aerial photography could provide an accurate composite base map.
That supplemental work was completed in June of 1993. The area of coverage was to elevation
6,440' in the dam/reservoir area and along a strip of land intended as the emergency spillway
route. Mapping was done to the standard accuracy appropriate for 2' contours at a 1 " = 200'
scale and provided as hard copy and in digital form. This permitted the generation of maps with
various scales, contour intervals and different plan features. Color contract prints of the aerial
photographs were also provided to help identify natural and manmade features both within and
outside the actual mapped area.
Based on this new mapping, a revised elevation-area-capacity characteristic curve for the
reservoir was developed. This new characteristics curve, shown in Figure 3-3 is a composite
made from the original project mapping, the 1991 aerial mapping, and the 1993 aerial mapping
performed as a part of the current study. While most of the desired plan and topographic
information was obtained from this mapping, there was a need for site surveying to provide
additional detail in certain defined areas. Peter Epp and Associates performed the site
surveying which included the following:
1. , Survey of the horizontal alignment and vertical elevations along the crest of the
existing dam for the purpose of establishing the dam settlement/remaining camber.
2. Survey of the horizontal and vertical positioning of borings and test pits
accomplished during the June/July 1993 supplemental geotechnical investigation.
of four downstream cross sections
3. Survey of the horizontal and vertical positioning
of Elkhead Creek, channel thalweg and hydraulic grade line.
This new composite aerial based mapping and site surveying provides the mapping basis
for the present feasibility study.
3-8