C153705 Feasibility Study

• Contract/PO #: C153705
• Contractor Name: Beaver Reservoir Company
• Contract Type: Loan
• Water District: 59
• County: Gunnison
• Bill Number: SB 94-029
• Loan Projects - Doc Type: Feasibility Study

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

Beaver Reservoir Company
Attention: Mr. Neil Thompson
February 4, 1993
Page 5

nominal distance of 50 feet. SP data were collected along each transect using a station
spacing of 20 to 35 feet, depending on the total length of the transect.

SP data were collected using MicroGeophysics millivoItmeters. These units are high-
impedance systems that allow measurement of the SP value and the contact resistance
between the soil and the electrodes. Tinkor and Rasor Model 6B copper-copper sulfate
electrodes were used for the survey. Throughout the survey period, electrodes were fairly
stable with measured drifts of 8 mv or less.

SP data were collected in a series of short loops from a constant base station. Each loop
generally included the data collection for approximately 15 stations, and was generally one
hour duration or less, thus minimizing drift corrections.

Processing of the SP raw data included making drift corrections and normalizing each SP
value to the base station reference. After removaIofdrift, the SP values were seen to have
a small dependence on surface topography. Asinall correction for topography was applied
to the data removing the topographic effects. Corrected SP values for each line were then
plotted for the entire survey area and contoured (see Appendix C).

Contouring was accomplished using the computer software GEOSOFf. Inputs to the
contouring include relative station' Coordinates and values, a spacing for gridding the raw
data (generally 22 feet) and a contour interval. Color enhancing is accomplished using the
gridded values from the raw data.

Significant seepage patterns are typically linear features with anomalies on the order of 25-
75 millivolts. Corwin and Butler (1989) indicate that negative SP anomalies are often seen
in areas where seepage flow is entering an embankment, or where the seepage flow is
horizontal ,or descending; and that positive anomalies are often observed where flow is
ascending. toward the.surface or where surface seepage with outward current is occurring.

Although there are several small anomalies over the survey area, there are no anomalies
of sufficient amplitude to indicate substantial seepage conduits through the dam
embankment. Seepage conduits were generally not active because the water level in the
reservoir had been drawn down significantly at the time of our investigation. In general,
the majority of the survey area has a relatively flat SP response. The total range of
contoured values is from -14 mv to + 16 mv, or 30 mv. The general distribution of SP
values appear to be primarily related to embankment material thickness rather than to
preferential seepage patterns. With the exception of the right abutment and spillway area,

23067/LEIO 02-04-93(4:24pm)/RPT