SPDSS_Task34-3_Phase 3_StreambedConductanceTesting

• Title: SPDSS Task 34.3 - Streambed Conductance Testing - Phase 3
• Description: The objective of this Task 34 is to collect data needed to quantify the groundwater flux across the streambed in the South Platte River and significant tributaries.
• Decision Support - Doc Type: Task Memorandum
• Date: 6/28/2007
• DSS Category: Groundwater
• DSS: South Platte
• Basin: South Platte
• Contract/PO #: C153953
• Grant Type: Non-Reimbursable
• Bill Number: SB01-157, HB02-1152, SB03-110, HB04-1221, SB05-084, HB06-1313, SB07-122
• Prepared By: CDM

C = Shape factor dependent on sorting
d5o = Sieve size that retains 50 percent of the sample mass (mm)
j = Exponent dependent on sorting
Shepherd (1989) developed a series of coefficients (C) and exponents (j) based on
depositional environment of the sediments. Tl1e parameters provided by Shepherd
(1989) that appear to best match t11e Sout11 Platte Basin's depositional setting are those
for "immature" sediments in which C = 800 gallons per day/ ft (gpd/ ft), or 107
ft/day, and j = 1.5. The 'C and 'j' values were selected for this analysis based on the
recommendation of Shepherd (1989).
Based on the grain-size results, K values calculated using the Shepherd equation
range from 8 to approximately 816 ft/day with a geometric mean of 114 ft/day. The
geometric mean is more appropriate t11an the mean (average) value for parameters
that have alog-normal distribution like K. The highest K value in the data is derived
from a sample collected from the mainstem of the South Platte River near Julesburg
and fl1e lowest K value is derived from a sample collected from Beebe Draw. Tl1e
geometric mean K value from the mainstem of the South Platte and Cache la Poudre
Rivers is 184 ft/day. The geometric mean K value from the tributaries is 37 ft/day,
approximately 5 times smaller. As with any general equation applied to a particular
set of data, the results should only be considered as estimates. However, fl1e high K
values that result from this b ain-size analysis suggest that the streambed materials
will have a sinular K as the underlying alluvial aquifer materials. This result is
supported by observations of the generally medium to coarse-grained sandy
streambed materials, as described in Appendix B and as shown in grain-size curves in
Appendix C. The K values calculated from grain-size analyses are presented in Table
1.
3.3 Streambed Hydraulic Conductivity Testing Results
Streambed K field tests were performed at all SPDSS streambed conductance locations
as described above. Results are summarized in Table 1 and are shown in map format
uz Figure 9.
K values determined by field testing ranged from 0.3 ft/day at test site SC-18 on Box
Elder Creek to 505 ft/day at test site SC-14 at the Kersey Gage on the South Platte
mainstem. The geometric mean K of all measurements is 65 ft/day. The geometric
mean is more appropriate than the mean (average) value for parameters that have a
log-normal distribution like K. Tl1e geometric mean K of measurements along the
mainstem of the South Platte and Cache la Poudre Rivers is 137 ft/day. The
geometric mean K of measurements in t11e tributaries is 11 ft/day, indicating
relatively less permeable sediments in the tributaries. The hig11 K value calculated
from the field data suggests that fl1e streambed sediments will Have a similar K as the
underlying aquifer materials.
The K results based on field testing are in general agreement with results based on the
empirical (Shepherd 1989) equation of grain-size as described in the previous section.
However, there is a poor correlation between field based and grain-size based K
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