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SPDSS_Task34-3_Phase 3_StreambedConductanceTesting
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SPDSS_Task34-3_Phase 3_StreambedConductanceTesting
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Last modified
4/17/2013 9:16:47 AM
Creation date
6/9/2008 1:14:26 PM
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Decision Support Systems
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
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values at most sites. One possible reason for a poor correlation is t11at the Shepherd <br />equation is generalized to represent a best fit among several grain-size data sets that <br />Shepherd used to develop the empirical relationships, but may not be the best <br />possible match to fl1e data collected in this study. <br />Another possible reason for a poor K versus grain-size correlation is that the K tests <br />were conducted on sediment grain-sizes different than that of the samples collected <br />for grain-size analysis and subsequently used in t11e empirical equations altllougll the <br />testing locations were immediately adjacent to the field testing site. This suggests a <br />high degree of localized heterogeneity within a range of inches to feet in the <br />streambed sediments of the South Platte River and its tributaries. Given the <br />constantly changing streamflow velocities in the channel, localized scouring of the <br />streambed and shifting of the active stream channel in most locations, a highly <br />heterogenous sediment grain-size distribution and K test results is reasonable. <br />Regardless of the reasons for a poor correlation between the field and grain size-based <br />methods, the large K values derived from both methods generally agree with field <br />observations of coarse-grained streambed materials and fl1e lack of afine-grained <br />material layer in the streambed of the Sout11 Platte mainstem. This is an important <br />finding that is supported by field observations of the streambed materials and with <br />the test results. <br />3.4 Streambed Vertical Hydraulic Gradient Results <br />Vertical hydraulic gradient data are provided in Table 1 and are mapped in Figure 10. <br />Water levels monitored in the temporary piezometer installed at each location <br />generally indicated downward vertical gradient from the streams into the underlying <br />sediments. Of 131ocations where observable head differences existed during the field <br />activities, two sites had upward vertical gradients ranging from 0.003 to 0.017 ft/ft, <br />located on Plum Creek and Beebe Draw, respectively, iizdicating a gaining stream at <br />these locations. The rema>11>11g 11 sites wit11 detectable head differences 11ad <br />downward gradients ranging from -0.110 to -0.003 ft/ ft, wifll t11e greatest downward <br />gradient observed at the Weldona Gage and the least at the Atwood Gage. However, <br />further investigation of the data measurement from the Weldona Gage (SC-17) <br />indicates t11at its large downward head difference appears to be anomalous. The <br />magnitude of the gradient at this site is inconsistent with the large K value computed. <br />Field conditions suggest t11at the groundwater level in the piezometer may not Have <br />equilibrated. Therefore, the resulting vertical b adient at this location should probably <br />be ignored. Of note are fllat the two locations with the next-highest downward <br />vertical gradients (SC-02 and SC-06) are located near active gravel pits and municipal <br />wells that were pumping at the time the measurements were made, therefore these <br />vertical gradients are also suspect. <br />Of the remainiizg 17 sites, 13 did not Have a detectable head difference and at four <br />sites (all on tributaries) water levels uz the piezometer did not equilibrate duruzg <br />testing. The results indicate that during October and November of 2005, the South <br />Platte River and its tributaries were losing streams at many of fl1e locations tested. <br />T11e losing nature of t11e streams could be a result of lowered groundwater levels <br />Fnlal T~zsl: 3~TM 06-28-07.doc 11 <br />
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