Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ... Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment analyzed for the ADWR study represent a much greater density of data than obtained from previous studies and includes historical gravity data acquired by the USBR and USGS in the vicinity of Hawk Rock and the Powerline FRS. Laney and Hahn Depth to Bedrock The depth to bedrock developed by Laney and Hahn is based on limited data from wells drilled in the area, geologic mapping and gravity data. These data were developed on a regional scale and do not reflect local variations in the depth to bedrock. This interpretation does provide a reasonable estimation of the depth of the bedrock-alluvium interface on a large scale, but the presentation does not show variations at a scale that is useful for delineating earth fissure risk. The depth to bedrock presented in Laney and Hahn provides a better indication of the depth to bedrock than provided from Bouguer gravity data alone. However, the Laney and Hahn work is concerned mainly with broad, regional variations, and the 400-foot contour interval used is relatively large. Those data also indicate that the depth to bedrock is increasing to the northeast and south of Hawk Rock. The data indicate that the depth to bedrock is relatively shallow in the vicinity of the Powerline FRS and the Powerline FRS IDSM and increases to depths greater than 1,200 feet for most of the Vineyard Road FRS alignment, for the entire length of the Rittenhouse FRS, and along the Powerline Diversion. USBR Bedrock Elevation Data and Well Data As part of the original investigations of subsidence and earth fissuring for the CAP Canal that were performed by the USBR and USGS, contours of bedrock elevation were developed for the area around Hawk Rock, including much of the area around the Powerline FRS and the northern portion of the Vineyard Road FRS. These contours were developed from an interpretation of deep seismic refraction and gravity surveys performed by the USGS and USBR. These data were never finalized and did not undergo a full review process. However, the contours do provide an informed and valuable local interpretation of the shape of the bedrock-alluvium interface in this portion of the project. For purposes of the following discussion, the ground surface in the area is assumed to be at an elevation of 1,500 feet. Actual elevations within the project area generally range from an elevation of 1,560 feet in the vicinity of the CAP Canal to an elevation of over 1,600 feet at the Rittenhouse FRS crest. The USBR data indicate that the depth to bedrock at the bend in the Powerline FRS embankment at about Station 110+00 is about 300 feet below the ground surface (bgs). It should be noted that more dense or more cemented material was encountered at a depth of 330 feet bgs, and schist bedrock was encountered at a depth of 335 feet bgs. The USBR data indicate that the depth to bedrock from about Powerline Station 50+00 to 105+00 ranges from about 600 to 700 feet bgs, and that north of Station 110+00 the depth to bedrock increases rapidly. These data indicate that depth to bedrock increases very rapidly south of the Powerline FRS. Underneath the northern portions of the Vineyard Road FRS, the depth to bedrock ranges from about 800 feet bgs at Station 345+00 to about 1,700 feet bgs at Station 310+00 to approximately 1,500 feet bgs at Station 280+00. The next deep well to the southeast at about Vineyard Road FRS Station 271+00, encountered conglomerate at a depth of 650 feet bgs and granite at a depth of 1,770 feet bgs. Southward from this point, the data indicate that depth to bedrock continues to increase to depths greater than 2,000 feet bgs. USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 31
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment InSAR InSAR provides what is probably the best indication of the shape of the bedrock-alluvium interface in the vicinity of Hawk Rock due to its full map-view and 3D coverage. The most important aspect of this is the indication of the presence of buried bedrock ridges that radiate outward from Hawk Rock and how those implied buried ridges relate to the location of known earth fissures. Most of the known earth fissures and all the known fissures not in the immediate vicinity of Hawk Rock follow the apparent crest of these implied buried ridges. When comparing the InSAR data and USBR bedrock elevation data, it appears that the InSAR data more accurately indicate the location of the implied buried ridges. Geophysical Measurements As part of previous and ongoing investigations in the Powerline FRS area, geophysical studies provide some indirect measurement of the depth to bedrock. As important to the detection of bedrock in the subsurface profile are measurements that do not indicate bedrock is present within the depths of investigation. As part of this study, a deep resistivity sounding was performed at about the Powerline IDSM Station 21+00. The interpreted depth to bedrock from this sounding is approximately 1,000 feet bgs. Several deep resistivity profiles and deep ReMi profiles were performed as part of the Earth Fissure Risk Zone Investigation Report, Powerline and Vineyard Flood Retarding Structures, Pinal County, Arizona (AMEC 2006). A resistivity profile centered north-south on Powerline FRS Station 135+00 indicates that the depth to bedrock may be greater than 1,000 feet bgs, while a profile centered north-south on Powerline FRS Station 115+00 does not indicate bedrock within a depth of investigation of about 300 feet bgs. Deep ReMi profiles performed at about Powerline FRS Stations 85+00, 110+00, 115+00 and 135+00 indicate that the depth to bedrock is greater than 300 feet bgs at each of those locations. Other deep resistivity soundings performed as part of this investigation did not indicate bedrock within the depth of investigation of approximately the greatest electrode spacing of 1,000 feet. Those results are consistent with the historic deep wells SG-6, SG-10 and SG-12, where bedrock was encountered at depths greater than 1,600 feet, or not encountered at all, 5.1.2. Deep Alluvium Characteristics The magnitude of ground subsidence and the location of earth fissures throughout the project area are controlled by the interactions of groundwater decline and the material characteristics of the compressible alluvial basin materials on the geometry of the underlying alluvial sediment/bedrock interface. The thickness of the basin alluvial materials is highly variable across the project area, ranging from none at Hawk Rock (less than 1 mile west of the Powerline FRS) to depths of 1,600 feet bgs at the southern end of the Vineyard FRS and 2,000 feet bgs at the Rittenhouse FRS. Variations in subsidence appear to reflect variations in both the depth to bedrock and the composition of the deep basin materials. Areas of minimal subsidence include the vicinity of Station 110+00 on Powerline FRS and the vicinity of Station 292+00 on Vineyard Road FRS. While shallow bedrock is clearly an influence at Powerline FRS Station 110+00, shallow bedrock does not appear to be present at or near Vineyard Road FRS Station 292+00 to minimize subsidence in that area. Even though deeper bedrock depths provide more presumably compressible alluvial materials under the Vineyard FRS compared with the Powerline FRS, considerably more subsidence is occurring in the USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 32
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<strong>Powerline</strong> <strong>Flood</strong> Retarding Structure<br />
Pinal County, AZ<br />
Draft Supplemental Watershed <strong>Plan</strong><br />
<strong>and</strong> <strong>Environ</strong>mental <strong>Assessment</strong><br />
InSAR<br />
InSAR provides what is probably the best indication of the shape of the bedrock-alluvium<br />
interface in the vicinity of Hawk Rock due to its full map-view <strong>and</strong> 3D coverage. The most<br />
important aspect of this is the indication of the presence of buried bedrock ridges that radiate<br />
outward from Hawk Rock <strong>and</strong> how those implied buried ridges relate to the location of known<br />
earth fissures. Most of the known earth fissures <strong>and</strong> all the known fissures not in the immediate<br />
vicinity of Hawk Rock follow the apparent crest of these implied buried ridges. When comparing<br />
the InSAR data <strong>and</strong> USBR bedrock elevation data, it appears that the InSAR data more<br />
accurately indicate the location of the implied buried ridges.<br />
Geophysical Measurements<br />
As part of previous <strong>and</strong> ongoing investigations in the <strong>Powerline</strong> FRS area, geophysical studies<br />
provide some indirect measurement of the depth to bedrock. As important to the detection of<br />
bedrock in the subsurface profile are measurements that do not indicate bedrock is present within<br />
the depths of investigation.<br />
As part of this study, a deep resistivity sounding was performed at about the <strong>Powerline</strong> IDSM<br />
Station 21+00. The interpreted depth to bedrock from this sounding is approximately 1,000 feet<br />
bgs. Several deep resistivity profiles <strong>and</strong> deep ReMi profiles were performed as part of the Earth<br />
Fissure Risk Zone Investigation Report, <strong>Powerline</strong> <strong>and</strong> Vineyard <strong>Flood</strong> Retarding Structures,<br />
Pinal County, Arizona (AMEC 2006). A resistivity profile centered north-south on <strong>Powerline</strong><br />
FRS Station 135+00 indicates that the depth to bedrock may be greater than 1,000 feet bgs, while<br />
a profile centered north-south on <strong>Powerline</strong> FRS Station 115+00 does not indicate bedrock<br />
within a depth of investigation of about 300 feet bgs. Deep ReMi profiles performed at about<br />
<strong>Powerline</strong> FRS Stations 85+00, 110+00, 115+00 <strong>and</strong> 135+00 indicate that the depth to bedrock<br />
is greater than 300 feet bgs at each of those locations.<br />
Other deep resistivity soundings performed as part of this investigation did not indicate bedrock<br />
within the depth of investigation of approximately the greatest electrode spacing of 1,000 feet.<br />
Those results are consistent with the historic deep wells SG-6, SG-10 <strong>and</strong> SG-12, where bedrock<br />
was encountered at depths greater than 1,600 feet, or not encountered at all,<br />
5.1.2. Deep Alluvium Characteristics<br />
The magnitude of ground subsidence <strong>and</strong> the location of earth fissures throughout the project<br />
area are controlled by the interactions of groundwater decline <strong>and</strong> the material characteristics of<br />
the compressible alluvial basin materials on the geometry of the underlying alluvial<br />
sediment/bedrock interface. The thickness of the basin alluvial materials is highly variable across<br />
the project area, ranging from none at Hawk Rock (less than 1 mile west of the <strong>Powerline</strong> FRS)<br />
to depths of 1,600 feet bgs at the southern end of the Vineyard FRS <strong>and</strong> 2,000 feet bgs at the<br />
Rittenhouse FRS. Variations in subsidence appear to reflect variations in both the depth to<br />
bedrock <strong>and</strong> the composition of the deep basin materials.<br />
Areas of minimal subsidence include the vicinity of Station 110+00 on <strong>Powerline</strong> FRS <strong>and</strong> the<br />
vicinity of Station 292+00 on Vineyard Road FRS. While shallow bedrock is clearly an influence<br />
at <strong>Powerline</strong> FRS Station 110+00, shallow bedrock does not appear to be present at or near<br />
Vineyard Road FRS Station 292+00 to minimize subsidence in that area. Even though deeper<br />
bedrock depths provide more presumably compressible alluvial materials under the Vineyard<br />
FRS compared with the <strong>Powerline</strong> FRS, considerably more subsidence is occurring in the<br />
USDA- NRCS <strong>Jan</strong>uary <strong>2013</strong><br />
Kimley-Horn <strong>and</strong> Associates, Inc. Page 32
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