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