Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
Overview of Hydrogeologic Conditions
The project is located within the Sonoran region of the Basin and Range physiographic province
and is, in part, separated from the central Phoenix Valley by the bedrock highs of the Tempe
Butte area. The following sections discuss aspects of the hydrogeological setting important to an
appraisal of ground subsidence and earth fissuring.
Depth to Bedrock
With regard to the PVR area, the buried shape and proximity of the bedrock-alluvium contact are
likely the most dominant influences on where earth fissures have formed and where they may
form in the future. The Powerline FRS and nearby Hawk Rock are regions where the gradient of
the bedrock-alluvium interface is quite pronounced and the thickness of alluvium is variable.
These conditions result in a strong potential for differential subsidence, a prerequisite for earth
fissure formation. Bedrock is exposed at the ground surface at Hawk Rock, which is located
about 4,000 feet west of the bend in the Powerline FRS embankment near Station 110+00. Hawk
Rock is the surficial expression of a much larger buried mass of bedrock that partially underlies
the Powerline FRS. Understanding this bedrock geometry is one of the most important factors
for delineating earth fissure risks for the PVR Rehabilitation or Replacement Project.
Data indicate that the depth to bedrock at the bend in the Powerline FRS embankment at about
Station 110+00 is close to or slightly greater than about 300 feet below the ground surface (bgs),
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 portion 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. A deep well 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.
InSAR provides what is probably the best indication of the shape of the bedrock-alluvium
interface in the vicinity of Hawk Rock and the Powerline FRS 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 near Hawk Rock, and all the
known fissures not in the immediate vicinity of Hawk Rock, follow the apparent crest of these
implied buried ridges. When comparing InSAR data and U.S. Bureau of Reclamation bedrock
elevation data, it appears that InSAR data more accurately indicate the location of the implied
buried ridges.
Deep Alluvial Characteristics
A review of selected geophysical data and geologic logs indicates that there are significant
lithologic variations present in the alluvial basin profile. The discussion below is summarized
from the Initial Subsidence and Earth Fissure Report (AMEC 2010b) and the reader is referred to
that report for a detailed discussion of the deep alluvial characteristics.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 42