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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
Earth Fissure Risk Delineation
Using the data and analytical techniques discussed above, earth Fissure Risk Zones (FRZ) were
developed. The risk zones were first delineated for the Powerline FRS and the northern portion
of the Vineyard Road FRS as part of a previous investigation. The risk zones were then updated
to account for the earth fissure discovered downstream of the Powerline FRS embankment at
about Station 115+45. The risk zones around the Powerline FRS, the proposed Powerline IDSM
and the northern third of the Vineyard Road FRS did not change as a result of this investigation.
However, the earth FRZ were extended south to include the remainder of the Vineyard Road
FRS and the Rittenhouse FRS and extended north to include the Powerline Diversion.
5.1.1. Geological Setting
The project site is within the Sonoran region of the Basin and Range physiographic province and
is, in part, structurally separated from the central Phoenix valley by the bedrock highs of the
Tempe Butte area. The study area lies within the Mesa-Chandler sub-basin, adjacent to the
Superstition and Goldfield Mountains, which are composed of metamorphic and igneous
bedrock. This sub-basin contains basin-fill deposits of the Salt River Valley that can be
subdivided into three units: lower alluvial unit (LAU), middle fine-grained unit (MFGU) and
upper alluvial unit (UAU).
The surficial geology for the PVR project is discussed in the Draft Supplemental Earth Fissure
Risk Technical Memorandum prepared by AMEC on behalf of the District. The following
sections discuss aspects of the geologic setting important to an appraisal of ground subsidence
and earth fissuring, such as depth to bedrock and deep alluvial characteristics.
Basin Geometry and Depth to Bedrock
The buried shape and proximity of the bedrock-alluvium contact are likely the most dominant
influences on where earth fissures form around Hawk Rock and near the Powerline FRS. These
are regions where the gradient of the bedrock-alluvium interface is quite pronounced, with the
thickness of the alluvium being variable. These conditions result in a differential subsidence
profile, a prerequisite to earth fissure formation. Bedrock is present at the surface at Hawk Rock,
which is about 4,000 feet west of the bend in the FRS embankment near Station 110+00.
Understanding this bedrock geometry is one of the most important factors for delineating earth
fissure risks for the project.
There are four data sets that either directly or indirectly estimate the depth to bedrock or shape of
the bedrock-alluvium interface in the vicinity of the project: 1) Bouguer gravity data, 2) depth to
bedrock, 3) bedrock elevations, 4) InSAR, and 5) geophysical measurements.
Bouguer Gravity Data
The Bouguer gravity anomaly represents corrected surficial gravity measurements. Differences
in the data represent differences in subsurface density. The most significant variation in
subsurface density is that between bedrock and overlying alluvium.
The gravity data indicate that the depth to bedrock increases to the north and northeast of Hawk
Rock. These data also indicate that the depth of bedrock and basin thickness increases to the
south and southeast of Hawk Rock.
ADWR is currently preparing a three-dimensional (3D) model of gravity data for the
Superstition Vistas area. This study includes the project and Hawk Rock areas. The gravity data
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 30