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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
analyses using both low-sun-angle aerial photography and high-resolution digital aerial imagery
for the purpose of identifying potential earth fissures have been performed at Powerline FRS by
AMEC in previous studies.
As part of the current study, aerial photographs and digital images were evaluated for purposes
of identifying features indicative of the presence of earth fissures. These features include
elongated fissure gullies, alignments of potholes and other small depressions, lineations in the
vegetative cover and subtle linear ground features caused by shading. No conclusive evidence for
earth fissuring was identified, except for the known earth fissure downstream of the Powerline
FRS embankment at about Station 115+45 and the known earth fissures in the vicinity of Hawk
Rock to the west of the Powerline FRS.
Geologic Reconnaissance
A ground reconnaissance was performed to inspect the project area for evidence of ground
subsidence or earth-fissure-related features. Select lineaments within the study area with the
potential to impact proposed facilities were observed from the ground. Very few lineaments
outside of the immediate vicinity of Powerline FRS were selected for direct ground observation.
No earth fissures or features indicative of possible earth fissures were observed at the project
site, excepting previously known earth fissures in the vicinity of the Powerline FRS and Hawk
Rock. As a consequence, slight adjustments were made to the alignment of some of the features
and other features were eliminated, particularly features that were identified as being cultural in
origin.
Deep Resistivity Profiling
Resistivity interpretations do not result in unique solutions; therefore, the interpreted results are
approximate. However, reasonable trends are apparent that can assist in understanding the basin
subsurface. Two-layer interpretations, typically used for a shallower and deeper interface, were
performed. Shorter array spacing data were used to develop two-layer interpretations of the
shallower subsurface. The longest array spacing data were used to develop two-layer
interpretations in the deeper subsurface, and when appropriate, for an intermediate interface or to
check against the possibility of high-resistivity deep bedrock. These interpretations provide
general resistivity values and ranges of anticipated depth interfaces within the basin subsurface
to depths of several hundred feet to as great as about 1,000 feet.
Future Subsidence Prediction
Based on the subsurface information available from literature, existing geophysical well logs and
from the surface geophysical measurements, simplified basin vertical profiles were developed to
estimate historic subsidence and the potential for future subsidence. It was assumed that basin
material compression leading to subsidence was limited to the portions of the upper or middle
alluvial units that were below the groundwater table; contributions of bedrock or the lower
alluvial unit were assumed to be negligible. Estimates of subsidence could then be calculated
from the resulting basin vertical profiles.
Based on these profiles, simplified estimates of subsidence were performed using the methods
and procedures discussed in the AMEC white paper “Characterization for Subsidence Modeling
and Percolation Theory–Based Modeling of Subsidence”.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 29