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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
are transported to the distal portions of fans. As gradients flatten toward the distal portions of fans,
sediments become finer grained and less permeable, and drainage channels often converge.
The Powerline, Vineyard Road, and Rittenhouse FRS have been the subject (particularly Powerline
and the northern 1/3 of Vineyard Road FRS) of previous geohazard investigations and analyses
including studies as part of this planning study. The Powerline and Vineyard Road FRS are located
in a region of local land subsidence. The purpose of the geohazard investigations were to evaluate
potential impacts of ground subsidence and earth fissuring on the existing structures and on the
selection and subsequent development of rehabilitation designs and/or replacement alternatives for
flood protection afforded by the Powerline, Vineyard Road and Rittenhouse Flood Retarding
Structures (FRS). The results, findings and conclusion of the studies were used by the project team
in its development and evaluation of dam rehabilitation and/or replacement alternatives.
There are known earth fissures in the project area near Hawk Rock and one known earth fissure
located immediately downstream of the Powerline FRS embankment at about Station 115+45.
There also is a high probability that the earth fissure near Station 115+45 extends beneath the
existing embankment. It is suspected that subsidence patterns at the Powerline FRS will continue
into the future, potentially causing further development of fissures in this area.
The studies have developed, through various techniques and methods, earth fissure risk zone
mapping for the PVR structures. Figure C-8 in Appendix C is the earth fissure risk zones
prepared for the PVR structures. As depicted in the risk zone map, Powerline FRS has portions of
the dam within all four zones. Vineyard Road FRS is within two zones, and Rittenhouse FRS falls
within one zone (the lowest probability zone). The zones are defined below.
In general terms, the earth fissure risk delineating process has evolved to include 4 general earth
FRZs that are broadly similar from site to site, although the exact definitions of the FRZs vary from
one site to another. The numbering system used to identify the four zones has changed through time
and from site to site, so a better understanding of the level of risk is provided by the actual
definition than by the zone number. The four FRZ definitions can be summarized as follows:
• Zone 1: An area with a known earth fissure(s), a high probability for the presence of earth
fissures without surface expression, or a high probability for the future development of earth
fissures.
• Zone 2: An area that in the future is likely to experience tensional ground strain that could
cause earth fissures, yet no earth fissures have been identified to date. An area with a
moderate probability for the future development of earth fissures.
• Zone 3: An area that has or is likely to experience some tensional ground strain, though not
great enough to result in earth fissures. An area with a low to moderate probability for the
future development of earth fissures.
• Zone 4: An area that has not, and is not expected to, experience tensional ground strain. An
area with a low probability for the future development of earth fissures.
The key terms used in the FRZ definitions are the presence of known fissures or the high, moderate,
low to moderate or low probability for the future development of earth fissures.
USDA- NRCS Page 4-2 January 2013
Kimley-Horn and Associates, Inc.