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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
to an approximately 8-foot-deep by 10-foot-wide channel. The channel is unlined and there is an
approximately 5-foot-high earthen embankment on the west side of the channel. The diversion
channel extends from the school properties to Baseline Road, crossing under U.S. Highway 60
through two 6-foot-high by 10-foot-wide concrete box culverts and crossing under Baseline Road
through three 4-foot-diameter corrugated metal pipes. The channel flows along the east side of the
Apache Creek Golf Course between U.S. Highway 60 and Baseline Road. The channel within this
area appears to be engineered and ranges from approximately 10 to 20 feet in width and 5 to 15 feet
in depth. The banks typically have slope ratios of 1H:1V to 1.5H:1V. In general, the bank on the
east side of the channel is higher than the bank on the west side of the channel, and the bank on the
east side of the channel is cut by a number of erosion gullies.
An exhibit of the Powerline North Diversion Channel is provided in Appendix C as Figure C-8.
2.3.6 POWERLINE FLOODWAY
Outflow from the Powerline, Vineyard Road and Rittenhouse FRSs eventually flow into the
Powerline Floodway. The Floodway is approximately 8.5 miles in length, extending from the
principal outlet of Powerline FRS to the East Maricopa Floodway (EMF) near the northwest corner
of the Williams Gateway Airport. The Powerline Floodway carries flows from the principal
spillways of both Powerline FRS and Vineyard Road FRS to the EMF.
2.4 INADEQUACIES OF STRUCTURE
The Arizona Department of Water Resources (ADWR) conducts inspections of jurisdictional dams
throughout the state. Powerline and Vineyard Road FRSs were inspected on January 30, 2012 and
Rittenhouse was inspected on January 31, 2012. ADWR issued a Notice of Safety Deficiency letter
for Powerline FRS. The following items were listed as Safety Deficiencies for Powerline FRS:
• Foundation Concerns
• Questionable Adequacy of Central Filter
ADWR identified one maintenance action item for the Powerline FRS. ADWR noted that the action
item as removing rodent burrowing on the structure in a timely manner.
ADWR has reviewed the size and hazard potential classifications for the dam. The classification
has not changed from intermediate size and high hazard potential.
2.4.1 FOUNDATION CONCERNS
In May 2007, an earth fissure was identified in close proximity to the downstream toe of the dam.
In light of the proximity of this earth fissure to the Powerline embankment, and the presence of
other documented earth fissures in the general vicinity of the dam, ADWR continues to classify
Powerline FRS as being in an “unsafe, non-emergency, elevated risk” condition. As such, the
Powerline FRS is one of ADWR’s highest priority unsafe dams in the state. On February 28, 2011,
ADWR approved the District’s permit application for the interim mitigation of fissure risk.
USDA- NRCS Page 2-4 January 2013
Kimley-Horn and Associates, Inc.