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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
2.3.2 EMBANKMENT AND RESERVOIR POOL
The FRS is a homogeneous earthen embankment 13,398 feet (2.54 miles) in length, with a
maximum height of 26.1 feet. The embankment was constructed with local borrow materials from
the upstream low flow channel and flood pool areas. The embankment crest elevation was designed
at 1,591.1 feet, and the auxiliary spillway was designed at 1,585.3 feet to provide 5.8 feet of
freeboard. The reservoir capacity is 4,194 acre-feet which provides for 175 acre-feet of sediment
and 4,019 acre-feet of flood water. At full capacity, the sediment pool area and flood pool area
would be 88 acres and 456 acres, respectively. The maximum recorded impoundment for the
Powerline FRS was 952 acre-feet with a flood stage of 11.0 feet on January 11, 1993. Other
historical storms of note include an October 1972 storm, where the reservoir stage may have
reached the auxiliary spillway crest and floodwaters may have flowed through the auxiliary
spillway. The FRS flood pool under normal operations is dry, only temporarily impounding water
in response to runoff events. The FRS does not have a permanent water pool.
A central filter was installed in the Powerline FRS in 1991 which extends along the entire length of
the embankment (from Stations 17+83 to 150+70). The filter has an average depth of 18 feet and a
maximum depth of 42 feet. The depth of the filter was established by the NRCS on the basis of its
1986 crack location investigation.
A typical section of the Powerline FRS embankment is provided in Appendix C as Figure C-1.
2.3.3 PRINCIPAL SPILLWAY
Flows entering the impoundment are routed through a low-flow channel to the principal spillway.
The spillway is an ungated, 156-foot long, 36-inch diameter RCP located at the southern abutment
of the FRS. The pipe is situated on a concrete cradle and five seepage cutoff collars are spaced at
20-foot intervals along the pipe. At the upstream inlet of the spillway, there is a trash rack to filter
debris and the spillway discharges into a concrete lined channel through an outlet structure. The
downstream outlet of the principal spillway features an energy dissipater, and the principal spillway
discharge capacity is 203 cubic feet per second (cfs) at the crest elevation of the auxiliary spillway.
An exhibit of the Powerline FRS principal spillway is provided in Appendix C as Figure C-2.
2.3.4 AUXILIARY SPILLWAY
The Powerline FRS auxiliary spillway is located at the southern abutment of the FRS. The
spillway is a 600-foot wide excavated earthen open channel with a capacity of 25,000 cfs. The
control section of the auxiliary spillway is a compacted, benched sill, located one foot above a level
upstream approach channel. The approach channel is on a constructed bend that directs flow
around the southern abutment of the FRS.
An exhibit of the Powerline FRS auxiliary spillway is provided in Appendix C as Figure C-3.
2.3.5 POWERLINE NORTH DIVERSION CHANNEL
The Powerline North Diversion Channel is located north of the FRS as shown in Appendix C. The
Powerline Diversion begins at Southern Avenue and flows between Desert Shadows Middle School
and Apache Junction High School. The diversion mostly consists of a gently sloping swale lined
with a 5-foot-wide section of concrete. Near the southern limit of the schools, the swale transitions
USDA- NRCS Page 2-3 January 2013
Kimley-Horn and Associates, Inc.