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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
implementation of a preferred alternative needed to address the identified inadequacies. In July
2009, the Sponsor initiated the planning phase. This Plan/EA is being prepared during this
planning phase.
The Powerline FRS is one of three earthen flood retarding structures (Vineyard Road FRS and
Rittenhouse FRS are the other two) located east of the Central Arizona Project (CAP) canal. The
three structures were originally designed by the NRCS to operate as an integrated system as the
principal spillways at Powerline FRS and Vineyard Road FRS both discharge into a common
channel – the Powerline Floodway. The Rittenhouse FRS principal spillway discharges into the
Vineyard Road FRS flood pool. The result is that all three principal spillways eventually flow into
the Powerline Floodway. The Powerline, Vineyard Road, and Rittenhouse FRSs Rehabilitation or
Replacement Project (Project) was initiated to formulate alternatives to mitigate identified dam
safety deficiencies at the three dams. One of the objectives in the formulation of alternatives for the
three dams was to maintain the integrated operational relationship of the three dams (or
alternatives) to discharge to the Powerline Floodway. This Supplemental Watershed Plan and
Environmental Assessment (Plan/EA) presents the alternatives evaluated and preferred alternative
for Powerline FRS. The Powerline FRS is a structural plan element of the Apache Junction-Gilbert
Watershed Work Plan. A separate Supplemental Watershed Plan/EA has been prepared for the
Vineyard Road FRS and Rittenhouse FRS as these two dams are structural elements of the
Williams-Chandler Watershed Work Plan.
2.3 EXISTING DAM CONDITION
2.3.1 LOCATION
The Powerline FRS is located in northwest Pinal County, Arizona. The structure is east of the City
of Mesa and south of the City of Apache Junction. The Powerline FRS is one of three flood
retarding structures designed and constructed by the NRCS to provide flood control benefits to
portions of Maricopa and Pinal County (the other two dams are Vineyard Road FRS and
Rittenhouse FRS). The contributing watersheds originate in the southwest-facing slopes of the
Superstition Mountains. The total watershed area contributing to the three structures is 147 square
miles and includes Weekes Wash and Siphon Draw. Queen Creek Wash is located south of the
structures. The Central Arizona Project Canal (CAP) is aligned west of the structures. The PVR
FRSs alignments extend south from Baseline Road to the Ocotillo Road alignment, approximately
12 miles in length. A project map for the Powerline FRS and associated features is provided in
Appendix B.
The 2.54-mile long Powerline structure, outlet works, and flood pool lie within the following
sections (Gila and Salt River Baseline & Meridian): Township 1 South, Range 8 East, Sections 5
and 8.
The FRS, outlet works and the flood pool are located on land that is owned by the State of Arizona.
The Sponsor has obtained easements from the State of Arizona to operate and maintain the dam and
appurtenant structures. The Powerline FRS provides 100-year flood protection from flood flows
originating from a 47-square-mile watershed. The City of Apache Junction and the State of
Arizona (State Land Department) have prepared or are in the process of preparing land use plans to
areas both upstream and downstream of the dam.
USDA- NRCS Page 2-2 January 2013
Kimley-Horn and Associates, Inc.