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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
The concept for Alternative 2 included reducing the existing structures to non-jurisdictional (by
ADWR definition) size with auxiliary spillway heights of six feet. The Powerline IDSM alignment
will be maintained for the reduced height Powerline structure. Upstream of the reduced structures,
detention basins sized for the 100-year event would be constructed outside of the mesquite bosques
to help preserve and protect the bosques. The basins would drain through the bosques and the basin
bottom depth will be controlled by the existing principal outlet location. The existing conditions
100-year 24-hour storage volume from the existing conditions 100-year HEC-1 models was used to
size each of the basins, in addition to the 100-year sediment volume. No FRS rehabilitation is
included.
To evaluate the required excavation volume, the existing flood volume behind the FRS up to the
spillway crest height of six feet was used as flood storage and was not included in the volume
required for basin excavation. The remaining required storage volume would be provided in the
excavated basins where the bottom depth of each basin is controlled by the existing principal outlet
location. The basins were sized such that the footprint of each basin was contained within the
existing modified easement. The estimated required basin areas and depths without freeboard are
289 acres and ten feet for Powerline, 385 acres and six feet for Vineyard Road, and 538 acres and
seven feet for Rittenhouse FRS. The basins will be excavated an additional three feet each in depth
to include the 100-year sediment storage.
The opinion of probable cost for converting each FRS to a basin and reducing the dam heights to
non-jurisdictional dams (as considered in the Level I analysis) is $190,690,000. This alternative
was eliminated from further detailed study in the workshop conducted on April 26, 2011 due to
high costs, operational complexity, lower flood protection level (100-year), and handling and
storage of excess earthwork spoils.
Decommissioning Powerline, Vineyard Road, and Rittenhouse FRS by Converting all Three Dams
to Levees that Drain south to a Basin that Outlets to Queen Creek (Alternative 3).
The concept for Alternative 3 included converting Powerline and Vineyard Road FRSs to levees to
meet FEMA and District criteria. Additionally, Rittenhouse FRS would be removed and a levee
would be constructed further west and adjacent to the CAP canal. A 100-year detention basin
would be constructed at the south end of the project with a gated outlet to Queen Creek Wash.
A new gated at-grade outlet would be constructed at the south end of the 100-year basin to drain the
basin to Queen Creek Wash. A new drainage channel would be constructed between the basin
outlet and a new CAP overchute near Queen Creek Wash. A gated trash rack would be provided at
the new outlet of the basin, and a new basin overflow spillway would be constructed to discharge
flow in the same general flowpath as the existing Rittenhouse auxiliary spillway. The Powerline
Floodway would not be used under this alternative, but may be utilized as an alternative outlet.
The invert elevation of the CAP overchutes at Queen Creek Wash is approximately 1,566 feet. In
comparison, the inverts of the principal spillway outlets at Powerline and Vineyard Road FRSs are
1,563.6 feet and 1,563.8 feet, respectively, lower than the invert elevation of the CAP overchutes at
Queen Creek Wash. The low point of the PVR system and the existing CAP overchutes at Queen
Creek Wash are separated by approximately 8.5 miles, so even if the overchutes were lowered three
feet, the flowpath south to Queen Creek Wash would be effectively flat. The primary constraint of
Alternative 3 is achieving positive drainage along the existing structure alignments to drain south to
USDA- NRCS Page 5-4 January 2013
Kimley-Horn and Associates, Inc.