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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
8.0 THE PREFERRED ALTERNATIVE
8.1 RATIONALE FOR ALTERNATIVE PREFERENCE
The Preferred Alternative is Alternative 2 – Decommissioning. The decommissioning will be
accomplished by replacing the Powerline FRS and Powerline IDSM with a 100-year channel to
discharge to the Vineyard Road FRS. The decommissioning alternative will meet the plan purpose
of flood prevention and the Sponsor’s formulation goal of providing continued flood protection and
eliminating the existing safety deficiencies associated with the Powerline FRS. The existing FRS
would no longer be defined as a jurisdictional dam by ADWR. A plan schematic/exhibit for
Alternative 2 is included in Appendix C.
The NRCS has identified in their memorandum with the subject of “NED Alternative and Preferred
Alternative for the PVR FRS Rehabilitation Project” (NRCS, Jan. 2013) that the NED alternative
(least-cost technically-feasible) is identified as consisting of the following major components and
filter options for the Powerline FRS, Vineyard Road FRS and Rittenhouse FRS:
• Replace Powerline FRS with 100-year channels with hardened segments;
• Raise and rehabilitate Vineyard Road FRS with the ‘hybrid” filter option;
• Convert Rittenhouse FRS to a levee.
The approximate total project cost for this alternative/filter options is $83M as noted in the Level
III report and is reported as the least cost alternative.
Consistent with P&G, the alternative that has the highest positive net benefits would be identified
as the NED Plan. However, per Section 502.2 of the NRCS National Watershed Program Manual,
“In cases where human life is at risk in the event of catastrophic failure of an existing dam that does
not meet current safety and performance standards, an overriding reason for an exception to the
NED plan requirement exists. To avoid seeking individual exceptions in such cases, the NED plan
is defined as the federally assisted alternative with the greatest net economic benefits.”
Section 5.2 presented a discussion of Alternatives 8B and 8A in relation to the Preferred
Alternative and the NED Plan. As noted Alternatives 8A and 8B were carried forward into a Level
III analysis (Level III Final Alternatives Summary Report – KHA January 2013). The Level III
analysis conducted further evaluations of Alternatives 8A and 8B. Both alternatives are identical in
the replacement of Powerline FRS with a new 100-year flood channel. The alternatives differ in the
treatment of Rittenhouse FRS and in the filter treatments for Vineyard Road FRS and Rittenhouse
FRS in Alternative 8B. The Level III report identified the Preferred Alternative as Alternative 8A
with the option of a ‘hybrid’ filter for the dam rehabilitation of Vineyard Road FRS, converting the
Rittenhouse FRS to a levee, and replacing Powerline FRS with a 100-year channel.
The Preferred Alternative and the NED Plan for the Powerline Channel is one in the same for this
Plan/EA. The Preferred Alternative and NED Plan for the Vineyard Road FRS and Rittenhouse
FRS are provided in the “Draft Supplemental Watershed Plan and Environmental Assessment for
the Vineyard Road FRS and Rittenhouse FRS” (Kimley-Horn January 2013).
USDA- NRCS Page 8-1 January 2013
Kimley-Horn and Associates, Inc.