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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
and continue to pose a significant risk of catastrophic failure until such time of the
structures’ eventual rehabilitation, replacement or removal from service.
Because both of the alternatives, including the No Federal Action/Future Without Project,
continue to provide flood protection throughout the project life, the benefits are same for each
alternative.
Benefits for the project were based upon the reduction of flood damages. Flood damages under
both a With Dams and Without Dams scenario were estimated. The difference in flood damages
between the two scenarios can be considered a benefit of retaining flood protection.
To analyze each dam as a single analysis unit, the downstream study area was reviewed by
project hydrologists to provide an approximate demarcation of flooding impacts attributable to
each dam. The study area was divided into four regions: one region for the floodplain of each
dam, and a fourth region where Vineyard FRS and Rittenhouse FRS have overlapping
floodplains (see Figure 1). Benefits for the overlapping floodplain were allocated by each FRS’s
percent control, measured by a comparison of total acre feet retained behind each structure. It
was determined that the storage capacity behind the two dams was nearly identical and benefits
were allocated accordingly at 50% for each dam.
3.4. Costs: Preferred Alternative and No Federal Action Alternative
Cost estimates for each alternative were developed by project engineers in conjunction with
NRCS and the Flood Control District of Maricopa County.
Average annual equivalent costs for each alternative were prepared. Costs were brought to
present value using the 2012 federal water projects discount rate of 3.750 percent, amortized
over the 103-year project analysis period, and described in annual terms for comparison to
project benefits. The net present value calculation assumed that flood protection would remain
in place for all alternatives throughout the project life. Under the No Federal Action Alternative,
Vineyard FRS was assumed to be reconstructed in project year 20, with Powerline and
Rittenhouse reconstructed in years 25 and 30, respectively. For the build alternatives,
construction was assumed to occur in three years. Project administration, permitting and
engineering costs for all alternatives were assumed to occur one year prior to construction.
Annual operation and maintenance costs (O&M)for the Preferred Alternative were estimated by
project engineers. For the No Federal Action alternative, O&M costs were assumed to be
approximately $512,000 per year until the dam is rehabilitated or replaced. After
rehabilitation/replacement, O&M costs were assumed to be equivalent to the Preferred
Alternative O&M estimates.
3.4.1. Benefit-Cost Ratio
For water and related land resources implementation studies, standards and procedures have been
established in formulating alternative plans. These standards and procedures are found in the
Economic and Environmental Principles and Guidelines for Water and Related Land Resources
Implementation Studies (P&G). According to P&G, an alternative that reasonably maximizes
net national economic development benefits is to be formulated. This alternative is to be
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 12