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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
identified as the National Economic Development (NED) plan. A comparison of the alternatives
was provided in the technical memorandum.
The Preferred Alternative (8A) is the plan that reasonably maximizes the net benefits and is
therefore identified as the NED plan. It can be noted that for all three dams, the net benefits
shown in the tables are higher for the No Federal Action alternative than for the Preferred
Alternative. This occurs because the No Federal Action alternative assumes that the dams would
not be rehabilitated or replaced until 20-30 years into the future. In the interim, the dams would
continue to provide flood protection benefits. Therefore, the costs of No Federal Action are
lower, while the benefits remain the same. However, the No Federal Action alternative does not
meet the purpose and need of the project because it does not remove the safety risk presented by
the aging dams for two decades into the future. This is judged to be an unacceptable risk to
human life and property by NRCS and the sponsor. For dam rehabilitation projects with risk to
human life, NRCS policy defines the NED Alternative as the “federally assisted alternative with
the greatest net economic benefits.” Therefore, the Preferred Alternative (8A) has been
identified as the NED alternative.
The Preferred Alternative results in a benefit to cost ratio of 0.1 to 1.0 for Powerline FRS, a ratio
of 0.1 to 1.0 for Vineyard FRS, and a ratio of 0.1 to 1.0 for Rittenhouse FRS. The benefit to cost
ratio for the dams are less than 1.0, but still provides the greatest net benefits (or fewest negative
net benefits) of any alternative that meets the purpose and need of the project.
3.4.2. Risks and Uncertainty
The areas of risk and uncertainty associated with the economic analysis include uncertainty
associated with the hydrologic and hydraulic modeling, simplifying assumptions used to run the
URB-1 model, and uncertainty related to the future changes in cost estimates as engineering
design progresses.
Hydrologic modeling over a wide swath of floodplain introduces an element of uncertainty in
estimating attenuation and resulting peak flows. The hydraulics analysis relies partially on
engineering judgment and experience to designate ineffective/effective flow areas over the wide
and shallow floodplain of the study area.
Several simplifying assumptions were used to create input for the URB-1 model. Parcel
elevations were determined using topographic contour data. Residential parcels were
proportionally assigned as either a single story or two-story residence based on the fieldview
determination that these two housing types were interspersed throughout the study area. Content
values for parcels were estimated based on a percentage of their tax assessed structure value.
The structure and contents value for some parcels were estimated using Marshall Valuation
Service data. While these assumptions represented the best methodology available within the
scope of the analysis, the model results may slightly overestimate or underestimate damages due
to the simplifying assumptions used to create the input data.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 13