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

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Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment 3.2. Hydraulic Model Hydraulic modeling of the downstream inundation area with and without the PVR structures was prepared using the FLO-2D flood routing model, version 2009.06. A 500-foot by 500-foot grid was developed for the computational domain area and ground elevations were obtained from the District’s 10-foot topography with supplemental 2-foot contour data sets from the District. Further details regarding the hydrologic and hydraulic modeling conducted for the project can be found in the following references: • “PVR Existing and Future Conditions Hydrology and Hydraulics Update Technical Report”, Kimley-Horn and Associates, Inc., November 2010. • “PVR Downstream Inundation Study”, JE Fuller, Inc., November 2010. Appendix A of the technical memorandum for floodplain delineation mapping shows the inundation limits for the 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year storm events for each scenario. 3.3. Benefits: Preferred Alternative and No Federal Action Alternative Benefits were estimated for the following two alternatives: • Alternative 8A: Replace Powerline FRS, Rehabilitate Vineyard Road FRS, and convert Rittenhouse FRS to a levee (Preferred Alternative) • No Federal Action Alternative (Future Without Project) The No Federal Action/Future Without Project represents the sponsor’s most likely course of action in the absence of federal funding. For this study, the “No Federal Action/Future Without Project Alternative” was defined as the following: The District would continue to operate and maintain the Powerline, Vineyard Road, and Rittenhouse Flood Retarding Structures under existing conditions until the end of the structures’ design lives. The structures would continue to have identified safety deficiencies and would not meet current NRCS and ADWR performance standards. Under this alternative, the structures would continue to provide flood protection until and if there is a sudden breach event. This condition is unacceptable to the District and NRCS. The District has indicated that without federal financial assistance, sufficient funds are not available to rehabilitate or replace the structures in a timely manner. The District recognizes, however, that there remains a need to address the existing safety deficiencies. Therefore, the District would eventually rehabilitate or replace the dams one at a time as non-federal funds become available. The District estimates that rehabilitation/replacement using local funds would occur (after approval of the Supplemental Watershed Plan) for the first structure in year 20, the second structure in year 25, and the third structure in year 30. This alternative is not acceptable to the Sponsors and local community as a viable solution, since the structures would continue to have safety and performance deficiencies USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 11
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
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