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 5.4 ALTERNATIVES DESCRIPTION Two alternative plans were identified for detailed study and are noted from this point on as Alternative 1 and Alternative 2 for the Powerline FRS. For clarification Alternative 2 is Alternative 8A/8B from the Level II and Level III reports. The difference between 8A and 8B is the treatment of Rittenhouse FRS. Rittenhouse is converted to a levee in 8A and is a rehabilitated dam in 8B. In both 8A and 8B, Powerline FRS is replaced with a 100-year channel to discharge to Vineyard Road FRS as described below. The description of the alternative below is only for Powerline FRS for this Plan/ EA. The two alternative plans for Powerline FRS are: • Alternative 1 – No Action Plan/Future Without Project: Powerline FRS remains in unsafe condition until Sponsor’s eventual Rehabilitation or Replacement of the dam. • Alternative 2 – Decommissioning: Convert the Powerline FRS into a channel to discharge into the rehabilitated Vineyard Road FRS. These alternatives are discussed in detail below. Cost estimates for the alternatives are provided in Appendix E. 5.4.1 ALTERNATIVE 1 – NO ACTION PLAN/FUTURE WITHOUT PROJECT The “No Action Alternative” is required under the NRCS National Watershed Manual for a dam rehabilitation planning project. This alternative is also known as the “No Action Alternative/Future Without Project Plan” which is defined as the most likely condition absent the Action Alternatives/Future With Project Plans under current consideration. Under the “No Action/Future Without Project Alternative” the Sponsor would continue to operate and maintain the Powerline, Vineyard Road, and Rittenhouse Flood Retarding Structures beyond the end of the structures’ design lives. The structures would continue to be operated as under existing conditions of the dams with the noted dam safety deficiencies. The structures would continue to have identified safety deficiencies and would not meet current NRCS and ADWR performance standards. The Sponsor has indicated that without federal financial assistance, sufficient funds are not available to rehabilitate or replace the structures in a timely manner. The Sponsor recognizes, however, that there remains a need to address the existing safety deficiencies. Therefore, the Sponsor 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 for the first structure in year 20, the second structure in year 25, and the third structure in year 30 (after the approval date of this Supplemental Watershed Plan). This alternative is not acceptable to the Sponsor and local community as a viable solution since it does not meet the purpose and need to reduce the risk of loss of life due to catastrophic dam failure. Action is needed to address public health and safety issues surrounding a flood control dam that does not meet existing safety and performance standards for a high hazard potential structure. The structures would continue to have safety and performance deficiencies and continue to pose a significant risk of catastrophic failure until eventual rehabilitation or replacement of the structures. The annual operation and maintenance costs for the No Action alternative are $212,700 (see NRCS Table 4 in Section 8.0). USDA- NRCS Page 5-10 January 2013 Kimley-Horn and Associates, Inc.
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment 5.4.2 ALTERNATIVE 2 –DECOMMISSIONING: CONVERT THE POWERLINE FRS INTO A CHANNEL TO DISCHARGE INTO THE VINEYARD ROAD FRS Alternative 2 would decommission the Powerline FRS and Powerline IDSM and replace the dam with a channel (for the purposes of nomenclature this channel is termed the Powerline Channel) designed for the 100-year flood event. The Powerline Channel would collect runoff from the upstream watershed including the Powerline North Diversion Channel, Weekes Wash, and Siphon Draw and would discharge to the Vineyard Road FRS. The Powerline Channel would discharge into Vineyard Road FRS east of the north Vineyard Road auxiliary spillway. There would be no impoundment of floodwaters with the Powerline Channel from the Powerline watershed. The existing dam embankment would be removed or breached and reduced in height to be a nonjurisdictional dam. The FRS would no longer meet the jurisdictional definition of a dam and then be removed from ADWR dam safety jurisdiction. The Powerline Channel would convey the 100- year storm and would provide a 100-year level of flood protection. A conceptual plan layout of Alternative 2 is provided in Appendix C as Figure C-5. The FRS is currently under the jurisdictional purview of ADWR. To remove the FRS from jurisdictional status would require decommissioning per Arizona Revised Statutes Section 45-1209. The Arizona Administrative Code Section R12-15-1209 Application to Breach or Remove a High or Significant Hazard Potential Dam outlines the requirements to reduce a high or significant downstream hazard potential dam to non-jurisdictional size. These regulations will need to be considered during final design of this alternative. The proposed Powerline Channel was sized using existing conditions 100-year, 24-hour peak discharge, as the 24-hour peak discharge was greater than the 6-hour peak discharge. The Powerline watershed features two proposed channels. The first is a collector channel running along the existing FRS alignment to capture inflow that would otherwise reach the structure. The second channel is specifically for Siphon Draw, the largest wash within the watershed. The Powerline Channel was initially sized using normal depth routing within HEC-1. The preliminary channel sizes were then input into a FLO-2D model for the alternative. The Powerline Channel was sized to convey a peak discharge of 12,500 cfs into Vineyard Road FRS. Where breakouts occurred from channel overtopping, the channel size was increased in width or depth in FLO-2D to provide additional capacity. The Powerline Channel bottom widths and geometries are summarized in Section 8.2. A typical section of the Powerline Channel is provided in Appendix C as Figure C-6. A construction cost for Alternative 2 is presented in Section 8. The cost estimate assumed that the entire embankment for the existing Powerline FRS and Powerline IDSM would be removed for a total of 704,000 cubic yards of material. The material is in the ownership of the Arizona State Land Department. For the purposes of the construction cost the material would be stockpiled on site. Locations of stockpiles would be determined in final design. The initial hydraulic analyses of the Powerline Channel indicate several locations may require small earthen dikes to provide the 1-foot minimum freeboard required for a 100-year channel. These dikes may be required at the Channel confluence with Siphon Draw and/or at the discharge location into the Vineyard Road FRS. This will be determined in final design. These embankments may be considered levees. If considered as levees, the embankments would be required meet USDA- NRCS Page 5-11 January 2013 Kimley-Horn and Associates, Inc.
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