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 of the channel crossing a yellow moderate FRZs will be hardened structures (concrete lined) with upstream and downstream permeable backfill trenches wrapped in geotextile. Channel segments in green FRZs will be earth-lined channels. The Powerline channel drains into the Vineyard Road FRS flood pool upstream of the north auxiliary spillway. The excess spoil material from the channel excavation is anticipated to be stockpiled within the Sponsor’s easement at locations to be determined with the land owner – Arizona State Land Department. 8.2.2 POWERLINE NORTH DIVERSION CHANNEL The section of the Powerline North Diversion Channel between Highway 60 and Baseline Road has several local washes entering the channel from the east. The confluence of the washes with the channel is experiencing erosion and local headcutting up into the washes. Stabilization of the channel banks and confluences with the local washes has been conceptually developed and an exhibit is provided in Appendix C. Construction cost estimates to stabilize the channel cross section have also been developed. The existing channel at Baseline Road is headcutting into the right-of-way of Baseline Road and into the Baseline Road embankment fill on the south side of the road. Stabilization of the channel at this location consists of rock-filled gabion baskets for grade control and rock riprap for scour protection. Exhibits showing the locations of the channel improvements are included in Appendix C as Figure C-8. The 48-in culverts under Baseline Road will be replaced with two 10-ft wide by 6-ft high box culverts similar to the crossing under US 60. 8.2.3 POWERLINE FRS EMBANKMENT REMOVAL The Preferred Alternative includes construction costs to remove the embankment for the Powerline FRS and the Powerline IDSM. The removal of the embankments would occur after construction of the new Powerline Channel. The location of the embankments would be graded, stabilized, and hydroseeded with native plant mix. The excess spoil material from the embankment removal is anticipated to be stockpiled within the Sponsor’s easement at locations to be determined with the land owner – Arizona State Land Department. 8.3 MITIGATION FEATURES Clearing and excavation of a new Powerline 100-year channel, improvements to the North Diversion Channel, and FRS and IDSM removal and construction of an O&M road on either side of the channel will temporarily disturb existing vegetation including portions of the existing mesquite bosque. Disturbed areas due to construction activities will be hydro-seeded with native plant mix. The mitigation may include planting new tall-pot mesquite trees (approximately 60 acres) and salvage of some mature mesquite trees and transplanting in designated areas (e.g., along Powerline Channel, and potential mitigation area at north end of Vineyard Road FRS). The Sponsor and NRCS will coordinate with ASLD regarding trees along the existing low flow channel along the Powerline FRS. Mitigation plans will be refined during final design. Weed prevention methods will be implemented during construction activities. These methods will be determined in final design but may include: use of certified weed free seed, sterilized straw mulch with a tackifier so the straw mulch holds onto the ground, washing vehicles off before they enter the site, and managing adjacent weed populations to the extent possible (to prevent seed drift). Also, the use of pre-emergent herbicide could be considered for any non-planted areas that USDA- NRCS Page 8-3 January 2013 Kimley-Horn and Associates, Inc.
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment will be plated with Decomposed Granite or Rip Rap. A further measure would be to not allow weeds to colonize any soil stockpile areas before the re-use of the stored soil on site. Erosion control measures, both temporary during construction and permanent measures will be installed. Erosion control will be specified in the NRCS contract specifications. Section 8.4 below states the Sponsors are responsible for a 402 National Pollutant Discharge Elimination System Permit (NPDES) for Stormwater from Construction Sites which includes the preparation of a Stormwater Pollution Prevention Plan (SWPPP). The installation activities will be required to conform with local and county dust control regulations and measures. Dust control will be specified in the NRCS contract specifications. 8.4 PERMITS AND COMPLIANCE The Sponsors are responsible for all permits, including a 402 National Pollutant Discharge Elimination System Permit (NPDES) for Stormwater from Construction Sites from the Arizona Department of Environmental Quality, and submittal Form SCS-ADS- 78 (Real Property Assurances). The final determination of the applicability of NPDES permitting requirements will be made upon completing the design of the proposed project; if applicable, the final construction contract will include NPDES planning and implementation as a responsibility of the construction contractor. Completion of the Final Watershed Plan-EA and a Finding of No Significant Impact will complete all requirements of NEPA. If during the installation cultural resources are discovered, all work activities associated with the installation activity will stop and procedures detailing cultural resources discovered during implementation will be implemented per NRCS contract specifications. All work will comply with Arizona State Level Agreement, NRCS General Manual 420 Part 401, and the National Cultural Resources Procedures Handbook [H 190 Part 601]. An encroachment permit may be required from the CAP for construction within the CAP canal right-of-way. 8.5 COSTS The estimated total installation cost for the Preferred Alternative is $23,893,800. This estimated total installation cost includes the cost of construction plus NRCS engineering and project administration costs, as well as the Sponsor’s engineering, and project administration costs. NRCS Tables 1 through 6 are included in this section of this Plan-EA and presents information about the Preferred Alternative. The costs shown in Tables 1 and 2 for the planned decommissioning work were estimated using a 2012 price level. Annual costs were based on a 2012 discount rate of 3.75% over a project life of 103-years. Table 3 summarizes important physical characteristics of the existing dam. Tables 4, 5, and 6 highlight the amortized or annual average dollar costs and benefits for the planned decommissioning work. An outline of the table contents follows: USDA- NRCS Page 8-4 January 2013 Kimley-Horn and Associates, Inc.
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DRAFT SUPPLEMENTAL WATERSHED PLAN A
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Powerline Flood Retarding Structure
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