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 FEMA levee certification requirements. FEMA levee certification requirements are located in Title 44 CFR 65.10. Minimal requirements include but are not limited to: a minimum of three feet of freeboard for the length of the levee above the base flood (100-year) elevation and an additional one foot on either side of structures (bridges, etc.); provisions for embankment erosion protection, embankment and foundation stability, and settlement analysis. The levees will also be required to be designed to meet US Army Corps of Engineers and/or NRCS standards for earthen levees. Portions of the Powerline Channel would be required to be hardened sections. These channel segments are located within the moderate fissure risk zones. Typical sections of the hardened channel sections are provided in Appendix C as Figure C-7. Alternative 2 would not require additional right-of-way or easements for the Powerline Channel. The channel would be located within the existing modified easement. The construction of the Powerline Channel and removal of the Powerline FRS and IDSM embankment potentially locally impact both environmental and cultural resources. The acres of existing mesquite bosque vegetation that may be directly affected have been quantified (approximately 120 acres). ). Mesquite bosque mitigation may include planting 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. The estimated cost of mesquite bosque mitigation for the PVR project is approximately $1,938,000. Clearing and excavation of the channel and construction of an O&M road on either side of the channel will disturb or remove existing vegetation. Disturbed areas will be hydroseeded and restored to a native plant community following construction. An estimation of the potential for impacting cultural resources has been completed for Alternative 2. The area of potential effect (APE) was determined and an estimate of the mitigation for cultural resources prepared. Sites in the APE that will not be subject to project-related impacts will be avoided by project implementation activities. Data recovery studies will be performed to mitigate impacts to all sites likely to suffer project-related impacts. Based on known sites in the vicinity, the majority of the cultural resources present would be either prehistoric sites without evidence of habitation (which might include scatters of artifacts on the surface, or artifacts with remnants of just one or a few roasting pits, agricultural features, and/or other non-habitation features) or historical sites (which might include homesteads, farming or ranching features, or simply scatters of historical artifacts). Most of the remaining sites would be smaller prehistoric habitation sites, with up to two relatively large prehistoric habitation sites within the total APE (for Powerline, Vineyard Road, and Rittenhouse FRSs). Based on experience with recent data recovery efforts in central and southern Arizona, the estimated cost for cultural resources mitigation for the PVR project is $3,010,000. The total installation cost for Alternative 2 (from NRCS Table 2 in Section 8.0) is $23,893,600. Total project costs include a contingency of 25%, additional permitting, engineering and construction management percentages. The Sponsor’s O&M costs are estimated at $212,700. USDA- NRCS Page 5-12 January 2013 Kimley-Horn and Associates, Inc.
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment 6.0 ENVIRONMENTAL CONSEQUENCES 6.1 EFFECTS OF ALTERNATIVE PLANS This section describes potential effects of the alternatives from Section 5.4 within each resource category noted as relevant to the proposed action in Table 3-1. 6.1.1 GEOLOGY The site geology and soils for the FRS embankment are subject to land subsidence leading to the potential for the development of earth fissures. Earth fissure risk zone mapping has been previously developed for the PVR project. Alternative 1 – No Action. This alternative would continue to operate the dam as under existing conditions with the modifications implemented with the Powerline IDSM. The IDSM has a 15 year design life. The site geology and soils would be a major consideration in Alternative 1 since the dam and IDSM would continue to be at risk to potential earth fissure development. Alternative 2 – Decommissioning. This alternative replaces the Powerline FRS with a channel. The existing dam embankment would be removed or lowered in height to make the remaining structure a non-jurisdictional dam. The Powerline Channel would direct flows from the Powerline watershed into the Vineyard Road FRS. The channel would be a hardened channel within the fissure risk zone and an earth-lined channel outside the fissure risk zone. Alternative 2 would have a short-term effect on soils by soil disturbance/grading activities. The site geology and soils would be a minor consideration for Alternative 2 as no floodwaters would be impounded. 6.1.2 PRIME AND UNIQUE FARMLAND Alternative 1 – No Action. This alternative would have a risk of catastrophic flooding from a dambreak of important agricultural lands, irrigation canals and diversions. Alternative 2 – Decommissioning. This alternative replaces the Powerline FRS with a channel and therefore there is no impoundment. The flows from the channel would be directed to the raised and rehabilitated Vineyard Road FRS. This alternative provides flood protection for downstream important agricultural lands. The risk of catastrophic flooding from a dambreak is removed. 6.1.3 SURFACE WATER QUANTITY Alternative 1 – No Action. This alternative would continue to operate the dam as under existing conditions with the modifications implemented with the Powerline IDSM. Alternative 1 does not safely pass the inflow design flood through the auxiliary spillway with the required freeboard. Alternative 2 – Decommissioning. This alternative would not impound water and therefore have no direct effect on impounded waters. Instead the channel would direct flows to the Vineyard Road FRS. The channel would be designed for the 100-year flood. The PMF is not required for Alternative 2. USDA- NRCS Page 6-1 January 2013 Kimley-Horn and Associates, Inc.
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