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 conveyed by the auxiliary spillway and would impact the CAP canal and downstream residential areas west of the dam. The dam would most likely remain designated as an “unsafe non-emergency elevated risk” structure. The Sponsors have stated that a constructed breach the dam is not an acceptable alternative. Alternative 2 would provide flood protection for greater than the 100-year event. Consistent with P&G, the alternative that has the highest positive net benefits would be identified as the NED Plan. However, per Section 502.2 of the NRCS National Watershed Program Manual, “In cases where human life is at risk in the event of catastrophic failure of an existing dam that does not meet current safety and performance standards, an overriding reason for an exception to the NED plan requirement exists. To avoid seeking individual exceptions in such cases, the NED plan is defined as the federally assisted alternative with the greatest net economic benefits.” The effects of the alternative plans in terms of the EQ and OSE accounts are summarized in Table 6-2 and Table 6-3, respectively below. SOILS Geology Table 6-2 Summary and Comparison of Candidate Plans – EQ Account Alternative 1 Alternative 2 No Federal Action Decommissioning Sponsor's Preferred/NED Plan IDSM design life 15 years Hardened channel segments within earth fissure risk zones Prime and Unique Farmland No effect No effect WATER Surface Water Quantity No effect Discharge to Vineyard Road FRS Floodplain Management Potential exposure the downstream floodplain to flooding risks from overtopping flood events or discharges through the auxiliary spillways above the 100-year event. Powerline flows directed to the Vineyard Road FRS and thus provide continued flood protection up to the 100-year event for downstream floodplain PLANTS Threatened and Endangered Species No effect No effect USDA- NRCS Page 6-10 January 2013 Kimley-Horn and Associates, Inc.
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment Table 6–2 Summary and Comparison of Candidate Plans – EQ Account (continued) PLANTS Vegetative Communities ANIMALS Fish and Wildlife Habitat Threatened and Endangered Species Migratory Birds/Bald and Golden Eagles HUMANS Aesthetics Alternative 1 Alternative 2 No Federal Action Decommissioning Sponsor's Preferred/NED Plan No effect No effect No effect No effect No effect Removal of existing vegetation within project footprint. Effect minimized due to post-construction hydro-seeding and other mitigation measures. Short-term minor adverse effect during construction No effect Mitigation for burrowing owl Moderate effects during construction, no long-term effect; provides opportunity for multi-use Flood Damages Cost, Sponsor Cost, NED Historic Properties/Cultural Resources Downstream properties, residences, business at risk from inundation from dam failure or dam overtopping Continued O&M costs No effect No effect Powerline channel discharge to Vineyard Road FRS. Reduced risk of downstream flood damages. Action will require significant expenditure of funds. Action will require significant expenditure of funds. 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. USDA- NRCS Page 6-11 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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