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 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. 2. Threatened and Endangered Species – Survey protocol for the Tucson shovel-nosed snake has not been published by the USFWS. Prior to construction the status of the Tucson shovel-nosed snake on the USFWS threatened, endangered, proposed, and candidate list will be re-verified (i.e. status changed from candidate to threatened) and coordination with the USFWS will be undertaken to determine if a survey protocol has been established and/or if a survey is required. If this species is observed construction will cease until measures can be taken to ensure the preservation of the snake and/or its habitat. Prior to start of construction, the project area will be canvassed for tortoises and, if any tortoises are discovered, AGFD guidelines (October 2007) will be followed. Prior to start of construction, a survey for burrowing owls will be conducted in the disturbance area following the AGFD protocol (January 2009). As a result of the actions described above, there will be no effect on federally threatened and endangered species or their critical habitat due to implementation of this alternative. The abovementioned mitigation measures will be utilized to minimize impacts to special status species. 3. Vegetative Community - Areas disturbed associated with construction activities will be hydro-seeded with native mix. Approximately 120 acres of mesquite bosque will be impacted. Mesquite bosque 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. Major Conclusions: The decommissioning of the Powerline FRS and replacement of the dam with a channel will remove the dam from ADWR jurisdiction. The channel will continue to provide 100- year flood protection for the downstream benefitted area. This alternative will reduce the risk of catastrophic failure of the existing structure and thus reduce the risk of loss of life and property. No significant adverse environmental effects will result from implementation of this project. Areas of Controversy: None. Issues to be Resolved: The Powerline channel will need to meet Flood Control District and NRCS design requirements for open channel flow. If portions of the channel include embankment for the purpose of providing channel freeboard, the embankments may need to meet the Federal Emergency Management Agency’s (FEMA) requirements for levee certification. Evidence of Unusual Congressional or Local Interest: None This report is in compliance with executive orders, public laws, and other statues governing the formulation of water resource projects. USDA- NRCS Page 1-5 January 2013 Kimley-Horn and Associates, Inc.
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment 2.0 PURPOSE AND NEED FOR ACTION 2.1 CHANGES REQUIRING PREPARATION OF A SUPPLEMENT The Powerline Flood Retarding Structure (FRS) is classified by the Arizona Department of Water Resources (ADWR) and NRCS as a high hazard structure that does not meet current safety and performance standards. The dam was originally classified by the Soil Conservation Service as a Class C dam. The dam has known deficiencies in the embankment (transverse and longitudinal cracking and erosion holes), located in an area with land subsidence, earth fissures and known inadequacies in auxiliary spillway capacity. There is a risk to downstream structures and inhabitants if no action is taken to rehabilitate or replace the dam. The NRCS addressed deficiencies due to transverse cracking by means of a repair project that was completed in June 1991. The repair project included installation of a central embankment filter along the dam. The project was funded under the NRCS Watershed Operations program. Current conditions indicate additional transverse and longitudinal cracks as well as erosion holes in the embankment are present. Remaining inadequacies in auxiliary spillway capacity are considered under this study. The Sponsor is presently implementing an interim dam safety measure (IDSM) to relocate a portion of the Powerline FRS embankment outside of an identified earth fissure high risk zone. The IDSM was designed by the Sponsor’s consultant and reviewed and approved by the NRCS and ADWR. The IDSM is presently under construction and is anticipated to be completed in September 2013. This Supplemental Watershed Plan and Environmental Assessment (Plan/EA) addresses the identified dam safety deficiencies (noted above) by evaluating alternatives to either rehabilitate or replace the Powerline FRS. The Plan/EA determines the feasibility of decommissioning the FRS and replacing the FRS with a constructed channel to provide for continued flood protection while meeting current applicable local, State, and Federal regulations. The changes being made are to replace the Powerline FRS and IDSM with an earthen flood control channel (Powerline Channel) designed for the 100-year flood event and to direct the flows from the channel to the Vineyard Road FRS flood pool. The purpose and need of the proposed project is to provide continued flood protection for the downstream benefitted area and 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. 2.2 BACKGROUND The FRS provides substantial flood protection benefits to portions of Maricopa and Pinal Counties, City of Mesa, State Lands, and surrounding areas. The FRS protects downstream public infrastructure including streets and highways, and residential and commercial properties. In June 2000, the Sponsor completed an Individual Structure Assessment for the Powerline FRS and in August 2000 the Sponsor completed an Alternatives Analysis Report for the Powerline FRS. In July 2009, the Sponsor entered into a Memorandum of Understanding with the Natural Resources Conservation Service (NRCS) to cooperatively conduct planning, design and USDA- NRCS Page 2-1 January 2013 Kimley-Horn and Associates, Inc.
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Powerline Flood Retarding Structure
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POWERLINE FRS TYPICAL SECTION
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Exhibit A-2 - Emergency Spillway Di
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