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 5.2.4. Recommendations Many of the outstanding issues associated with earth fissure risk zoning at the PVR FRSs could be addressed through a final design level of subsidence and earth fissure investigation. Such an investigation would likely include additional subsidence refraction profiling, deep resistivity soundings, seismic reflection profiling, future subsidence prediction models and updated stressstrain models. Through such an investigative process the existing earth fissure risk zones would be updated based on a better understanding of site conditions and predicted groundwater declines. This evaluation would include the forthcoming ADWR update to the modeled gravity and depth to bedrock in the area and the updated predictions for groundwater declines as presented in ADWR’s Modeling Report No. 22 (Hipke, 2010). A quantitative risk assessment could also be considered as a means of quantifying the earth fissure risk associated with each earth fissure risk zone, although there would still likely be a large degree of uncertainty at the end of this assessment. This assessment could be supplemented by targeted failure modes and effects analyses that would address specific identified failure modes and lead to the best solution for mitigating the failure mode(s). For the alternatives evaluation, AMEC recommends the following: Powerline FRS: Structures located within moderate earth fissure risk zones at the Powerline FRS should be planned as hardened structures. Additionally, due to the level of uncertainty, the moderate risk zones in this area are on the higher end of moderate as opposed to the lower end of moderate. It should be anticipated that the planned IDSM will need to be hardened as part of the rehabilitation project. Vineyard Road FRS: Based on our current understanding of earth fissure risk conditions at the Vineyard Road FRS, the section of embankment located within the moderate earth fissure risk zone does not need to be hardened; however, this area should be the subject of a robust monitoring program. Our recommendation is based on our current understanding of the subsurface profile in this area and the results of recent InSAR data. This data suggests the moderate risk zone at the Vineyard Road FRS is on the lower end of moderate as opposed to the higher end of moderate. It is noted that dam concepts involving earth fissure risk zones mitigated through hardened dam segments and cutoff walls have been taken through the FMEA process (workshop and report) as part of this study and no potential failure modes have been identified for these concepts. It is recommended that any additional concept to be considered for final alternative selection involving earth fissure risk zones without hardened dam segments and/or without cutoffs (such as an embankment dam only with long-term monitoring) should also be taken through an FMEA workshop prior to alternative selection. USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 45
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment 6. ENVIRONMENTAL STUDIES This section provides a summary of the environmental reviews and analyses conducted by Kimley-Horn for the PVR project. The purpose of the environmental investigations was to identify the existing environmental conditions of the PVR structures within the modified easement area and an adjacent buffer area around the easement limits. The environmental evaluation included an overview of the site biology, cultural resources, hazardous materials, social population/demographics, and Section 404 of the Clean Water Act planning considerations. The environmental studies are documented in the following reports. • “Powerline, Vineyard Road and Rittenhouse Flood Retarding Structures Rehabilitation or Replacement Project - Biological Evaluation”. (Kimley-Horn July, 2010) • “Powerline, Vineyard Road and Rittenhouse Flood Retarding Structures Rehabilitation or Replacement Project – Social Population and Demographics Existing Conditions Study”. (Kimley-Horn July, 2010) • “Powerline, Vineyard Road and Rittenhouse Flood Retarding Structures Rehabilitation or Replacement Project – Cultural Resources Existing Conditions Study”. (Kimley-Horn July, 2010) • “Powerline, Vineyard Road and Rittenhouse Flood Retarding Structures Rehabilitation or Replacement Project – Hazardous Materials Technical Memorandum”. (Kimley-Horn July, 2010) • “Powerline, Vineyard Road and Rittenhouse Flood Retarding Structures Rehabilitation or Replacement Project – Significant Nexus Analysis”. (Kimley-Horn May, 2012) 6.1. Biological Evaluation The purpose of the biological evaluation was to document the existing habitat conditions of the project area, and document any species or species specific habitat observed during field biological reconnaissance visits. This assessment focused on gathering general habitat information for the project area and the potential for any threatened, endangered, candidate, or other sensitive species or associated species habitat to occur within the project area. 6.1.1. Habitat Considerations The majority of the project area is flat and sparsely vegetated, with the majority of the vegetation occurring along the ephemeral washes. Very dense mesquite bosques, containing velvet mesquite (Prosopis velutina), blue paloverde (Parkinsonia florida), catclaw acacia (Acacia greggii), and cottonwoods (Populus fremontii) (very rare) are present near and adjacent to the FRSs. The mesquite bosques provide valuable food, shelter, and travel corridors for multiple mammal, avian, and reptile species. Additionally, a higher plant species diversity was observed within the mesquite bosques. The majority of the mesquites observed within the bosque ranged from 20-30 feet in height. The width of the mesquite bosque ranged from 200 feet to over 650 feet. 6.1.2. Threatened and Endangered Species The Arizona Game and Fish Department’s (AGFD) Heritage Data Management System (HDMS) provided a list of special status species that have been documented as occurring within 5 miles of the project area. Additionally, the U.S. Fish and Wildlife Service (USFWS) threatened, USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 46
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Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

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