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 Future Subsidence Predictions by Scenarios (continued) Estimated Future Subsidence (feet) Base Case Scenario - No Recharge Facility Scenario 2 - With Recharge Facility Location 2030 2100 2030 2100 South Rittenhouse FRS Approximately Station 50+00 -0.1 0.2 -0.1 -0.2 Earth Fissure Risk Delineation The risk zones were first delineated for the Powerline FRS and the northern portion of the Vineyard Road FRS as part of a previous investigation. The risk zones were then updated to account for the earth fissure discovered downstream of the Powerline FRS embankment at about Station 115+45. The risk zones were further discussed in relation to the proposed Powerline IDSM. The risk zones around the Powerline FRS, the proposed Powerline IDSM and the northern third of the Vineyard Road FRS did not change as a result of this investigation. However, the earth FRZ was extended south to include the remainder of the Vineyard Road FRS and the Rittenhouse FRS and extended north to include the Powerline Diversion. The definitions for the earth FRZ are presented below. FRZ 1: Earth fissures are present and will likely continue to occur in the future as evidenced through the application of multiple investigative methods, including published and unpublished mapping, photolineament analysis, geophysical surveys, ground reconnaissance and trenching. FRZ 2: Conditions for the past development of earth fissures are present through multiple lines of evidence; however, earth fissures have not been positively identified. A high probability of future development of earth fissures is present. Evidence supporting the possible presence of concealed fissures and probability of development of fissuring in the future includes strain calculations and predictions from subsidence modeling, InSAR and geodetic survey data, and photolineament analysis, as well as proximity to and trends of known earth fissures and the presence of known seismic anomalies, soil discontinuities and large transverse cracks in the FRS embankments. FRZ 3: A probability of future earth fissure formation is present, but the risk is less than in FRZ 2, unless future differential subsidence occurs in FRZ 2, coupled with the currently elevated horizontal tensional strain. Evidence supporting this designation includes InSAR data, photolineament analysis, subsidence modeling and geodetic survey data. FRZ 4: A low probability of future earth fissure formation exists. Evidence suggests that no significant tensional strain will develop from future subsidence. A portion of the Powerline FRS embankment is located in FRZ 1 and in FRZ 2. Portions of the Powerline FRS, the proposed Powerline IDSM, and the Vineyard Road FRS are located within FRZ 3. The Rittenhouse FRS, the Powerline Diversion, most of the Vineyard Road FRS, and portions of the Powerline FRS are located within FRZ 4. The earth FRZ were approximated to reflect the earth fissure risk posed to the project through 2025, should current subsidence trends continue. To be conservative, the earth fissure risk USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 37
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment delineations do not anticipate recharge from the proposed Superstition Mountains Recharge Facility. Recharge from the proposed facility would likely not affect the risk delineation in the vicinity of Powerline FRS, but it may have an effect on the risk delineation at Vineyard Road FRS. 5.1.5. Conclusions and Recommendations The major concern relating to subsidence and earth fissuring is focused on the Powerline FRS. There is a known earth fissure immediately downstream of about Powerline FRS Station 115+45, and there is a high probability that the earth fissure is also present beneath the FRS embankment. The earth fissure risk is highest in the area around the Powerline FRS and the portion of the Vineyard Road FRS located in FRZ 3. Predicted future subsidence is greatest in the area of the Powerline FRS, particularly portions to the north of the bend at about Powerline Station 110+00. Subsidence is predicted to continue in this area and will complicate future plans regarding the Powerline Diversion as it could potentially impact the grade of the channel and make draining water to the south challenging. Any future alternative options that require moving water from north to south in this area will also have to address the potential that future subsidence will impact the grade of channels or other infrastructure. Future subsidence predictions indicate that the potential for 1 to 2 feet of additional subsidence is likely in area north of about Powerline Station 110+00 and along the proposed IDSM alignment. Other portions of the project are not anticipated to experience large amounts of subsidence in the future, and are anticipated to have a low risk for development of earth fissures. Results of this study indicate that the portion of the Vineyard Road FRS located in FRZ 3 may have a lower risk for earth fissures than previously thought. It is recommended that the District consider performing a future investigation to determine if the earth fissure risk in this zone is less than currently classified. It is recommended that any future investigation performed include close evaluation of available monitoring data, additional InSAR analysis, additional geophysical profiling, additional subsidence prediction, and updating the stress-strain model in this area. 5.2. Final Earth Fissure Risk Zoning Memorandum The purpose of this memorandum was to address NRCs comments about subsidence and earth fissure risk zoning at the Powerline, Vineyard Road and Rittenhouse (PVR) Flood Retarding Structures (FRSs) as they relate to the PVR FRS Rehabilitation or Replacement Project. This memorandum starts with a general discussion of ground subsidence and earth fissuring due to groundwater withdrawals. This is followed by discussions of the investigative and modeling methods used to predict future subsidence and the risk of earth fissuring, and the history of earth fissure risk zoning at flood control structures in the Phoenix area. The memorandum culminates with a discussion of earth fissure risk zoning at PVR and responses to specific questions that NRCS has posed regarding PVR earth fissure risk zoning. 5.2.1. Subsidence and Earth Fissuring Due To Groundwater Withdrawal Land subsidence and the development of earth fissures at the PVR FRSs, and at other flood control structures in the Phoenix area, are primarily a result of groundwater level declines.. USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 38
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DRAFT SUPPLEMENTAL WATERSHED PLAN A
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Powerline Flood Retarding Structure
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Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

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