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 4.21.1 HYDROLOGIC FAILURE MODES The July 2002 FMEA identified one hydrologic potential failure mode. This is: • Failure from Overtopping of Powerline FRS (Category I). Hydrologic studies conducted for Powerline, Vineyard, and Rittenhouse FRSs indicated there is a potential for Powerline FRS to be overtopped by the full PMF storm event. The overtopping may be on the order of a maximum of 0.0 to 1.3 feet for a total discharge duration of 2.8 hours. The studies also indicate there may be a discharge from the emergency spillway for the 100-year event. Overtopping of Powerline FRS would occur at the low point on the crest (approximately Station 90+00). Flow would overtop the dam at that point and as flows increased, erosion of the crest and downstream slope would occur and potentially lead to a breach of the dam. 4.21.2 STRUCTURE FAILURE MODES The July 2002 FMEA identified five structure related potential failure modes. These are: • Failure from a Potential Earth Fissure(s) through the Dam Embankment in Association with a Significant Flooding Event (Category I). The presence of existing earth fissures in the vicinity and adjacent to Powerline FRS demonstrates the potential for a new earth fissure to manifest itself at the dam embankment. An earth fissure could cause a structural failure of the embankment by opening a crack through the structure. Alternatively, the fissure could potentially undermine the dam embankment by abruptly causing separation of structure at the foundation with the dam bridging the fissure. If the fissure occurred in association with a large flood the structural failure from the fissure would provide a path for a seepage erosion breach. In such a case there would be minimal loss of water. This failure mode was considered by the FMEA team to be the most significant threat to Powerline FRS. • Failure from Piping through Transverse Crack Causing Breach of Dam in Association with a Major Flooding Event (Category I). Potential for impounded water to infiltrate and flow into a transverse crack(s) existing in embankment prior to the major flooding event or a transverse crack that develops in association with the flooding. The transverse crack allows the entry of great enough flow of water to initiate seepage erosion and breaching failure. Two modes were identified for failure: Mode A – failure through a large (wide) transverse crack that extends across the central filter, and Mode B – failure through entry of flow through multiple upstream transverse cracks, water flows to/through a flaw in the central filter and begins seepage erosion (and loss of central filter material) through a downstream transverse crack. • Failure from Potential Piping Around/Or from 12-inch Corrugated Metal Pipe Outlet at Station 141+00 (Category II). Infiltration of water into embankment material around pipe and soil is carried away due to seepage erosion along culvert or piping leading to an erosional tunnel, caving or breaching. • Failure from Potential Erosion of Abutment and/or Emergency Spillway During Spillway Discharges (Category II). The emergency spillway is a limited service earth-lined spillway. Spillway discharges from extreme events could potentially damage the spillway floor and left abutment due to erosion. USDA- NRCS Page 4-13 January 2013 Kimley-Horn and Associates, Inc.
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment • Failure from Potential Embankment Slope Instability (Category II). Potential embankment slope failure due to internal pressure in the central filter. 4.22 CONSEQUENCES OF DAM FAILURE Dambreak analyses were completed by JE Fuller/Hydrology & Geomorphology, Inc. in June 2007 on behalf of the District and LTM Engineering, Inc. The report titled “Emergency Action Plan for Powerline, Vineyard Road, & Rittenhouse Flood Retarding Structures” (JEF, 2007) documented the inundation areas downstream from the three dams for emergency spillway discharges and from dambreaks. Copies of the flood inundation mapping from the Emergency Action Plan for Powerline FRS are included in Appendix C. • The flood wave generated from a dambreach failure at Powerline FRS has the potential to impact major public infrastructure (Loop 202, Williams Gateway Airport) and existing/planned residential areas. • The flood wave from the auxiliary spillway (discharge of 21,300 cfs) would impact existing and planned residential areas east of the dam. Major public infrastructure is also within the flood hazard area (Ironwood Road and Williams Gateway Airport). The population-at-risk (PAR) downstream of the Powerline FRS was conservatively estimated in the economics study conducted in support of this Plan/EA. The PAR was estimated and documented in the “Economics Evaluation Technical Memorandum” (Gannett Fleming, January 2013). The PAR is estimated to be 27,000 people. This estimate was worst case based on simultaneous full occupancy of the commercial buildings, schools, residences, etc. The methodology for determining the PAR is provided in the “Investigation and Analysis Report” provided in Appendix D of this Plan/EA. 4.23 AESTHETICS A Site Analysis for the project study area extending out one mile around the PVR dams is documented in the report titled “Powerline, Vineyard Road and Rittenhouse Flood Retarding Structures Rehabilitation or Replacement Project - Site Analysis Report” (EPG, Inc. February, 2010). The Site Analysis identified site constraints and opportunities for the future development of flood hazard mitigation solutions and for potential multi-use functions within the project study area. Some of the most visually striking and identifiable landforms within the project boundary are the PVR dams themselves. The PVR dams total 11.6 miles in length and have respective maximum heights of 26.1 feet, 16.5 feet, and 24.1 feet. The structures provide 12,036 acre-feet of flood storage and 100-year flood protection for 147 square miles. Recommendations for the rehabilitation of the flood retarding structures (if a rehabilitation alternative is selected) should consider the functioning of the interconnected system, including its three embankments, three reservoirs, four auxiliary spillways, and one floodway (Powerline Floodway). Atop the FRS dams, spectacular panoramic views are afforded to additional identifiable and noteworthy landforms such as the Superstition Mountains and Tonto National Forest to the northeast, the Usery and Goldmine Mountains to the northwest, and the San Tan Mountains to the southwest. The remaining prevailing landform comprises large, open flat areas, and jagged edged USDA- NRCS Page 4-14 January 2013 Kimley-Horn and Associates, Inc.
Page 1 and 2: DRAFT SUPPLEMENTAL WATERSHED PLAN A
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Powerline Flood Retarding Structure
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Signal Butte Rd Meridian Rd Ironwoo
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POWERLINE FRS TYPICAL SECTION
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Ironwood Rd Gantzel Rd Mountain Vie
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Figure 22 Figure 21 - Conceptual Mu
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Exhibit A-2 - Emergency Spillway Di
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Price Price Price Dobson Alma Schoo
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Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

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