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 1. INTRODUCTION The Powerline Flood Retarding Structure (FRS) was designed and constructed by the United States Department of Agriculture – Natural Resources Conservation Service (NRCS) (formerly the Soil Conservation Service – SCS) in 1968 under the authority of the Watershed Protection and Flood Prevention Act (Public Law 566) as amended. The FRS was constructed as part of the Apache Junction-Gilbert Watershed Protection and Flood Prevention Project. The FRS is operated and maintained by the Flood Control District of Maricopa County (District). The 2.5 mile long, 21 foot high earth dam provides flood protection to downstream residents, structures, and infrastructure from flood flows originating from the 47 square mile watershed that lies east of the dam. The FRS has developed safety deficiencies and is approaching the original 50-year project lifetime. In May 2007, an earth fissure was identified in close proximity to the downstream toe of the dam. In light of the proximity of this earth fissure to the Powerline embankment, and the presence of other documented earth fissures in the general vicinity of the dam, the Arizona Department of Water Resources (ADWR) classifies Powerline FRS as being in an “unsafe, nonemergency, elevated risk” condition. As such, the Powerline FRS is one of ADWR’s highest priority unsafe dams in the state. The District is currently constructing the Powerline Interim Dam Safety Measure (IDSM). The IDSM consists of a new alignment for a portion of the Powerline FRS based on the results of past investigations indicating a high probability of an earth fissure beneath the existing embankment. In 2009, the District requested Federal planning and implementation assistance for long-term solutions to the identified dam safety deficiencies. This Supplemental Watershed Plan and Environmental Assessment (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 Powerline FRS is one of three dams (the other two are Vineyard Road FRS and Rittenhouse FRS) that were designed and constructed by the NRCS located along the Central Arizona Project canal in Pinal County. The structures were designed to operate as an integrated passive system. Each FRS has a principal spillway consisting of reinforced concrete pipe and each FRS has an earthen, open-channel auxiliary spillway, with the exception of Vineyard Road FRS, which has two auxiliary spillways. The principal spillways for both the Powerline and Vineyard Road FRS discharge into a common channel called the Powerline Floodway. Rittenhouse FRS functions as a cascading reservoir during principal spillway operation by discharging into the flood pool reservoir behind Vineyard Road FRS, which outfalls to the Powerline Floodway. The result is that all three principal spillways eventually flow into the Powerline Floodway. However, the auxiliary spillways of the FRSs do not cascade and do not outfall to the Powerline Floodway. The above information is presented as the investigations, studies, and reports for the Powerline, Vineyard Road and Rittenhouse Flood Retarding Structures Rehabilitation or Replacement Project was conducted on a joint approach rather than on the individual dams and watersheds. The following sections below are summaries of those studies and reports for the PVR project. USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 1
Powerline Flood Retarding Structure Pinal County, AZ Draft Supplemental Watershed Plan and Environmental Assessment 2. HYDROLOGY AND HYDRAULICS This section of the “Investigations and Analyses Report” provides a summary of the hydrologic and hydraulics analysis in previous investigations as well as studies conducted by Kimley-Horn for the PVR project. 2.1. Previous Studies Previous existing conditions of the PVR FRS watersheds have been studied as part of the following investigations: • “Desert Drive Area Study Existing Conditions Hydrology” (DDA Existing Conditions Hydrology) by the Arizona State Land Department (ASLD, 2007) o The Arizona State Land Department conducted an existing conditions hydrology study for an area of State Lands known as Desert Drive Area (DDA). The Desert Drive Area study includes the PVR structures and their contributing watersheds. The purpose of the hydrology study was to determine the rainfall runoff response for the Desert Drive Area under existing conditions. o HEC- 1 models were developed for the l00-year, 200-year, 500-year, and PMF storm events. The 100-year, 200-year and 500-year storm events were analyzed for 6-hour and 24-hour storm durations. The PMF storms were analyzed for the 6-hour and 72-hour storms. For the Desert Drive Area downstream of the PVR structures, only the 100-year, 6-hour and 24-hour models were developed. • “Desert Drive Area Study Existing Conditions Inundation and Sedimentation Analysis for PVR Structures (DDA Inundation and Sedimentation) by the Arizona State Land Department (ASLD 2008). This is Volume II of the Desert Drive Area study. This study utilized FLO-2D software to create a two dimensional model of the discharges entering the PVR structures. The FLO- 2D model was completed in order to evaluate the interconnected character of the PVR Structures and to determine the existing condition inundation limits of the structures for the 100-year, 24-hour storm and the 6-hour and 72-hour PMF events (full and half). The results of the analysis provide overtopping depths and durations for each of the storms evaluated. 2.2. Hydrologic Design Criteria The rehabilitation or replacement of the PVR structures must meet the most critical design criteria between NRCS, ADWR and District hydrologic design criteria. The project must meet both District and NRCS requirements for flood protection and District, NRCS and ADWR requirements for dam safety. NRCS design criteria, flood protection, and dam safety requirements are documented in “Earth Dams and Reservoirs,” Technical Release No. 60, dated July 2005. ADWR design criteria and dam safety requirements are documented in the “Arizona Administrative Code Title 12, Chapter 15, Article 12 – Dam Safety Procedures” dated June 12, 2000. Per NRCS criteria for High Hazard Dams, the principal spillway design flood is the 100-year and 10-day storm, and the principal spillway must have: a) the capacity to prevent the auxiliary spillway from functioning more frequently than the 100 year event. ADWR requires that the USDA- NRCS January 2013 Kimley-Horn and Associates, Inc. Page 2
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