Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
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<strong>Powerline</strong> <strong>Flood</strong> Retarding Structure<br />
Pinal County, AZ<br />
Draft Supplemental Watershed <strong>Plan</strong><br />
<strong>and</strong> <strong>Environ</strong>mental <strong>Assessment</strong><br />
1. INTRODUCTION<br />
The <strong>Powerline</strong> <strong>Flood</strong> Retarding Structure (FRS) was designed <strong>and</strong> constructed by the United<br />
States Department of Agriculture – Natural Resources Conservation Service (NRCS) (formerly<br />
the Soil Conservation Service – SCS) in 1968 under the authority of the Watershed Protection<br />
<strong>and</strong> <strong>Flood</strong> Prevention Act (Public Law 566) as amended. The FRS was constructed as part of the<br />
Apache Junction-Gilbert Watershed Protection <strong>and</strong> <strong>Flood</strong> Prevention Project. The FRS is<br />
operated <strong>and</strong> maintained by the <strong>Flood</strong> <strong>Control</strong> District of Maricopa County (District). The 2.5<br />
mile long, 21 foot high earth dam provides flood protection to downstream residents, structures,<br />
<strong>and</strong> infrastructure from flood flows originating from the 47 square mile watershed that lies east<br />
of the dam.<br />
The FRS has developed safety deficiencies <strong>and</strong> is approaching the original 50-year project<br />
lifetime. In May 2007, an earth fissure was identified in close proximity to the downstream toe<br />
of the dam. In light of the proximity of this earth fissure to the <strong>Powerline</strong> embankment, <strong>and</strong> the<br />
presence of other documented earth fissures in the general vicinity of the dam, the Arizona<br />
Department of Water Resources (ADWR) classifies <strong>Powerline</strong> FRS as being in an “unsafe, nonemergency,<br />
elevated risk” condition. As such, the <strong>Powerline</strong> FRS is one of ADWR’s highest<br />
priority unsafe dams in the state. The District is currently constructing the <strong>Powerline</strong> Interim<br />
Dam Safety Measure (IDSM). The IDSM consists of a new alignment for a portion of the<br />
<strong>Powerline</strong> FRS based on the results of past investigations indicating a high probability of an<br />
earth fissure beneath the existing embankment. In 2009, the District requested Federal planning<br />
<strong>and</strong> implementation assistance for long-term solutions to the identified dam safety deficiencies.<br />
This Supplemental Watershed <strong>Plan</strong> <strong>and</strong> <strong>Environ</strong>mental <strong>Assessment</strong> (<strong>Plan</strong>/EA) determines the<br />
feasibility of decommissioning the FRS <strong>and</strong> replacing the FRS with a constructed channel to<br />
provide for continued flood protection while meeting current applicable local, State, <strong>and</strong> Federal<br />
regulations.<br />
The <strong>Powerline</strong> FRS is one of three dams (the other two are Vineyard Road FRS <strong>and</strong> Rittenhouse<br />
FRS) that were designed <strong>and</strong> constructed by the NRCS located along the Central Arizona Project<br />
canal in Pinal County. The structures were designed to operate as an integrated passive system.<br />
Each FRS has a principal spillway consisting of reinforced concrete pipe <strong>and</strong> each FRS has an<br />
earthen, open-channel auxiliary spillway, with the exception of Vineyard Road FRS, which has<br />
two auxiliary spillways. The principal spillways for both the <strong>Powerline</strong> <strong>and</strong> Vineyard Road FRS<br />
discharge into a common channel called the <strong>Powerline</strong> <strong>Flood</strong>way. Rittenhouse FRS functions as<br />
a cascading reservoir during principal spillway operation by discharging into the flood pool<br />
reservoir behind Vineyard Road FRS, which outfalls to the <strong>Powerline</strong> <strong>Flood</strong>way. The result is<br />
that all three principal spillways eventually flow into the <strong>Powerline</strong> <strong>Flood</strong>way. However, the<br />
auxiliary spillways of the FRSs do not cascade <strong>and</strong> do not outfall to the <strong>Powerline</strong> <strong>Flood</strong>way.<br />
The above information is presented as the investigations, studies, <strong>and</strong> reports for the <strong>Powerline</strong>,<br />
Vineyard Road <strong>and</strong> Rittenhouse <strong>Flood</strong> Retarding Structures Rehabilitation or Replacement<br />
Project was conducted on a joint approach rather than on the individual dams <strong>and</strong> watersheds.<br />
The following sections below are summaries of those studies <strong>and</strong> reports for the PVR project.<br />
USDA- NRCS <strong>Jan</strong>uary <strong>2013</strong><br />
Kimley-Horn <strong>and</strong> Associates, Inc. Page 1