Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
Alternative 1 – No Action: In this alternative the Powerline FRS and IDSM would remain in place
and would continue to provide flood protection up to the 100-year event, given assumed future
conditions on the upstream watershed to be the same as existing conditions, until year 2017 (end of
design life). The Powerline FRS would not meet all of the standards set by ADWR and the NRCS
for public safety and reliability. The previously described inadequacies in dam foundation, central
filter, auxiliary spillway capacity, and embankment cracking would remain. At the end of the
design life period, the Sponsors would continue to operate and maintain the dam until such time
that available construction funding becomes available to replace the dam. Therefore, Alternative 1
is not acceptable to the Sponsor and local community as a viable solution since it does not meet the
purpose and need to reduce the risk of loss of life due to catastrophic dam failure. The structures
would continue to have safety and performance deficiencies and continue to pose a significant risk
of catastrophic failure until eventual rehabilitation or replacement of the structures.
Alternative 2 – Decommissioning: The alternative eliminates impoundment by the Powerline FRS
so that it will no longer need to conform to dam safety criteria or performance standards.
Alternative 2 provides a new channel that replaces the Powerline FRS. The channel will convey
floods up to the 100-year event to the Vineyard Road FRS. Therefore, Alternative 2 meets the
project purpose and the Sponsor’s formulation goals.
6.2.1 RISK AND UNCERTAINTY
A variety of factors contribute to the potential for flood control structure failure, including storm
event intensity, control structure construction materials and techniques, and operation and
maintenance activities. These uncertainties described below will be addressed during the design
phase of the project.
Under Alternative 1 – No Action, the Sponsor (District) would continue to operate and maintain the
dam in its present condition and in accordance with the current Emergency Action Plan. The
Powerline IDSM is an interim measure (design life of 15 years) to mitigate the potential for an
earth fissure near or under the Powerline FRS. The alignment/location of the IDSM is based on
current investigations and analyses for projected future earth fissure risk zones. The District will
continue the on-going monitoring program at Powerline FRS for indications of further subsidence
and potential of earth fissure development.
The dam would continue to have structural inadequacies in auxiliary spillway capacity. As a result,
there is a risk of dam break until the FRS is decommissioned.
Current hydrologic analytical results indicates the FRS and auxiliary spillway is not capable of
passing the planning 6-hr PMF (for both existing and future land use conditions), which overtops
the dam by 0.6 foot. The dam experiences no flow in the auxiliary spillway for any of the existing
conditions multi-frequency (2-year through 500-year) events. The future conditions land use for
Powerline FRS results in auxiliary spillway flow only during the 500-year, 24-hour storm event at a
depth of 0.5 ft.
The erosion analysis using the SITES model indicates that the auxiliary spillway erodes during the
stability design hydrograph and breaches during the existing and future conditions freeboard design
hydrographs. As a result there remains a risk of breach of the auxiliary spillway.
USDA- NRCS Page 6-6 January 2013
Kimley-Horn and Associates, Inc.