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

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.