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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
5.2.4. Recommendations
Many of the outstanding issues associated with earth fissure risk zoning at the PVR FRSs could
be addressed through a final design level of subsidence and earth fissure investigation. Such an
investigation would likely include additional subsidence refraction profiling, deep resistivity
soundings, seismic reflection profiling, future subsidence prediction models and updated stressstrain
models. Through such an investigative process the existing earth fissure risk zones would
be updated based on a better understanding of site conditions and predicted groundwater
declines. This evaluation would include the forthcoming ADWR update to the modeled gravity
and depth to bedrock in the area and the updated predictions for groundwater declines as
presented in ADWR’s Modeling Report No. 22 (Hipke, 2010).
A quantitative risk assessment could also be considered as a means of quantifying the earth
fissure risk associated with each earth fissure risk zone, although there would still likely be a
large degree of uncertainty at the end of this assessment. This assessment could be supplemented
by targeted failure modes and effects analyses that would address specific identified failure
modes and lead to the best solution for mitigating the failure mode(s).
For the alternatives evaluation, AMEC recommends the following:
Powerline FRS:
Structures located within moderate earth fissure risk zones at the Powerline FRS should
be planned as hardened structures. Additionally, due to the level of uncertainty, the
moderate risk zones in this area are on the higher end of moderate as opposed to the
lower end of moderate. It should be anticipated that the planned IDSM will need to be
hardened as part of the rehabilitation project.
Vineyard Road FRS:
Based on our current understanding of earth fissure risk conditions at the Vineyard Road
FRS, the section of embankment located within the moderate earth fissure risk zone does
not need to be hardened; however, this area should be the subject of a robust monitoring
program. Our recommendation is based on our current understanding of the subsurface
profile in this area and the results of recent InSAR data. This data suggests the moderate
risk zone at the Vineyard Road FRS is on the lower end of moderate as opposed to the
higher end of moderate.
It is noted that dam concepts involving earth fissure risk zones mitigated through hardened dam
segments and cutoff walls have been taken through the FMEA process (workshop and report) as
part of this study and no potential failure modes have been identified for these concepts. It is
recommended that any additional concept to be considered for final alternative selection
involving earth fissure risk zones without hardened dam segments and/or without cutoffs (such
as an embankment dam only with long-term monitoring) should also be taken through an FMEA
workshop prior to alternative selection.
USDA- NRCS January 2013
Kimley-Horn and Associates, Inc. Page 45