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

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
The future behavior of groundwater elevations remains one of the most challenging and
uncertain parameters to quantify and delineate earth fissure risks. These uncertainties tend to
result in more conservative delineation of the potential risks in order to reduce the risk imparted
by the uncertainties.
5.2.2. Investigative and Modeling Methods
Investigative Methods
There is a lack of published guidelines for the delineation of earth fissure risks, with the first
guideline document for subsidence and earth fissure investigations being published in 2011
(Arizona Land Subsidence Interest Group 2011). Most of the investigative techniques and
guidelines in this document were pioneered by AMEC as part of their work on District projects
throughout the Phoenix area. The investigative methods have evolved through time and the
Initial Subsidence and Earth Fissure Report, PVR Planning Study (AMEC 2010b) and the
Supplemental Earth Fissure Risk Report, Powerline FRS Interim Dam Safety Measure (AMEC
2010c) represent the most recently completed, large-scale subsidence and earth fissure
investigations in the Phoenix area as of the publication of this memorandum.
Detailed discussions of the methodologies utilized are included in these reports. In addition,
AMEC has recently developed Procedural Documents (AMEC 2011b) that describe in detail the
procedures for 11 methods of investigation for earth fissure risk delineation.
Modeling Methods
Sub-basins in the Salt River Valley are generally subdivided into three alluvial units: the upper
alluvial unit (UAU), the fine-grained middle alluvial unit (MAU), and the lower alluvial unit
(LAU) Prokopovich 1983; Laney and Hahn 1986). In order to model future subsidence,
simplified basin vertical profiles are developed to estimate historical subsidence and predict the
potential for future subsidence. Within the Phoenix area, it is assumed that basin material
compression leading to subsidence is limited to the portions of the UAU and/or MAU that are
below the groundwater table (i.e., saturated). Estimates of subsidence are then calculated using
the basin vertical profiles.
Based on these profiles, simplified estimates of subsidence are performed using the methods and
procedures discussed in AMEC’s white paper “Characterization for Subsidence Modeling and
Percolation Theory–Based Modeling of Subsidence” included in Appendix E of the Initial
Subsidence and Earth Fissure Report (AMEC 2010b). Calculation of subsidence is based on a
concept of increased loading due to falling groundwater levels with resulting increases in
effective stress on the compressible sections of basin alluvium (each section is typically 100 feet
in thickness) whose compression (and resulting ground subsidence) is a function of the moduli of
the basin alluvium sections. This approach uses percolation theory (PT) to model relationships of
basin density and modulus in basin alluvial sections. The resulting modulus profiles are variable
(increasing with depth) and nonlinear through the compressible section of the basin alluvial
profile.
The general process of calculating a subsidence estimate involves several tasks. Initial basin
alluvial section densities and moduli are a function of the applied effective stress on the alluvial
sections before groundwater declines began in the model. An increase in effective stress
calculated from an increment of groundwater level decline corresponding to a historic period is
then applied to the model. From this increase in effective stress loading, the resulting increment
USDA- NRCS January 2013
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