I-10 Twin Peaks Traffic Interchange, Environmental Assessment

Affected Environment and Environmental Impacts October 2005
to the east and west of the Santa Cruz River. Slopes angle toward the Santa Cruz
River. Slopes increase in the eastern portion of the study area near the foothills of
the Santa Catalina Mountains and Tortolita Mountains. The Santa Cruz River and
floodplain is the dominant topographic feature in the project area. With the
exception of the Santa Cruz River, there are no unique or important topographic
features in the study area.
Impacts
No Build Alternative
The no build alternative would have no impact on topography in the project area.
Preferred Alternative
Although the preferred alternative would construct an elevated roadway and
bridges over the Santa Cruz River, I-10, and the UPRR, existing slopes and other
topographic features in the project area, would not be affected. The low flow
channel of the Santa Cruz River would be widened to compensate for the roadway
and bridge embankment fill placed within the high flow Santa Cruz River channel
(see Floodplain Section, page 4-14); however, these impacts would not affect the
Santa Cruz River flows or functions.
Mitigation
As described in the previous paragraph, no mitigation measures for topography are
necessary, if the preferred alternative were constructed.
Soils
Existing Conditions
Soil types were identified for the study area through a review of comprehensive
maps compiled by the United States Department of Agriculture, Natural Resources
Conservation Service (NRCS) compiled in 2003, and the Arizona Agricultural
Experiment Station in 1969. As would be expected in a fluvial depositional
system, soils vary widely throughout the study area, often changing over distances
of tens of feet. Soils are predominantly of the Anthony, Agua and Grabe Series.
Anthony series consists of well-drained sandy loams to gravelly sandy loams.
These soils are formed in mixed material that was deposited on flood plains and
alluvial fans by rivers and streams. Slopes are 0 to 5 percent. Permeability of these
soils is moderately rapid and the hazard of erosion is moderate. These soils are
found throughout the study area.
Agua Series soils consist of well-drained fine sandy loams about 2 feet thick over
fine sand. These soils formed in mixed material that was deposited on flood plains
by rivers and streams. Deposits of the Aqua series are found on the flood plain of
the Santa Cruz River. Slopes are generally level and runoff is slow. Hence, the
hazard of erosion is slight to moderate.
Interstate 10 Traffic Interchange at
Twin Peaks/Linda Vista
4-5
Project No.: NH-010-D (AIW)
TRACS No.: 10 PM 236 H5838 01D