I-10 Twin Peaks Traffic Interchange, Environmental Assessment

Affected Environment and Environmental Impacts October 2005
year floodplain with sheet flow flooding where average depths are less than 1 foot;
in the 100-year floodplain with stream flooding where the contributing drainage is
less than one square mile; or, protected from 100-year flood events by levees. The
FIRMs for the study area are shown in Figure 4-2.
Impacts
No Build Alternative
Under the no build alternative, no affects to floodplains would occur.
Preferred Alternative
Based on the information contained in the FIRMs for the study area, the proposed
improvements constitute actions within the limits of the base floodplain; therefore,
compliance with EO 11988 and 23 CFR 650 must be demonstrated. These actions
within the 100-year floodplain include construction of: twin bridges spanning the
low flow channel of the Santa Cruz River; bridge and roadway embankments
within the high flow Santa Cruz River channel; bridge and roadway embankments
on the east side of I-10; a new signalized intersection at Twin Peaks Road/El
Camino de Mañana/Linda Vista Boulevard; and, reconstruction of Linda Vista
Boulevard. The floodplain analysis conducted for these improvements is discussed
in the following paragraphs.
The risks associated with the Santa Cruz River bridges described in Chapter 3,
Alternatives, were analyzed. Based on the scour analysis, the Town of Marana
would design the bridges’ substructures and foundations for these scour depths;
therefore, risks to the structures would be minimized.
Incompatible use or development within the floodplain would not be facilitated by
the proposed project. Any developments within the area must comply with the
Town of Marana or Pima County zoning and floodplain ordinances. The area east
of the proposed improvements is within the Tortolita Basin as defined by Pima
County. The County has designated this area as a critical basin, which requires that
developments provide sufficient stormwater detention to reduce predevelopment
peak flows.
Several improvement alternatives were developed and their associated impacts to
the 100-year floodplain were analyzed. Alternatives considered, but eliminated
from further consideration, included: 1) the no build alternative; 2) bridges
spanning the high flow channel of the Santa Cruz River; and, 3) bridges spanning
the low flow channel of the Santa Cruz River. These alternatives were eliminated
because they either did not address the stated purpose and need of the proposed
improvements (no build alternative) or they would increase flood elevations over the
allowable 0.1 foot according to ADOT and Pima County design standards (the two
bridge alternatives).
Interstate 10 Traffic Interchange at
Twin Peaks/Linda Vista
4-16
Project No.: NH-010-D (AIW)
TRACS No.: 10 PM 236 H5838 01D