I-10 Twin Peaks Traffic Interchange, Environmental Assessment

Affected Environment and Environmental Impacts October 2005
of the Santa Catalina Mountains while other minor washes originate in the
Tortolita Mountains located to the northeast of the study area. According to the
Drainage Report, runoff from Tortolita Mountains to the Santa Cruz River is
impeded by the UPRR, the I-10 westbound frontage road, and the I-10 mainline.
The UPRR is located on the upstream side of the westbound frontage road, and,
because the existing cross drainage structures are undersized, this causes flow
from the upstream watersheds to pond at the drainage structures. Excess flows not
conveyed by cross drainage structures continue to the northwest along the
upstream side of the UPRR to the next cross drainage structure. This pattern is
consistent throughout the study area, and continues beyond the limits of the study
area.
Additional cross drainage structures are located under the I-10 frontage roads and
mainline. Generally there is adequate drainage capacity to prevent the 50-year
rainfall event from ponding on the I-10 frontage roads and mainline because of a
combination of: 1) the metering of flows to the roadways by the UPRR drainage
structures; 2) the capacities of the I-10 frontage road and I-10 mainline drainage
structures; and 3) the capacities of the roadside ditches which transmit flows along
the roadways to the northwest. In one area north of the Twin Peaks Road TI and
one location south of the Twin Peaks Road TI, however, runoff exceeds the
capacity of this system. In these locations, water can flow over the westbound
frontage road and the I-10 mainline during a heavy rainfall event.
There are no cross drainage structures along El Camino de Mañana and Linda
Vista Boulevard within the study area and stormwater runoff currently overtops
the surface of these roadways.
Impacts
No Build Alternative
Under the no build alternative, no impacts to surface water resources would result
and no improvements to the drainage facilities near I-10 would occur; however,
drainage improvements would occur as a part of the ultimate freeway
improvements proposed in the I-10 General Plan. According to PAG’s 2025
Regional Transportation Plan (as amended), I-10 is proposed to consist of 8 lanes
by 2025 from the Pinal/Pima County line to the I-10/I-19 Interchange. However,
these improvements are not programmed (planned and funded) in PAG’s 2005-
2009 Transportation Improvement Program; therefore, it may be assumed that
these improvements would be programmed between the years 2010 and 2025. As a
result, the potential for a heavy rainfall event to cause water to flow over the
westbound frontage road and the I-10 mainline in the project area would remain
until these improvements were completed.
Under the no build alternative, stormwater runoff that currently flows over the
surface of El Camino de Mañana and Linda Vista Boulevard would not be routed
under the roadways. As a result, surface flows that may damage the structure of
the roadways and pose a safety challenge to motorists would continue.
Interstate 10 Traffic Interchange at
Twin Peaks/Linda Vista
4-10
Project No.: NH-010-D (AIW)
TRACS No.: 10 PM 236 H5838 01D