I-10 Twin Peaks Traffic Interchange, Environmental Assessment

Affected Environment and Environmental Impacts October 2005
As a result, a modified design alternative was developed and analyzed. This
modified design consisted of longer bridges with shorter embankments, and an
accompanying widening of the low flow channel of the Santa Cruz River. The
proposed design (the preferred alternative) consisted of proposed 750-foot long
twin bridges over the low flow channel and a widening of the low flow channel to
a 610-foot top width at the bridge crossing. The modeled channel widening
transitioned back to the existing 400-foot width over a distance of approximately
1,500 feet upstream and downstream of the bridge crossing. The modeled
widening was assumed to occur entirely on the west side of the existing low flow
channel because the existing bank protection on the west side of the channel
exhibits undercutting and is in need of repairs. This alternative would not increase
flood elevations by more than 0.1 foot; therefore, this alternative was advanced for
preliminary design and environmental investigation.
The alternatives and analyses within the floodplain were developed in consultation
with several local, state, and federal water resource and floodplain management
agencies. Meetings or telephone conversations with ADOT and the U.S. Army
Corps of Engineers (Corps) were held to obtain current information on
development and proposed actions in the affected watersheds. These discussions
helped also guide the preliminary alternatives design. The resulting analytical
reports were shared with the Town of Marana Floodplain Coordinator and the
Arizona State Floodplain Coordinator; therefore, the preferred bridge alternative is
consistent with existing watershed and floodplain management programs.
Although removal of vegetation during construction would occur, the Town of
Marana would minimize vegetation removal and would develop a revegetation
plan that would improve the value of the vegetation in the disturbed area of the
floodplain. This is discussed in the Threatened and Endangered Species Section
(page 4-26). As a result, the quality of habitat to be used by wildlife, would be
improved with no reduction in the ability of the floodplain to mitigate the 100-year
flooding event.
As discussed above, the preferred bridge alternative is the only practicable
alternative. The project must be located within the floodplain to enhance
transportation connectivity across the Santa Cruz River. Although other
alternatives were considered, these alternatives were not practicable because they
could not meet freeboard requirements or they resulted in an unacceptable increase
in flood elevations. The preferred bridge alternative conforms to applicable State
and local floodplain protection standards.
Mitigation
The preferred alternative would implement measures to minimize floodplain
impacts to the proposed improvements and to minimize floodplain impacts caused
by the action. Based on the bridge analysis, the substructure and foundation of the
bridge would be designed for appropriate scour depths to minimize risks to the
structures. Twin Peaks Road, Linda Vista Boulevard, and El Camino de Mañana
Interstate 10 Traffic Interchange at
Twin Peaks/Linda Vista
4-18
Project No.: NH-010-D (AIW)
TRACS No.: 10 PM 236 H5838 01D