I-10 Twin Peaks Traffic Interchange, Environmental Assessment

Affected Environment and Environmental Impacts October 2005
Nation’s floodplains; 2) to avoid longitudinal encroachments, where practicable;
3) to avoid significant encroachments, where practicable; 4) to minimize impacts
of highway agency actions which adversely affect base floodplains; 5) to restore
and preserve the natural and beneficial floodplain values that are adversely
impacted by highway agency actions; 6) to avoid support of incompatible
floodplain development; 7) to be consistent with the intent of the Standards and
Criteria of the National Flood Insurance Program, where appropriate; and 8) to
incorporate “A Unified National Program for Floodplain Management” of the
Water Resources Council into FHWA procedures.
To determine compliance with EO 11988 and 23 CFR 650, the Federal Emergency
Management Agency (FEMA) floodplains map of the project area was obtained
and a location hydraulic study for the proposed improvements was conducted. The
elements of this study are contained within the Interstate 10 Traffic Interchange at
Twin Peaks/Linda Vista Santa Cruz River at Twin Peaks Road Existing Hydraulics
Report, dated January 14, 2004 and the Interstate 10 Traffic Interchange at Twin
Peaks/Linda Vista Santa Cruz River at Twin Peaks Road Preliminary Bridge
Hydraulics Report, July 8, 2004. The reports are summarized in the following
discussion; however, readers wishing a greater level of detail are referred to the
full reports.
Existing Conditions
The Santa Cruz River has been modified extensively within the study area to
reduce the threat of flooding to nearby developments. According to the
Continental Ranch Specific Plan, from Cortaro Road north for approximately 3
miles (downstream), the Continental Ranch area developer constructed an
approximately 2,000-foot wide floodway, which is referred to as the high flow
channel of the Santa Cruz River. Within this floodway, the perennial flow of the
Santa Cruz River is maintained within a bank-protected channel of approximately
400 feet in width, which is referred to as the low flow channel. The low flow
channel contains the approximate flows of a 10-year rainfall event and the high
flow channel contains the flows of the 100-year rainfall event.
Flood Insurance Rate Maps (FIRM) have been prepared and published by FEMA
for the Twin Peaks study area (FIRM 0419C1015K and 0419C1605K effective
date February 8, 1999). The portion of the project area within the high flow
channel of the Santa Cruz River is located in Zone AE. Zone AE is defined as
areas of 100-year floodplain with base flood elevations. Within the project area,
the base flood elevations vary from 2088 feet above sea level in the northern
project area to 2135 feet in the southern project area. The entire project area east of
the UPRR is located in Zone AH. Zone AH is defined as areas of 100-year
floodplain with shallow flooding of an average depth of between one and three
feet. Twin Peaks Road from Coachline Boulevard east to the high flow channel of
the Santa Cruz River is located in Zone X, as are portions of the area from the east
bank of the Santa Cruz River high flow channel to the eastbound I-10 frontage
road. Zone X is defined as areas: outside the 100-year flood plain; within the 100-
Interstate 10 Traffic Interchange at
Twin Peaks/Linda Vista
4-15
Project No.: NH-010-D (AIW)
TRACS No.: 10 PM 236 H5838 01D