Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...

Powerline Flood Retarding Structure
Pinal County, AZ
Draft Supplemental Watershed Plan
and Environmental Assessment
of the channel crossing a yellow moderate FRZs will be hardened structures (concrete lined) with
upstream and downstream permeable backfill trenches wrapped in geotextile. Channel segments in
green FRZs will be earth-lined channels. The Powerline channel drains into the Vineyard Road
FRS flood pool upstream of the north auxiliary spillway. The excess spoil material from the
channel excavation is anticipated to be stockpiled within the Sponsor’s easement at locations to be
determined with the land owner – Arizona State Land Department.
8.2.2 POWERLINE NORTH DIVERSION CHANNEL
The section of the Powerline North Diversion Channel between Highway 60 and Baseline Road has
several local washes entering the channel from the east. The confluence of the washes with the
channel is experiencing erosion and local headcutting up into the washes. Stabilization of the
channel banks and confluences with the local washes has been conceptually developed and an
exhibit is provided in Appendix C. Construction cost estimates to stabilize the channel cross
section have also been developed.
The existing channel at Baseline Road is headcutting into the right-of-way of Baseline Road and
into the Baseline Road embankment fill on the south side of the road. Stabilization of the channel at
this location consists of rock-filled gabion baskets for grade control and rock riprap for scour
protection. Exhibits showing the locations of the channel improvements are included in Appendix
C as Figure C-8. The 48-in culverts under Baseline Road will be replaced with two 10-ft wide by
6-ft high box culverts similar to the crossing under US 60.
8.2.3 POWERLINE FRS EMBANKMENT REMOVAL
The Preferred Alternative includes construction costs to remove the embankment for the Powerline
FRS and the Powerline IDSM. The removal of the embankments would occur after construction of
the new Powerline Channel. The location of the embankments would be graded, stabilized, and
hydroseeded with native plant mix. The excess spoil material from the embankment removal is
anticipated to be stockpiled within the Sponsor’s easement at locations to be determined with the
land owner – Arizona State Land Department.
8.3 MITIGATION FEATURES
Clearing and excavation of a new Powerline 100-year channel, improvements to the North
Diversion Channel, and FRS and IDSM removal and construction of an O&M road on either side of
the channel will temporarily disturb existing vegetation including portions of the existing mesquite
bosque. Disturbed areas due to construction activities will be hydro-seeded with native plant mix.
The mitigation may include planting new tall-pot mesquite trees (approximately 60 acres) and
salvage of some mature mesquite trees and transplanting in designated areas (e.g., along Powerline
Channel, and potential mitigation area at north end of Vineyard Road FRS). The Sponsor and
NRCS will coordinate with ASLD regarding trees along the existing low flow channel along the
Powerline FRS. Mitigation plans will be refined during final design.
Weed prevention methods will be implemented during construction activities. These methods will
be determined in final design but may include: use of certified weed free seed, sterilized straw
mulch with a tackifier so the straw mulch holds onto the ground, washing vehicles off before they
enter the site, and managing adjacent weed populations to the extent possible (to prevent seed
drift). Also, the use of pre-emergent herbicide could be considered for any non-planted areas that
USDA- NRCS Page 8-3 January 2013
Kimley-Horn and Associates, Inc.