3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
stronger than ice formed in the main body of the inlet, where salinity is greater
(Michel 1978). Outgoing tides carry ice out of the Knik Arm and mix it with ice from other
sources in the main body of Cook Inlet before it flows back into Knik Arm. Ice also develops
on the tidal flats during tidal inundations, particularly as the tide flows out from the flats. The
resulting frozen material comprises the majority of any land-fast ice that may be encountered
in Knik Arm.
Ice also forms on structures placed into either the tidal flats or offshore, much in the same
manner as beach ice. Cold metal pilings are particularly susceptible to ice formation, and
successive layers of structural ice coatings will form enlarged “ice collars” that increase the
cross-sectional width of such pilings. When these “collars” thaw, they may release essentially
intact and are thus capable of damage to their host structure (KABATA 2006r).
3.8.1.2.4 Hydrology – sedimentation
Bluff erosion and glacially fed rivers are the primary contributors to the sediment load of
Knik Arm waters. The Knik and Matanuska Rivers contribute the largest suspended load to
Knik Arm, with average summer loads (mid-May to mid-October) estimated at 6.84 million
tons and 5.45 million tons, respectively. Additionally, the bluffs along Knik Arm are
believed to provide a significant contribution to the inlet water sediment load
(KABATA 2006r). The bluffs originally formed as a result of tidal forces cutting through the
area following glacial withdrawal (Lade 1985), and are being continuously eroded by wind,
rain, slope failure, and intermittent exposure of the toe to wave action.
Smith et al. (2005) conclude that Knik Arm has two distinct types of sediment transport in
the vicinity of the proposed crossing alignment. Sandy bed sediment is transported along the
bed surface, and finer silt and clay particles remain in suspension and are carried past the
corridor of the proposed crossing location by prevalent strong tidal currents.
KABATA (2006r) and Smith et al. (2005) report that sand is the dominant material of the
bed in the vicinity of the proposed crossing and is transported through this area in large
patches of mobile sand waves. Areas of coarser material do exist (e.g., the west channel sideslope),
which suggests that currents in this area are consistently strong enough to sweep
smaller particles away. Little of the Study Area appears naturally prone to accumulation of
silt and clay. Isolated areas dominated by silt appear in deeper areas of the channel and on the
southeast tidelands of the proposed crossing corridor.
In areas of the Knik Arm where water velocity is sufficiently low, such as areas of shallow
water or around natural or human-made protrusions, suspended fines can settle from the
water column and deposit on the seafloor. Protrusions into Knik Arm function essentially like
groins, with sediment accumulating until stable “fillets” are formed on both sides. One
example in the vicinity of the Study Area is the Port MacKenzie dock, which protrudes
850 feet from the shoreline to a water depth of about 40 feet. The sediment fillets that formed
on both sides extend about 1,000 feet along the original shoreline and appear to have reached
a stable form within 2 years after dock construction.
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