3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
Additional information on these gravel sources is provided in the Knik Arm Crossing
Gravel Findings Technical Report (KABATA 2006m).
• East bluff and Anchorage alternatives. The upper part of the 70-foot-high bluff near the
east end of the proposed bridge contains about 20 feet of interbedded sand, gravel, and
peat associated with Elmendorf Moraine, which is underlain by sand and clay of the
Bootlegger Cove Formation. The bluffs along the proposed Below-the-Bluff Roadway and
Government Hill contain 20 to 85 feet of outwash sands and gravels from the Naptowne
Glaciation, underlain by the Bootlegger Cove Formation (KABATA 2006j, 2006n; Updike
and Carpenter 1986). The sensitive clay unit of the Bootlegger Cove Formation occurs
between about mean sea level and 50 feet in elevation along the Knik Arm and Ship Creek
bluffs (Updike and Carpenter 1986). Surficial deposits in Ship Creek valley consist of up
to 25 feet of gravelly sand alluvium underlain by Bootlegger Cove silty clay
(KABATA 2006n).
Knik Arm subsurface sediment
Like the deposits onshore, geologic units beneath Knik Arm mostly resulted from successive
periods of glaciation, leaving behind a complex mix of Quaternary till, moraine, outwash,
and marine deposits that exhibit limited lateral continuity (Figure 3.27). Deposits from the
Knik and Naptowne Glaciations extend partially or completely beneath Knik Arm, and are
overlain by younger marine sand. These deposits have been divided into four units in
geotechnical studies completed in 2004 for the proposed KAC project. From roughly
youngest to oldest, they include:
• Marine sand. Shannon & Wilson (KABATA 2006j) indicate the presence of up to 40 feet
of loose to medium-dense marine sand at the seafloor surface across much of the crossing.
Locally, these deposits thin on the east side of Knik Arm to less than 10 or 15 feet, are
very thin or absent in the deepest part of the channel, and are absent on the west side. The
sands are somewhat mobile and tend to shift over time with changing currents and tides.
This deposit likely formed during the Holocene sea level rise as sediments were deposited
on top of the eroded Bootlegger Cove Formation seaward of the Knik Arm bluffs.
• Glacial till or moraine deposits. A very dense glacial till-type of deposit underlies the
marine sand on the west and east sides of Knik Arm, and appears to have been eroded
away in the center of the channel. This unit extends into both bluffs and may be the
equivalent of Knik Glaciation deposits on the west side of Knik Arm and the lower
Bootlegger Cove Formation on the east side (MSB 1995a; KABATA 2006j; Updike and
Carpenter 1986).
• Glacial lake clays or marine/alluvial sands. A thick section of glacial lake clays and
marine/alluvial sands underlie the till unit on the sides of Knik Arm and beneath the loose
marine deposits in midchannel. This unit is thin or absent near the west bluff, thickens to
over 200 feet of dense sand beneath the Knik Arm main channel, and
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