3.0 Affected Environment - Knik Arm Bridge and Toll Authority

Knik Arm Crossing DraftFinal EIS
Affected Environment
3.8.1.2 Knik Arm
3.8.1.2.1 Hydrology – tides
Knik Arm experiences the second-largest tidal range in the world—approximately 30 feet.
Tides are semidiurnal, with two unequal high tides and two unequal low tides per day. The
range of tides at Anchorage is 29 feet and the observed extreme low water is 6.4 feet below
MLLW (Table 3-26). Tidal energy is the most dominant force driving water circulation in
Knik Arm. Because of predominantly shallow depths, tides within Knik Arm have a much
larger range than in the main body of Cook Inlet (KABATA 2006r).
Table 3-26. Anchorage tidal data
Tidal measure
Abbreviation
Magnitude
(in feet with
respect to MLLW)
Highest observed water level (October 24, 1980) NA a 34.6
Mean higher high water MHHW 29.2
Mean high water MHW 28.4
Mean sea level MSL 16.5
Mean tide level MTW 15.3
Mean low water MLW 2.2
Mean lower low water MLLW 0.0
Lowest observed water level (December 25, 1999) NA a -6.4
a not applicable
Source: NOAA 2004
3.8.1.2.2 Hydrology – currents
Currents in the Study Area are primarily influenced by tidal flow. However, freshwater
inputs into the Knik Arm result in a stronger ebb (westerly) flow than flood (easterly) flow
(KABATA 2006r). Maximum current speeds in Knik Arm, observed during spring ebb tide,
exceed 7 knots (12 feet/second) and are approximately 6 knots (10 feet/second) within the
Study Area. Maximum current speeds persist for about an hour before subsiding to near-zero
speed and then reversing. The time between slack water times is about 6.2 hours.
3.8.1.2.3 Hydrology –sea ice
When air temperatures begin to drop below 20°F, sea ice begins to form in Knik Arm as
frazil (loose, randomly oriented ice crystals), but will not persist because of wind or tidal
mixing of the water column. By late November, ice in Knik Arm typically will be either new
or frazil-formed “shuga,” or pancake ice sheets (up to 4 inches thick). Shuga ice can cover up
to 50 percent of the area of Upper Cook Inlet by the end of December. In spite of the
coverage, ice in Knik Arm rarely forms columnar layers of crystals because of tidal
turbulence. Nonetheless, ice rafts (formed from fusion of these pancake ice sheets) and brash
ice (ice that has been broken up by turbulent tidal movement) as thick as 2½ feet thick has
been reported in the Upper Cook Inlet area (Mulherin et al. 2001).
Ice forming in Knik Arm will typically not be as strong as the river ice entering from the
Knik and Matanuska Rivers or from other contributing drainages, but will be comparably
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