Hydraulic Effects of Proposed Knik Arm Crossing

Knik Arm, an extension of Upper Cook Inlet, with the world's second highest tides, seasonal ice cover, and prodigious sediment load, presents unique challenges to the design and construction of a highway crossing. The current concept for a proposed Knik Arm Crossing (KAC) structure includes earthen-fill approaches from either side, connected by an 8,180 ft (2,493 m) pile-supported bridge, to span the 12,500-ft (3,810 m) width of Knik Arm at the preferred alignment. Described herein are the methods, results and conclusions of analyses performed for an assessment of the effects of KAC on the hydraulic environment of Knik Arm. Principal effects include increases in maximum current speed of ≤10% within 1.3 mi. ( 2.2 km) of KAC. Except for expected sediment accumulations on both sides of the earthen-fill portions of KAC, no significant changes in sedimentation processes are expected beyond the immediate vicinity of KAC.