3-D Model Observations
During the week of 4-7 November 2002, engineers from the Alaska District
participated in tests using the 3-D Cook Inlet flow table model. All of the large-
scale flow structures observed in the idealized models during flood and ebb tide
were seen in the 3-D model. This included flow separation at the major
headlands (Point Woronzof, Point MacKenzie, and Cairn Point), large slow-
moving gyres in the lee of the headlands, and areas of ebb-flow reduction at the
Port of Anchorage.
Previously, flood flow tests in the idealized models indicated a strong cross-
channel bottom flow originating just north of Point Woronzof and crossing the
inlet diagonally toward Port MacKenzie. The mechanism for this transport is
local flow acceleration and flow separation at Point Mackenzie which decreases
the local water elevation and creates a cross-channel momentum inbalance that is
alleviated by cross-channel mass transport at the bottom. During the idealized
model tests it was hypothesized that the vertical sides of the different depth
terraces might unrealistically be contributing to the cross-channel flow.
However, the same cross-channel transport occurred in the 3-D model at
approximately the same location as observed in the idealized model. In the 3-D
model it seemed that the cross-channel flow was weaker, but this may have been
the result of reduced model discharge used in the 3-D model.
Impacts related to shifting of the Fire Island shoals were examined by
placing new shoals fabricated out of floral clay into the main channel on the west
side of Fire Island. This modified the local flow patterns, but no detectable
changes seemed to occur farther upstream in the vicinity of the Port of
Anchorage. However, the shifting shoals might create conditions suitable for
additional entrainment of fine sediment from the mud flats which in turn could
result in increased dredging requirements at the Port of Anchorage.
A key discovery of the idealized model tests was the formation of an area of
reduced flow velocity in the lee of Cairn Point during ebb tide. This was thought
to be the primary cause for siltation at the Port of Anchorage. Simulation of
maximum ebb flow over the actual bathymetry of the 3-D model generated the
same area of reduced flow at the Port of Anchorage as was observed in the
idealized models. Figure 67 shows ebb currents moving surface tracers past
Cairn Point. Most of the tracer has already moved downstream with the
exception of the tracer particles caught in the gyre formed in the lee of Cairn
Point. Within the gyre the tracer particles slowly circulated in a counter-
clockwise direction. Fine sediment entrained in the water would have ample
opportunity to deposit on the bottom.
Dye injected near the bottom in the reduced-flow region adjacent to the Port
of Anchorage also exhibited lengthy residence times before moving on
downstream as shown in Figure 68. Dye injected upstream of Cairn Point near
the east bank of the inlet ended up being trapped in the gyre south of the point.
The extent of flow separation and entrainment at Cairn Point during ebb flow
was estimated and sketched on the aerial photograph shown in Figure 69. During
flood flow in the model, surface tracers and subsurface dye moved freely
northward past the port indicating that the volume of deposited sediment would
be significantly less during flood tide.
Chapter 7 Three-Dimensional Cook Inlet Model