The main concern related to incorrect boundary friction is flow moving
around bends where the centrifugal acceleration is countered in part by bottom
friction. After observing flow characteristics for a flood flow simulation, the
model was dried, and a coat of rubber cement was applied to the Plexiglas in a
rough, thick layer with cross-channel brush strokes. When dried, the glue layer
provided a rough surface. Observations of flow patterns with the roughened
boundary were not noticeably different from those previously observed.
However, it is important to note this was not a rigorous test, and this issue should
system.
Conclusions from Cook Inlet 3-D Model
The 3-D model of Cook Inlet featured actual bathymetry reproduced at very
small scale and with significant geometric distortion. Nevertheless, the large-
scale flow features such as flow separation at major headlands and gyre
formations resembled known flow behavior observed at the study site. The
capability of extending the study area in both directions by adding and removing
sections enabled more replication of the critical upstream boundary conditions,
thus assuring better simulation of flow conditions in the region of interest. The
conclusions drawn from the 3-D model task are as follows:
a. Tidal flow in upper Cook Inlet features large, 3-D, gyres formed by flow
separation at major headlands. The gyres exhibited vertical and lateral
mixing, and regions of reduced flow velocity magnitude corresponded
well with shoal areas observed in the prototype.
b. Turbulent flow separation at Cairn Point during ebb tide was shown to be
instrumental in causing shoaling of the Port of Anchorage. This shoaling
mechanism was first observed in the idealized models, but there was
concern that the terracing of bathymetry in the idealized models might be
contributing to the flow pattern. The 3-D model confirmed this was not
the case.
c. Dye injection during flood and ebb tide demonstrated that dredge
disposal practices could be improved by choice of disposal sites and
timing dump releases to correspond with tidal flow direction. This
reduces the possibility of dredged material being immediately carried
back into the harbor area.
d. Dredging along the dock face without leaving a pathway to the deeper
channel could promote trapping of suspended sediment and increased
sedimentation in the harbor region.
e. Modifying the Fire Island shoals did not appear to affect flow patterns
farther upstream around Anchorage during flood tide. However, this
conclusion is based only on visual observation without any quantifying
measurements.
f.
The smooth boundaries of the physical model might impede
development of fully turbulent flow, and thus represent a scale effect.
However, addition of surface roughness did not have any impact on the
overall flow conditions observed in the model. This may be the result of
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Chapter 7 Three-Dimensional Cook Inlet Model
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