they still produced large-scale turbulent flow features known to exist in upper
Cook Inlet. Furthermore, the location and areal extent of the gyres and eddies
were qualitatively the same as in the prototype. The following conclusions were
drawn from the idealized model task based on visual observations, limited
velocity measurements (not included in report), and discussions among the test
participants:
a. Turbulent flow in Cook Inlet stemming from tidal flow separation at
major headlands introduces substantial turbulence into the flow regime
with some 3-D effects. Vertical mixing was evident as were cross-
channel flows due to water elevation differentials.
b. It appeared that the turbulent flow separation at Cairn Point during
ebb tide was a significant factor in the sedimentation of the Port of
Anchorage, which lies in the lee of the point. Current velocites were
reduced to the point that suspended fine sediments had time to settle in
the port area.
c. The apparent importance of the 3-D flow components to shoaling at the
Port of Anchorage may make application of two-dimensional (2-D),
depth-averaged tidal flow numerical models problematic.
Consequently, natural processes such as shifting of major shoals could
have significant impact that might lead to additional maintenance
dredging requirements. Being able to forecast these requirements by
monitoring shoal development would be an important achievement.
e. The terraced bathymetry of the idealized models appeared to inhibit
water exchange between depth levels. More accurate bathymetry will
most certainly produce more reliable flow patterns.
f.
Geometric distortion of the idealized models will have some impact on
the turbulent flow structures, although this impact is difficult to quantify.
In general, any vertically-directed turbulent velocities will be greater in
the model than in the prototype. However, the steeper slopes in the
distorted model will tend to generate less vertical fluid motion. Surface
flow characteristics of flow separation and entrainment are in good
similitude, but the 3-D structure farther down in the water column may
have a scale effect. Flow around bends in the absence of flow separation
is reasonably well simulated in a distorted model, and includes more
physical reality than depth-averaged numerical models. However,
bottom roughness may play a critical role in correct similitude of river
bend flows.
g. Idealized flow table models provide useful qualitative insight and semi-
quantitative results at a modest cost. The key points are to be aware of
model limitations and assure that observed flows in the model resemble
those observed in nature.
Based on the May 2002 experiments with the idealized models, the Alaska
District decided to fund construction of another large-scale flow table model
featuring actual bathymetry. Description of the 3-D flow table model and study
results are documented in Chapter 7.
22
Chapter 3 Idealized Cook Inlet Models
Previous Page
