h. The terraced bathymetry of the idealized models appeared to inhibit
water exchange between depth levels. Nevertheless, idealized flow table
models provide useful qualitative insight and semiquantitative results at a
modest cost. The key points are to be aware of model limitations and to
assure that observed flows in the model resemble those observed in
nature.
i.
Tidal flow leaving the harbor across a vertical transition is redirected
seaward closer to shore where the transition step is highest. Flow
leaving the harbor over a sloping transition remained straight and mostly
uniform. Neither transition should cause an increase of suspended
sediment deposition potential as flow leaves the harbor region.
j.
Tidal flow entering the harbor across a vertical transition creates a
turbulent wake region that has suspended sediment deposition potential
in the immediate area downstream of the step. In addition, fluid is
advected laterally along the vertical cut which would move sediment
shoreward toward the mooring area. Replacing the vertical transition
with a sloping transition significantly reduced the turbulent wake
resulting in smoother flow patterns and less likelihood of sediment
deposition.
The value of small-scale, 3-D physical models was amply demonstrated by the
Alaska District engineers who used the model to understand more fully some of
the probable reasons for sedimentation at the Port of Anchorage, and to develop
cost-saving dredging procedures at the port. Being able to visualize the flow
with dyes and tracers provided insight and instant feedback to enhance their
understanding about the primary flow processes.
Turbulence scale effects study conclusions
a. 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.
b. Evaluation of potential turbulent scale effects in proposed distorted
physical models requires good understanding of dominant flow patterns
in the prototype along with knowledge of boundaries where flow
separation and/or turbulence generation might occur. Evaluation must
be site-specific.
c. Theoretical analysis showed that flow around bends in the absence of
flow separation is reasonably well simulated in a distorted model, and the
physical model includes more physical reality than depth-averaged
numerical models. However, bottom roughness may play a critical role
in correct similitude of river bend flows.
d. Flow separation and turbulence generated at vertical edges will not have
a scale effect in geometrically distorted models because vertical turbulent
fluctuations are small.
e. Flow separation at vertical edges with constraining downstream jetties
will be similitude in distorted models with good correspondence in
resulting flow patterns in the main flow and in the entrainment region.
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Chapter 8 Summary and Conclusions
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