EM 1110-2-1100 (Part II)
30 Apr 02
(d) Circulation patterns are typically measured by photographing the movement of floats. Model
exchange coefficients and flushing efficiencies are measured by adding dye to the water at the start of tests
and then measuring the decrease of the dye concentration during subsequent tidal cycles. While exchange
coefficient values from the model and model circulation patterns do not precisely equate to prototype
conditions, these harbor model studies can provide a basis for comparing alternative design features for a
proposed harbor or guidance in modifying an existing harbor.
(e) Table II-7-4 lists, for ease of comparison, the relative advantages of physical and numerical models
for harbor flushing and circulation studies. The table gives a general comparison, but for a specific
application, modelling experts should be consulted before deciding whether to use a numerical or physical
model or some combination of both.
Table II-7-4
Advantages of Physical and Numerical Models
Physical Models
Provide good visual demonstration of flow patterns
Some three-dimensional effects can be represented relatively easily
Intricate harbor boundaries can be easily simulated
Numerical Models
Wind stress and Coriolis effects can be simulated
Lower model development and maintenance costs
Easy to store model for future use
Easy to adjust or expand model boundary conditions
Extensive output data can easily be obtained
(3) Field studies.
(a) Limited field studies at an existing harbor may be conducted to obtain sufficient data to calibrate a
numerical model of that harbor so the model can be run to investigate a range of other tide and wind
conditions. This would provide a detailed look at the flushing and circulation characteristics of the harbor
and some insight into possible remedial efforts that may be necessary. Or the model may then be run to
evaluate proposed modifications of the harbor.
(b) An alternative is to run more extensive field studies as the sole effort to evaluate conditions at a
harbor. This would generally be more costly than the hybrid field-model approach, but it may provide some
detail that cannot be achieved from model studies alone.
(c) Also, field studies have been done to support the general development of physical and numerical
modelling techniques for the study of harbor flushing and circulation.
(d) Field measurements include those that define the hydrodynamics of a harbor and supplementary
measurements to quantify harbor flushing. The former include measurements of tide levels inside and outside
the harbor, current velocity measurements at the entrance to quantify flow rates into and out of the harbor,
and flow velocity measurements throughout the harbor and/or drogue studies to define circulation patterns
in the harbor. If tidal flushing is the primary concern, these measurements would be conducted on days when
the wind velocity is low. Otherwise, a directional anemometer would also be used to measure the wind speed
and direction.
(e) To determine exchange coefficients throughout the harbor and the harbor's flushing efficiency, the
harbor would be uniformly seeded with a harmless detectable solute such as a fluorescent dye and then
Harbor Hydrodynamics
II-7-53
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