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CHAPTER 6. SEDIMENT TRANSPORT MODELS
measurements of sediment transport rates is the most difficult because field
methods for making these types of measurements are still being developed.
There will be some of situations for which no prototype data exist, and
in these instances it is advisable to conduct a scaling series. The same test
condition is run at different scales to see if the scaling is correct. However,
extrapolation of model results to prototype should not be done without
careful thought and analysis. Kamphuis and Nairn (1984) discussed the
validity of scale model extrapolation in reference to models of sand islands.
They stated that extrapolation appeared valid for short prototype-scale
time periods.
The importance of movable-bed model verification to the engineering
problem being studied was summed up by Motta (1986) when he said:
The reproduction of known prototype conditions in the present
and the past constitutes the experimental evidence that scale effects which may have occurred have not damaged the study in
reference.
6.6.2 Movable-Bed Model Measurements
In movable-bed models, it is important to document bed evolution with
periodic measurements of model bathymetry. Obtaining spatial measurements of bed elevations in the model is very laborious unless the procedure
has been automated and the soundings are recorded by computers. Threedimensional models in basins require a large volume of water, and it is not
practical to drain the facility every time intermediate bathymetry measurements are needed. Furthermore, the draining process itself can cause strong
water currents that may alter the bed features. Intermediate bed surveys
can be made from a supporting grid suspended over the model, and the
soundings can be referenced to the grid coordinates.
Three-dimensional bed elevation contours can be documented by slowly
draining the basin water to known depths and marking the position of the
waterline on the exposed portions of the bed using string. Photographing
the contours from overhead provides a record of the bathymetric change.
Sometimes sufficient contrast between the exposed sand and the water
makes string placement unnecessary.
Tracer models conducted with a veneer of material can be surveyed
similar to fully mobile-bed models, however, this is usually not warranted
because the results are still considered qualitative unless the model has
undergone extensive verification. Documentation of injection-type tracer
models is normally performed photographically because water clarity usually is excellent, and the sediment paths and deposition areas are readily
CHAPTER 6. SEDIMENT TRANSPORT MODELS
measurements of sediment transport rates is the most difficult because field
methods for making these types of measurements are still being developed.
There will be some of situations for which no prototype data exist, and
in these instances it is advisable to conduct a scaling series. The same test
condition is run at different scales to see if the scaling is correct. However,
extrapolation of model results to prototype should not be done without
careful thought and analysis. Kamphuis and Nairn (1984) discussed the
validity of scale model extrapolation in reference to models of sand islands.
They stated that extrapolation appeared valid for short prototype-scale
time periods.
The importance of movable-bed model verification to the engineering
problem being studied was summed up by Motta (1986) when he said:
The reproduction of known prototype conditions in the present
and the past constitutes the experimental evidence that scale effects which may have occurred have not damaged the study in
reference.
6.6.2 Movable-Bed Model Measurements
In movable-bed models, it is important to document bed evolution with
periodic measurements of model bathymetry. Obtaining spatial measurements of bed elevations in the model is very laborious unless the procedure
has been automated and the soundings are recorded by computers. Threedimensional models in basins require a large volume of water, and it is not
practical to drain the facility every time intermediate bathymetry measurements are needed. Furthermore, the draining process itself can cause strong
water currents that may alter the bed features. Intermediate bed surveys
can be made from a supporting grid suspended over the model, and the
soundings can be referenced to the grid coordinates.
Three-dimensional bed elevation contours can be documented by slowly
draining the basin water to known depths and marking the position of the
waterline on the exposed portions of the bed using string. Photographing
the contours from overhead provides a record of the bathymetric change.
Sometimes sufficient contrast between the exposed sand and the water
makes string placement unnecessary.
Tracer models conducted with a veneer of material can be surveyed
similar to fully mobile-bed models, however, this is usually not warranted
because the results are still considered qualitative unless the model has
undergone extensive verification. Documentation of injection-type tracer
models is normally performed photographically because water clarity usually is excellent, and the sediment paths and deposition areas are readily