6.1. INTRODUCTION TO MOVABLE-BED MODELS
239
a. Knowledge of the character of the dominant forces, and
b. An understanding of the dominant response mechanisms of
the sediment
Dean (1985) went on to point out that
. the state of knowledge regarding
movable-bed modeling of the nearshore zone is largely based on empiricism”.
In the absence of fundamental knowledge of the dominant processes
and associated sediment response necessary to develop scale relationships,
movable-bed scale models can be used to investigate the effects of certain parameters in systematic ways to establish general behavior patterns
(Gourlay 1980). Alternately, the researcher can abandon the idea of reproducing the dominant physical processes and instead attempt to maintain
similarity of important observed engineering characteristics such as beach
profile shape or longshore transport rates (Hudson et al. 1979).
Regardless of the approach taken to develop scaling relationships for
movable-bed models, the nearly unanimous opinion among researchers is
that it is important to verify the scaling laws by reproducing prototypescale events. Preferably, the scale model should be validated using field
data, but often this is not practical, and large-scale laboratory results must
suffice. Only after validation can credence be given to the model results, and
then only for situations which seem to be governed by the same processes
that were assumed dominant during the validation testing. That is to say,
for example, a movable-bed model validated for surf zone sediment response
is not necessarily valid for application outside the surf zone where different
physical mechanisms may be governing the transport of sediment.
This leads to the axiom
Scale laws should be derived with a main requirement
of invariability of the scale for the material transport
over the entire area of the model concerned. (Bijker
1967).
Under such constraints, situations where sediment is transported by significantly different physical mechanisms in different regions usually cannot be
modeled simultaneously except at prototype scale.
In spite of the problems associated with the use of movable-bed physical models, researchers still strive to improve our capabilities with these
engineering tools. Dean (1985) summarized the role of physical models
(including movable-bed models) by stating that they will continue to be
important engineering tools for several decades because:
a. They do not require mathematical quantification and representation of the physical processes as do numerical models.
239
a. Knowledge of the character of the dominant forces, and
b. An understanding of the dominant response mechanisms of
the sediment
Dean (1985) went on to point out that
. the state of knowledge regarding
movable-bed modeling of the nearshore zone is largely based on empiricism”.
In the absence of fundamental knowledge of the dominant processes
and associated sediment response necessary to develop scale relationships,
movable-bed scale models can be used to investigate the effects of certain parameters in systematic ways to establish general behavior patterns
(Gourlay 1980). Alternately, the researcher can abandon the idea of reproducing the dominant physical processes and instead attempt to maintain
similarity of important observed engineering characteristics such as beach
profile shape or longshore transport rates (Hudson et al. 1979).
Regardless of the approach taken to develop scaling relationships for
movable-bed models, the nearly unanimous opinion among researchers is
that it is important to verify the scaling laws by reproducing prototypescale events. Preferably, the scale model should be validated using field
data, but often this is not practical, and large-scale laboratory results must
suffice. Only after validation can credence be given to the model results, and
then only for situations which seem to be governed by the same processes
that were assumed dominant during the validation testing. That is to say,
for example, a movable-bed model validated for surf zone sediment response
is not necessarily valid for application outside the surf zone where different
physical mechanisms may be governing the transport of sediment.
This leads to the axiom
Scale laws should be derived with a main requirement
of invariability of the scale for the material transport
over the entire area of the model concerned. (Bijker
1967).
Under such constraints, situations where sediment is transported by significantly different physical mechanisms in different regions usually cannot be
modeled simultaneously except at prototype scale.
In spite of the problems associated with the use of movable-bed physical models, researchers still strive to improve our capabilities with these
engineering tools. Dean (1985) summarized the role of physical models
(including movable-bed models) by stating that they will continue to be
important engineering tools for several decades because:
a. They do not require mathematical quantification and representation of the physical processes as do numerical models.