1.5. OVERVIEW OF HYDRAULIC PHYSICAL MODELS
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Three-dimensional movable-bed models are much rarer, due in part to
the high costs associated with these models. Examples include studies of
erosion of oil drilling sand islands, littoral drift induced by oblique wave
approach, formation of sand spits, three-dimensional ripple formation, and
scour holes in the vicinity of structures.
A compromise between the fixed-bed and movable-bed model occurs
when sediment tracer materials are used in fixed-bed models as a qualitative indicator of sediment erosion and deposition areas.
Both fixed-bed and movable-bed physical models can be either sitespecific or generic in purpose. Site-specific models pertain to actual prototype situations, and as mentioned, they can often be difficult to conduct
correctly. Also, site-specific model results are strictly applicable only to the
prototype situation being modeled. A site-specific model is in the design
model category.
On the other hand, generic physical models are somewhat simplified or
idealized models in which response data are collected for a variety of hydrodynamic forcing conditions. These data are then used to develop or verify
theoretical and/or empirical relationships between the flow characteristics
and the process response, or to calibrate or verify a numerical model formulation. Design guidance derived from generic model study results may
have wide applicability. Generic physical models can be either validation
models, process models, or design models.
A final characteristic that can be assigned to either fixed-bed or movablebed models is whether the model is “short-term” or “long-term”. Shortterm models examine response of the project or physical system to shortduration (hours to days), high intensity events, such as storms. Long-term
models determine system changes that occur over extended time periods
(days to years). Short-term physical models are far more practical to conduct.
1.5.4 Important Notes About Physical Models
It is easy to understand how previous successes by others in physical hydraulic modeling often lure unsuspecting investigators into a false sense of
infallibility regarding their own laboratory studies. We must always bear
in mind that two necessary requirements must be first met before placing stock in physical model results: (a) equivalence between prototype and
model must be met to the extent possible under the constraints of the
study, and (b) model data must be properly interpreted in view of the
known model shortcomings (Keulegan 1966).
Svendsen (1985) gave two good rules to help guide us while evaluating
physical model results.
13
Three-dimensional movable-bed models are much rarer, due in part to
the high costs associated with these models. Examples include studies of
erosion of oil drilling sand islands, littoral drift induced by oblique wave
approach, formation of sand spits, three-dimensional ripple formation, and
scour holes in the vicinity of structures.
A compromise between the fixed-bed and movable-bed model occurs
when sediment tracer materials are used in fixed-bed models as a qualitative indicator of sediment erosion and deposition areas.
Both fixed-bed and movable-bed physical models can be either sitespecific or generic in purpose. Site-specific models pertain to actual prototype situations, and as mentioned, they can often be difficult to conduct
correctly. Also, site-specific model results are strictly applicable only to the
prototype situation being modeled. A site-specific model is in the design
model category.
On the other hand, generic physical models are somewhat simplified or
idealized models in which response data are collected for a variety of hydrodynamic forcing conditions. These data are then used to develop or verify
theoretical and/or empirical relationships between the flow characteristics
and the process response, or to calibrate or verify a numerical model formulation. Design guidance derived from generic model study results may
have wide applicability. Generic physical models can be either validation
models, process models, or design models.
A final characteristic that can be assigned to either fixed-bed or movablebed models is whether the model is “short-term” or “long-term”. Shortterm models examine response of the project or physical system to shortduration (hours to days), high intensity events, such as storms. Long-term
models determine system changes that occur over extended time periods
(days to years). Short-term physical models are far more practical to conduct.
1.5.4 Important Notes About Physical Models
It is easy to understand how previous successes by others in physical hydraulic modeling often lure unsuspecting investigators into a false sense of
infallibility regarding their own laboratory studies. We must always bear
in mind that two necessary requirements must be first met before placing stock in physical model results: (a) equivalence between prototype and
model must be met to the extent possible under the constraints of the
study, and (b) model data must be properly interpreted in view of the
known model shortcomings (Keulegan 1966).
Svendsen (1985) gave two good rules to help guide us while evaluating
physical model results.