6.3. BEDLOAD-DOMINATED TRANSPORT MODELS
271
Kamphuis (1974, 1975, 1985) provided a very thorough analysis of each
model and the scale effects that arise when certain dimensionless products are not preserved between the prototype and model. These references
should be consulted for in-depth study of bedload-dominated movable-bed
models.
Initiation of Sediment Transport
In steady-flow movable-bed models, similitude of the Shields parameter
(densimetric Froude number) is considered important if incipient sediment
motion is a critical aspect of the modeling effort. However, authors disagree on whether the Shields parameter should be conserved in unsteady
coastal flows because the processes are much more complex. This is particularly true during flow reversals when flow in the bottom boundary layer
is expected to transition to laminar flow briefly.
Dalrymple (1989) argued for bedload models that preserve the Shields
criterion because sediment transport, sediment suspension, ripple geometry, and beach recovery processes have all been expressed in terms of the
Shields parameter. Oumeraci (1984) stated that dissimilar Shields parameters between prototype and model adversely affects similitude of incipient
motion of the sediment, and the time scale for sediment motion is exaggerated (waves take longer to move the sediment). He also stated that
dissimilar Shields parameters would give different ripple formations which
in turn would distort bottom friction so that approaching model waves
would be more attenuated in the model8.
Motta (1986) said there is little interest in the rigorous similarity of
the sediment motion threshold conditions on a flat bed because ripples will
always be present in the prototype (and most likely in the model). Dean
(1985) reasoned that similitude of Shields parameter is not important for
beach processes because sediment mobilization is by breaking wave turbulence and not shear stress. He also stated that scaling the sediment fall
speed approximately satisfies the Shields criterion.
Similitude of Bedforms
The formation of bedforms (ripples) in a movable-bed model was studied by
Mogridge (1974) in the laboratory using a wave flume (small scale) and an
oscillating wave tunnel (near prototype scale). Natural sand and lightweight
granular material were used as movable-bed sediments. Mogridge stated
that correct reproduction of bedforms is necessary in the model to assure
correct bedload transport rates and wave attenuation.
The influence of bottom friction on shoaling wave heights is a hot topic in itself.
271
Kamphuis (1974, 1975, 1985) provided a very thorough analysis of each
model and the scale effects that arise when certain dimensionless products are not preserved between the prototype and model. These references
should be consulted for in-depth study of bedload-dominated movable-bed
models.
Initiation of Sediment Transport
In steady-flow movable-bed models, similitude of the Shields parameter
(densimetric Froude number) is considered important if incipient sediment
motion is a critical aspect of the modeling effort. However, authors disagree on whether the Shields parameter should be conserved in unsteady
coastal flows because the processes are much more complex. This is particularly true during flow reversals when flow in the bottom boundary layer
is expected to transition to laminar flow briefly.
Dalrymple (1989) argued for bedload models that preserve the Shields
criterion because sediment transport, sediment suspension, ripple geometry, and beach recovery processes have all been expressed in terms of the
Shields parameter. Oumeraci (1984) stated that dissimilar Shields parameters between prototype and model adversely affects similitude of incipient
motion of the sediment, and the time scale for sediment motion is exaggerated (waves take longer to move the sediment). He also stated that
dissimilar Shields parameters would give different ripple formations which
in turn would distort bottom friction so that approaching model waves
would be more attenuated in the model8.
Motta (1986) said there is little interest in the rigorous similarity of
the sediment motion threshold conditions on a flat bed because ripples will
always be present in the prototype (and most likely in the model). Dean
(1985) reasoned that similitude of Shields parameter is not important for
beach processes because sediment mobilization is by breaking wave turbulence and not shear stress. He also stated that scaling the sediment fall
speed approximately satisfies the Shields criterion.
Similitude of Bedforms
The formation of bedforms (ripples) in a movable-bed model was studied by
Mogridge (1974) in the laboratory using a wave flume (small scale) and an
oscillating wave tunnel (near prototype scale). Natural sand and lightweight
granular material were used as movable-bed sediments. Mogridge stated
that correct reproduction of bedforms is necessary in the model to assure
correct bedload transport rates and wave attenuation.
The influence of bottom friction on shoaling wave heights is a hot topic in itself.