6.3. BEDLOAD-DOMINATED TRANSPORT MODELS
263
Lightweight Model Scale Effects
Using lightweight materials as model sediment keeps the grain size Reynolds
number and the densimetric Froude number similar between prototype and
model, and this places the prototype and model sediments at the same place
on the Shields diagram (Figure 6.3). We should therefore expect that incipient motion of the model sediment will be correctly simulated. However,
lightweight model sediments cause distortions in the other important scale
factors; and consequently, there are significant scale effects, as pointed out
by Kamphuis (1975, 1991).
a. Incorrect scaling of relative density results in inaccurate particle accelerations. This leads to underestimation of sediment transport rates and the possibility that particles may
go into suspension earlier than in the prototype, thus altering the mode of transport in the model.
b. Lightweight model sediment is relatively heavier when not
submerged, and this results in a piling up of material at
the shoreline (Oumeraci 1984). Particles lifted onto the
beach are much more difficult to move, and this may have
an impact on transport in the rest of the model. Therefore, lightweight models cannot be used to simulate bedload transport phenomena such as beach accretion.
c. The relative length scale is not correctly scaled because the
lightweight particles are quite a bit larger than they should
be. This causes sediment movement in the model to be
relatively less than what should occur. Another factor to
be considered is that bedforms, such as ripples, depend on
the relative length ratio; and these bedforms will not be
properly reproduced in the lightweight model. Incorrect
bedforms will modify the roughness and turbulence, which
may affect sediment transport. Kamphuis (1985) stated
that he felt not maintaining the prototype-to-model ratio
of relative length lead to the greatest amount of scale effect,
but he was somewhat optimistic because he recommended
that this should be evaluated in the model.
d. Movable-beds made of lightweight sediment are relatively
more porous because the particles are larger than they
should be, and this increased porosity will enable relatively
more wave energy to be absorbed by the model bed.
e. Liquefaction of the movable bed will occur sooner than it
should in the lightweight model. This is a problem if
263
Lightweight Model Scale Effects
Using lightweight materials as model sediment keeps the grain size Reynolds
number and the densimetric Froude number similar between prototype and
model, and this places the prototype and model sediments at the same place
on the Shields diagram (Figure 6.3). We should therefore expect that incipient motion of the model sediment will be correctly simulated. However,
lightweight model sediments cause distortions in the other important scale
factors; and consequently, there are significant scale effects, as pointed out
by Kamphuis (1975, 1991).
a. Incorrect scaling of relative density results in inaccurate particle accelerations. This leads to underestimation of sediment transport rates and the possibility that particles may
go into suspension earlier than in the prototype, thus altering the mode of transport in the model.
b. Lightweight model sediment is relatively heavier when not
submerged, and this results in a piling up of material at
the shoreline (Oumeraci 1984). Particles lifted onto the
beach are much more difficult to move, and this may have
an impact on transport in the rest of the model. Therefore, lightweight models cannot be used to simulate bedload transport phenomena such as beach accretion.
c. The relative length scale is not correctly scaled because the
lightweight particles are quite a bit larger than they should
be. This causes sediment movement in the model to be
relatively less than what should occur. Another factor to
be considered is that bedforms, such as ripples, depend on
the relative length ratio; and these bedforms will not be
properly reproduced in the lightweight model. Incorrect
bedforms will modify the roughness and turbulence, which
may affect sediment transport. Kamphuis (1985) stated
that he felt not maintaining the prototype-to-model ratio
of relative length lead to the greatest amount of scale effect,
but he was somewhat optimistic because he recommended
that this should be evaluated in the model.
d. Movable-beds made of lightweight sediment are relatively
more porous because the particles are larger than they
should be, and this increased porosity will enable relatively
more wave energy to be absorbed by the model bed.
e. Liquefaction of the movable bed will occur sooner than it
should in the lightweight model. This is a problem if