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CHAPTER 6. SEDIMENT TRANSPORT MODELS
region, and the resultant sediment transport in the model most likely will
not be in similitude.
The geometric distortion required by some models introduces greater
wave reflection from the beach and the longshore bars, which impacts the
incoming waves and possibility influences the result. Therefore, the modeler
should attempt to minimize model distortion if at all practical. In movablebed models the sediment’s angle of repose may influence the resulting profile
if the length scale distortion becomes excessive.
Lightweight model sediments have not seen much use in recent years,
even though the scaling relationships can accommodate the lighter particles. Paul, et al. (1972) studied equilibrium profile development using
lightweight model sediment, and they concluded that a major scale effect
occurs because the lightweight particles accelerate faster, and this increases
the horizontal extent of the particle trajectory. This scale effect is particularly pronounced in the surf zone and foreshore where high fluid accelerations occur. In the offshore region, the lightweight particles were moved at
relatively greater depths.
As previously mentioned, accretionary episodes will not be correctly
modeled in a suspension-dominated model when the model is less than
prototype scale. This concerns us because even during erosional phases
some waves in the irregular wave train may have accretionary characteristics
which will not be in similitude with the prototype. However, validation
obtained for the geometrically undistorted sediment fall parameter model
seems to indicate this scale effect can be neglected for models that are
chiefly erosional.
Conley and Inman (1992) reported that micro-scale details of sediment
suspension in the fluid-granular boundary layer observed in the field under the crests of near-breaking waves were not reproduced in laboratory
experiments conducted at near-prototype scale. They concluded that permeability of the seafloor was a critical factor contributing to the observed
difference, and additional tests were planned using a wave tank with a
ventilated bottom.
Movable-bed models do introduce several laboratory effects in addition
to the effects common to fixed-bed models. Chestnutt (1975) examined
the impacts of initial beach slope and distance from the beach to the wave
board in a series of large-scale tests using regular waves. Differences in the
rate of beach erosion were noted for different initial slopes, as might be
expected. Hughes and Fowler (1990a) summarized some of the conflicting
opinions on the importance of initial slope. It appears that a beach will
still reach the same equilibrium shape inside the surf zone irrespective of
the initial slope. Of course, realistic model simulations should begin with a
reasonable approximation of an equilibrium beach profile, thus the storm-
CHAPTER 6. SEDIMENT TRANSPORT MODELS
region, and the resultant sediment transport in the model most likely will
not be in similitude.
The geometric distortion required by some models introduces greater
wave reflection from the beach and the longshore bars, which impacts the
incoming waves and possibility influences the result. Therefore, the modeler
should attempt to minimize model distortion if at all practical. In movablebed models the sediment’s angle of repose may influence the resulting profile
if the length scale distortion becomes excessive.
Lightweight model sediments have not seen much use in recent years,
even though the scaling relationships can accommodate the lighter particles. Paul, et al. (1972) studied equilibrium profile development using
lightweight model sediment, and they concluded that a major scale effect
occurs because the lightweight particles accelerate faster, and this increases
the horizontal extent of the particle trajectory. This scale effect is particularly pronounced in the surf zone and foreshore where high fluid accelerations occur. In the offshore region, the lightweight particles were moved at
relatively greater depths.
As previously mentioned, accretionary episodes will not be correctly
modeled in a suspension-dominated model when the model is less than
prototype scale. This concerns us because even during erosional phases
some waves in the irregular wave train may have accretionary characteristics
which will not be in similitude with the prototype. However, validation
obtained for the geometrically undistorted sediment fall parameter model
seems to indicate this scale effect can be neglected for models that are
chiefly erosional.
Conley and Inman (1992) reported that micro-scale details of sediment
suspension in the fluid-granular boundary layer observed in the field under the crests of near-breaking waves were not reproduced in laboratory
experiments conducted at near-prototype scale. They concluded that permeability of the seafloor was a critical factor contributing to the observed
difference, and additional tests were planned using a wave tank with a
ventilated bottom.
Movable-bed models do introduce several laboratory effects in addition
to the effects common to fixed-bed models. Chestnutt (1975) examined
the impacts of initial beach slope and distance from the beach to the wave
board in a series of large-scale tests using regular waves. Differences in the
rate of beach erosion were noted for different initial slopes, as might be
expected. Hughes and Fowler (1990a) summarized some of the conflicting
opinions on the importance of initial slope. It appears that a beach will
still reach the same equilibrium shape inside the surf zone irrespective of
the initial slope. Of course, realistic model simulations should begin with a
reasonable approximation of an equilibrium beach profile, thus the storm-