272
CHAPTER 6. SEDIMENT TRANSPORT MODELS
Mogridge reported that flow-induced ripples were best simulated by using a model sediment having the same density as the prototype, and a
median grain size diameter that was reduced from prototype size using the
model length scale. These are the essential ingredients of the Best Model.
In situations where it is impossible to scale the sediment by Nl, Mogridge
(1974) recommended a procedure whereby distortion of the grain size is
introduced for a lightweight sediment and the resulting bedform is scaled
to prototype using horizontal and vertical length scale factors determined
from equations and a nomogram presented in his paper. The lightweight
sediment will have a diameter greater than prototype grain diameter, and
Mogridge stated that bedform distortion would be small if material selection provided similitude of the densimetric Froude number (i.e., the
Lightweight Model or the Densimetric Froude Model).
Yalin (1972) presented scaling relationships for modeling dunes formed
by bedload transport under unidirectional tidal current flows. The scaling
relationship for flow velocity was derived by considering a "... generalized
friction equation which takes into account the influence of both skin friction and form drag.” Yalin’s suggested scaling is intended for use with
lightweight sediments selected to match the grain size Reynolds number and densimetric Froude number of the prototype sediment (i.e., a
Lightweight Model).
Bedload Model Morphological Scale
Determination of morphological time scales in movable-bed physical models
of bedload-dominated processes is very subjective, and these time scales
are best determined by comparing model response time to known prototype
response (Kamphuis 1975). Kamphuis (1974) presented prototype-to-model
scale ratios for offshore sediment transport rate (per unit width) and for
littoral transport rate in the nearshore region. These transport rate scale
factors were a combination of other model scale ratios, plus they contained
a factor that represented the total scale effect due to nonsimilitude of one or
more of the dimensionless parameters given in Eqn. 6.4. Generally, the scale
effect factor is unknown and only can be determined by direct comparison
of prototype and model sediment transport rates.
Once the prototype-to-model transport rate scale has been empirically
determined, the morphological time scale can be estimated as
_ NxNzN{1 p)
(6.67)
/v
'
where
CHAPTER 6. SEDIMENT TRANSPORT MODELS
Mogridge reported that flow-induced ripples were best simulated by using a model sediment having the same density as the prototype, and a
median grain size diameter that was reduced from prototype size using the
model length scale. These are the essential ingredients of the Best Model.
In situations where it is impossible to scale the sediment by Nl, Mogridge
(1974) recommended a procedure whereby distortion of the grain size is
introduced for a lightweight sediment and the resulting bedform is scaled
to prototype using horizontal and vertical length scale factors determined
from equations and a nomogram presented in his paper. The lightweight
sediment will have a diameter greater than prototype grain diameter, and
Mogridge stated that bedform distortion would be small if material selection provided similitude of the densimetric Froude number (i.e., the
Lightweight Model or the Densimetric Froude Model).
Yalin (1972) presented scaling relationships for modeling dunes formed
by bedload transport under unidirectional tidal current flows. The scaling
relationship for flow velocity was derived by considering a "... generalized
friction equation which takes into account the influence of both skin friction and form drag.” Yalin’s suggested scaling is intended for use with
lightweight sediments selected to match the grain size Reynolds number and densimetric Froude number of the prototype sediment (i.e., a
Lightweight Model).
Bedload Model Morphological Scale
Determination of morphological time scales in movable-bed physical models
of bedload-dominated processes is very subjective, and these time scales
are best determined by comparing model response time to known prototype
response (Kamphuis 1975). Kamphuis (1974) presented prototype-to-model
scale ratios for offshore sediment transport rate (per unit width) and for
littoral transport rate in the nearshore region. These transport rate scale
factors were a combination of other model scale ratios, plus they contained
a factor that represented the total scale effect due to nonsimilitude of one or
more of the dimensionless parameters given in Eqn. 6.4. Generally, the scale
effect factor is unknown and only can be determined by direct comparison
of prototype and model sediment transport rates.
Once the prototype-to-model transport rate scale has been empirically
determined, the morphological time scale can be estimated as
_ NxNzN{1 p)
(6.67)
/v
'
where