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CHAPTER 6. SEDIMENT TRANSPORT MODELS
6.3.5 Densimetric Froude Model Requirements
The Densimetric Froude Model (DFM) is similar to the Lightweight Model
except that it relinquishes preservation of the grain size Reynolds number
scale in order to pick up more flexibility in specifying model parameters.
The scale of the grain size Reynolds number is found in general form
by solving the requirement N^ = 1 for jVv, and substituting into the
expression for Nr, to get
(Ny.Nd\'' *
Nd
Nr- = \~) n:
(6.55)
or
JVfl. =
(6.56)
Once again, it is difficult to specify a general fall speed scale without
details of the selected model sediment. Thus, no relative fall speed scale
can be given.
Densimetric Froude Model Shear Stress and Current Scales
The scale for sediment shear stress is found from the requirement, Npt = 1,
and the definition, Nv. = NTbINp, i.e.,
N N
o' ^ = ^Nd
(6.57)
To assure that bottom shear stress has the same scale as the short-wave
bottom shear stress, we equate Eqn. 6.57 with the stress given by Eqn. 6.14
(where Ny = 1, and a flat bottom is assumed so that Nk, = Nj) to give
Ny,Nd = (NL)'^(Nd)3/4
(6.58)
which is reduced to
N * N d = NL
(6.59)
From Eqn. 6.59 it is recognized that the modeler is free to choose two
of the three scales given in the equation. Whichever choice is made, all
the other scale ratios can be given in terms of the two selected. Of course
there are limits on selection of model sediment density, and this restricts
the range of parameters somewhat.
For the case of choosing Nd and Nl, Eqn. 6.59 is rewritten as
(6.60)