6.3. BEDLOAD-DOMINATED TRANSPORT MODELS
267
and Eqn. 6.56 becomes
NK. = N'L/sN'd',a
(6.61)
The necessary velocity scales for correct current reproduction in the
Offshore DFM and Inshore DFM are already given in terms of NL and Nd
by Eqns. 6.21 and 6.22, respectively (with Nk, = Nd).
Alternately, the scale ratios for the Densimetric Froude Model could be
derived in terms of Nl and N7i .
Densimetric Froude Model Scale Effects
The Densimetric Froude Model is more flexible because the model engineer
has the freedom to select two scale ratios instead of just one as in the
Best Model and the Lightweight Model] however, payment for this luxury
is extracted by having to deal with all the scale effects inherent in the
Lightweight Model, plus the viscous scale effects mentioned for the Best
Model. See the previous discussions on LWM and BM scale effects for
more detail.
Currents simulated in the DFM are distorted, and this can produce unwanted scale effects as mentioned in Chapter 4 (Short-Wave Model Boundary Similitude and Long- Wave Model Boundary Similitude).
6.3.6
Sand Model Requirements
Finally, Kamphuis (1974, 1975) described the Sand Model (SM). It fulfills
the requirement that the model sediment have the same density as the prototype, and many believe that using sand as the model sediment is preferable for coastal sediment transport models (Dalrymple 1989). Assuming
that water is used in the model, this means
Np.l, = N1. =1
(6.62)
The modeler has the freedom of selecting the length scale (Nl) and the
sediment diameter scale (Nd) with the rest of the scale ratios being defined
in terms of these two scales.
Sand Froude Model Shear Stress and Current Scales
Because there is no requirement in the Sand Model to maintain the sediment
shear stress scale, then Nr.
1, and shear stresses are defined as the
short-wave bottom shear stress, given by Eqn. 6.14. The scale ratio of
densimetric Froude number is found by substituting the short-wave shear
stress expression (with Ny = 1 and Nk, = Nj for a flat bed) into the
expression for Nk. (Eqn. 6.27) to get
267
and Eqn. 6.56 becomes
NK. = N'L/sN'd',a
(6.61)
The necessary velocity scales for correct current reproduction in the
Offshore DFM and Inshore DFM are already given in terms of NL and Nd
by Eqns. 6.21 and 6.22, respectively (with Nk, = Nd).
Alternately, the scale ratios for the Densimetric Froude Model could be
derived in terms of Nl and N7i .
Densimetric Froude Model Scale Effects
The Densimetric Froude Model is more flexible because the model engineer
has the freedom to select two scale ratios instead of just one as in the
Best Model and the Lightweight Model] however, payment for this luxury
is extracted by having to deal with all the scale effects inherent in the
Lightweight Model, plus the viscous scale effects mentioned for the Best
Model. See the previous discussions on LWM and BM scale effects for
more detail.
Currents simulated in the DFM are distorted, and this can produce unwanted scale effects as mentioned in Chapter 4 (Short-Wave Model Boundary Similitude and Long- Wave Model Boundary Similitude).
6.3.6
Sand Model Requirements
Finally, Kamphuis (1974, 1975) described the Sand Model (SM). It fulfills
the requirement that the model sediment have the same density as the prototype, and many believe that using sand as the model sediment is preferable for coastal sediment transport models (Dalrymple 1989). Assuming
that water is used in the model, this means
Np.l, = N1. =1
(6.62)
The modeler has the freedom of selecting the length scale (Nl) and the
sediment diameter scale (Nd) with the rest of the scale ratios being defined
in terms of these two scales.
Sand Froude Model Shear Stress and Current Scales
Because there is no requirement in the Sand Model to maintain the sediment
shear stress scale, then Nr.
1, and shear stresses are defined as the
short-wave bottom shear stress, given by Eqn. 6.14. The scale ratio of
densimetric Froude number is found by substituting the short-wave shear
stress expression (with Ny = 1 and Nk, = Nj for a flat bed) into the
expression for Nk. (Eqn. 6.27) to get