Sediment Grain Size 



145. All tests conducted as part of this test series used the same sand for the model movable-bed 

 sediment; therefore, no direct tests involving perturbation of the sediment fall speed value were performed. 

 However, it is possible to qualify the impact of different grain sizes in the model by examining similar 

 model experiments conducted by Schulz (1985). 



146. Schulz (1985) conducted movable-bed model simulations aimed at reproducing the same GWK 

 prototype regular wave experiment as described in Part III of this report. Schulz's experiments were 

 conducted at an undistorted length scale of 1:10 with hydrodynamics scaled according to the Froude 

 criterion. Tests were conducted using three different grain sizes (0.18, 0.35, and 0.70 mm) to simulate the 

 prototype grain size of 0.33 mm. 



147. Figures 20 and 21 give intermediate and equilibrium comparisons of Schultz's 0.18-mm and 

 0.35-min sand tests with the prototype condition. In both figures, the solid line is the model result scaled 

 to prototype dimensions, and the dashed line is the prototype. For intermediate comparisons, the number 

 of waves is different because profiling was performed at times different from in the prototype; hence, the 

 closest profiles in terms of number of waves were chosen for comparison. The bottom plot on each figure 

 represents the equilibrium condition, which occurred much later in the scaled model than in the prototype. 



148. Neither the 0.18-mm nor the 0.35-mm grain size achieved a good match for the prototype profile 

 evolution. The test compared in Figure 21 represents the use of model sediment nearly the same size as the 

 prototype, which would be much coarser than what would be recommended using the scaling guidance 

 presented in this report. The test compared in Figure 20 used a finer sediment than the prototype, but one 

 that was still coarser than the scaling guidance would recommend. 



149. At the 1:10 scale used by Schulz, the fall speed parameter scaling guidance (Equation 6) gives Nw 

 = 3.16 and would require a model sediment fall speed of 1.4 cm/sec to correspond to the prototype 

 sediment (see Table 1). Model sand having a median diameter of about 0.12 mm would be required to 

 adhere to the fall speed parameter scaling criteria with length scale equal to 1:10. Sand finer than the 

 0.18 mm used in Schulz's experiment (Figure 20) would be mobilized easier and could move toward the 

 offshore region under the generated wave conditions. 



150. Qualitatively, it can be concluded that large perturbations in the model sediment fall speed 

 parameter due to grain size can greatly influence the movable-bed response. Similar conclusions cannot be 

 made regarding the effects of small perturbations in grain size. It may be that perturbations in the 

 sediment fall speed resulting from variations in water temperature or mean grain size may also contribute 

 to the overall experimental error (see Part IV). 



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