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The model developed in Part A is verified and validated from 6 points of view.
1. The traditional Shields diagram, a sensitivity analysis
2. Exposure and protrusion levels
3. Shear velocity and shear stress
4. The ratio between the friction velocity and the terminal settling velocity
5. Stages of entrainment
6. Laminar main flow
It is proposed to distinguish 4 different Shields curves:
1. The Shields curve based on spheres in a turbulent main flow.
2. The Shields curve based on natural sands and gravels in a turbulent main flow.
3. The Shields curve in a laminar main flow for spheres.
4. The Shields curve in a laminar main flow for natural sand and gravels.
The general conclusions of this research are:
• The basic Shields curve can be determined by applying the sliding entrainment mechanism, with a friction angle of 30°, an exposure level of 0.5 (protrusion level of 0.3), a turbulence intensity factor of n=3, a lift coefficient of 0.415 and the drag coefficient of spheres.
• Using a reasonable bandwidth for the properties, like friction angle, lift coefficient and turbulence intensity, most of the scatter in the data found, can be explained.
• For natural sands and gravels a modified drag coefficient should be applied, based on the angularity of the particles.
• In the laminar region entrainment is dominated by drag and turbulence, while in the turbulent region this is dominated by drag and lift.
• Up to an exposure level of 0.6 sliding is the main entrainment mechanism, while for higher exposure levels rolling will occur.
• Laminar and turbulent main flow result in two different entrainment curves, based on the presence of turbulence. For laminar main flow a turbulence intensity factor of 0.5 has been found to correlate well with the measurements.
• The model developed correlates well with datasets of many independent researchers.