184
CHAPTER 5. COASTAL STRUCTURE MODELS
5.2.2 Rubble-Mound Model Lab and Scale Effects
Laboratory Effects
Physical models of rubble-mound structures have many of the same laboratory effects as discussed in Chapter 4 in connection with short-wave,
fixed-bed hydrodynamic models. The reader should refer to the section
Short-Wave Model Laboratory and Scale Effects in Chapter 4 with rubblemound structure models in mind.
Viscous Scale Effects in Rubble-Mound Models
Perhaps the most important scale effect associated with physical models of
rubble-mound structures2 * is the viscous forces associated with flow through
the underlayers and core of the structure. At typical scales employed for
rubble-mound stability models, viscous scale effects are not a problem in the
primary, and often secondary, armor layer because the Reynolds number
based on the characteristic dimension of the armor unit is sufficiently large
to insure fully turbulent flow. However, in some underlayers and in the
core material, there is a possibility that the flow Reynolds number may fall
below a value considered critical for avoiding scale effects.
2 For some topics in this Chapter, the term “rubble-mound” should be taken to include
all structures that are constructed by building a mound with many individual units
(usually randomly placed) so that a porous structure results. This includes structures
constructed with concrete armor units.
Dai and Kamel (1969) combined a series of small-scale tests conducted
at the Waterways Experiment Station with large-scale test data to investigate the viscous scale effect in terms of flow Reynolds number for smooth
and rough quarrystone and quadripods. Velocity in the Reynolds number
for the armor layer was represented as Vw = \/gH, so the Reynolds number
was expressed in the form
(5.31)
p
where La is the characteristic armor unit length which is often determined
as
(5.32)
Stability numbers for each condition were calculated as the parameter given
by Eqn. 5.19. Figure 5.3 presents Dai and Kamel’s test results. They found
that similarity of stability number (labeled Ns on the figure) was obtained
for Reynolds numbers greater than 3 x 104.
CHAPTER 5. COASTAL STRUCTURE MODELS
5.2.2 Rubble-Mound Model Lab and Scale Effects
Laboratory Effects
Physical models of rubble-mound structures have many of the same laboratory effects as discussed in Chapter 4 in connection with short-wave,
fixed-bed hydrodynamic models. The reader should refer to the section
Short-Wave Model Laboratory and Scale Effects in Chapter 4 with rubblemound structure models in mind.
Viscous Scale Effects in Rubble-Mound Models
Perhaps the most important scale effect associated with physical models of
rubble-mound structures2 * is the viscous forces associated with flow through
the underlayers and core of the structure. At typical scales employed for
rubble-mound stability models, viscous scale effects are not a problem in the
primary, and often secondary, armor layer because the Reynolds number
based on the characteristic dimension of the armor unit is sufficiently large
to insure fully turbulent flow. However, in some underlayers and in the
core material, there is a possibility that the flow Reynolds number may fall
below a value considered critical for avoiding scale effects.
2 For some topics in this Chapter, the term “rubble-mound” should be taken to include
all structures that are constructed by building a mound with many individual units
(usually randomly placed) so that a porous structure results. This includes structures
constructed with concrete armor units.
Dai and Kamel (1969) combined a series of small-scale tests conducted
at the Waterways Experiment Station with large-scale test data to investigate the viscous scale effect in terms of flow Reynolds number for smooth
and rough quarrystone and quadripods. Velocity in the Reynolds number
for the armor layer was represented as Vw = \/gH, so the Reynolds number
was expressed in the form
(5.31)
p
where La is the characteristic armor unit length which is often determined
as
(5.32)
Stability numbers for each condition were calculated as the parameter given
by Eqn. 5.19. Figure 5.3 presents Dai and Kamel’s test results. They found
that similarity of stability number (labeled Ns on the figure) was obtained
for Reynolds numbers greater than 3 x 104.