5.2. RUBBLE-MOUND STRUCTURES
189
Friction Scale Effects in Rubble-Mound Structure Models
Bottom friction scale effects are possible in a coastal structure model if
the wave propagation distance is very long. This is typically not a consideration for rubble-mound structure models because of the relatively large
length scales. A theoretical correction method for regular waves is given in
Chapter 4 in the section Short-Wave Model Laboratory and Scale Effects.
The other friction scale effect arises from the contact friction between
adjacent armor units. In prototype rubble-mound structures, contact frictional forces are usually considered negligible compared to the dominant
forces affecting the structure’s response to wave action (unless the structure
is built of artificial armor units specifically designed to provide frictional interlocking of units). However, in a small-scale physical model, the frictional
forces between units may not be in similitude with the prototype because
the armor unit surface can be relatively rougher than the large-scale units.
Few systematic studies of the contact friction scale effect have been
reported, and the standard practice is to reduce the friction between armor units as much as possible by making the model units smooth. As
mentioned, painting armor stones or units with enamel paint provides a
smoother surface, as well as making identification of damage areas easier.
Hudson and Davidson (1975) noted that slightly conservative stability results would be provided if the model units are relatively smoother than the
prototype. They also noted that the surface roughness of the underlayer
material (usually crushed basalt or limestone) will decrease slightly through
wear if the material is re-used over time.
Aeration Scale Effects in Rubble-Mound Structure Models
Hall (1990) conducted an experimental program that examined the entrainment and movement of air bubbles pushed into the voids of rubble-mound
models by waves breaking directly on the structure and by flow separation
as water moves rapidly past the solid armor units. Hall noted that entrained air bubbles would not be similitude in small-scale physical models
because of lack of similarity of the Weber number between prototype and
model. This results in air bubbles that are relatively larger in the model
than in the prototype, which in turn leads to too much energy dissipation
in the model. Therefore, the total energy dissipation on the rubble-mound
slope will be greater than it should be, and wave runup will be somewhat
affected.
Although this scale effect is not understood well enough to provide quantification of the scale effect or empirical correction techniques, Hall was able
identify trends to aid in future research. These trends were given as
189
Friction Scale Effects in Rubble-Mound Structure Models
Bottom friction scale effects are possible in a coastal structure model if
the wave propagation distance is very long. This is typically not a consideration for rubble-mound structure models because of the relatively large
length scales. A theoretical correction method for regular waves is given in
Chapter 4 in the section Short-Wave Model Laboratory and Scale Effects.
The other friction scale effect arises from the contact friction between
adjacent armor units. In prototype rubble-mound structures, contact frictional forces are usually considered negligible compared to the dominant
forces affecting the structure’s response to wave action (unless the structure
is built of artificial armor units specifically designed to provide frictional interlocking of units). However, in a small-scale physical model, the frictional
forces between units may not be in similitude with the prototype because
the armor unit surface can be relatively rougher than the large-scale units.
Few systematic studies of the contact friction scale effect have been
reported, and the standard practice is to reduce the friction between armor units as much as possible by making the model units smooth. As
mentioned, painting armor stones or units with enamel paint provides a
smoother surface, as well as making identification of damage areas easier.
Hudson and Davidson (1975) noted that slightly conservative stability results would be provided if the model units are relatively smoother than the
prototype. They also noted that the surface roughness of the underlayer
material (usually crushed basalt or limestone) will decrease slightly through
wear if the material is re-used over time.
Aeration Scale Effects in Rubble-Mound Structure Models
Hall (1990) conducted an experimental program that examined the entrainment and movement of air bubbles pushed into the voids of rubble-mound
models by waves breaking directly on the structure and by flow separation
as water moves rapidly past the solid armor units. Hall noted that entrained air bubbles would not be similitude in small-scale physical models
because of lack of similarity of the Weber number between prototype and
model. This results in air bubbles that are relatively larger in the model
than in the prototype, which in turn leads to too much energy dissipation
in the model. Therefore, the total energy dissipation on the rubble-mound
slope will be greater than it should be, and wave runup will be somewhat
affected.
Although this scale effect is not understood well enough to provide quantification of the scale effect or empirical correction techniques, Hall was able
identify trends to aid in future research. These trends were given as