5.3. SLOPING IMPERMEABLE STRUCTURES
215
3 x IO3, which was stated to correspond to a critical model wave height of
0.5 m. Oumeraci stated that there are generally no scale effects associated
with the location of wave impact; but due to differences in wave runup, this
is a higher probability that plunging wave impacts will occur on a waterfree portion of a sloping model structure. This will lead to proportionately
higher pressures when scaled to prototype. He also stated that steeper
slopes and smaller models increase this scale effect.
Scale effects also occur in wave runup on sloping impermeable structures
because of the inability to scale roughness effects in smaller models. This
results in smaller values of wave runup compared to those under prototype
conditions (Oumeraci 1984). This scale effect can be minimized by making
model slopes very smooth, and conducting tests at the largest possible
scale. Interestingly, Fiihrboter (1986) reported a slight decrease in regular
wave runup as wave height increased in his tests. This is contrary to what
would be expected from surface roughness effects. Fiihrboter attributed the
decrease in runup to increased aeration of the water which causes higher
internal friction.
An indication of the length scales where the wave runup scale effect is
negligible was given by Jensen (1989), who reported a good comparison of
overtopping rates between prototype scale and models having length scales
of 1:8 and 1:10.
Kostense and Boer (1984) studied the effect of model scale on the stability of sloping revetments consisting of concrete blocks laid over an impermeable slope. Tests were conducted at four different length scales ranging
between Nl — 3.6 and 10. Both regular and irregular waves were used in the
tests. Kostense and Boer found that block stability was not affected within
the scale range tested, but they noted that contact friction between blocks
could create increased stability in the model. To be on the safe side, the
best practice would be to avoid friction between non-interlocking blocks in
the model. Kostense and Boer recognized that scaling of low permeability
underlayer materials, such as sand and clay, represents a potential problem
that needs further study.
Finally, there is a minor density scale effect that arises when using a
fresh-water model to represent a salt-water prototype. This scale effect
is generally neglected as minor, and no corrections are needed unless the
smooth slope is built of slab-like blocks that are held in place by their own
self-weight, in which case the block weight can be scaled using Eqn. 5.25.
215
3 x IO3, which was stated to correspond to a critical model wave height of
0.5 m. Oumeraci stated that there are generally no scale effects associated
with the location of wave impact; but due to differences in wave runup, this
is a higher probability that plunging wave impacts will occur on a waterfree portion of a sloping model structure. This will lead to proportionately
higher pressures when scaled to prototype. He also stated that steeper
slopes and smaller models increase this scale effect.
Scale effects also occur in wave runup on sloping impermeable structures
because of the inability to scale roughness effects in smaller models. This
results in smaller values of wave runup compared to those under prototype
conditions (Oumeraci 1984). This scale effect can be minimized by making
model slopes very smooth, and conducting tests at the largest possible
scale. Interestingly, Fiihrboter (1986) reported a slight decrease in regular
wave runup as wave height increased in his tests. This is contrary to what
would be expected from surface roughness effects. Fiihrboter attributed the
decrease in runup to increased aeration of the water which causes higher
internal friction.
An indication of the length scales where the wave runup scale effect is
negligible was given by Jensen (1989), who reported a good comparison of
overtopping rates between prototype scale and models having length scales
of 1:8 and 1:10.
Kostense and Boer (1984) studied the effect of model scale on the stability of sloping revetments consisting of concrete blocks laid over an impermeable slope. Tests were conducted at four different length scales ranging
between Nl — 3.6 and 10. Both regular and irregular waves were used in the
tests. Kostense and Boer found that block stability was not affected within
the scale range tested, but they noted that contact friction between blocks
could create increased stability in the model. To be on the safe side, the
best practice would be to avoid friction between non-interlocking blocks in
the model. Kostense and Boer recognized that scaling of low permeability
underlayer materials, such as sand and clay, represents a potential problem
that needs further study.
Finally, there is a minor density scale effect that arises when using a
fresh-water model to represent a salt-water prototype. This scale effect
is generally neglected as minor, and no corrections are needed unless the
smooth slope is built of slab-like blocks that are held in place by their own
self-weight, in which case the block weight can be scaled using Eqn. 5.25.