5.4. VERTICAL-WALL STRUCTURES
217
Hudson, et al. (1979) pointed out that the character and magnitude of
the shock pressures developed during wave breaking on impervious vertical
walls can vary tremendously, and they list the following important physical
factors that contribute to shock pressure:
1. Wave characteristics including wave dimensions, angle of
approach, water depth at the structure’s toe, seaward bottom slope, and structure’s reflective characteristics
2. Concentration of entrained air in the water as the wave
strikes the face of the structure, and the pressures in the
entrained air bubbles
3. Pressures in air pockets that are trapped between the structure and the wave front
4. Pressures in air cushions formed by wave fronts that allow
the air to escape upward or laterally
A dimensional analysis for breaking wave shocks and impacts on vertical walls is given in Hudson, et al. (1979), and the resulting functional
relationship shows an extremely complex interaction of water, compressed
air, and capillary forces. This makes it difficult to analyze potential sources
of scale effects and to develop empirical corrections.
It is believed that
Models of vertical-wall structures should be designed
and operated according to the Froude criterion and the
model results scaled to prototype values using approximate methods suggested by Lundgren (1969).
Lundgren (1969) described three types of breaking waveforms that can
cause shock pressure intensities greater then pressures induced by similarsized nonbreaking waves: ventilated shock, compression shock, and hammer
shock. Figure 5.7 illustrates these three possibilities.
Ventilated Shock. If the wave front approaches the vertical-wall structure in a manner similar to the top illustration of Figure 5.7 so the air is
ventilated rather than being trapped, then ventilated shock pressures occur.
The primary forces in this phenomenon are inertial and gravitational (Hudson, et al. 1979), so the impulses and pressures can be scaled to prototype
values with sufficient accuracy using the Froude impulse scale
JVimpu.. =
(NO7'2
(5.50)
and the Froude pressure scale
NP = NywNL
(5.51)
217
Hudson, et al. (1979) pointed out that the character and magnitude of
the shock pressures developed during wave breaking on impervious vertical
walls can vary tremendously, and they list the following important physical
factors that contribute to shock pressure:
1. Wave characteristics including wave dimensions, angle of
approach, water depth at the structure’s toe, seaward bottom slope, and structure’s reflective characteristics
2. Concentration of entrained air in the water as the wave
strikes the face of the structure, and the pressures in the
entrained air bubbles
3. Pressures in air pockets that are trapped between the structure and the wave front
4. Pressures in air cushions formed by wave fronts that allow
the air to escape upward or laterally
A dimensional analysis for breaking wave shocks and impacts on vertical walls is given in Hudson, et al. (1979), and the resulting functional
relationship shows an extremely complex interaction of water, compressed
air, and capillary forces. This makes it difficult to analyze potential sources
of scale effects and to develop empirical corrections.
It is believed that
Models of vertical-wall structures should be designed
and operated according to the Froude criterion and the
model results scaled to prototype values using approximate methods suggested by Lundgren (1969).
Lundgren (1969) described three types of breaking waveforms that can
cause shock pressure intensities greater then pressures induced by similarsized nonbreaking waves: ventilated shock, compression shock, and hammer
shock. Figure 5.7 illustrates these three possibilities.
Ventilated Shock. If the wave front approaches the vertical-wall structure in a manner similar to the top illustration of Figure 5.7 so the air is
ventilated rather than being trapped, then ventilated shock pressures occur.
The primary forces in this phenomenon are inertial and gravitational (Hudson, et al. 1979), so the impulses and pressures can be scaled to prototype
values with sufficient accuracy using the Froude impulse scale
JVimpu.. =
(NO7'2
(5.50)
and the Froude pressure scale
NP = NywNL
(5.51)