440
CHAPTER 7. LABORATORY WAVE GENERATIO1
boundaries.
Wave absorbers are broadly classed into passive absorbers that dam}
incident wave motion by a variety of techniques, and active absorbers that
move in response to the incident waves. These absorber types are briefly
summarized in the following sections.
Passive Absorbers
Traditionally, wave absorption at model boundaries has consisted of placing
gentle slopes (less than 1:10), porous material, or screens in front of the
boundary to dissipate a large fraction of the incident wave energy. These
“passive absorbers” can be designed to be effective over a specified range of
wave conditions, but they often require a substantial length to reduce the
reflection below 10%. Thus, passive wave absorbers can use up valuable
space in a wave flume or basin.
A thorough survey of wave absorbers by Ouellet and Datta (1986) indicated that the most popular passive wave absorber is a constant-slope beach
constructed of gravel or stones. Although these fixed absorbers prove effective in reducing wave reflection, they are not easily moved, making them
less practical for model basins where frequent boundary changes may be
needed. There have been a number of theoretical and experimental studies
addressing the absorption properties of various passive wave absorber configurations, and a review (in chronological order) of some of these studies
is presented in the following paragraphs.
Straub, Bowers, and Herbich (1957) briefly reviewed earlier studies on
wave reflection and absorption, and then presented experimental results
plotting reflection coefficient12 versus slope angle for different values of wave
steepness. Their experiments included impermeable slopes, crushed rock
slopes, wire mesh slopes, and several composite slopes. Results indicated
that crushed rock absorber slopes must be less than about 1:4 to keep
reflections less than 10%.
Goda and Ippen (1963) theoretically analyzed and tested wave absorbers
composed of vertical mesh screens aligned normal to the direction of wave
propagation. Four different screen absorbers where tested under deepwater
wave conditions, and results compared favorably with theory. Reflection
was shown to be dependent upon screen spacing, but not so dependent
on the number of screens “provided the number is fairly large. ” They also
stated that the screen absorber must be at least as long as the wavelength
of the incident wave.
The reflection coefficient is Hr/Hi where Hi is the incident monochromatic wave
height and Hr is the reflected wave height.
CHAPTER 7. LABORATORY WAVE GENERATIO1
boundaries.
Wave absorbers are broadly classed into passive absorbers that dam}
incident wave motion by a variety of techniques, and active absorbers that
move in response to the incident waves. These absorber types are briefly
summarized in the following sections.
Passive Absorbers
Traditionally, wave absorption at model boundaries has consisted of placing
gentle slopes (less than 1:10), porous material, or screens in front of the
boundary to dissipate a large fraction of the incident wave energy. These
“passive absorbers” can be designed to be effective over a specified range of
wave conditions, but they often require a substantial length to reduce the
reflection below 10%. Thus, passive wave absorbers can use up valuable
space in a wave flume or basin.
A thorough survey of wave absorbers by Ouellet and Datta (1986) indicated that the most popular passive wave absorber is a constant-slope beach
constructed of gravel or stones. Although these fixed absorbers prove effective in reducing wave reflection, they are not easily moved, making them
less practical for model basins where frequent boundary changes may be
needed. There have been a number of theoretical and experimental studies
addressing the absorption properties of various passive wave absorber configurations, and a review (in chronological order) of some of these studies
is presented in the following paragraphs.
Straub, Bowers, and Herbich (1957) briefly reviewed earlier studies on
wave reflection and absorption, and then presented experimental results
plotting reflection coefficient12 versus slope angle for different values of wave
steepness. Their experiments included impermeable slopes, crushed rock
slopes, wire mesh slopes, and several composite slopes. Results indicated
that crushed rock absorber slopes must be less than about 1:4 to keep
reflections less than 10%.
Goda and Ippen (1963) theoretically analyzed and tested wave absorbers
composed of vertical mesh screens aligned normal to the direction of wave
propagation. Four different screen absorbers where tested under deepwater
wave conditions, and results compared favorably with theory. Reflection
was shown to be dependent upon screen spacing, but not so dependent
on the number of screens “provided the number is fairly large. ” They also
stated that the screen absorber must be at least as long as the wavelength
of the incident wave.
The reflection coefficient is Hr/Hi where Hi is the incident monochromatic wave
height and Hr is the reflected wave height.