98
CHAPTER 4. HYDRODYNAMIC MODELS
linear theory transformation function. Svendsen (1985) discussed these and
other laboratory effects in wave tanks, and Chapter 7 presents methods
of mechanical wave generation to overcome some of the abovementioned
laboratory effects.
The boundaries in wave tanks constrain the hydrodynamics to be essentially two-dimensional. However, this effect is so obvious that the experimenter naturally considers this constraint when the decision is made
to employ a wave tank. A not-so-obvious boundary effect is caused by rereflection of waves by the wave board. Waves are generated and propagate
down the wave tank until they reach either a structure or beach on the
far end. Some wave energy is reflected seaward (toward the board), just
as happens in nature. However, in nature the reflected waves continue out
into the ocean, whereas in the wave tank, they are again reflected back
toward the beach. This laboratory effect is dealt with in a number of ways.
• Energy dissipating beaches comprised of gentle slopes and
rubberized “horsehair” mats are used to minimize reflection to less than 5% of the incident wave height.
• Experiments are conducted as a series of wave bursts with
each burst of waves ending before re-reflected waves can
again reach the testing section of the wave tank.
• Active wave absorption is implemented at the wave board
to detect and absorb unwanted reflected wave energy.
If waves are generated by wind in a two-dimensional tank, a potential laboratory effect occurs near the sidewalls where the model boundary
condition requires that the normal water particle velocity becomes zero.
Longuet-Higgins (1990) showed that waves near the sidewall may be as
much as \/2 times steeper than those near the center of the tank.
Experiments and models in wave basins suffer primarily because of their
finite width and the effects of side boundaries on the hydrodynamic processes. Obliquely approaching waves create longshore currents that result
in rip currents along the basin side wall. This causes large scale circulation
within the wave basin; and therefore, testing must be limited to regions
unaffected by the side wall effects (Dalrymple 1989). This problem can be
partially resolved by using training walls to direct waves and by allowing
alongshore-flowing water to pass out of the model at the side wall while at
the same time introducing an equal volume of water at the opposite side
wall to conserve water mass (Dalrymple 1989). Dalrymple, et al. (1977)
analyzed three types of experimental basin configurations in terms of the
wave-induced mean basin circulation. The theoretical longshore velocity
profiles within the surf zone for the three basin types were nearly identical
CHAPTER 4. HYDRODYNAMIC MODELS
linear theory transformation function. Svendsen (1985) discussed these and
other laboratory effects in wave tanks, and Chapter 7 presents methods
of mechanical wave generation to overcome some of the abovementioned
laboratory effects.
The boundaries in wave tanks constrain the hydrodynamics to be essentially two-dimensional. However, this effect is so obvious that the experimenter naturally considers this constraint when the decision is made
to employ a wave tank. A not-so-obvious boundary effect is caused by rereflection of waves by the wave board. Waves are generated and propagate
down the wave tank until they reach either a structure or beach on the
far end. Some wave energy is reflected seaward (toward the board), just
as happens in nature. However, in nature the reflected waves continue out
into the ocean, whereas in the wave tank, they are again reflected back
toward the beach. This laboratory effect is dealt with in a number of ways.
• Energy dissipating beaches comprised of gentle slopes and
rubberized “horsehair” mats are used to minimize reflection to less than 5% of the incident wave height.
• Experiments are conducted as a series of wave bursts with
each burst of waves ending before re-reflected waves can
again reach the testing section of the wave tank.
• Active wave absorption is implemented at the wave board
to detect and absorb unwanted reflected wave energy.
If waves are generated by wind in a two-dimensional tank, a potential laboratory effect occurs near the sidewalls where the model boundary
condition requires that the normal water particle velocity becomes zero.
Longuet-Higgins (1990) showed that waves near the sidewall may be as
much as \/2 times steeper than those near the center of the tank.
Experiments and models in wave basins suffer primarily because of their
finite width and the effects of side boundaries on the hydrodynamic processes. Obliquely approaching waves create longshore currents that result
in rip currents along the basin side wall. This causes large scale circulation
within the wave basin; and therefore, testing must be limited to regions
unaffected by the side wall effects (Dalrymple 1989). This problem can be
partially resolved by using training walls to direct waves and by allowing
alongshore-flowing water to pass out of the model at the side wall while at
the same time introducing an equal volume of water at the opposite side
wall to conserve water mass (Dalrymple 1989). Dalrymple, et al. (1977)
analyzed three types of experimental basin configurations in terms of the
wave-induced mean basin circulation. The theoretical longshore velocity
profiles within the surf zone for the three basin types were nearly identical