14.4 Waves at Coral Reefs
433
Since wave heights are governed by prevailing water depths, depth limited
wave height criteria are essential for determining wave heights for structures,
and for sedimentological and geomorphological processes on coral reefs. Waves
propagate from the deep ocean onto the reef slope, break on the reef as plunging
breakers and then propagate over the reef top as a succession of bores, before
reforming into stable oscillatory waves at some distance from the reef edge.
Data collected during the REEF88 experiment (Hardy et at., 1990) indicate
that the largest stable wave had Hmax/h ~ 0.6, achieved by an individual
wave in a random wave train propagating over a horizontal bed. Again this is
considerably lower than the usual oceanographic and engineering criterion.
Gourlay (1994) has published the results of laboratory experiments on wave
transformation over a natural reef profile and provided further evidence that
maximum depth-limited wave heights on a horizontal reef were consistent with
the REEF88 experiment results. Sulaiman et at. (1994) has reported the results
of experiments on maximum wave heights on the horizontal reef flat at Sanur
Beach in Bali. For a non-dimensional water depth range 1.7 x 10- 4 < h / (gT2) <
0.17, the ratio of maximum wave height to water depth is always smaller than
0.6, except for a very few cases when it is about 0.65.
It is important to note that the lower limit of (H / h )max ratio is applicable to
those locations on coral reef platforms where the distance from the reef edge
is sufficient for waves to reform into oscillatory waves, after dissipating energy
due to breaking. Close to the reef edge, waves with larger (H /h)max ratio can
exist, but these would be turbulent breaking waves attenuating rapidly with
distance from the reef edge.
14.4.3 Water Circulation on Coral Reefs
Energy of waves propagating over the reef slope, tides and wind contribute to
the water movement through coral reefs and within coral reef lagoons. Wave
breaking and a rapid transformation of energy at the reef edge result in a sea
level set-up and drive strong currents. The features of the observed circulation
are usually reproduced using a numerical model in which a pattern depth integrated circulation in the lagoon is predicted (Prager, 1991; Wolanski et at.,
1993; David and Kjerfve, 1998; Kraines et at., 1998). However, the details
of circulation are strongly controlled by relative importance of major forcing
mechanisms, i. e. waves, tides, ocean currents, local lagoon pattern and lagoon
bathymetry. For example, large Laguna de Terminos, in Mexico, is mostly
controlled by tides which are responsible for 70% of the water level variability,
and approximately 95% of the current variability in the lagoon's inlets.
In smaller lagoons, the significance of wave motion can be more substantial. The measurements by Massel and Brinkman (1998) in the lagoon behind
Ningaloo Reef, the largest fringing coral reef system in Australia extending 300
km along the Western Australian coast, showed no evidence of inflow into the
lagoon other than via flow over the reef top. The continual outflow of water
from the lagoon, independent of tidal phase, indicates that wave pumping is a
433
Since wave heights are governed by prevailing water depths, depth limited
wave height criteria are essential for determining wave heights for structures,
and for sedimentological and geomorphological processes on coral reefs. Waves
propagate from the deep ocean onto the reef slope, break on the reef as plunging
breakers and then propagate over the reef top as a succession of bores, before
reforming into stable oscillatory waves at some distance from the reef edge.
Data collected during the REEF88 experiment (Hardy et at., 1990) indicate
that the largest stable wave had Hmax/h ~ 0.6, achieved by an individual
wave in a random wave train propagating over a horizontal bed. Again this is
considerably lower than the usual oceanographic and engineering criterion.
Gourlay (1994) has published the results of laboratory experiments on wave
transformation over a natural reef profile and provided further evidence that
maximum depth-limited wave heights on a horizontal reef were consistent with
the REEF88 experiment results. Sulaiman et at. (1994) has reported the results
of experiments on maximum wave heights on the horizontal reef flat at Sanur
Beach in Bali. For a non-dimensional water depth range 1.7 x 10- 4 < h / (gT2) <
0.17, the ratio of maximum wave height to water depth is always smaller than
0.6, except for a very few cases when it is about 0.65.
It is important to note that the lower limit of (H / h )max ratio is applicable to
those locations on coral reef platforms where the distance from the reef edge
is sufficient for waves to reform into oscillatory waves, after dissipating energy
due to breaking. Close to the reef edge, waves with larger (H /h)max ratio can
exist, but these would be turbulent breaking waves attenuating rapidly with
distance from the reef edge.
14.4.3 Water Circulation on Coral Reefs
Energy of waves propagating over the reef slope, tides and wind contribute to
the water movement through coral reefs and within coral reef lagoons. Wave
breaking and a rapid transformation of energy at the reef edge result in a sea
level set-up and drive strong currents. The features of the observed circulation
are usually reproduced using a numerical model in which a pattern depth integrated circulation in the lagoon is predicted (Prager, 1991; Wolanski et at.,
1993; David and Kjerfve, 1998; Kraines et at., 1998). However, the details
of circulation are strongly controlled by relative importance of major forcing
mechanisms, i. e. waves, tides, ocean currents, local lagoon pattern and lagoon
bathymetry. For example, large Laguna de Terminos, in Mexico, is mostly
controlled by tides which are responsible for 70% of the water level variability,
and approximately 95% of the current variability in the lagoon's inlets.
In smaller lagoons, the significance of wave motion can be more substantial. The measurements by Massel and Brinkman (1998) in the lagoon behind
Ningaloo Reef, the largest fringing coral reef system in Australia extending 300
km along the Western Australian coast, showed no evidence of inflow into the
lagoon other than via flow over the reef top. The continual outflow of water
from the lagoon, independent of tidal phase, indicates that wave pumping is a