36
Reef Environments
Most reef systems are situated in zones of permanent and vigorous wave
stress, caused mainly by the action of trade winds, which have a general
western direction and speed of 2-10 m s -1. The height of waves induced by
such winds is 1-4 m. Trade winds of average strength, inducing waves 2-3 m
high, act on a medium-sized atoll with the power of half a million hp (von
Arx 1948). The average power of a medium-high wave acting upon a
windward reef equals 1-4 x 10 g ergs- 1 (Roberts 1974). During storms the
power of the wave stress increases many times. A storm of medium strength
with a wind velocity of 10-20 mls drives waves whose height near the reef
edge will reach 5-8 m. Many reef regions suffer constantly from the
periodical action of destructive tropical storms (hurricanes or typhoons)
when the speed of wind exceeds 20 mls and the height of waves reaches
1O-15m (Stoddart 1969, 1974; Woodley et al. 1981; Kjerfve and Dinnel
1983). Wave stress is among the most powerful physical factors acting upon
the reef ecosystems, including its geomorphology, zonal distribution of its
structural elements, distribution of carbonate material and topography of its
biotopes (Roberts 1974; Heckel 1974; Adey 1978; Geister 1977; Bradbury
and Young 1981; Dollar 1982). This influence is especially displayed in the
frontal zone of windward flats and on the outer slopes.
Interacting with the elements of a complex relief of windward reefs the
oncoming waves are transformed, changing their periodicity and height
(Thornton et al. 1976). Largely under the action of waves on the outer
windward slope a system of buttresses and channels is formed which actually
compose a system of natural wave-brakers, called also the spoor and groove
system (Munk and Sargent 1948). Interacting with waves, they dissipate
their energy, thus decreasing their destructive stress upon the reef
construction and the biota. The degree of development of spoor and groove
systems on reefs directly depends upon the wave stress acting upon them
(Roberts 1974). The dissipation of wave energy proceeds also by their
interaction with the channels which cut through the outer frontal part of the
reef flat. Often these channels are actually the prolongations of grooves of
the buttress system (Fig. 2.2). The channels with langths up to 100-150 m
actually serve as a trap for waves, where they finally loose their energy. The
waves loose a part of their energy in supporting an elevated sea-level above
the submerged flat where the pressure is maintained by water pumped up by
waves through the channels and grooves. This difference between sea level
above the flat and that of the ocean is about one-fifth of the height of
oncoming waves. It means that at an average height of waves of 2-4 m this
elevation of level above the reef flat could be 40-60 cm (Munk and Sargent
1948; Tait 1972). But during storms with wave heights of 8-10 m it could
reach 1.5-2 m, and could exceed the height of the tide itself. The wave
stress as well as the differences in sea-level caused by it induce powerful
pulses of currents, having speeds up to 1.5-2ms- 1 (Roberts et al. 1975,
1983; Atkinson et al. 1981; Church et al. 1985; cf. Fig. 2.3). Such currents
easily drag along not only gravel and rubble but also large coral heads,
Reef Environments
Most reef systems are situated in zones of permanent and vigorous wave
stress, caused mainly by the action of trade winds, which have a general
western direction and speed of 2-10 m s -1. The height of waves induced by
such winds is 1-4 m. Trade winds of average strength, inducing waves 2-3 m
high, act on a medium-sized atoll with the power of half a million hp (von
Arx 1948). The average power of a medium-high wave acting upon a
windward reef equals 1-4 x 10 g ergs- 1 (Roberts 1974). During storms the
power of the wave stress increases many times. A storm of medium strength
with a wind velocity of 10-20 mls drives waves whose height near the reef
edge will reach 5-8 m. Many reef regions suffer constantly from the
periodical action of destructive tropical storms (hurricanes or typhoons)
when the speed of wind exceeds 20 mls and the height of waves reaches
1O-15m (Stoddart 1969, 1974; Woodley et al. 1981; Kjerfve and Dinnel
1983). Wave stress is among the most powerful physical factors acting upon
the reef ecosystems, including its geomorphology, zonal distribution of its
structural elements, distribution of carbonate material and topography of its
biotopes (Roberts 1974; Heckel 1974; Adey 1978; Geister 1977; Bradbury
and Young 1981; Dollar 1982). This influence is especially displayed in the
frontal zone of windward flats and on the outer slopes.
Interacting with the elements of a complex relief of windward reefs the
oncoming waves are transformed, changing their periodicity and height
(Thornton et al. 1976). Largely under the action of waves on the outer
windward slope a system of buttresses and channels is formed which actually
compose a system of natural wave-brakers, called also the spoor and groove
system (Munk and Sargent 1948). Interacting with waves, they dissipate
their energy, thus decreasing their destructive stress upon the reef
construction and the biota. The degree of development of spoor and groove
systems on reefs directly depends upon the wave stress acting upon them
(Roberts 1974). The dissipation of wave energy proceeds also by their
interaction with the channels which cut through the outer frontal part of the
reef flat. Often these channels are actually the prolongations of grooves of
the buttress system (Fig. 2.2). The channels with langths up to 100-150 m
actually serve as a trap for waves, where they finally loose their energy. The
waves loose a part of their energy in supporting an elevated sea-level above
the submerged flat where the pressure is maintained by water pumped up by
waves through the channels and grooves. This difference between sea level
above the flat and that of the ocean is about one-fifth of the height of
oncoming waves. It means that at an average height of waves of 2-4 m this
elevation of level above the reef flat could be 40-60 cm (Munk and Sargent
1948; Tait 1972). But during storms with wave heights of 8-10 m it could
reach 1.5-2 m, and could exceed the height of the tide itself. The wave
stress as well as the differences in sea-level caused by it induce powerful
pulses of currents, having speeds up to 1.5-2ms- 1 (Roberts et al. 1975,
1983; Atkinson et al. 1981; Church et al. 1985; cf. Fig. 2.3). Such currents
easily drag along not only gravel and rubble but also large coral heads,