40
Keet !c,nvlronments
Their velocity and direction change, these being dependent upon the height
and on the phase of the tide. They move the water mass back and forth.
During the incoming tide water goes into the lagoon first through the
passages, then also up to the flat. During the ebb the current goes out from
the lagoon. These inflowing currents are especially strong in the passages of
the ring reefs and over their leeward reefs (von Arx 1948; Ludington 1979).
The velocity of tidal currents could be high (up to 30-80cm S-l) even on
reefs with relatively small tides of 20-40 cm (Maxwell 1968; Roberts et al.
1975; Thompson and Golding 1982). An average velocity of outflow currents
over leeward reefs of an atoll could be 20-40cms- 1 (Ludington 1979), but
in the passages it could be much more - up to 2-5 m S-l (von Arx 1948;
Michel et al. 1972). The velocity of currents in passages of the elevated
Fanning atoll was 40-80cm S-l (Gallagher et al. 1971). Over the flat and in
channels of barrier reefs the average velocities of tidal currents varied within
30-50cms-\ but in passages they could be up to 2ms- 1 (Maxwell 1968;
Roberts et al. 1975; Cresswel and Greig 1978; Maragos 1978; Wolanski and
Jones 1980; Geer and Kjerfve 1982; Thompson and Golding 1982; Marsden
1983). The periodicity of these currents sequential to the periodicity of tides
(Fig. 2.3). On barrier reefs along the continental shelf the long-period
barotrophic tidal waves, while causing the fluctuations of sea-level, also
induce the long-period oscillations of currents with periods of 10 to 30 days
(Cresswell and Greig 1978; Wolanski and Bennet 1983; Wolanski and
Pickard 1983b). In lagoons of the ring reefs the main driving force of
currents is the wind stress (Atkinson et al. 1981). Some influence on the
lagoonal circulation, especially in shallow lagoons, is exerted also by wavedriven currents crossing the windward flat and tidal currents going through
the channels (Frith 1981). In deep lagoons their influence is limited to zones
near the windward flat and close to the passages (Fig. 2.5). In the surface
layer the wind-driven drift prevails, while in deeper layers the compensation
currents are induced. The speed of surface currents in lagoons varied within
5 to 20cms- 1 (von Arx 1948; Ludington 1979; Atkinson et al. 1981). But in
large atolls with deep and vast lagoons they could attain 40 cm S-l and more
(Gladfelter et al. 1978). In the lagoon of the Enivetok atoll the velocity of
wind driven currents, as measured by the use of dye, was 50cms- 1 ,
attaining sometimes even 1.4cm S-l (Odum and Odum 1955). In lagoons of
WInd
Fig. 2.5. Scheme of currents in the lagoon of atoll. (After von Arx 1948)
Keet !c,nvlronments
Their velocity and direction change, these being dependent upon the height
and on the phase of the tide. They move the water mass back and forth.
During the incoming tide water goes into the lagoon first through the
passages, then also up to the flat. During the ebb the current goes out from
the lagoon. These inflowing currents are especially strong in the passages of
the ring reefs and over their leeward reefs (von Arx 1948; Ludington 1979).
The velocity of tidal currents could be high (up to 30-80cm S-l) even on
reefs with relatively small tides of 20-40 cm (Maxwell 1968; Roberts et al.
1975; Thompson and Golding 1982). An average velocity of outflow currents
over leeward reefs of an atoll could be 20-40cms- 1 (Ludington 1979), but
in the passages it could be much more - up to 2-5 m S-l (von Arx 1948;
Michel et al. 1972). The velocity of currents in passages of the elevated
Fanning atoll was 40-80cm S-l (Gallagher et al. 1971). Over the flat and in
channels of barrier reefs the average velocities of tidal currents varied within
30-50cms-\ but in passages they could be up to 2ms- 1 (Maxwell 1968;
Roberts et al. 1975; Cresswel and Greig 1978; Maragos 1978; Wolanski and
Jones 1980; Geer and Kjerfve 1982; Thompson and Golding 1982; Marsden
1983). The periodicity of these currents sequential to the periodicity of tides
(Fig. 2.3). On barrier reefs along the continental shelf the long-period
barotrophic tidal waves, while causing the fluctuations of sea-level, also
induce the long-period oscillations of currents with periods of 10 to 30 days
(Cresswell and Greig 1978; Wolanski and Bennet 1983; Wolanski and
Pickard 1983b). In lagoons of the ring reefs the main driving force of
currents is the wind stress (Atkinson et al. 1981). Some influence on the
lagoonal circulation, especially in shallow lagoons, is exerted also by wavedriven currents crossing the windward flat and tidal currents going through
the channels (Frith 1981). In deep lagoons their influence is limited to zones
near the windward flat and close to the passages (Fig. 2.5). In the surface
layer the wind-driven drift prevails, while in deeper layers the compensation
currents are induced. The speed of surface currents in lagoons varied within
5 to 20cms- 1 (von Arx 1948; Ludington 1979; Atkinson et al. 1981). But in
large atolls with deep and vast lagoons they could attain 40 cm S-l and more
(Gladfelter et al. 1978). In the lagoon of the Enivetok atoll the velocity of
wind driven currents, as measured by the use of dye, was 50cms- 1 ,
attaining sometimes even 1.4cm S-l (Odum and Odum 1955). In lagoons of
WInd
Fig. 2.5. Scheme of currents in the lagoon of atoll. (After von Arx 1948)