Hydrodynamics and Thermohaline Features of Reef Waters
39
Fig. 2.4. Scheme of interaction of a ring reef with the coming oceanic current. (After
Hamner and Hauri 1981)
of the windward slope induce also strong currents in the moat at 3-6 m
depth with velocities up to 1-1.S m S-I. Where the moat is crossed by
channels cutting the reef flat, the flows from it go up these channels and
through them into the lagoon (Marsh et al. 1981). At the leeward side of the
reef the encircling currents form a complex system of reciprocal eddies,
which enhance the turbulence and thus also induce the vertical transport of
deeper waters, bringing up the nutrients (Wolanski 1982; Fig. 2.4). These
local upwellings, caused by the interactions of reef constructions with the
oceanic current, result in the so-called island mass effect, the effect of
enhanced productivity of plankton in waters near islands in zones of large
quasi-permanent oceanic currents (Gilmartin and Relevante 1974; Hamner
and Hauri 1981; cf. Sect. 3.2.2).
The action of wave stress as a driving force in hydrodynamic processes
manifests itself mainly in the windward flat at the expense of dissipation of
wave energy. As mentioned before, the sea-level in its frontal zone is
supported 20-40 cm higher than in the surrounding ocean at wave heights of
1.S-4 m. This difference induces permanent currents across the flat to the
lagoon with average velocities of 20-40 m s -I, while that of the pulses along
the channels could be 1-Z m (von Arx 1948; Roberts 1974; Maragos 1978;
Marsh et al. 1981; Atkinson et al. 1981). The strength of these currents
varies within the diurnal tide cycle (Fig. 2.3). It increases at high tide and
decreasses at the ebb-tide. According to the last-mentioned authors the
velocity of current crossing the windward flat of the Enivetok atoll varied
within 10 to ISO cm s -1. The rate of water transport during high tide was 1.S
and during the ebb - O.OS (an average of 0.S6m 3 s- 1 m- 1 ). During one tide
phase via windward flat ca. 66 x 10 8 m 3 of water or around 60% 'of its total
daily debit arrived into the lagoon. For the Bikini atoll the corresponding
share was 30% (von Arx 1948).
On reefs of atolls and on barrier reefs, situated in areas of a weak wave
stress, the main driving force of flows over the flat are the tide currents.
39
Fig. 2.4. Scheme of interaction of a ring reef with the coming oceanic current. (After
Hamner and Hauri 1981)
of the windward slope induce also strong currents in the moat at 3-6 m
depth with velocities up to 1-1.S m S-I. Where the moat is crossed by
channels cutting the reef flat, the flows from it go up these channels and
through them into the lagoon (Marsh et al. 1981). At the leeward side of the
reef the encircling currents form a complex system of reciprocal eddies,
which enhance the turbulence and thus also induce the vertical transport of
deeper waters, bringing up the nutrients (Wolanski 1982; Fig. 2.4). These
local upwellings, caused by the interactions of reef constructions with the
oceanic current, result in the so-called island mass effect, the effect of
enhanced productivity of plankton in waters near islands in zones of large
quasi-permanent oceanic currents (Gilmartin and Relevante 1974; Hamner
and Hauri 1981; cf. Sect. 3.2.2).
The action of wave stress as a driving force in hydrodynamic processes
manifests itself mainly in the windward flat at the expense of dissipation of
wave energy. As mentioned before, the sea-level in its frontal zone is
supported 20-40 cm higher than in the surrounding ocean at wave heights of
1.S-4 m. This difference induces permanent currents across the flat to the
lagoon with average velocities of 20-40 m s -I, while that of the pulses along
the channels could be 1-Z m (von Arx 1948; Roberts 1974; Maragos 1978;
Marsh et al. 1981; Atkinson et al. 1981). The strength of these currents
varies within the diurnal tide cycle (Fig. 2.3). It increases at high tide and
decreasses at the ebb-tide. According to the last-mentioned authors the
velocity of current crossing the windward flat of the Enivetok atoll varied
within 10 to ISO cm s -1. The rate of water transport during high tide was 1.S
and during the ebb - O.OS (an average of 0.S6m 3 s- 1 m- 1 ). During one tide
phase via windward flat ca. 66 x 10 8 m 3 of water or around 60% 'of its total
daily debit arrived into the lagoon. For the Bikini atoll the corresponding
share was 30% (von Arx 1948).
On reefs of atolls and on barrier reefs, situated in areas of a weak wave
stress, the main driving force of flows over the flat are the tide currents.