high salinity water sinking to the seabed (10
À8 ms
À1
), and
thermally driven flows (10
À7 ms
À1
) may be present in
some situations. Density driven circulation has been
described at a large scale by Leclerc et al. (1999).
Groundwater generated hydraulic gradients resulting
from recharge of the freshwater lens on reefs with coral
cays (see Coral Cays – Geohydrology) can generate slow
internal flow (10
À7 ms
À1
). However, in some circumstances where the Ghyben-Herzberg model of the freshwater lens applies, water exits the freshwater lens at the
island shoreline, resulting in local flows potentially at least
an order of magnitude higher. On fringing reefs, or on
reefs with other elevated land adjacent (such as raised
coral islands), flows within the adjacent reef framework
may be orders of magnitude higher (Parnell, 1986) than
in the typical coral cay case. Where the tidal range is
significant and the reef flat drains, hydraulic gradients
can be significantly greater and water speeds within the
framework similarly large.
The primary effect of waves, on the other hand,
although capable of causing horizontal flows in surface
layers, is to provide a pumping mechanism in the vertical,
an important process in the mixing of surface waters and
shallow pore waters. Buddemeier and Oberdorfer (1988)
suggest speeds of up to 10
À2 ms
À1
. They demonstrate that
a 10 s period wave has the potential to move about
2 m
3 per m
2 in and out of the reef surface.
Low frequency, high intensity events such as major
storms, tectonic movements, and tsunami can cause pressures resulting in very significant water flows within the
framework and within island sediments. For example,
there were many accounts of water bubbling up through
the reef framework, and water pressure from underneath
causing hard tile floors in houses on coral cay islands to
buckle during the 2004 Indian Ocean tsunami (Australian
Government Mission and the Maldives Marine Research
Centre, 2005). Such events may also cause changes to
the structure of individual reefs, permanently altering the
hydraulics.
Experimental studies
In separate studies using fluorescent tracers within the
Great Barrier Reef Marine Park, Parnell (1986) reported
water movement within a fringing reef flat in the order
of 40 m day
À1
, and Oberdorfer and Buddemeier (1986)
reported flows of between 0.2 m day À1 and 400 m day À1
on a mid shelf reef, but generally being in the order of
10 m day
À1
. Both studies recorded considerable vertical
movement within the framework. Most water movement
resulted from bulk flow through high permeability zones
(voids and rubble) with flows in less permeable framework being much slower. Oberdorfer and Buddemeier
(1986) comment on the striking similarities in the results
of the two studies, despite the considerable differences in
the characteristics of the study locations, concluding that
the dominant factor was the heterogeneity of reef sediments. The mid-shelf study also demonstrated large fluxes
of water across the reef surface (in the order of
10–1,000 m
3 of water per m
2 per year).
Numerous studies have concentrated on pore water
chemistry, showing that transport in the surface layers is
higher than at greater depths, possibly indicating the
importance of wave and current driven transport (Tribble
et al., 1992; Falter and Sansone, 2000). Some studies have
shown extremely short residence times in the upper few
meters of the reef framework, attributing the results to
wave driven oscillations and “stable head differences of
uncertain origin” (Tribble et al., 1992, 1290). McCullough
and Land (1992) noted the importance of flow through
high permeability zones to reef diagenetic processes.
Summary
A number of processes result in hydraulic gradients that
drive internal circulation on coral reefs at a range of time
and space scales, including wave setup, lagoon drainage,
tides, and other regional currents, density gradients,
groundwater flow, and oscillatory waves. Some flows,
such as wave driven pore-water circulation can be very
fast, resulting in large water turnover between surface
and subsurface sediments, over relatively small areas,
whereas at the other end of the spectrum, slow flows
driven by other mechanisms affect large volumes of
framework, over periods extending from years to geologic
time. Field studies have indicated flow through reef framework in the order of tens of meters per day, with water
moving in high permeability zones such as cavities and
rubble dominating bulk flow.
Bibliography
Australian Government Mission and the Maldives Marine Research
Centre, 2005. An assessment of damage to maldivian coral reefs
and baitfish populations from the Indian ocean tsunami.
Canberra: Commonwealth of Australia, p. 67.
Buddemeier, R. W., and Oberdorfer, J. A., 1988. Hydrogeology and
hydrodynamics of coral reef pore waters. In Proceedings Sixth
International Coral Reef Symposium, Vol. 2, pp. 485–490.
Falter, J. L., and Sansone, F. J., 2000. Shallow pore water sampling
in reef sediments. Coral Reefs, 19, 93–97.
Leclerc, A. M., Baptiste, P. J., Texier, D., and Broc, D., 1999.
Density-induced water circulations in atoll coral reefs:
A numerical study. Limnology and Oceanography, 44,
1268–1281.
Li, Y. H., and Gregory, S., 1974. Diffusion of ions in sea water and
in deep-sea sediments. Geochimica et Cosmochimica Acta, 38,
703–714.
McCullough, M. L., and Land, L. S., 1992. Dynamic hydrology and
diagenesis of a submerged Pleistocene fringing-Reef, Discovery
Bay, Jamaica. Marine Geology, 104, 139–151.
Oberdorfer, J. A., and Buddemeier, R. W., 1986. Coral-reef
hydrology – Field studies of water-movement within a barrierreef. Coral Reefs, 5, 7–12.
Parnell, K. E., 1986. Water-movement within a fringing-reef flat,
Orpheus Island, North-Queensland, Australia. Coral Reefs, 5,
1–6.
Tribble, G. W., Sansone, F. J., Buddemeier, R. W., and Li, Y. H.,
1992. Hydraulic exchange between a coral-reef and surface
sea-water. Geological Society of America Bulletin, 104,
1280–1291.
INTERNAL CIRCULATION
609
À8 ms
À1
), and
thermally driven flows (10
À7 ms
À1
) may be present in
some situations. Density driven circulation has been
described at a large scale by Leclerc et al. (1999).
Groundwater generated hydraulic gradients resulting
from recharge of the freshwater lens on reefs with coral
cays (see Coral Cays – Geohydrology) can generate slow
internal flow (10
À7 ms
À1
). However, in some circumstances where the Ghyben-Herzberg model of the freshwater lens applies, water exits the freshwater lens at the
island shoreline, resulting in local flows potentially at least
an order of magnitude higher. On fringing reefs, or on
reefs with other elevated land adjacent (such as raised
coral islands), flows within the adjacent reef framework
may be orders of magnitude higher (Parnell, 1986) than
in the typical coral cay case. Where the tidal range is
significant and the reef flat drains, hydraulic gradients
can be significantly greater and water speeds within the
framework similarly large.
The primary effect of waves, on the other hand,
although capable of causing horizontal flows in surface
layers, is to provide a pumping mechanism in the vertical,
an important process in the mixing of surface waters and
shallow pore waters. Buddemeier and Oberdorfer (1988)
suggest speeds of up to 10
À2 ms
À1
. They demonstrate that
a 10 s period wave has the potential to move about
2 m
3 per m
2 in and out of the reef surface.
Low frequency, high intensity events such as major
storms, tectonic movements, and tsunami can cause pressures resulting in very significant water flows within the
framework and within island sediments. For example,
there were many accounts of water bubbling up through
the reef framework, and water pressure from underneath
causing hard tile floors in houses on coral cay islands to
buckle during the 2004 Indian Ocean tsunami (Australian
Government Mission and the Maldives Marine Research
Centre, 2005). Such events may also cause changes to
the structure of individual reefs, permanently altering the
hydraulics.
Experimental studies
In separate studies using fluorescent tracers within the
Great Barrier Reef Marine Park, Parnell (1986) reported
water movement within a fringing reef flat in the order
of 40 m day
À1
, and Oberdorfer and Buddemeier (1986)
reported flows of between 0.2 m day À1 and 400 m day À1
on a mid shelf reef, but generally being in the order of
10 m day
À1
. Both studies recorded considerable vertical
movement within the framework. Most water movement
resulted from bulk flow through high permeability zones
(voids and rubble) with flows in less permeable framework being much slower. Oberdorfer and Buddemeier
(1986) comment on the striking similarities in the results
of the two studies, despite the considerable differences in
the characteristics of the study locations, concluding that
the dominant factor was the heterogeneity of reef sediments. The mid-shelf study also demonstrated large fluxes
of water across the reef surface (in the order of
10–1,000 m
3 of water per m
2 per year).
Numerous studies have concentrated on pore water
chemistry, showing that transport in the surface layers is
higher than at greater depths, possibly indicating the
importance of wave and current driven transport (Tribble
et al., 1992; Falter and Sansone, 2000). Some studies have
shown extremely short residence times in the upper few
meters of the reef framework, attributing the results to
wave driven oscillations and “stable head differences of
uncertain origin” (Tribble et al., 1992, 1290). McCullough
and Land (1992) noted the importance of flow through
high permeability zones to reef diagenetic processes.
Summary
A number of processes result in hydraulic gradients that
drive internal circulation on coral reefs at a range of time
and space scales, including wave setup, lagoon drainage,
tides, and other regional currents, density gradients,
groundwater flow, and oscillatory waves. Some flows,
such as wave driven pore-water circulation can be very
fast, resulting in large water turnover between surface
and subsurface sediments, over relatively small areas,
whereas at the other end of the spectrum, slow flows
driven by other mechanisms affect large volumes of
framework, over periods extending from years to geologic
time. Field studies have indicated flow through reef framework in the order of tens of meters per day, with water
moving in high permeability zones such as cavities and
rubble dominating bulk flow.
Bibliography
Australian Government Mission and the Maldives Marine Research
Centre, 2005. An assessment of damage to maldivian coral reefs
and baitfish populations from the Indian ocean tsunami.
Canberra: Commonwealth of Australia, p. 67.
Buddemeier, R. W., and Oberdorfer, J. A., 1988. Hydrogeology and
hydrodynamics of coral reef pore waters. In Proceedings Sixth
International Coral Reef Symposium, Vol. 2, pp. 485–490.
Falter, J. L., and Sansone, F. J., 2000. Shallow pore water sampling
in reef sediments. Coral Reefs, 19, 93–97.
Leclerc, A. M., Baptiste, P. J., Texier, D., and Broc, D., 1999.
Density-induced water circulations in atoll coral reefs:
A numerical study. Limnology and Oceanography, 44,
1268–1281.
Li, Y. H., and Gregory, S., 1974. Diffusion of ions in sea water and
in deep-sea sediments. Geochimica et Cosmochimica Acta, 38,
703–714.
McCullough, M. L., and Land, L. S., 1992. Dynamic hydrology and
diagenesis of a submerged Pleistocene fringing-Reef, Discovery
Bay, Jamaica. Marine Geology, 104, 139–151.
Oberdorfer, J. A., and Buddemeier, R. W., 1986. Coral-reef
hydrology – Field studies of water-movement within a barrierreef. Coral Reefs, 5, 7–12.
Parnell, K. E., 1986. Water-movement within a fringing-reef flat,
Orpheus Island, North-Queensland, Australia. Coral Reefs, 5,
1–6.
Tribble, G. W., Sansone, F. J., Buddemeier, R. W., and Li, Y. H.,
1992. Hydraulic exchange between a coral-reef and surface
sea-water. Geological Society of America Bulletin, 104,
1280–1291.
INTERNAL CIRCULATION
609