Reef Flats
Sediment Dynamics
Sediments, Properties
Shingle Ridges
Soils of Low Elevation Coral Structures
Steers, James Alfred (1899–1987)
Stoddart, David Ross (1937–)
Temperature Change: Bleaching
Tropical Cyclone/Hurricane
Tsunami
Unvegetated Cays
Vegetated Cays
Wave Shoaling and Refraction
Waves and Wave-Driven Currents
CORAL CAYS – GEOHYDROLOGY
Kevin E. Parnell
James Cook University, Townsville, QLD, Australia
Synonyms
Atoll island hydrology; Reef island geohydrology
Definition
The geohydrology of coral cays relates to the movement
of water through the island and reef framework under
coral cays, particularly with respect to the characteristics
of the freshwater resource resulting from density differences of freshwater and saltwater as affected by the composition and permeability of the reef framework in the
context of its evolutionary history.
Introduction
The earliest work on geohydrology applicable to coral
cays was undertaken separately by Ghyben and Herzberg
in the late 1800s and early 1900s, who determined the
shape and thickness of a freshwater lens that forms under
coral cays once they reach a minimum size. The relationship, which is based on the different densities of freshwater and saltwater, is expressed in the Ghyben–Herzberg
equation:
z ¼
r f
ðr s À r f Þ
h;
where h is the distance above sea level to the water table
(phreatic surface), z is the distance below sea level to the
freshwater–saltwater interface, and r s and r f are the densities of saltwater and freshwater, respectively. Using
densities of 1.00 g cm
À3 for freshwater and 1.025 g cm À3
for saltwater gives the often quoted relationship
z ¼ 40 h:
The Ghyben–Herzberg model makes an assumption of
a single layer homogeneous medium, and a system in
hydrostatic equilibrium, with no mixing of fresh and salt
water, giving a sharp transition between the freshwater
and saltwater. This model is normally implemented with
the Dupuit assumption of horizontal flow (Oberdorfer
et al., 1990) and is frequently applied in resource assessments of potable water for human use on inhabited coral
cay islands.
In reality, this model makes assumptions that are clearly
not valid in most coral reef environments. Tidal fluctuations
of the water level are assumed to be negligible, water movement within the lens is assumed to result entirely from
recharge-induced changes to the hydraulic head, outflow
from the freshwater lens required to maintain mass-balance
is assumed to take place at the island margin, and mixing
within the framework caused by various water movements
and pressure gradients (such as tidal mixing) is not considered. Perhaps most significantly, the assumption of
a homogeneous medium rarely holds. In particular, differences in the reef framework above and below the Pleistocene solution unconformity (sometimes called the Thurber
discontinuity) typically found 6–25 below the current reef
flat level in tectonically stable areas, means that the model
is fundamentally flawed. In general, a very broad transition
zone between fresh and saltwater can be expected.
The dual aquifer model
The framework below the Pleistocene unconformity is
typically highly karstified with high porosity and permeability, due to exposure during much of the period since
the last interglacial approximately 120,000 years ago
(Vacher, 1997). Seawater can move effectively through
the karstified framework, effectively truncating the freshwater lens at the unconformity (Figure 1). The hydraulic
conductivity of the Holocene framework is probably
1 or 2 orders of magnitude less than that of the Pleistocene
framework (Oberdorfer et al., 1990; Woodroffe and
Falkland, 1997).
A two-layer (dual-aquifer) model with a moderately
permeable Holocene reef framework overlying a highpermeability Pleistocene aquifer, proposed by Wheatcraft
and Buddemeier (1981) has been developed and used by
others in modeling and investigations of cay groundwater
resources (Ayers and Vacher, 1986; Herman et al., 1986;
Oberdorfer et al., 1990; Underwood et al., 1992; Griggs
and Petersen, 1993; Bailey et al., 2009; and others). These
studies have found a quantitative agreement between
model results and physical and chemical field data, giving
some confidence in the approach. Tidal flow through the
Pleistocene framework has been shown to be particularly
significant, overwhelming any density differentials that
may limit mixing in the Pleistocene aquifer. Short-term
vertical water movements of 1 m or less driven by horizontally directed (in the Pleistocene aquifer) and vertically
directed (in the Holocene aquifer; Herman et al., 1986)
tidal pulses control the nature of transition zone mixing
(Underwood et al., 1992). Other short-term climatic
events, such as storms, can also be reflected in water
movements that cause mixing in the reef framework.
Clearly, the use of the Ghyben–Herzberg–Dupuit model
254
CORAL CAYS – GEOHYDROLOGY
Sediment Dynamics
Sediments, Properties
Shingle Ridges
Soils of Low Elevation Coral Structures
Steers, James Alfred (1899–1987)
Stoddart, David Ross (1937–)
Temperature Change: Bleaching
Tropical Cyclone/Hurricane
Tsunami
Unvegetated Cays
Vegetated Cays
Wave Shoaling and Refraction
Waves and Wave-Driven Currents
CORAL CAYS – GEOHYDROLOGY
Kevin E. Parnell
James Cook University, Townsville, QLD, Australia
Synonyms
Atoll island hydrology; Reef island geohydrology
Definition
The geohydrology of coral cays relates to the movement
of water through the island and reef framework under
coral cays, particularly with respect to the characteristics
of the freshwater resource resulting from density differences of freshwater and saltwater as affected by the composition and permeability of the reef framework in the
context of its evolutionary history.
Introduction
The earliest work on geohydrology applicable to coral
cays was undertaken separately by Ghyben and Herzberg
in the late 1800s and early 1900s, who determined the
shape and thickness of a freshwater lens that forms under
coral cays once they reach a minimum size. The relationship, which is based on the different densities of freshwater and saltwater, is expressed in the Ghyben–Herzberg
equation:
z ¼
r f
ðr s À r f Þ
h;
where h is the distance above sea level to the water table
(phreatic surface), z is the distance below sea level to the
freshwater–saltwater interface, and r s and r f are the densities of saltwater and freshwater, respectively. Using
densities of 1.00 g cm
À3 for freshwater and 1.025 g cm À3
for saltwater gives the often quoted relationship
z ¼ 40 h:
The Ghyben–Herzberg model makes an assumption of
a single layer homogeneous medium, and a system in
hydrostatic equilibrium, with no mixing of fresh and salt
water, giving a sharp transition between the freshwater
and saltwater. This model is normally implemented with
the Dupuit assumption of horizontal flow (Oberdorfer
et al., 1990) and is frequently applied in resource assessments of potable water for human use on inhabited coral
cay islands.
In reality, this model makes assumptions that are clearly
not valid in most coral reef environments. Tidal fluctuations
of the water level are assumed to be negligible, water movement within the lens is assumed to result entirely from
recharge-induced changes to the hydraulic head, outflow
from the freshwater lens required to maintain mass-balance
is assumed to take place at the island margin, and mixing
within the framework caused by various water movements
and pressure gradients (such as tidal mixing) is not considered. Perhaps most significantly, the assumption of
a homogeneous medium rarely holds. In particular, differences in the reef framework above and below the Pleistocene solution unconformity (sometimes called the Thurber
discontinuity) typically found 6–25 below the current reef
flat level in tectonically stable areas, means that the model
is fundamentally flawed. In general, a very broad transition
zone between fresh and saltwater can be expected.
The dual aquifer model
The framework below the Pleistocene unconformity is
typically highly karstified with high porosity and permeability, due to exposure during much of the period since
the last interglacial approximately 120,000 years ago
(Vacher, 1997). Seawater can move effectively through
the karstified framework, effectively truncating the freshwater lens at the unconformity (Figure 1). The hydraulic
conductivity of the Holocene framework is probably
1 or 2 orders of magnitude less than that of the Pleistocene
framework (Oberdorfer et al., 1990; Woodroffe and
Falkland, 1997).
A two-layer (dual-aquifer) model with a moderately
permeable Holocene reef framework overlying a highpermeability Pleistocene aquifer, proposed by Wheatcraft
and Buddemeier (1981) has been developed and used by
others in modeling and investigations of cay groundwater
resources (Ayers and Vacher, 1986; Herman et al., 1986;
Oberdorfer et al., 1990; Underwood et al., 1992; Griggs
and Petersen, 1993; Bailey et al., 2009; and others). These
studies have found a quantitative agreement between
model results and physical and chemical field data, giving
some confidence in the approach. Tidal flow through the
Pleistocene framework has been shown to be particularly
significant, overwhelming any density differentials that
may limit mixing in the Pleistocene aquifer. Short-term
vertical water movements of 1 m or less driven by horizontally directed (in the Pleistocene aquifer) and vertically
directed (in the Holocene aquifer; Herman et al., 1986)
tidal pulses control the nature of transition zone mixing
(Underwood et al., 1992). Other short-term climatic
events, such as storms, can also be reflected in water
movements that cause mixing in the reef framework.
Clearly, the use of the Ghyben–Herzberg–Dupuit model
254
CORAL CAYS – GEOHYDROLOGY