in the 1970s, sufficient to cause some of the shoreline
changes experienced during this period (Flood, 1986).
(d) High-energy, low-frequency events
Cyclonic events can cause erosion or accretion on reef
islands depending on the tidal conditions and sediment availability at time of impact. If tides are high,
erosion usually results; if low, then accretion may
occur (Flood, 1986). Storm surges that exacerbate erosion on mainland sites have far less an impact on cays
on offshore reefs, which generally lack the gradual
shoaling of mainland bays and the funneling effect of
coastal embayments. Many examples have been
described from the GBR (Flood, 1980, 1981, 1986;
Flood and Jell, 1977; Hopley, 1972). Cyclone Emily
(1972) with a central pressure of 985 hPa produced
surge levels >2 m on the Queensland mainland near
Gladstone, but only 0.8 m as it passed over offshore
Heron Island (Hopley, 1972). On nearby One Tree
Reef, around 27 m of rubble was added to the ramparts
on the southeast side of the island as this cyclone struck
at low tide. In contrast, Cyclone Winifred (982 hPa) in
1986 passed over Green Island at high tide producing
70 m of erosion on the unstable spit. At low latitudes
where the Coriolis force is insufficient to generate
mature cyclones, swell waves from cyclones at slightly
higher latitudes can still affect island shores.
Tsunamis can be experienced in many reefal areas.
The 2004 Indian Ocean event (see Tsunami) caused
washovers in several island groups (e.g., Maldives,
Chagos) and although leaving erosional scars of up
to 2 m and depositional sand sheets extending inland,
the overall geomorphological impact was not as great
as might have been expected.
All of these natural changes can be exacerbated by anthropogenic activities (see Infrastructure and Reef Islands;
Engineering on Coral Reefs with Emphasis on Pacific
Reefs). Groynes, rock walls, boat channels, and changes
to reef flat ecology can all result in changes to beach morphology and sediment budgets with erosion being the
most common result.
Future for reef cays
Cays are dynamic landforms that are mostly low and composed of unconsolidated sediments. They are widely
perceived as particularly vulnerable to climate and sealevel changes projected as imminent consequences of
anthropogenic activities (IPCC, 2007). Some believe that
they are more resilient – that their dynamic nature will
allow them to adjust. Some of the key potential impacts
of sea level and climate changes on reef islands are schematically represented in Figure 9 and discussed further
below.
Sea-level rise
Early concerns that sea-level rise will simply drown reef
islands are now considered oversimplistic, with island
response reflecting the complex interplay of numerous
physical, biological, and chemical factors. On some reefs,
rising sea levels will allow larger waves to propagate further through more of each tidal cycle, increasing the transport of available sediment to islands. On reef flats affected
by falling late Holocene sea levels, sediment deposits have
accumulated and these may be mobilized shoreward –
Hopley (1996) suggested that a sea-level rise of just
0.5 m would achieve this on many reefs. Hopley (1996)
also modeled carbonate budgets for an idealized reef flat
affected by a 0.5-m and 1.8-m rise by 2100 and showed
that under the lower rate almost the entire reef would vertically accrete and reef morphology would not significantly change (see Climate Change: Impact of Sea-Level
Rise on Reef Flat Zonation and Productivity). However,
at the higher rate the algal zone transformed to coral cover,
markedly increasing calcium carbonate production. The
enhanced carbonate productivity would yield sediments
suitable for reef-island construction, and because greater
depth improves shoreward sediment transport, Hopley
concluded that faster sea-level rise at first may be more
beneficial for reef-island sustainability than a slower rate.
Many reef geomorphologists agree that projected sealevel rise will substantially rework unconsolidated sediments and initially maintain reef-island mass (e.g., Kench
and Cowell 2002).
The issue is more complicated, however, as elevation is
critical on these low islands. Cay buildup is largely controlled by the characteristics of waves reaching the beach,
with berm height – the height of the beach above mean
high water – dependent on wave run-up. In a detailed
study at Raine Island, Gourlay and Hacker (1991) found
that the berm height was controlled by the wave run-up
during the highest spring tides and calculated that the
island may vertically accrete by an amount larger than
the sea-level rise if reef flat aggradation lags behind the
rate of sea-level rise and larger waves can reach the beach.
However, given the diversity of reef-island morphologies,
evolutionary histories, geographical settings, and human
pressures, the future for reef islands may be diverse.
Late-Holocene emergence which has been demonstrated to be important to the formation and longer term
stability of at least some cays has been discussed above.
These factors would lose their influence as sea level
increases. Uncertainties exist about future changes in
storm frequency and intensity, but increases in both are
possible. Higher wave energy might increase shoreline
erosion, increase the frequency of inundation events, and
threaten the quality and maintenance of groundwater aquifers and the long-term habitability of many reef islands.
On Pacific Ocean, Indian Ocean, and Caribbean reef
islands, settlements and vital infrastructure are almost
without exception located close to the beach, where they
are vulnerable to inundation, erosion, and other coastal
hazards that may compromise the socioeconomic wellbeing of island communities. Understanding of some
aspects of these interactions has advanced remarkably in
recent decades. However, significant gaps still exist which
limit capacity to confidently predict the long-term future
250
CORAL CAY CLASSIFICATION AND EVOLUTION
changes experienced during this period (Flood, 1986).
(d) High-energy, low-frequency events
Cyclonic events can cause erosion or accretion on reef
islands depending on the tidal conditions and sediment availability at time of impact. If tides are high,
erosion usually results; if low, then accretion may
occur (Flood, 1986). Storm surges that exacerbate erosion on mainland sites have far less an impact on cays
on offshore reefs, which generally lack the gradual
shoaling of mainland bays and the funneling effect of
coastal embayments. Many examples have been
described from the GBR (Flood, 1980, 1981, 1986;
Flood and Jell, 1977; Hopley, 1972). Cyclone Emily
(1972) with a central pressure of 985 hPa produced
surge levels >2 m on the Queensland mainland near
Gladstone, but only 0.8 m as it passed over offshore
Heron Island (Hopley, 1972). On nearby One Tree
Reef, around 27 m of rubble was added to the ramparts
on the southeast side of the island as this cyclone struck
at low tide. In contrast, Cyclone Winifred (982 hPa) in
1986 passed over Green Island at high tide producing
70 m of erosion on the unstable spit. At low latitudes
where the Coriolis force is insufficient to generate
mature cyclones, swell waves from cyclones at slightly
higher latitudes can still affect island shores.
Tsunamis can be experienced in many reefal areas.
The 2004 Indian Ocean event (see Tsunami) caused
washovers in several island groups (e.g., Maldives,
Chagos) and although leaving erosional scars of up
to 2 m and depositional sand sheets extending inland,
the overall geomorphological impact was not as great
as might have been expected.
All of these natural changes can be exacerbated by anthropogenic activities (see Infrastructure and Reef Islands;
Engineering on Coral Reefs with Emphasis on Pacific
Reefs). Groynes, rock walls, boat channels, and changes
to reef flat ecology can all result in changes to beach morphology and sediment budgets with erosion being the
most common result.
Future for reef cays
Cays are dynamic landforms that are mostly low and composed of unconsolidated sediments. They are widely
perceived as particularly vulnerable to climate and sealevel changes projected as imminent consequences of
anthropogenic activities (IPCC, 2007). Some believe that
they are more resilient – that their dynamic nature will
allow them to adjust. Some of the key potential impacts
of sea level and climate changes on reef islands are schematically represented in Figure 9 and discussed further
below.
Sea-level rise
Early concerns that sea-level rise will simply drown reef
islands are now considered oversimplistic, with island
response reflecting the complex interplay of numerous
physical, biological, and chemical factors. On some reefs,
rising sea levels will allow larger waves to propagate further through more of each tidal cycle, increasing the transport of available sediment to islands. On reef flats affected
by falling late Holocene sea levels, sediment deposits have
accumulated and these may be mobilized shoreward –
Hopley (1996) suggested that a sea-level rise of just
0.5 m would achieve this on many reefs. Hopley (1996)
also modeled carbonate budgets for an idealized reef flat
affected by a 0.5-m and 1.8-m rise by 2100 and showed
that under the lower rate almost the entire reef would vertically accrete and reef morphology would not significantly change (see Climate Change: Impact of Sea-Level
Rise on Reef Flat Zonation and Productivity). However,
at the higher rate the algal zone transformed to coral cover,
markedly increasing calcium carbonate production. The
enhanced carbonate productivity would yield sediments
suitable for reef-island construction, and because greater
depth improves shoreward sediment transport, Hopley
concluded that faster sea-level rise at first may be more
beneficial for reef-island sustainability than a slower rate.
Many reef geomorphologists agree that projected sealevel rise will substantially rework unconsolidated sediments and initially maintain reef-island mass (e.g., Kench
and Cowell 2002).
The issue is more complicated, however, as elevation is
critical on these low islands. Cay buildup is largely controlled by the characteristics of waves reaching the beach,
with berm height – the height of the beach above mean
high water – dependent on wave run-up. In a detailed
study at Raine Island, Gourlay and Hacker (1991) found
that the berm height was controlled by the wave run-up
during the highest spring tides and calculated that the
island may vertically accrete by an amount larger than
the sea-level rise if reef flat aggradation lags behind the
rate of sea-level rise and larger waves can reach the beach.
However, given the diversity of reef-island morphologies,
evolutionary histories, geographical settings, and human
pressures, the future for reef islands may be diverse.
Late-Holocene emergence which has been demonstrated to be important to the formation and longer term
stability of at least some cays has been discussed above.
These factors would lose their influence as sea level
increases. Uncertainties exist about future changes in
storm frequency and intensity, but increases in both are
possible. Higher wave energy might increase shoreline
erosion, increase the frequency of inundation events, and
threaten the quality and maintenance of groundwater aquifers and the long-term habitability of many reef islands.
On Pacific Ocean, Indian Ocean, and Caribbean reef
islands, settlements and vital infrastructure are almost
without exception located close to the beach, where they
are vulnerable to inundation, erosion, and other coastal
hazards that may compromise the socioeconomic wellbeing of island communities. Understanding of some
aspects of these interactions has advanced remarkably in
recent decades. However, significant gaps still exist which
limit capacity to confidently predict the long-term future
250
CORAL CAY CLASSIFICATION AND EVOLUTION