et al., 1994; Daly and Brander, 2006), with energy dissipation due to bottom friction contributing significantly
to, and perhaps dominating, the overall energy reduction
(Lugo-Fernandez et al., 1998a; Lowe et al., 2005).
A relatively small proportion of the wave energy reaches
the back reef or lagoon. Tidal range is an important
controlling factor (Hardy et al., 1991a, b) because of its
influence on the water depth over the reef crest. At low
tide, little or no wave energy crosses the reef flat. At mid
tide, most waves break, energy is dissipated, and some
waves re-form as bores, passing over the reef flat as solitary waves and reforming (if a lagoon is present). At high
tide, waves may break and reform, or under low energy
conditions or in situations with a high tidal range, waves
may pass over the reef crest onto the reef flat or into the
lagoon, and travel to the beach with little loss of energy.
Lugo-Fernandez et al. (1998b) reported an average 62%
wave attenuation between the fore-reef and the reef crest
and 90% attenuation between the fore-reef and the backreef lagoon, in the low tidal range environment of St
Croix., with wave attenuation being significantly higher
at low tide. Following breaking, a broadening of the wave
spectrum occurs (Young, 1989; Hardy and Young, 1991).
A maximum for the wave limiting parameter (wave
height/mean water depth) of about 0.55 as reported by
Gourlay (1994), Nelson (1994), and Kench and Brander
(2006) is applicable to fringing reefs.
Some studies have demonstrated that infragravity
waves, perhaps generated by shelf resonance, can be
significant (Roberts and Suhayda, 1983), although their
overall importance remains unclear (Lugo-Fernandez
et al., 1998c). Low frequency resonance is also described
by Peguignet et al. (2009), who hypothesize that during
high energy events, resonance leads to a significant
increase in wave energy reaching the shoreline.
Wave setup is of prime importance in driving water
across the reef crest into the lagoon or onto the reef flat
on fringing reefs. Lowe et al. (2009) found wave setup
to be roughly proportional to offshore wave energy flux,
above a lower threshold where setup was minimal. Jago
et al. (2007), in a study on the (non-fringing) Lady Elliot
Island (Great Barrier Reef ) reef, demonstrated secondary
wave setup on the beach, a factor not incorporated into
previous models. The effects of setup on reef water circulation is described by Gourlay (1996a, b) and Gourlay and
Colleter (2005). Symonds et al. (1995) showed reef and
lagoonal currents were proportional to off-reef wave
height after tidal effects had been removed. Hearn
(1999) introduced the concept of current depth coefficients on the basis of studies in Kaneohe Bay and on
Ningaloo Reef, predicting a linear relationship between
the height of incoming waves and the speed of the current
across the reef and through the lagoon. However, although
modeling studies of wave setup on reefs now abound, field
verification remains limited, and important assumptions
have not been tested. Importantly, Hearn (1999) points
out that variable wave friction which is dependent on
depth means that the hydrodynamics of fringing reef
systems is highly non-linear, and this is a severe restriction
for laboratory or modeling studies.
Many studies emphasize the importance of channels for
water returning to the open sea (e.g., Roberts et al., 1975;
Roberts, 1981; Marsh et al., 1981; Yamano et al., 1998;
Lugo-Fernandez et al., 2004). Setup generated flow drives
water into the lagoon (where one exists), and balance is
maintained by preferential return flow through channels
(Lowe et al., 2009), a process analogous to rip formation
on beaches (Roberts, 1981). Longshore currents develop
to carry water to the channels. Velocities in channels
up to 1.5 ms
À1 are reported by Coronado et al. (2007),
with maximum currents within the lagoon being
0.6 ms
À1
, which is in the same order as reported by other
studies (e.g., Marsh et al., 1981). Tamura et al. (2007)
show that although flows towards channels are expected
to dominate when wave-generated flow into the lagoon
is significant, the same circulation pattern exists even
when wave energy is small, despite the presence of tidal
currents. Jago et al. (2007) note that it is generally
assumed that a return to the mean water level landward
of the reef to the open ocean water level takes place by
flows through a lagoon. In cases where a lagoon does
not exist, as is the case on many fringing reefs, it is
suggested that alongshore variation in wave setup drives
along reef flows which have the same function as crossreef directed flows through lagoon openings.
Wind-generated circulation
When wave energy is low, circulation can be dominated
by flows generated by trade-winds. Roberts (1981) reports
that currents of 0.5–0.8 ms
À1 for durations of 2–6 s are
common under such conditions, although typical wind
generated flows are in the range of 0.10–0.25 ms
À1 . Presto
et al. (2006), in a study on Molokai, Hawaii, showed that
trade-wind generated flows dominated circulation and
sediment flux. In a study on the Puerto Morelas fringing
reef system, Mexico, Coronado et al. (2007) found trade
wind generated flows were small, and concluded that wind
influence through wind-wave generation was the most
effective wind influence. Trade-wind generated waves
were also reported to increase sediment suspension by
Storlazzi et al. (2004).
More generally, on wide reef flats or in wide lagoons,
high-frequency wind waves can be generated, the frequency and direction of which can be quite different from
those of waves that have traveled from the reef edge
(Kench and Brander, 2006).
Topographically controlled circulation
At a regional scale, circulation on fringing reef shores is
dominated by larger scale circulation related to features
such as shelf waves and tides, modified locally by the
influence of waves (Monismith, 2007). In these cases,
where fringing reefs interact with regional circulation,
reefs will generate large scale three dimensional eddies,
similar to those shed by any topographic feature in moving
428
FRINGING REEF CIRCULATION
to, and perhaps dominating, the overall energy reduction
(Lugo-Fernandez et al., 1998a; Lowe et al., 2005).
A relatively small proportion of the wave energy reaches
the back reef or lagoon. Tidal range is an important
controlling factor (Hardy et al., 1991a, b) because of its
influence on the water depth over the reef crest. At low
tide, little or no wave energy crosses the reef flat. At mid
tide, most waves break, energy is dissipated, and some
waves re-form as bores, passing over the reef flat as solitary waves and reforming (if a lagoon is present). At high
tide, waves may break and reform, or under low energy
conditions or in situations with a high tidal range, waves
may pass over the reef crest onto the reef flat or into the
lagoon, and travel to the beach with little loss of energy.
Lugo-Fernandez et al. (1998b) reported an average 62%
wave attenuation between the fore-reef and the reef crest
and 90% attenuation between the fore-reef and the backreef lagoon, in the low tidal range environment of St
Croix., with wave attenuation being significantly higher
at low tide. Following breaking, a broadening of the wave
spectrum occurs (Young, 1989; Hardy and Young, 1991).
A maximum for the wave limiting parameter (wave
height/mean water depth) of about 0.55 as reported by
Gourlay (1994), Nelson (1994), and Kench and Brander
(2006) is applicable to fringing reefs.
Some studies have demonstrated that infragravity
waves, perhaps generated by shelf resonance, can be
significant (Roberts and Suhayda, 1983), although their
overall importance remains unclear (Lugo-Fernandez
et al., 1998c). Low frequency resonance is also described
by Peguignet et al. (2009), who hypothesize that during
high energy events, resonance leads to a significant
increase in wave energy reaching the shoreline.
Wave setup is of prime importance in driving water
across the reef crest into the lagoon or onto the reef flat
on fringing reefs. Lowe et al. (2009) found wave setup
to be roughly proportional to offshore wave energy flux,
above a lower threshold where setup was minimal. Jago
et al. (2007), in a study on the (non-fringing) Lady Elliot
Island (Great Barrier Reef ) reef, demonstrated secondary
wave setup on the beach, a factor not incorporated into
previous models. The effects of setup on reef water circulation is described by Gourlay (1996a, b) and Gourlay and
Colleter (2005). Symonds et al. (1995) showed reef and
lagoonal currents were proportional to off-reef wave
height after tidal effects had been removed. Hearn
(1999) introduced the concept of current depth coefficients on the basis of studies in Kaneohe Bay and on
Ningaloo Reef, predicting a linear relationship between
the height of incoming waves and the speed of the current
across the reef and through the lagoon. However, although
modeling studies of wave setup on reefs now abound, field
verification remains limited, and important assumptions
have not been tested. Importantly, Hearn (1999) points
out that variable wave friction which is dependent on
depth means that the hydrodynamics of fringing reef
systems is highly non-linear, and this is a severe restriction
for laboratory or modeling studies.
Many studies emphasize the importance of channels for
water returning to the open sea (e.g., Roberts et al., 1975;
Roberts, 1981; Marsh et al., 1981; Yamano et al., 1998;
Lugo-Fernandez et al., 2004). Setup generated flow drives
water into the lagoon (where one exists), and balance is
maintained by preferential return flow through channels
(Lowe et al., 2009), a process analogous to rip formation
on beaches (Roberts, 1981). Longshore currents develop
to carry water to the channels. Velocities in channels
up to 1.5 ms
À1 are reported by Coronado et al. (2007),
with maximum currents within the lagoon being
0.6 ms
À1
, which is in the same order as reported by other
studies (e.g., Marsh et al., 1981). Tamura et al. (2007)
show that although flows towards channels are expected
to dominate when wave-generated flow into the lagoon
is significant, the same circulation pattern exists even
when wave energy is small, despite the presence of tidal
currents. Jago et al. (2007) note that it is generally
assumed that a return to the mean water level landward
of the reef to the open ocean water level takes place by
flows through a lagoon. In cases where a lagoon does
not exist, as is the case on many fringing reefs, it is
suggested that alongshore variation in wave setup drives
along reef flows which have the same function as crossreef directed flows through lagoon openings.
Wind-generated circulation
When wave energy is low, circulation can be dominated
by flows generated by trade-winds. Roberts (1981) reports
that currents of 0.5–0.8 ms
À1 for durations of 2–6 s are
common under such conditions, although typical wind
generated flows are in the range of 0.10–0.25 ms
À1 . Presto
et al. (2006), in a study on Molokai, Hawaii, showed that
trade-wind generated flows dominated circulation and
sediment flux. In a study on the Puerto Morelas fringing
reef system, Mexico, Coronado et al. (2007) found trade
wind generated flows were small, and concluded that wind
influence through wind-wave generation was the most
effective wind influence. Trade-wind generated waves
were also reported to increase sediment suspension by
Storlazzi et al. (2004).
More generally, on wide reef flats or in wide lagoons,
high-frequency wind waves can be generated, the frequency and direction of which can be quite different from
those of waves that have traveled from the reef edge
(Kench and Brander, 2006).
Topographically controlled circulation
At a regional scale, circulation on fringing reef shores is
dominated by larger scale circulation related to features
such as shelf waves and tides, modified locally by the
influence of waves (Monismith, 2007). In these cases,
where fringing reefs interact with regional circulation,
reefs will generate large scale three dimensional eddies,
similar to those shed by any topographic feature in moving
428
FRINGING REEF CIRCULATION