L
LAGOON CIRCULATION
Harry H. Roberts
1
, Alexis Lugo-Fernández
2
1
Louisiana State University, Baton Rouge, LA, USA
2
Physical Sciences Unit, New Orleans, LA, USA
Synonyms
Atoll lagoon; Backreef lagoon
Definition
A lagoon is a shallow and generally elongate body of
water parallel to the coastline and separated from the open
sea by a natural barrier. In siliciclastic sediment systems,
this barrier is usually a barrier island, but in carbonate systems, the barrier consists of islands such as the Florida
Keys, or more frequently, the barrier is a coral-algal reef
of the fringing or barrier type. With atolls, the lagoons
can be quite deep and generally circular instead of elongated, and may or may not be associated with a land mass.
Fringing and barrier reef lagoons
Water circulation in carbonate lagoons is a product of
many variables including lagoon dimensions, continuity
of the reef barrier, wind speed and direction, wave-swell
height and direction, tides, and, to a lesser extent, salinity
and water temperature variations. Large-scale ocean currents and high-energy events may also contribute to
across-the-reef flow and lagoon circulation (e.g.,
Coronado et al., 2007). The relevance of each force
depends on the interplay between its strength and duration
(e.g., wave height, tidal range, and wind speed), reef
geometry (water depth and width of the reef crest-flat,
and bottom roughness), and the morphology and openness
of the lagoon. Figure 1 schematically illustrates two
end-member types of shallow, reef-fronted lagoons where
the length of the lagoon (L) is usually less than 10–15 km
but can extend up to hundreds of kilometers alongshore,
such as Ningaloo reef of Western Australia (Hearn and
Parker, 1988), while lagoon width (W ) is generally less
than a kilometer or two. These dimensions are typical of
fringing reef lagoons both in the Atlantic – Caribbean
and in Pacific reef provinces. The reef crest-flat depth h c
(Figure 1b) affects the way tides, waves, winds, and storm
surge interact with the reef (Gourlay, 1994), and the reef
crest-flat width (D c ), another relevant aspect of reef geometry, also has an impact on wave setup and across-the-reef
flow (Symonds et al., 1995; Massel and Brinkman, 2001;
Figure 1b). The wider the reef crest-flat (large D c ), the
more attenuation of cross-reef current velocities and flux
into the backreef lagoon.
One reef-lagoon type is characterized by a rather continuous reef with no major breaks. Except in areas where
salinities are reduced by freshwater runoff entering the
lagoon (Tamura et al., 2007) or in locales where
hypersaline conditions develop because of extremely high
evaporation rates, e.g., Red Sea (e.g., Friedman, 1968) or
Arabian Gulf (e.g., Gunatilaka et al., 1984), waters in
fringing reef lagoons are well mixed, have short flushing
times (hours–few days, e.g., Hearn and Parker, 1988),
and have rather simple circulation patterns. Flow into the
lagoon shown in Figure 1a is primarily from water-level
setup and overtopping associated with wave breaking on
the reef front or crest. The resulting water-level gradient
between the reef front and backreef lagoon drives a net
flux of ocean water lagoonward. Wave setup in submerged
reefs (h c > 0) is approximately 50% of H 0 when h c <
0.7H 0 but ~10% of H 0 when h c > H 0 where H 0 represents
the off-reef or incident wave height (Gourlay, 1993;
Gourlay, 1996a). However, when H 0 < 0.4 h c , there is
no setup (Gourlay, 1996b). Figure 1b illustrates wave
setup on the reef front and the water-level gradient into
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011
LAGOON CIRCULATION
Harry H. Roberts
1
, Alexis Lugo-Fernández
2
1
Louisiana State University, Baton Rouge, LA, USA
2
Physical Sciences Unit, New Orleans, LA, USA
Synonyms
Atoll lagoon; Backreef lagoon
Definition
A lagoon is a shallow and generally elongate body of
water parallel to the coastline and separated from the open
sea by a natural barrier. In siliciclastic sediment systems,
this barrier is usually a barrier island, but in carbonate systems, the barrier consists of islands such as the Florida
Keys, or more frequently, the barrier is a coral-algal reef
of the fringing or barrier type. With atolls, the lagoons
can be quite deep and generally circular instead of elongated, and may or may not be associated with a land mass.
Fringing and barrier reef lagoons
Water circulation in carbonate lagoons is a product of
many variables including lagoon dimensions, continuity
of the reef barrier, wind speed and direction, wave-swell
height and direction, tides, and, to a lesser extent, salinity
and water temperature variations. Large-scale ocean currents and high-energy events may also contribute to
across-the-reef flow and lagoon circulation (e.g.,
Coronado et al., 2007). The relevance of each force
depends on the interplay between its strength and duration
(e.g., wave height, tidal range, and wind speed), reef
geometry (water depth and width of the reef crest-flat,
and bottom roughness), and the morphology and openness
of the lagoon. Figure 1 schematically illustrates two
end-member types of shallow, reef-fronted lagoons where
the length of the lagoon (L) is usually less than 10–15 km
but can extend up to hundreds of kilometers alongshore,
such as Ningaloo reef of Western Australia (Hearn and
Parker, 1988), while lagoon width (W ) is generally less
than a kilometer or two. These dimensions are typical of
fringing reef lagoons both in the Atlantic – Caribbean
and in Pacific reef provinces. The reef crest-flat depth h c
(Figure 1b) affects the way tides, waves, winds, and storm
surge interact with the reef (Gourlay, 1994), and the reef
crest-flat width (D c ), another relevant aspect of reef geometry, also has an impact on wave setup and across-the-reef
flow (Symonds et al., 1995; Massel and Brinkman, 2001;
Figure 1b). The wider the reef crest-flat (large D c ), the
more attenuation of cross-reef current velocities and flux
into the backreef lagoon.
One reef-lagoon type is characterized by a rather continuous reef with no major breaks. Except in areas where
salinities are reduced by freshwater runoff entering the
lagoon (Tamura et al., 2007) or in locales where
hypersaline conditions develop because of extremely high
evaporation rates, e.g., Red Sea (e.g., Friedman, 1968) or
Arabian Gulf (e.g., Gunatilaka et al., 1984), waters in
fringing reef lagoons are well mixed, have short flushing
times (hours–few days, e.g., Hearn and Parker, 1988),
and have rather simple circulation patterns. Flow into the
lagoon shown in Figure 1a is primarily from water-level
setup and overtopping associated with wave breaking on
the reef front or crest. The resulting water-level gradient
between the reef front and backreef lagoon drives a net
flux of ocean water lagoonward. Wave setup in submerged
reefs (h c > 0) is approximately 50% of H 0 when h c <
0.7H 0 but ~10% of H 0 when h c > H 0 where H 0 represents
the off-reef or incident wave height (Gourlay, 1993;
Gourlay, 1996a). However, when H 0 < 0.4 h c , there is
no setup (Gourlay, 1996b). Figure 1b illustrates wave
setup on the reef front and the water-level gradient into
David Hopley (ed.), Encyclopedia of Modern Coral Reefs, DOI 10.1007/978-90-481-2639-2,
# Springer Science+Business Media B.V. 2011