shore-parallel zones of differing exposure indices that
increase towards the shore. Macrotidal coasts (say, with
tidal ranges >4 m, and locally over 10 m) tend to have relatively wide tidal flats, particularly if the tidal flats have
a very low topographic gradient, and the exposure
between low and high water results in wide shore-parallel
zones within a tidal flat of differing exposure indices
increasing towards the shore. In this latter situation, the
gradient of salinity of the tidal flat is markedly evident
between MSL and high water and follows the shoreparallel exposure indices.
In combination with sediment type, the tidal flat slope
determines how rapidly during a low tide the tidal flat
drains by vertical infiltration or by direct runoff. Tidal flat
slope also determine how rapidly tidal flat water changes
from phreatic (with its water table at, or close to the tidal
flat surface) to vadose (the wetted sediment above
a falling water table at or close to the tidal flat surface).
Topographically very low-gradient tidal flats, particularly
if they are muddy and not sandy, drain water relatively
slowly such that there is a longer residency time of pore
water and pellicular water in the surface and hence
a longer period for evaporation. Steeper flats, particularly
if they are sandy, tend to drain water more quickly with
shorter residency time for surface pore water and
pellicular water and less time for direct evaporation.
Sediment types also determine how rapidly the tidal flat
drains during low tide. Sandy substrates are more hydraulically transmissive than mud substrates and drain water
more rapidly laterally and vertically. Because of their
low transmissivity, surface layers of mud and muddy sand
retain water for a longer period, are subject to evaporative
processes for longer periods, and tend to salinize more
rapidly than sand. While tidal flats that are composed
wholly of mud or wholly of sand will manifest relatively
uniform but different shore-parallel gradients of salinity
in groundwater and pellicular water in response to inundation, evaporation and transpiration, tidal flat sediments
that vary from sand to mud in cross-tidal-flat gradients,
or mosaics of sand and mud will exhibit more complicated
salinity patterns as the sediment types become additional
controlling factors.
The shore-parallel zones of increasing groundwater and
pellicular water salinity from MSL to high water in
response to decreasing inundation and increasing evaporation and transpiration and their resultant salinity gradients
can be disrupted by the hydrological changes effected by
vertical sedimentary shoaling to levels above the high
water mark and by discordant (or heterogeneous) stratigraphy such as sandbars and cheniers. In humid environments with excess rainfall, the sedimentary sequence
may vertically shoal (accrete) to levels of high water and
then be capped by peat (Coleman et al., 1970). Peat,
sustained by rainfall, may continue to accrete vertically
to form peat mounds to levels above high tide. Such peat
mounds, being above the level of high tides, are
replenished by rainfall and store freshwater as subsurface
mounds which discharge into the adjoining tidal flats
diluting and perturbating the pattern of tidal flat salinity.
Similarly, a discordant body of topographically emergent
sand on a muddy tidal flat (e.g., a sandy chenier that has
emerged above high water) will not be hydrologically
and hydrochemically influenced by flood-tide recharge
but by rainfall. The chenier, on receiving rainfall, stores
freshwater as a subsurface lens which, during times of
elevated water tables after rain, can discharge into the surrounding tidal flat, perturbating the salinity gradient (i.e.,
the increase in salinity from MSL to high water on the surrounding tidal flat). The extent of the perturbation/dilution
by the freshwater stored in a chenier on the salinity of the
tidal flat depends on where the chenier is located. The
most profound effect is by cheniers and their freshwater
lenses on the higher parts of tidal flats where there is the
most marked contrast between freshwater and the tidal flat
salinity.
As such, tidal flats generally have a gradient of increasing salinity from low-tidal zones to high-tidal zones
(Figure 2).
While there may be a general trend of increasing tidal
flat salinity from low-tidal levels to the high-tide mark,
this can be reversed by freshwater seepage along the margin of the hinterland. Freshwater seepage can manifest as
discharge along the upper tidal flat surface as surface
flows or springs or as subterranean flow along appropriate
stratigraphic conduits or sheets.
Tidal flat salinity gradients
Examples are provided of tidal flat salinity gradients in
Figures 3 and 4 from a tropical macrotidal estuarine
muddy tidal flat where rainfall is $1,548 mm per annum
and evaporation is $2,800 mm per annum and from subtropical (near-temperate) microtidal estuarine sand-andmud tidal flat where rainfall is $880 mm per annum and
evaporation is $1,300 mm per annum (Figure 4). In both
cases the open estuarine waters are near marine in salinity.
In the tropical macrotidal estuarine example, the tidal flat
salinity increases from 35 to 40 ppt at MSL to $ >180
near the level of the highest astronomical tide (HAT). In
the subtropical microtidal estuarine example, the tidal flat
salinity increases from 35 ppt at MSL to $60 ppt at mean
high water spring tide (MHWS), but is diluted by freshwater seepage along the hinterland edge (see below). In areas
of seasonal high rainfall, and where the tidal flats are
underlain by sand and bordered by a hinterland of sand,
the hinterland may be an aquifer storing and seasonally
discharging freshwater by seepage into the tidal flat –
while there is a salinity gradient from low-tidal to hightidal level, it seasonally changes from saline to brackish
in the higher parts of the tidal flat over the dry to wet
season.
In estuaries where there is a strong seasonal to perennial
delivery of freshwater by surface seepage or by subterranean seepage from supratidal environments (Semeniuk,
1983; Cresswell, 2000), the gradient of increasing salinity
upslope can be locally reversed from hypersaline to
680
TIDAL FLAT SALINITY GRADIENT
increase towards the shore. Macrotidal coasts (say, with
tidal ranges >4 m, and locally over 10 m) tend to have relatively wide tidal flats, particularly if the tidal flats have
a very low topographic gradient, and the exposure
between low and high water results in wide shore-parallel
zones within a tidal flat of differing exposure indices
increasing towards the shore. In this latter situation, the
gradient of salinity of the tidal flat is markedly evident
between MSL and high water and follows the shoreparallel exposure indices.
In combination with sediment type, the tidal flat slope
determines how rapidly during a low tide the tidal flat
drains by vertical infiltration or by direct runoff. Tidal flat
slope also determine how rapidly tidal flat water changes
from phreatic (with its water table at, or close to the tidal
flat surface) to vadose (the wetted sediment above
a falling water table at or close to the tidal flat surface).
Topographically very low-gradient tidal flats, particularly
if they are muddy and not sandy, drain water relatively
slowly such that there is a longer residency time of pore
water and pellicular water in the surface and hence
a longer period for evaporation. Steeper flats, particularly
if they are sandy, tend to drain water more quickly with
shorter residency time for surface pore water and
pellicular water and less time for direct evaporation.
Sediment types also determine how rapidly the tidal flat
drains during low tide. Sandy substrates are more hydraulically transmissive than mud substrates and drain water
more rapidly laterally and vertically. Because of their
low transmissivity, surface layers of mud and muddy sand
retain water for a longer period, are subject to evaporative
processes for longer periods, and tend to salinize more
rapidly than sand. While tidal flats that are composed
wholly of mud or wholly of sand will manifest relatively
uniform but different shore-parallel gradients of salinity
in groundwater and pellicular water in response to inundation, evaporation and transpiration, tidal flat sediments
that vary from sand to mud in cross-tidal-flat gradients,
or mosaics of sand and mud will exhibit more complicated
salinity patterns as the sediment types become additional
controlling factors.
The shore-parallel zones of increasing groundwater and
pellicular water salinity from MSL to high water in
response to decreasing inundation and increasing evaporation and transpiration and their resultant salinity gradients
can be disrupted by the hydrological changes effected by
vertical sedimentary shoaling to levels above the high
water mark and by discordant (or heterogeneous) stratigraphy such as sandbars and cheniers. In humid environments with excess rainfall, the sedimentary sequence
may vertically shoal (accrete) to levels of high water and
then be capped by peat (Coleman et al., 1970). Peat,
sustained by rainfall, may continue to accrete vertically
to form peat mounds to levels above high tide. Such peat
mounds, being above the level of high tides, are
replenished by rainfall and store freshwater as subsurface
mounds which discharge into the adjoining tidal flats
diluting and perturbating the pattern of tidal flat salinity.
Similarly, a discordant body of topographically emergent
sand on a muddy tidal flat (e.g., a sandy chenier that has
emerged above high water) will not be hydrologically
and hydrochemically influenced by flood-tide recharge
but by rainfall. The chenier, on receiving rainfall, stores
freshwater as a subsurface lens which, during times of
elevated water tables after rain, can discharge into the surrounding tidal flat, perturbating the salinity gradient (i.e.,
the increase in salinity from MSL to high water on the surrounding tidal flat). The extent of the perturbation/dilution
by the freshwater stored in a chenier on the salinity of the
tidal flat depends on where the chenier is located. The
most profound effect is by cheniers and their freshwater
lenses on the higher parts of tidal flats where there is the
most marked contrast between freshwater and the tidal flat
salinity.
As such, tidal flats generally have a gradient of increasing salinity from low-tidal zones to high-tidal zones
(Figure 2).
While there may be a general trend of increasing tidal
flat salinity from low-tidal levels to the high-tide mark,
this can be reversed by freshwater seepage along the margin of the hinterland. Freshwater seepage can manifest as
discharge along the upper tidal flat surface as surface
flows or springs or as subterranean flow along appropriate
stratigraphic conduits or sheets.
Tidal flat salinity gradients
Examples are provided of tidal flat salinity gradients in
Figures 3 and 4 from a tropical macrotidal estuarine
muddy tidal flat where rainfall is $1,548 mm per annum
and evaporation is $2,800 mm per annum and from subtropical (near-temperate) microtidal estuarine sand-andmud tidal flat where rainfall is $880 mm per annum and
evaporation is $1,300 mm per annum (Figure 4). In both
cases the open estuarine waters are near marine in salinity.
In the tropical macrotidal estuarine example, the tidal flat
salinity increases from 35 to 40 ppt at MSL to $ >180
near the level of the highest astronomical tide (HAT). In
the subtropical microtidal estuarine example, the tidal flat
salinity increases from 35 ppt at MSL to $60 ppt at mean
high water spring tide (MHWS), but is diluted by freshwater seepage along the hinterland edge (see below). In areas
of seasonal high rainfall, and where the tidal flats are
underlain by sand and bordered by a hinterland of sand,
the hinterland may be an aquifer storing and seasonally
discharging freshwater by seepage into the tidal flat –
while there is a salinity gradient from low-tidal to hightidal level, it seasonally changes from saline to brackish
in the higher parts of the tidal flat over the dry to wet
season.
In estuaries where there is a strong seasonal to perennial
delivery of freshwater by surface seepage or by subterranean seepage from supratidal environments (Semeniuk,
1983; Cresswell, 2000), the gradient of increasing salinity
upslope can be locally reversed from hypersaline to
680
TIDAL FLAT SALINITY GRADIENT