ensure the full 18.6-year lunar nodal cycle is included in
the analysis. The tidal datum is the average water level
over a tidal epoch of water height at a particular tidal
phase.
A multitude of tidal datums are currently in use within
different counties. The common tidal datums are:
• Highest astronomical tide (HAT): The highest water
level predicted to occur under any combination of astronomical conditions.
• Mean high water springs (MHWS) and mean low water
neaps (MHWN): The average of all high waters at the
time of spring or neap tide, respectively. This is applicable only in regions with semidiurnal tides. In mixed and
diurnal tidal regions, mean higher high water (MHHW)
and mean lower high water (MLHW) are used, which
are the mean of the higher or lower of the two daily high
waters, respectively.
• Mean high water (MHW): The average of all high
waters.
• Mean sea level (MSL): The arithmetic mean of hourly
observations of water level.
• Mean low water springs (MLWS) and mean low water
neaps (MLWN): The average of all low waters at the
time of spring or neap tide, respectively. In mixed and
diurnal tidal regions, mean higher low water (MHLW)
and mean lower low water (MLLW) are used, which
are the mean of the higher or lower of the two daily
low waters, respectively.
• Mean low water (MLW): The average of all low waters.
• Lowest astronomical tide (LAT): The lowest water level
which can be predicted to occur under any combination
of astronomical conditions. LAT is often used to define
chart datum (CD), which is the water level which depths
displayed on a nautical chart are referenced to.
Bibliography
National Oceanic and Atmospheric Administration, 2000. Tidal
datums and their applications. NOAA Special Publication NOS
CO-OPS 1. Available from: http://tidesandcurrents.noaa.gov/
publications/tidal_datums_and_their_applications.pdf.
Cross-references
Mean Sea Level
Tidal Ranges
Tides
TIDAL FLAT
Vic Semeniuk
V & C Semeniuk Research Group, Warwick,
WA, Australia
Synonyms
Mud flat; Sand flat
Definition
A tidal flat is a low-gradient surface, underlain by mud,
muddy sand, or sand, which is exposed during low tide.
Definitional limits, zones, and characteristics
of tidal flats
Before ecological aspects of tidal flats are described, some
background information on the variety of tidal flats is provided from the perspective of their limits, zones, geomorphology, sedimentology, hydrology, and salinity, as these
will determine, for a given climate, the types of habitats
developed, the biota inhabiting them, and ecological processes that will operate therein.
The concept and definition of a tidal flat varies with
author and between disciplines. Tidal flats, in the broadest
sense, are tidally exposed low-gradient surfaces, underlain
by a variety of substrates, and variably vegetated depending
on climate and location of the surface relative to mean sea
level (Semeniuk, 2005). They can be free of vegetation or
inhabited by mangroves, saltmarsh, or seagrass. In the
narrowest sense, for some authors, tidal flats are the
vegetation-free tidally exposed low-gradient surfaces usually occurring between MSL and low tide though, even in
this case, these types of flats may have a sparse ground
cover of seagrass. Some authors may incorrectly exclude
flats vegetated by mangroves (mangal) and saltmarsh, but
the vegetated parts of tidally exposed low-gradient surfaces
are still tidal flats. Although they have the same geomorphic
expression as traditional tidal flats, extensively exposed
rock pavements cut to low-gradient flats in the intertidal
zone, and gravel-floored flats are generally not considered
to be tidal flats. The consensus on tidal flats is that the tidally
exposed surface is underlain by soft sediment (sand, muddy
sand, or mud).
The moon and sun, the rotation of the earth, the regional
bathymetry, and the shape of the coast are major factors in
determining tidal hydrodynamics, the magnitude of tides,
and the forcing of the ebb and flow of tides (Cartwright,
1999). Water levels also can be affected by barometric
pressure and wind. The major effect of the moon and the
sun is to generate spring tides (the larger tidal oscillations
approximately coinciding with a new moon and full moon
or sun-earth-moon syzygy) and neap tides (the smaller
tidal oscillations approximately coinciding with the first
quarter moon and third quarter moon). Tidal zones
are identified as follows: equinoctial low water spring tide
(¼ELWS), a spring tide low water occurring twice yearly
during the equinoctial period; mean low water spring tide
(¼MLWS), the mean position of low water of spring tides;
mean low water neap tide (¼MLWN), the mean position
of low water of neap tides; MSL, the mean position of
water level between low water and high water; mean high
water neap tide (¼MHWN), the mean position of high
water during neap tides; mean high water spring tide
(¼MHWS), the mean position of high water during spring
tides; and equinoctial high water spring tide (¼EHWS),
a spring tide high water occurring twice yearly during
TIDAL FLAT
665
the analysis. The tidal datum is the average water level
over a tidal epoch of water height at a particular tidal
phase.
A multitude of tidal datums are currently in use within
different counties. The common tidal datums are:
• Highest astronomical tide (HAT): The highest water
level predicted to occur under any combination of astronomical conditions.
• Mean high water springs (MHWS) and mean low water
neaps (MHWN): The average of all high waters at the
time of spring or neap tide, respectively. This is applicable only in regions with semidiurnal tides. In mixed and
diurnal tidal regions, mean higher high water (MHHW)
and mean lower high water (MLHW) are used, which
are the mean of the higher or lower of the two daily high
waters, respectively.
• Mean high water (MHW): The average of all high
waters.
• Mean sea level (MSL): The arithmetic mean of hourly
observations of water level.
• Mean low water springs (MLWS) and mean low water
neaps (MLWN): The average of all low waters at the
time of spring or neap tide, respectively. In mixed and
diurnal tidal regions, mean higher low water (MHLW)
and mean lower low water (MLLW) are used, which
are the mean of the higher or lower of the two daily
low waters, respectively.
• Mean low water (MLW): The average of all low waters.
• Lowest astronomical tide (LAT): The lowest water level
which can be predicted to occur under any combination
of astronomical conditions. LAT is often used to define
chart datum (CD), which is the water level which depths
displayed on a nautical chart are referenced to.
Bibliography
National Oceanic and Atmospheric Administration, 2000. Tidal
datums and their applications. NOAA Special Publication NOS
CO-OPS 1. Available from: http://tidesandcurrents.noaa.gov/
publications/tidal_datums_and_their_applications.pdf.
Cross-references
Mean Sea Level
Tidal Ranges
Tides
TIDAL FLAT
Vic Semeniuk
V & C Semeniuk Research Group, Warwick,
WA, Australia
Synonyms
Mud flat; Sand flat
Definition
A tidal flat is a low-gradient surface, underlain by mud,
muddy sand, or sand, which is exposed during low tide.
Definitional limits, zones, and characteristics
of tidal flats
Before ecological aspects of tidal flats are described, some
background information on the variety of tidal flats is provided from the perspective of their limits, zones, geomorphology, sedimentology, hydrology, and salinity, as these
will determine, for a given climate, the types of habitats
developed, the biota inhabiting them, and ecological processes that will operate therein.
The concept and definition of a tidal flat varies with
author and between disciplines. Tidal flats, in the broadest
sense, are tidally exposed low-gradient surfaces, underlain
by a variety of substrates, and variably vegetated depending
on climate and location of the surface relative to mean sea
level (Semeniuk, 2005). They can be free of vegetation or
inhabited by mangroves, saltmarsh, or seagrass. In the
narrowest sense, for some authors, tidal flats are the
vegetation-free tidally exposed low-gradient surfaces usually occurring between MSL and low tide though, even in
this case, these types of flats may have a sparse ground
cover of seagrass. Some authors may incorrectly exclude
flats vegetated by mangroves (mangal) and saltmarsh, but
the vegetated parts of tidally exposed low-gradient surfaces
are still tidal flats. Although they have the same geomorphic
expression as traditional tidal flats, extensively exposed
rock pavements cut to low-gradient flats in the intertidal
zone, and gravel-floored flats are generally not considered
to be tidal flats. The consensus on tidal flats is that the tidally
exposed surface is underlain by soft sediment (sand, muddy
sand, or mud).
The moon and sun, the rotation of the earth, the regional
bathymetry, and the shape of the coast are major factors in
determining tidal hydrodynamics, the magnitude of tides,
and the forcing of the ebb and flow of tides (Cartwright,
1999). Water levels also can be affected by barometric
pressure and wind. The major effect of the moon and the
sun is to generate spring tides (the larger tidal oscillations
approximately coinciding with a new moon and full moon
or sun-earth-moon syzygy) and neap tides (the smaller
tidal oscillations approximately coinciding with the first
quarter moon and third quarter moon). Tidal zones
are identified as follows: equinoctial low water spring tide
(¼ELWS), a spring tide low water occurring twice yearly
during the equinoctial period; mean low water spring tide
(¼MLWS), the mean position of low water of spring tides;
mean low water neap tide (¼MLWN), the mean position
of low water of neap tides; MSL, the mean position of
water level between low water and high water; mean high
water neap tide (¼MHWN), the mean position of high
water during neap tides; mean high water spring tide
(¼MHWS), the mean position of high water during spring
tides; and equinoctial high water spring tide (¼EHWS),
a spring tide high water occurring twice yearly during
TIDAL FLAT
665