The tide, as defined by the Australian Hydrographic Service glossary, is the periodic rise and fall of the water of oceans, seas, bays, etc., caused mainly by the gravitational interactions between the Earth, Moon and Sun. The tidal inundation regime is the frequency of tidal inundation and is a fundamental driver of wetland function.
Tidal inundation defines and delineates intertidal and subtidal areas. The intertidal area is between the highest astronomical tide (HAT) and the lowest astronomical tide (LAT) and experiences fluctuating inundation and exposure to air. The subtidal area is below LAT and experiences permanent inundation.
The duration and frequency of inundation (e.g. diurnal, weekly, fortnightly, monthly, annual, decadal, episodic) and the spatial extent is important as it may affect desiccation and stress.
Over a lunar month, the highest tides and lowest tides occur roughly every 14 days, at the new and full moons, when the gravitational pull of the moon and the sun are in line and are termed spring tides which align to higher and lower tidal planes.
The terms “tidal planes”, “tidal datums”, and “tidal levels” may be used interchangeably, although some draw a distinction - for example, a “plane” implies a two-dimensional surface extending over a given region. A tidal datum plane is a plane of reference for elevations, determined from the rise and fall of the tides. Various tidal planes may be derived, and each is designated by a definite name, as, for example, the plane of mean high water, the plane of half-tide level, the plane of lower low water.
Tidal planes are a product of and response to topography. Currently tidal planes are almost impossible to predict at the habitat scale; they are best applied at seascape scales and above.
The duration of the tidal cycle changes along the Queensland coast for example, south from Lindeman Island tidal planes are semidiurnal (i.e. usually two high and two low tides each day), whereas in the Torres Strait and the Gulf of Carpentaria tidal planes are diurnal (usually one high and one low tide per day).
Although tidal planes can be important legal boundaries, the mapping is often based on models. Biotic patterns are often better indicators of tidal inundation, because inundation determines a range of water and sediment chemical properties that influence biota, e.g. salinity and oxidation-reduction (redox).
Patterns of marine plants can approximate some of the tidal planes. For example, marine plant ecosystems grow below HAT and are mapped as ‘Estuarine’ by the state wetland mapping and as occurring on Land Zone 1 (tidal flats and beaches) in Regional Ecosystem (RE) mapping. Mangroves generally grow above mean sea level (MSL) and species such as marine couch (Sporobolus virginicus) often help to identify the upper tidal limit or Highest Astronomical Tide (HAT). However freshwater influence (either groundwater or riverine) can modify patterns, especially at the upper tidal limits.
Tidal influence may also be independent of water salinity as the tidal water moving up and down in the upper estuary (including areas of riverine flow) or in groundwater influenced ecosystems may be fresh water. It may be difficult to determine whether such areas are being tidally influenced or not and for the purposes of classification these areas are categorised as ‘indeterminate’ until otherwise confirmed in the field.
Queensland Intertidal and Subtidal Classification Scheme
Inundation falls under the 'Hydrology' theme of attributes for the Intertidal and Subtidal Classification Scheme and represents the tidal inundation regime.
Modelled HAT surfaces are based on LiDAR.
Inundation frequency are based on multiple imagery time series. The spectral bands of light attenuate through the water column exponentially but at different rates depending on the depth or presence of land (various products derive from a 30 year Landsat series of 30m pixel size – seascape scale – produced by: Geosciences Australia; various commercial remote sensing companies).
Attribute category table - Inundation
Last updated: 16 July 2019
This page should be cited as:
Tidal inundation, WetlandInfo 2019, Department of Environment and Science, Queensland, viewed 31 January 2020, .