You are here: ANRA home » Natural resource topics » Coasts » Estuary processes
This web site is no longer being updated
In this section you will find detailed information, including national overviews and state and regional level assessments, from the 2000-2002 National Land and Water Resources Audit theme assessments.
Coasts - Understanding Processes - Australia
Australia
Processes
A Process-Based Classification
The Australian Geological Survey Organisation developed a process-based classification covering physical forces (wave, tide and river energies) driving the form and function of Australian estuaries and coastal waterways. Geomorphic characteristics and sedimentary environments have been mapped for 405 of Australia's modified coastal waterways.
Australian estuaries were classified into six subclasses according to the wave-, tide- and river-energies that shape them and their overall geomorphology. The geomorphic definition of an estuary is different to the one used for the condition assessment. Only 28 Australian coastal waterways are considered estuaries using the geomorphic definition. The remainder are deltas (19), strandplains (5) or tidal creeks (35)
- 17 Australian estuaries are wave-dominated estuaries
- 11 Australian estuaries are tide-dominated estuaries
- 10 Australian estuaries are wave-dominated deltas
- 9 Australian estuaries are tide-dominated deltas
A seventh subclass "others" (includes: Drowned River Valleys, Embayments and Coastal Lakes/Lagoons/Creeks. Strandplains and Tidal Creeks are indicative of very low river-energy, and their joint dominance in the data set (reflects the fact that Australia is a dry continent, with relatively little river runoff.
Classification of Coastal Subsystems - Divided into six subclasses
Classification of coastal systems divided into six subclasses
The position of each subclass with respect to one another depends on the relative influence of wave, tide and river energies. All coastal systems are thus distinguished based on the relative wave/tide power (the x-axis) and river energy (y-axis). Deltas (WDD = wave-dominated delta; TDD = tide-dominated delta) have relatively high river energy and therefore occupy the uppermost regions of the triangle. Strandplains and tidal flats occupy the base of the triangle and are characterised by relatively low river energy. Estuaries, from a geomorphologic perspective, refer to those located in the middle of the triangle.
Map:Map of Australia showing the classification of 780 estuaries into subclasses based on their geomorphology.
The northern coast has the highest number of tide-dominated coastal systems. The south-eastern coast is characterised by wave-dominated estuaries. South Australia is dominated tidal flat/creeks due to the arid climate and the lack of significant fluvial discharge. The ratio of tide-dominated to wave-dominated systems in Western Australia is 2:1; the tide-dominated systems occur in the north of the state, whilst the wave-dominated systems occur in the southwest.
Table: Estuary processes and functions and considerations for management.
Different estuary types exhibit naturally different behaviours in terms of sediment trapping efficiency, turbidity and circulation and sedimentation. Human activities can change the way an estuary functions. The natural processes within an estuary need to be understood for effective management. For instance, the sediment trapping efficiency of wave-dominated estuaries is high. Therefore wave-dominated estuaries are more likely to silt up and accumulate sediments. This is important information if navigation is a priority use in this estuary.
Conceptual Models
The following generic 3D-conceptual models illustrate the general characteristics of wave- and tide-dominated estuaries and deltas and the way they transport sediment and cycle nitrogen. The models present a simple, yet holistic picture of the physical and biological processes. The models are tools that should assist managers and stakeholders with the development of coastal waterway management plans, including monitoring protocols. It is important to note that a particular estuary may exhibit different characteristics due to seasonality.
Sediment transport
Turbidity can be considered a management concern for estuaries and coastal waterways because significant increase in suspended material in the water column limits photosynthesis which in turn impacts seagrass habitat and phytoplankton viability.
Nitrogen cycling
Nitrogen is an important nutrient controlling phototrophic growth in many Australian estuaries. Denitrification is the microbial conversion of nitrogen (N) to nitrogen gas (N2) within the sediment and is a self-cleansing mechanism by which water bodies can rapidly rid themselves of N derived from point- and non-point sources within the catchment. Nitrogen gas is unavailable biologically and is vented to the atmosphere. Ocean upwellings are believed to be an important source of nitrogen for Tasmanian estuaries.
Wave Dominated Estuary
Wave-dominated estuaries are distinguished by higher wave energy at the mouth compared to tide energy. In the middle of the estuary, total energy is low because waves cannot penetrate the estuary, and because tidal energy is dissipated on the ebb- and flood-tide deltas.
Figure: Geomorphic and sedimentary facies model of wave-dominated estuaries
Sediment processes
Waves transport sediment from the sea towards the estuary and build a barrier at the mouth. Tidal currents transport sediment into the estuary to form flood and ebb tidal deltas that extend seaward and landward of the inlet. Landward of the barrier and flood/ebb tide deltas is a low-energy, deep central basin where fine sediment accumulates. Waves and tidal currents deposit fine sediment on the edge of the central basin to form intertidal flats, saltflats and saltmarshes. Sandy beaches can also form. Sediment from the catchment is deposited in the main channel, on the floodplain, and can be transported into the estuary to form a fluvial bay-head delta that extends into the central basin.
Nitrogen processes
Particulate and dissolved nitrogen (total nitrogen) enters the estuary from both point and non-point sources. Some deposition and burial of particulate nitrogen occurs on flanking saltmarshes. Decomposition of organic matter within the sediment produces dissolved inorganic nitrogen which is transported into the central basin of the estuary, where biological uptake (phytoplankton, seagrass and macrophytes) occurs along the way if residence times are long enough, and if temperature and light levels are suitable. Denitrification within the sediment converts nitrate to N2 gas. The N2 escapes from the system to the atmosphere.
Tide Dominated Estuary
Tide-dominated estuaries are distinguished by relatively high tidal energy at the mouth compared with wave energy. Inside the estuary, wave energy is reduced over extensive tidal sand banks. Total energy rises to a maximum where the difference between the effects of constriction by the funnel-shaped entrance (tidal-amplification) and effects of dissipation by sediment shoals is greatest. Total energy rises near the river because of constriction at the head. Mangroves with interspersed tidal drainage channels commonly flank tide-dominated estuaries.
Figure: Geomorphic and sedimentary facies model of tide-dominated estuaries
Sediment processes
In the funnel-shaped mouth, strong tidal currents transport coarse sediment into the estuary and build elongate tidal sand banks that extend to the zone of maximum total energy. Near the tidal limit, where the channel is characterised by a sinuous river channel pattern, total energy is at a minimum. Sediment of mixed river and marine origin accumulates here. Intertidal flats, mangroves, and saltflat/saltmarshes occur extensively along the sides of the channel. Tide-dominated estuaries are naturally turbid because of the strong tidal currents.
Nitrogen processes
Particulate and dissolved nitrogen (total nitrogen) enters the estuary from both point and non-point sources. Tidal movements on the flanks of the estuary transport nitrogen onto the intertidal flats, where some of the dissolved inorganic nitrogen is converted to particulate nitrogen through the activity of benthic micro-algae. Nutrient uptake is driven by high rates of plant growth and microbial activity. However due to turbidity, phytoplankton productivity is limited. N-fixation is active in the root-zone and contributes to the DIN pool. Some N is liberated to the atmosphere as N2 gas through denitrification. PN is processed by biota such as crabs, or it is exported to the coastal waters as leaf litter and fine particulate matter. Most PN is exported back into the estuarine channel during the ebb tide.
Wave Dominated Delta
Wave-dominated deltas are characterised by relatively high wave energy at the mouth compared to tide energy, and are distinguished from wave-dominated estuaries by their high river energy. Total energy at the mouth is high because of high wave energy at the coast. Total energy declines immediately landward of the mouth because wave energy is dissipated on the barrier. The dominance of river energy further landward means total energy is relatively high along the channel. Maximum tidal energy occurs in the constricted inlet mouth. At the mouth, waves transport sediment towards the entrance and build a sub aerial barrier. Sediment transported from the catchment by the river is deposited on the floodplain, forming levees and back swamps, and in the main channel. River sediment is transported directly to the mouth because the channel connects the rivers catchment with the ocean. Relatively strong river energy causes net seaward-directed sediment transport. Coarse sediment deposited near the inlet forms flood/ebb tide deltas.
Figure: Geomorphic and sedimentary facies model of wave-dominated deltas
Sediment processes
Fine and coarse sediment enter the estuarine system from the catchment, depending on river flow and sediment supply. Suspended fine sediment, and coarse sediment is moved along the bottom of the channels downstream (as bedload), due to unimpeded river flow within the delta. Some lateral deposition of both types of sediment can occur, including the development of coarse sediment point bar deposits, and deposition of fine sediment (during flood events) on the floodplain. Limited deposition and resuspension occurs on intertidal flats and saltmarshes if present. The majority of deposition occurs at the mouth of the delta, and results in the export of sediment into the marine environment. Fine suspended sediment is generally exported, with some flocculation occurring over the salinity gradient. Bedload accumulation of coarser sediment can occur, and may form an ebb tidal delta within the entrance of the estuary. High wave energy results in the distribution of sediment along the coastline proximal to the delta, forming a barrier bar.
Nitrogen processes
Biological uptake by plants of dissolved inorganic nitrogen (DIN) occurs on the flanks of the river channel. Intertidal flats and mangroves often occur, and can influence nutrient dynamics as per tide-dominated estuaries. However, they may play a smaller role. Particulate nitrogen (PN) is buried in saltmarsh facies during king tides, or during periods of high fluvial flow. Some PN is exported back into the estuarine channel during the ebb tide. The majority of the river-borne TN is transported from the delta by strong downstream displacement. Lower turbidities allow for its assimilation by phytoplankton in the marine environment. DIN uptake by seagrass growth may occur at the mouth of the delta.
Tide Dominated Delta
Tide-dominated deltas are characterised by relatively high tide energy at the mouth compared with wave energy, and are distinguished from tide-dominated estuaries by high river energy. Tidal energy is greatest slightly landward of the mouth due to constriction by the funnel shaped mouth. Wave energy is dissipated on shoals seaward of the mouth, and declines rapidly landwards. River energy remains moderate to high along the channel, but drops off significantly as the channel widens towards the mouth. Inside the mouth, moderately-strong tidal currents transport coarse sediment into the channel from offshore and build elongate tidal sand banks. These banks only extend a short distance into the channel because tidal energy is dissipated by channel friction. Extensive areas of intertidal flats, mangroves, and saltflat/saltmarshes occur along the sides of the channel.
Figure: Geomorphic and sedimentary facies model of tide-dominated deltas
Sediment processes
Fine and coarse sediment enter the estuarine system from the catchment, depending on river flow and sediment supply. Suspended fine sediment, and coarse sediment (as bedload) are transported downstream. Some lateral deposition of both types of sediment can occur, including the development of coarse sediment point bar deposits, and floodplain deposition of fine sediment (during flood events). Fine and coarse sediment are deposited onto the flanking intertidal flats, mangrove and saltmarsh environments, in a similar manner to processes described for tide dominated estuaries. The majority of deposition occurs at the mouth of the delta, and results in the export of sediment into the marine environment. Fine suspended sediment is generally exported, with some flocculation occurring over the salinity gradient. Bedload accumulation of coarser sediment can occur, and may form an ebb tidal delta within the entrance of the estuary, and tidal sand banks may form due to sediment resuspension and recycling. Sediment transported by tidal currents accumulates on the delta front, causing the gradual progradation of the delta.
Nitrogen processes
Biological uptake by plants of dissolved inorganic nitrogen (DIN) occurs on the flanks of the river channel and may be an important sink for N within the delta. Intertidal flats and mangrove facies influence nutrient dynamics in a similar way to that described for tide-dominated estuaries. Particulate N (PN) is buried in saltmarsh facies during king tides, or during periods of high river flow. Some PN can be exported back into the estuarine channel during the ebb tide. The majority of the TN load is transported from the delta by strong downstream displacement. Lower turbi../images/. Some circulation and re-suspension of nutrients also occurs. DIN uptake by seagrass growth may occur at the mouth of the delta.
Further information
- View or download the Australian Geological Survey Organisation report "A geoscience perspective for improved and integrated resource management" [PDF 17.2 MB]
- Click here to download the Estuary Assessment 2000 [PDF 9.3 MB]
Key reference:
Dalrymple R. W., Zaitlin B. A. & Boyd R. (1992). Estuarine facies models: conceptual basis and stratigraphic implications. Journal of Sedimentary Petrology, 62 (6):1130-1146.
Link to the Map Maker to make a map using this information.
Before you download
Most publications are downloadable as PDF files. Adobe Acrobat Reader is required to view PDF files.
If you are unable to access a publication, please contact us to organise a suitable alternative format.
Key
Links to an another web site
Opens a pop-up window