Figure 48: Sedimentary environments in Moreton Bay, Queensland

Notes:

1. Wave-dominated estuaries are distinguished by relatively high wave energy at the mouth compared to tide energy.
2. Near the mouth, total energy is high due to the summation of high wave and tide energies.
3. Near the head, total energy is high due to high river energy. River energy declines downstream due to a reduction in downstream hydraulic gradient.
4. In the middle of the estuary, total energy is low because waves can not penetrate the estuary, and because tidal energy is dissipated on the ebb- and flood-tide deltas.
5. 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.
6. Landward of the barrier and flood/ebb tide deltas is a low-energy relatively deep central basin. The central basin is the main sink for fine sediment.
7. Waves and tidal currents deposit fine sediment on the edge of the central basin to form intertidal flats, and saltflats/saltmarshes. Mangroves are common along margins. Sandy beaches can also form.
8. 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.

Notes:

1. Near the mouth, total energy is high because both tidal energy is high and wave energy is moderate.
2. Inside the estuary, wave energy is reduced over extensive tidal sand banks, thus decreasing total energy.
3. Total energy rises to a maximum where the difference between the effects of construction by the funnel-shaped entrance (tidal-amplification) and effects of dissipation by sediment shoals is greatest.
4. Further headward, total energy falls to a minimum because friction created by the sediment shoals becomes greater than tidal amplification.
5. total energy rises in the river-dominated zone because of constriction at the head.
6. 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.
7. 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.
8. Intertidal flats, mangroves, and saltflat/saltmarshes occur extensively along the sides of the estuarine channel (woodroffe et al. 1989).
9. Tide dominated estuaries are naturally turbid because of the strong tidal currents.

Notes:

1. Wave-dominated deltas are characterised by relative high wave energy at the mouth compared to tide energy, and are distinguished from wave-dominated estuaries by high river energy.
2. Total energy at the mouth is high because of high wave energy at the coast.
3. 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.
4. Maximum tidal energy occurs in the considered inlet mouth.
5. At the mouth, waves transport sediment towards the entrance and build a sub aerial barrier.
6. Sediment transported from the catchment by the river is deposited on the floodplain, forming levees and back swamps, and in the main channel.
7. River sediment is transported directly to the mouth because the channel connects the rivers catchment with the ocean.
8. Relatively strong river energy causes net seaward-directed sediment transport. Coarse sediment deposited near the inlet forms flood/ebb tide deltas.

Notes:

1. 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.
2. Tidal energy is greatest slightly landward of the mouth due to construction by the funnel shaped mouth.
3. Wave energy is dissipated on shoals seaward of the mouth, and declines rapidly lamdwards.
4. River energy remains moderate to high along the channel, but drops off significantly as the channel widens towards the mouth.
5. 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.
6. Extensive areas of intertidal flats, mangroves, and saltflat/ saltmarshes occur along the sides of the channel.

Notes:

1. Fine and coarse sediment enter the estuary from the catchment, depending on river flow and sediment supply.
2. The majority of coarse material is deposited at the head of the estuary, due to a reduction of river flow velocity and therefore sediment transport capacity. Some reworking and redeposition of material by tidal currents also occurs.
3. Fine sediment undergoes both deposition and erosion in intertidal flats, aided by biological activity such as burrowing. Coarser material is also deposited on flanking environments by tidal currents and flood events. A general trend of slow growth of intertidal sedimentary environments is observed.
4. Large quantities of suspended sediment are characteristic of tide-dominated estuaries, and a dynamic relationship exists between deposition, flocculation, resuspension and transport of sediment. Quantities of fine and coarse sediment can pool temporarily within the channel.
5. Mangrove sedimentary environments, with interspersed tidal drainage channels, commonly flank tide-dominated estuaries, and serve as a depocentre for fine and flocculated sediment. Tidal asymmetry (high energy flood and lower energy ebb), baffling by vegetation, and percolation of tidal water through animal burrows result in the deposition of fine sediment, and allow for the replacement of intertidal flats by mangrove.
6. Saltflat sedimentary environment experience inundation by king tides, and some deposition of fine sediment can occur. Ebb tide waters often flow through tidal drainage channels. Quantities of fine and coarse sediment can also be derived from the catchment and deposited during storm events.
7. Accumulation of coarse bedload material can occur within the mouth of the estuary, forming tidal sand banks. This material tends to be unstable and is redistributed in large quantities during storms. Seagrasses are able to colonise and fix the sediment to an extent, also mangrove colonisation can occur on larger sand banks.
8. Very little sediment is exported from the estuary overall, due to net landward transport driven by tidal action. The majority of sediment export occurs during flood events.

Notes:

1. Nitrogen (particulate and dissolved;TN) enters the estuaries system from point- and non-point sources from within the catchment.
2. Tidal movements on the flanks of the estuary transport particulate nitorgen (PN) and dissolved inorganic nitrogen (DIN) onto the intertidal flats, where some of the dissolved inorganic notrogen is converted to particulate nitrogen through the activity of benthic micro algae.
3. Mangrove sediment is a net sink for dissoloved inorganic nitrogen and particulate nitrogen. Nutrient uptake is driven by high rates of plant growth and microbial activity . N-fixation is active in the root-zone and contributes to the dissolved inorganic nitrogen pool. Some nitrogen is liberated to the atmosphere as N2 gas through denitrification. Particulate nitrogen is processed by biota such as crabs, or it is exported to the copastal waters as leaf litter and fine particulate matter. In the coastal waters it may be redistributed during ebb tides.
4. Small amounts of particulate nitrogen are buriewd in salt flats during king tides. Most particulate nitrogen is exported back into the estuarine channel during the ebb tide.
5. Particulate nitrogen and dissolved inorganic nitrogen exist within the water column. However due to turbidity, phytoplanktonproductivity is limited. Circulation and resuspension of particulate nitrogen occurs in this zone. Particulate nitrogen is probably reworked during the resuspension process, and dissolved inorganic nitrogen can be remineralised to the water column.
6. A proprtion of the dissolved inorganic nitrogen reaches the less turbid zone at the mouth of the estuary where phytoplankton convert it to particulate nitrogen.
7. Seagrasses, which colonise the tidal sand banks near the mouth of the estuary, also process dissolved inorganic nitrogen, in the same manner as that described for wave-dominated estuaries.
8. Typically, only moderate quantities of the total nitrogen load are exported to the marine environment, however, this may be significant during flood events.