Sediment transport in wave-dominated deltas

Block diagram of sediment transpoert in wave-dominated estuaries

Sediment transport processes in wave-dominated deltas

1. Fine and coarse sediment input from the catchment

Fine and coarse sediment enters the delta from the catchment. catchment and climatic conditions, and the volume of river input. However, the amount of terrigenous sediment delivered to wave-dominated deltas is usually relatively large. Seasonal and climate factors dominate the function of deltas, with episodic high-flow events causing intense flushing, sedimentation, and erosion in the main channels and floodplain (Eyre et al., 1999).

2. Deposition of fines in freshwater wetlands

Limited deposition of fine sediment (including clays, muds and organic material) occurs upon the floodplain during high flow events (Jones et al., 1993). This is enhanced by the baffling effects of floodplain vegetation associated with swamp and marsh areas, and leads to slow vertical accretion of the floodplain. Some lateral deposition of sediment can occur, including the development of coarse sediment point-bar deposits.

3. Deposition and resuspension

Some tidal deposition and burial of fine sediment occurs on flanking intertidal environments, aided by the baffling effects of vegetation such as saltmarshes and mangroves. Coarse sediment (such as sands and gravels) may also accumulate here during flood/high flow conditions. Some burial and resuspension of sediment can also occur within intertidal flats. Flanking environments typically play a much smaller role in deltas in comparison to estuaries, as there is very little space remaining for deposition within deltas.

4. Downstream transport of fines

Wave-dominated deltas are characterised by net seaward-directed sediment transport, associated with the relatively high river discharge and relative absence of available accommodation space for sediment deposition (Bhattacharya et al., 1992). Consequently, fine suspended sediment, and coarse sediment (as bedload) is moved downstream along the bottom of the deltaic channels, due to unimpeded river flow. Some lateral deposition of both types of sediment can occur, including the development of coarse sediment point-bar deposits. Deposition may be limited at the bottom of channels due to scouring by strong currents.

5. Export of sediment

The majority of deposition occurs seaward of the delta mouth, and results in the net export of sediment into the marine environment (Jones et al., 1993, Hume et al., 1993). Fine suspended sediment is generally transported offshore, with some flocculation occurring over the salinity gradient. Coarser sediment tends to accumulate close to the entrance of the delta, although this material is generally redistributed by wave action (Melville, 1984, Cooper, 1993). The sediment trapping capacity of wave-dominated deltas is low, because most terrigenous sediment is exported to the marine environment and lost to the coastal sediment budget (Davies, 1974, Roy et al., 1977, Hacker, 1988).

6. Washover during storm events

High wave energy results in the distribution of sediment along the coastline proximal to the delta, forming a barrier (Melville, 1984, Otvos, 2000, Roy et al., 2001). In situations where the river delivers sediment to the coast faster than waves are able to transport it away, the coastline tends to slowly prograde into the marine environment, forming a ‘coastal protuberance’.