Sediment input to coastal waterways varies greatly around Australia, and is often site specific. Transport of sediments by wind and water is a natural process that shapes the geomorphology of beaches and estuaries, but anthropogenic factors can change the spatial and temporal patterns of movement, as well as the quantity and type of sediment transported.
Catchment modification is one of the major drivers of change, and generally increases sediment input. High proportions of annual sediment budgets can come from the catchment and be transported during flood flows (Hossain & Eyre 2002). Sedimentation of navigation channels is also a major management issue for ports and harbours. Coastal lagoons and shallow coastal waters are among the most vulnerable habitats, because of their limited volumes and rates of water exchange with the ocean. Sediment input and nutrient input are closely linked processes, and many coastal waterways with elevated sediment input also suffer from nutrient enrichment (see Nutrient pollution).
Sedimentation influences a range of taxa, including mangroves, saltmarshes, corals and other sessile invertebrates, seagrasses, phytoplankton, microphytobenthos, and infauna (Todd et al. 2015). Impacts of excess sediments on pelagic (open-ocean) species include smothering, gill irritation and reduced light penetration, whereas impacts on benthic (ocean-floor) species arise from changes in grain size distribution, light penetration and seabed depth. Increased density of fine suspended sediments can also influence the behaviour and functioning of vertebrates (e.g. fish, turtles and dugongs), or have indirect impacts through changes in prey abundance.
In addition to sediment input, increased resuspension of bottom sediments can have significant ecological impacts (Knott et al. 2009), change water quality parameters and alter patterns of sediment transport. Sediment resuspension is exacerbated by vessel movements in shallow waters, dredging operations, and changes in hydrodynamics because of coastal engineering. Resuspension of contaminated sediments is particularly concerning because it releases contaminants into the water column, increasing their bioavailability and bioaccumulation (Hedge et al. 2009).
For the Great Barrier Reef coast, the Reef Plan report cards (2014) present sediment, nutrient and pesticide loads to the coast, and include modelled estimates of how they have changed because of catchment management works. Modelling of river plumes has been done for the Great Barrier Reef (e.g. Kroon et al. 2012; Álvarez-Romero et al. 2013; Fabricius et al. 2014, 2016). Although some dredge plume modelling has been done, it is generally not published in peer-reviewed literature (but see review by McCook et al. 2014). Sediment and nutrient transport to coastal waters is understood to be one of the major ongoing threats to the health of the Great Barrier Reef. Measures to reduce sediment inputs in Queensland are currently under way, although it is likely to take many years to achieve any outcomes (Reef Water Quality Protection Plan Secretariat 2014).
Sediment inputs from several north coast rivers in New South Wales have been studied by Eyre and Ferguson (2006), among others, but there is no ongoing comprehensive monitoring of sediment inputs to waterways of New South Wales. Research on storm water in Sydney Harbour (Birch & Rochford 2010) and plumes in selected locations near Sydney (Birch & O’Hea 2007) provides some insight into the role of urbanisation in sediment transport. A study of 184 New South Wales estuaries found that total suspended sediments were 16 to 10,594 per cent higher in areas of significant human land use, compared with relatively undisturbed catchments (Roper et al. 2011).
Assessment of the impacts of catchment management decisions on sediment loads requires either modelling of source inputs or monitoring across decades. Such long-term monitoring programs are economically costly, and have only been implemented at a small number of locations (e.g. Great Barrier Reef, Port Phillip Bay, Moreton Bay, Nepean–Hawkesbury River, Swan River and Blackwood River). More information on the effect of sediment transport on bedded sediments is crucial, as they are often the dominant habitat type and are good indicators of integrated environmental condition over time (Burton & Johnston 2010).
Coastal sediments around Australia have recently been mapped in the Coastal Sediment Compartments Project, which aims to improve coastal risk assessment by classifying coast based on landforms and patterns of sediment movement. It uses 3 levels to capture processes at different scales, and each level is suitable for different types of decision-making:
- primary—based on the influence of large landforms and offshore processes; suitable for regional planning or positioning of large-scale engineering such as ports
- secondary—based on medium landforms and regional sediment processes; useful for smaller engineering or local planning decisions
- tertiary—based on individual beaches; suitable for very small projects that are unlikely to restrict sediment movement, such as deciding the exact location of a groyne or sea wall within a broader management plan.
The outlook for sediment transport varies around Australia. Sediment quality is improving in some urban areas, particularly where organic inputs have declined because of changed waste management and/or the restoration of riparian vegetation, but coastal sprawl into previously undeveloped areas is increasing sediment and contaminant transport. Continued land clearing and poor agricultural practices result in increased transfer of terrestrial sediments to aquatic systems. Management should focus on combined sediment and nutrient input reductions, since the 2 inputs often co-occur, and can have compounding or interactive effects on sediment and water quality.