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Sediment load

Before European settlement and clearance, river flow in coastal catchments was more even, and flood peaks were lower. Most commentators estimate that sediment discharge by rivers to the marine environment was much lower, and that the increase following the institution of European farming practices was of the order 2-10 times.

While the amounts of soil loss would vary greatly with many factors, but especially soil type, slope and rainfall intensity, clearance of native vegetation has everywhere been a major precursor to erosion.

Table 3.6 Land cover change in Australia (000 km2), 1788 to 1988, by major vegetation types

Vegetation type

Area in 1788 (km2)

Area in 1988 (km2)

Forest

690 000

390 000

Woodland

1570000

1070 000

Open Woodland

1650000

2000000

Shrubland

3 080000

2880000

Grassland/ Pasture

500000

1150000

Source: SEAC 1996, p. 6-11.

Clearance led to degradation of the soil structure through loss of organic matter and surface packing, infiltration was reduced and run-off increased, leading to erosion through sheet wash, stream bank erosion and gullying. The major disturbance of the native vegetation/soil complex, which had formerly held the soil on the land, resulted in major loss of topsoil within the catchments of humid Australia. Since vegetation cover is vital for soil retention, land use affects the erosion rate: because many factors are involved a great range of sediment yield values are found. Some indicative values are given in table 3.7. Wasson (1997)

Table 3.7 Soil loss on sloping land

Land management system

Region

Soil loss on sloping land (t/ha/year)

Tropical cropping, e.g. sugar, pineapple

Queensland, NT

100-500

Cereal cropping

Southern Queensland, South Australia

1-50

Forested catchments

South-eastern Australia

0-11

Pastures (well managed)

Southern Australia

50-100

Bare fallow

Southern Australia

50-100

110-50 after bushfires

Source: SEAC 1996

estimated that currently the total yield of sediments delivered to the coast is 244 million tonnes/year.

However, land use and vegetation cover are only two of the factors affecting sediment quantities delivered to the coast. Soil loss occurs through gullying, stream bank erosion and sheet and rill erosion. Networks of gullies often form quickly on cleared land in humid Australia, with the networks enlarging for decades until they reach maturity. Estimates suggest that gullies (while their networks are growing) may well be the most important form of erosion: in the uplands of New South Wales and Victoria, gullied catchments yield about eight times more sediment than ungullied ones, regardless of land use (Neil & Galloway 1989).

Wasson (1997) noted that in small catchments sediment yield varies with hill slope and form. Thus a steep hillslope with gullies leading straight to a creek may deliver sediment rapidly to the creek, but where the gully leads to a floodplain the storage of sediment within the floodplain may be of the order of hundreds of years. Changes in run-off, and consequently river discharge, also have channel effects, altering channel slope and pattern. Thus, for example, a rise in discharge will usually lead to channel widening, a greater meander curve and a greater meander amplitude. Channel changes erode materials stored in the valley floor and floodplain; the silts and clays eroded in this way are moved rapidly through the system to estuaries and the sea, while the sands and gravels move more slowly in the form of slugs of material transported at times of flood. The journey time of such coarse sediment slugs to the sea may be slow, but because of the sediment bulk the effects may be locally overwhelming for estuarine and marine life forms. Very large bodies of coarse sediment may be moved in stages down rivers from sand and gravel works and other types of mining. The 1996 State of the Environment Report (SEAC 1996, p. 7-15) mapped areas of major channel change in south-eastern Australia. The incision of valley floors was reported throughout the south-eastern highlands of New South Wales and Victoria, mainly in the headwaters of the Murray-Darling catchment. Channel widening was reported on the slopes and coastal plains of New South Wales. River changes associated with sand and gravel works are marked around Sydney and Brisbane.

Concentration of suspended solids in estuaries and nearshore waters

To the casual observer, turbidity in marine waters is one of its more obvious qualities. We quickly notice the outflow of muddy river waters, or the plume of sediment behind a dredge. The term turbidity refers to suspended material: silt, clay, organic fragments, microalgae and plankton. Turbidity is measured by filtration to give concentration in milligrams per litre, or estimated in nephelometric turbidity units (NTU) using a Secchi disc.

The turbidity of coastal waters varies greatly with circumstances. Large floods deliver large amounts of sediment, but there is not a straightforward relationship of rainfall total and sediment quantity. Heavy rain following drought may cause erosion peaks from land surfaces and creek banks. Building, clearance, fires or dirt road construction often cause erosion, sending giant sediment slugs down channels to the sea.

Dispersion within an estuary or at the shore may depend on local wave turbulence or tidal flow. Measured levels of suspended solids in coastal waters are rare, and those that exist show variation in one location of several orders of magnitude over time. SEAC (1996, p. 8-39) reported measured values in the sheltered waters of Barker Inlet, South Australia, averaging 25 mg/L, falling to 3.5 mg/L in calm weather and rising to 300 mg/L in a storm.

 
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