2 Background
2.1 Sediment and the Bowen Broken Bogie Catchment
The Bowen Broken Bogie( BBB) Catchment produces 25 % of the total fine sediment load discharged to the Great Barrier Reef( GBR) 3 and represents some of the highest sediment concentrations and loads in the GBR. The current annual average sediment loads from the BBB are estimated to be 7.5 times the natural rate( Bartley et al., 2015). Grazing lands are the primary source of total suspended sediment( TSS) loads in the BBB. Gully erosion( hillslope and alluvial) is considered the dominant source of TSS( 65 %), followed by hillslope( 27 %) and streambank( 8 %) erosion. These proportions vary between sub catchments, but gully erosion is the dominant sediment source, except in the Broken River sub catchment where hillslope erosion dominates( 65 %).
Four types of erosion contribute to the sediment loads. These are:
1. Hillslope gully erosion( Figure 2) is the most prevalent form of gully erosion( the BBB catchment contains around 2,750km of gullies( Waterhouse, Greiner, Bainbridge, & King, 2017)). Hillslope gully erosion has linear or branching features along hillslope drainage lines. In this type of gully the overland or subsurface water flow over the gully headwall causes the headwall to migrate upslope. Wilkinson, Kinsey-Henderson, Hawdon, Ellis, and Nicholas( 2013) note that there is extensive literature related to the management of this type of gully;
2. Hill slope erosion includes rilling and sheetwash erosion, and commonly surrounds gully erosion in the BBB( Wilkinson et al., 2013);
3. Streambank erosion contributes about 8 % of BBB loads, although riparian tree cover is largely intact in the BBB( Waterhouse et al., 2017). There is some evidence of severe stream bank erosion at a number of existing and abandoned mine sites in the upper sections of the little Bowen sub-catchment; and
4. Alluvial gullies( Figure 3). These occur in deep alluvial soils which result in gullies which are not necessarily aligned with overland flow pathways. Like hillslope gullies, these gullies are probably initiated by localised vegetation disturbance along drainage lines. Once these gullies are established, mass failures at their edges can see them spread independent of surface runoff.
There is considerable evidence that gully erosion became much more widespread in south-east Australian landscapes following the introduction of livestock grazing, and associated degradation of ground vegetation cover( Prosser, Rutherfurd, Olley, Young, & Wallbrink, 2001).
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The Bowen sub-catchment delivers 43 % of the Burdekin TSS. The Bowen and the Upper Burdekin sub catchments have the highest rates of accelerated erosion relative to pre-European rates at 7.5 and 3.6 times respectively( Bartley, Croke, Bainbridge, Austin, & Kuhnert, 2015).
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