4.2.2.1 Sources of soil erosion by process type
Gully erosion Gully features are the largest source of soil erosion in the BBB catchment ( see Section 4.2.3 ; Wilkinson et al . 2013 ). The estimated contributions of gully erosion to sediment export are documented in Wilkinson et al . ( 2015a ) showing that the BBB has one of the highest contributions per hectare and per kilometre of gully erosion relative to other catchments in the Burdekin region , along with the Lower Burdekin ( upper parts ) and Don catchments have the highest contributions . This is not surprising , given the nature of soils in the BBB ( see section 2.3 ). This is also supported by the 2015 Source Catchments modelling ( McCloskey et al . 2017a ). Environmental factors associated with gully frequency in the area have been assessed , and suggest that lithology and soil data are the most useful predictors of gully erosion , and topographic data is also a moderately useful predictor of gully frequency ( Bui et al . 2017 ).
The current estimate of gully presence in the Burdekin Region is shown in Figure 18 and Bowen catchment specifically in Figure 19 ( Tindall et al . 2014 ). Present records were based on visual assessment of high resolution satellite imagery and aerial photographs at test locations , along with subsequent predictive modelling for the remaining catchment . Gully presence was expressed in seven ordinal classes from very low to very high in 5km x 5km grid units . The mapping has not been completed for the coastal catchments . Using this data , it is estimated that 60 per cent of the Burdekin Basin has low or very low gully densities , and a large proportion of this area is in the Cape Campaspe and Belyando catchments and the southern half of the Suttor catchment . Only 3 per cent of the Burdekin Basin has high to very high gully density , with most of these areas are in the Upper Burdekin ( 54 per cent ), BBB catchment ( 22 per cent ) and northern Suttor catchments ( 12 per cent ). It is noted that most gullies occur within the first 1.5 metres elevation above drainage lines and gully changes tend to be episodic and event driven ( Beutel et al . 2015 ).
In an assessment of the areas that are most prone to gully erosion , Wilkinson et al . ( 2015a ) has identified that duplex soils classified as Chromosols were found to be most prone to gullying . The top layer is sandier than the subsoil , and the clay-rich subsoil impedes water infiltration into the deeper soil profile and thus promotes overland flow and lateral subsurface flow .
It is estimated that there are approximately 34,100 kilometres of gullies in the Burdekin Region ( Wilkinson et al . 2015a ; Figure 18 and Figure 19 based on Tindall et al . 2014 , Darr et al . unpublished and NLWRA 2001 ; calculated using gully presence divided by 10 and multiplied by unit area ). Analysis of the gully lengths in individual catchments shows that the BBB catchment contains around 2,750 kilometres of gullies , which is in the lower range of the total proportion of gully length among the catchments ( 8 per cent ), but highly relevant given the location below the Burdekin Dam , and size of this catchment ( Figure 19 ).
Rill and scald erosion Rilling and sheetwash erosion in scalded areas on hillslopes ( degraded areas denuded of vegetation ) are most extensive in the Belyando‐Suttor catchments ( Karfs et al . 2009 ), but also commonly surround gully networks in the Bowen ( Wilkinson et al . 2013 ) and Upper Burdekin catchments ( Bartley et al . 2010a , 2010b ), where the rainfall erosivity and terrain slopes are higher than further inland from the coast .
Streambank erosion Streambank erosion is considered to contribute ~ 8 per cent to end of catchment sediment loads in the BBB catchment ( NQ Dry Tropics , 2016 ; McCloskey et al . 2017a ). However , it is not clear how much stream bank erosion has changed over time . Recent studies such as Brooks et al . ( 2014 ) suggest that the presence of woody vegetation , particularly on in‐channel features is a major factor controlling bank erosion rates in Queensland rivers . In the Burdekin Basin , and the BBB catchment specifically , tree cover in riparian zones is
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