Final LDC WQ Report | Page 88

Key Findings

Gully mapping in the BBB A total of 22,311 active gullies were mapped within the study area (~ 3,500 km 2 ), which have a cumulative active area ( currently eroding , shown within the ‘ hard margin ’ in the example in Figure 37 of 4,620 ha . This represents 1.3 % of the land area captured within the LiDAR . Gully soft margins ( where not all the mapped area is currently eroding ) encompass 5.4 % of the total area .
Gullies are not equally distributed through the BBB catchment . There are obvious hotspots which are clear in certain landscape units and key soil types ( outlined further in full report ).
Fine sediment estimates from the mapping showed that :
• The estimated total fine sediment (< 20 µ m ) yield ( mean annual rainfall corrected for the last 20 years ) for this study from all gullies was 1,300 kt / yr ± 614 kt / yr .
• The mean fine sediment proportion contribution was 56.3 % for all alluvial gullies and 48.6 % for hillslope gullies .
• The fine sediment contributions from hillslope ( 687 kt ) and alluvial gullies ( 624 kt ) appeared to be roughly equal ( within the error margins of the data ), however there were more hillslope gullies ( 12,293 ) than alluvial gullies ( 10,018 ). The specific yield was higher for alluvial gullies .
Figure 37 : Example of gullies mapped with a soft margin ( not all of which is currently eroding ) and a hard margin ( the actively eroding part ).
Sediment yields The sediment yield from individual gullies was highly skewed with a small number of large active alluvial gully systems contributing a disproportionate quantum of the fine sediment load , as illustrated in Figure 38 . The fine sediment yield curve shows a highly skewed distribution with a small number of high yielding gullies contributing 30 % of the catchment fine sediment yield ( yellow bar ). Of the top 15 % of the cumulative yield
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