identify active gullies from a single LiDAR derived DEM – using aspects of the terrain roughness 2 . This has been calibrated to DEM of difference data and can now be used as a tool to compare gully activity across large areas where LiDAR data exists .
Gully volume can also be used to provide an estimate of total gully sediment yield and , where the initiation date is known , the lifetime average sediment yield . However , gully lifetime sediment yields are influenced by the prior land surface reconstruction assumptions , which would very rarely be flat . An objective Prior Land Surface reconstruction method has been developed where LiDAR data is available which picks up remnant surfaces within the gully and bases the reconstruction on the surrounding terrain 3 .
NESP Tropical Water Quality Hub research also identified several additional key factors for selecting gullies for rehabilitation :
• ‘ Economies of scale ’ will be important in selecting a gully for rehabilitation . It may be more efficient to treat several gullies with slightly lower baseline yield if they are near to each other , or particularly if they are near to a high yielding gully . It will likely be more cost-effective in terms of transport costs and access issues , as well as design and monitoring costs .
• Landholder ‘ buy-in ’ is critical . The buy-in includes permission to access sites and undertake the works , landholder support for post-rehabilitation site management , including grazing management , and the acceptance that gully rehabilitation is a useful investment of public funds .
• Upslope catchment area is important . If a hillslope gully is large but has almost reached its natural extent and only has a small upslope catchment area , then it may not be cost-effective to actively treat this site , as it has almost reached natural maturity . Using controlled grazing management to reduce side-wall erosion may be sufficient . On the other hand , if there is a relatively small but active hillslope gully with large upslope catchment area , it may be suitable to treat as there is a large potential sediment saving by preventing future erosion .
• In some circumstance ’ s significant erosion can persist via surface rainsplash erosion , even where gullies have reached their full extent . Surface capping may be required at these sites .
From the project partners perspective in the LDC Project , the following additional operational issues have also been identified :
• Recognition of the planning and preparation needed to obtain permits and approvals to undertake gully rehabilitation , and more recently , COVID approvals . Weed permits are also required at some sites .
• Ability to access to sites , particularly for large machinery , and access during the wet season .
• Consideration of the overall budget including the peripheral costs that go towards design , permit approvals , monitoring and maintenance .
These additional factors are critical in guiding future investment decisions and must be considered in conjunction with the biophysical characteristics of the BBB catchments when encouraging adoption of particular practices or landscape remediation activities .
2
Daley et al ., 2021
3
See Daley et al ., 2021 for more detail on this approach
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