6.1.2 Managing gullies : Recent learnings The Australian Government Reef Trust program has targeted gully remediation across the GBR catchmetns with the greatest investment in the priority areas including the BBB catchment . The projects are listed in Section 8 . The Reef Trust IV Gully Toolbox ( Wilkinson et al ., 2016 ) provides a useful reference for knowledge of current treatment options that are applicable to the BMIP . This is most relevant to the treatment of smaller scale erosion features over large areas . In addition , treatments for large scale alluvial gullies will be required for the BMIP , but there has been limited application of these experiences for GBR outcomes to date .
Large alluvial gully systems are a significant contributor to the sediment load of GBR catchment rivers , and should be considered more as major point sources rather than diffuse sources that can be managed through catchment-wide land management programmes ( Brooks et al ., 2016 ). Management of alluvial gullies were considered as part of the mix of options identified in a 3 day workshop about alluvial gully restoration held at Collinsville as part of NESP Project 2.1.10 . The conclusions are relevant to the BMIP , including the following highlights .
• There was broad agreement that standard approaches to land management that involve the implementation of grazing BMPs and small scale , low cost , interventions will not be sufficient on their own to significantly reduce sediment and nutrient yields from large , active alluvial gully complexes . This is not to say that these approaches will not be part of the management solution mix , tailored to individual gully complexes .
• Many examples of gullies were observed in the field that had little or no overland flow entering the gully head . Therefore strategies that rely on modifying the rainfall : runoff ratio through grazing and pasture improvement will have little effect on the sediment yields of these gullies . Such gullies , some of which are migrating at rates of up to 2 m per year , are driven by the erosion of the exposed dispersive and / or slaking soils associated with direct rainfall impacting on the gully itself . Fluvial scour associated with flows concentrated within the gully itself also contributes significantly to secondary gully erosion processes and needs to be a major focus of management treatments . Where gullies also have an overland flow contribution , erosion rates are likely to be even higher than situations without an overland flow contribution , hence in these circumstances overland flow management will also need to be part of the management mix .
• It was agreed that the current rates of alluvial gully activity may not be directly related to current land management ( be it good or poor ). These large alluvial gullies are a legacy issue that may have been activated when cattle and other disturbance processes were first introduced into the landscape up to 150 years ago triggering the gully erosion .
• There was broad agreement that there is a huge diversity of alluvial gully types and processes - even within a fairly confined spatial area - and hence the management strategies employed to rehabilitate and stabilise these gullies will need to be carefully tailored to the individual site conditions and dominating processes .
• The diversity of alluvial gully form , function and activity is a function of a range of factors , including :
-Soil / sediment heterogeneity -Local topography -Regional topography -Local hydrology , including piping / tunneling -Regional hydrology – including the hydrological interaction with the mainstream channel into which the gully is draining .
There was broad agreement that more work is required to fully understand how these factors contribute to gully form , processes and rates of sediment and nutrient production ; hence how this dictates the management strategy to be employed at each site .
- 47 -