The findings presented in this report provide the basis for several key conclusions relevant to future project implementation in the BBB . These are presented below by the primary erosion source .
• Gully erosion is the dominant erosion source of fine sediment throughout the BBB ( refer Figure 45 ) contributing 81 % of all fine sediment at end of stream within the Source Catchment model estimates ( compared to 10 % streambank and 9 % hillslope ).
• With some 22,000 actively eroding gullies within the BBB ( refer section 5.1.3 ), sub-catchment prioritisation is less imperative , although gully fine sediment load is lowest in the Broken and the Bogie sub-catchments ( refer Figure 41 ).
• The Gully Prioritisation study undertaken by Brooks et al . ( 2020 ) showed that clustering sites is most cost-effective ( refer section 5.1 ), and clustering can assist in delivering more sediment outcomes within a sub catchment . This was seen through LDC Project implementation where 10 of 23 gullies produced two-thirds of the fine sediment from the Bowen sub-catchment and this sub-catchment in turn produced 60 % of total fine sediment savings reported in the LDC Project ( refer Figure 43 ).
• Gullies within the top 15 % of the cumulative fine sediment yield curve produce 64 % of the sediment from alluvial gullies ( refer section 5.1.3 ). Interestingly , only half of LDC Project landholders surveyed ( 51 %) were positive that large scale remediation is a cost-effective way to improve water quality ( refer section 6 ), therefore , communication of gully design and transparency in costs with the landholders at regular intervals is required for long term landholder buy-in , including longer term land management agreements .
• The average cost-effectiveness of all gully remediation could be expected to be near $ 120 / t / yr using the LDC Cost Effectiveness tool ( refer Figure 4 ). Large scale ( type 3 , alluvial ) gullies engineered for fine sediment reductions achieved cost-effectiveness around one-third of this average .
• Linking hillslope practice change to as much of the gully catchment where practical will increase the likelihood of longer-term water quality outcomes ( refer section 4.3.6 ) and is a regular request from the BBB community ( refer section 6 ).
• Whilst not as a high priority in terms of fine sediment abatement potential per individual site , smaller gully remediation ( type 1 , hillslope gullies ) still represent just under half (~ 49 %) of the estimated total fine sediment reductions from gullies within the BBB ( refer section 5.1.3 ) and may present opportunities for landholders ( especially those with access to machinery ) to apply lowercost interventions .
• Importantly , for all future gully works , landholder satisfaction and attitudes need to be carefully managed . Current data indicates that those who have contracted gully remediation works are less positive about the LDC Project demonstrating new ways of remediating gullies for improved water quality outcomes than those who have not had gully contracts ( refer section 6 ). This should be considered carefully across all scales of gully projects into the future .
• Streambank sediment reductions could be a priority action within the Bowen River sub-catchment ( refer Figure 45 ) with sediment load within the Bowen River sub-catchment alone almost equal to the hillslope sediment load for all sub-catchments combined ( 120,000 tonnes compared to 145,000 tonnes ). Note that the substantial areas of alluvial gullies along the main streamlines of this subcatchment may influence how the model represents streambank erosion areas ( i . e ., areas of low cover close to streamlines ), however , managing land near the streamline is applicable to either erosion type .
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