Figure 1. Total fine sediment reduction by project type and erosion source. Inset shows the proportion of the total project area for each project type.
These estimates have been calculated using two methods:
1) The pollutant reduction component of the Alluvium/Great Barrier Reef Foundation (GBRF) investment tool for hillslope and streambank erosion management projects;
2) The Reef Trust Gully Toolbox method for gully erosion management projects. The LDC Water Quality Report 2020 (Waterhouse et al., 2020) highlights that a number of assumptions underlay these calculations, therefore these figures should be treated as the best available estimate of sediment reductions to date.
Preventing sediment from reaching the Great Barrier Reef
Each wet season sediment is washed into local waterways and out to the Great Barrier Reef (GBR).
Most sediment is very fine, and can stay suspended for a long time and can travel great distances. Valuable topsoil is lost from production, and increased concentrations on the reef can be harmful to seagrasses and corals.
Landholders in the BBB have completed 69 on-ground water quality practice changes, and it is estimated that these have contributed a fine sediment reduction of 6,154 tonnes per year from reaching the GBR.
Of this, approximately half of the sediment savings are attributable to grazing land management changes on hillslopes and streambanks, and the other half as a result of gully remediation treatments across a broad range of scales, as shown in the graphs above.
The table below also highlights the relatively small area of intervention in the gully management projects compared to the large sediment savings that these can achieve - 60 per cent of the sediment savings over only 4 per cent of the project area.
Table 1. Estimated sediment reductions (tonnes) from projects completed in the LDC Project to date.