Assessing the cost-effectiveness of gully and streambank rehabilitation sites for reducing river basin sediment exports to the Great Barrier Reef

Reducing river basin sediment loads is a common goal of erosion management programs, yet resources are limited. Research has focused on identifying sediment source areas and processes and defining erosion control practices. However, achieving targeted and efficient sediment management requires decision support systems based on cost-effectiveness. A method was developed to prospectively assess the cost-effectiveness of rehabilitating gully and streambank sites, being the cost to reduce future sediment yield by 1 tonne per year. Sediment reduction was calculated as the untreated yield multiplied by the effectiveness of erosion treatments, which ranged between 10 and 80 percent. The method was applied to 273 sites in river basins flowing to the Great Barrier Reef coast. Cost-effectiveness varied by 2 to 3 orders of magnitude, driven mainly by untreated sediment loads and treatment costs. Cost-effectiveness was better at sites with low unit-area treatment costs, large areas and high sediment connectivity to the basin outlet. Revegetation was more cost effective than structural treatments. Streambank sites were more cost-effective than gullies. Rapid erosion was more cost-effectively treated than slow erosion. Assessment during management planning helped to avoid sites with poor cost-effectiveness and to match treatment intensity to erosion severity. Despite these gains, material sediment reductions require large investments. The cost to reduce GBR sediment loads by 25% of current loads is at least AUD$1 billion. Concurrent land use intensification and climate change could potentially increase sediment loads. Better estimation of historical erosion rates and further monitoring of treatment effectiveness will reduce uncertainties in the approach.