Streambank-Derived Sediment Delivery to the Great Barrier Reef from the Burnett River over Multiple Time Scales: Implications for Sediment Management

Abstract:

Degradation of the Great Barrier Reef (GBR) is in part, the result of the delivery of fine-grained, terrestrial sediment from catchments draining to the Coral Sea. The Burnett River, draining the GBR experienced severe flooding in 2011 and 2013 with the latter flood breaking all historical records. Huge quantities of sediment were transported to the Coral Sea. The purpose of this study was to quantify the magnitude of sediment eroded from the channel banks during the floods, long term bank-erosion rates, and cost-effective protection measures. System-wide analysis combined with numerical modeling using the Bank-Stability and Toe-Erosion Model (BSTEM) was used to determine annual bank-erosion rates over periods ranging from 4.5 to 100 years.

Analysis of 2009 and 2013 aerial imagery revealed that 47.3 Mt (10.4 MT/y) of sediment was eroded from the banks of the lower 300 km of the Burnett River. Erosion of these bank materials does not equate to an equal volume exported to the Coral Sea, as an unknown proportion is deposited. It can be assumed, however, that the majority of the fine-grained materials (54%) were transported out to sea. Long-term simulations (42 years) were conducted using BSTEM, to compare longer term averages with those determined by the catchment model SedNet. Over this longer time period, annual bank-erosion rates were about 3.1 Mt/y, about 18 times greater than the value predicted by SedNet. Assuming 100 years of simulation and using an empirical relation between the length of BSTEM simulations and calculated erosion rates, a conservative value for the annual rate of bank erosion is 2.0 Mt/y. The BSTEM results and analysis of aerial imagery are within 10%. Bank erosion, instead of being a minor source of sediment representing 8% of the total, was found to be the single largest contributor, representing between 44 and 73% of the total annual sediment budget. In absolute terms, this is an increase in the reported average, annual rate of bank erosion from 0.175 Mt/y to 2.0 Mt/y. The significance of bank erosion to total, fine-grained sediment loads are drastically different than those reported for the Burnett and other Reef Catchments where fields and uplands are listed as the leading sediment contributors. These results pose important implications for sediment management and protection of the GBR.