Modelling and monitoring the sediment trapping efficiency and sediment dynamics of the Burdekin Falls Dam, Queensland, Australia

The Burdekin River, Queensland, Australia drains a catchment area of ~130,000 km and contributes approximately 30% of the total sediment supplied (~ 3.8 t) to the Great Barrier Reef Lagoon from all Great Barrier Reef catchments. Much of this watershed (~115,000 km) drains into the Burdekin Falls Dam, the largest dam reservoir in Queensland (1,860 GL capacity; 224 km surface area; average depth 8.3 m). Current SedNet and ANNEX modelling (a sediment and nutrient transport process model) of the Burdekin catchment suggests that the Burdekin Falls Dam (BFD) is a very efficient trap for sediment and particulate matter. However, some field studies have suggested a much lower trapping efficiency. Improved knowledge of the amount of sediment trapped by the Burdekin Falls Dam is crucial for stakeholders managing sediment loads to the Great Barrier Reef Lagoon. The SedNet model is used to assist the identification and prioritisation of areas for remedial works within the catchment. If the Burdekin Falls Dam traps the high amounts of suspended sediment as predicted by the SedNet model then works can essentially be prioritised in the catchment area below the dam, a much smaller area (15,000 km). A monitoring program was conducted over three wet seasons to estimate the trapping efficiency of the Burdekin Falls Dam and its variability over different flow events. We measured suspended sediment concentrations and particle size distribution in the Burdekin dam overflow and also in the large river catchments upstream and downstream of the dam to calculate sediment loads and examine sediment dynamics operating in the dam. We found that in moderate to large flow events, the Burdekin Falls Dam traps approximately 60% (±10%) of suspended sediment while in smaller flows the trapping efficiency is much higher (~80-90%). The results also show that the Upper Burdekin River arm of the catchment consistently contributes a large proportion of suspended sediments (>77%) delivered to the Burdekin Falls Dam even with the larger flows that occurred in the Cape and Belyando-Suttor catchments in the 2007/08 water year. Therefore we believe that the SedNet model is overestimating the sediment trapping efficiency of the Burdekin Falls Dam due to the trapping algorithm which is unsuitable for the Burdekin catchment area. Conversely, our calculations of sediment trapping appear to be higher than the estimates deduced from other field studies. Particle size distribution data show that the coarser sediment fraction >20 μm typically does not pass through the Burdekin Falls Dam.