Trapping efficiency of the Burdekin Falls Dam, North Queensland: Estimates from a three-year monitoring program

Research Report

Stephen E. Lewis¹, Zoë T. Bainbridge¹, Bradford S. Sherman², Jon E. Brodie¹ and Michelle Cooper³

¹Australian Centre for Tropical Freshwater Research, James Cook University, Townsville
²CSIRO Land and Water, Canberra
³Geoscience Australia, Canberra

ISBN 9781921359385
Published October 2009

MTSRF Project 3.7.2 - Connectivity and risk:  Tracing materials from the upper catchment to the reef

Introduction

The increasing global demand for water to support an ever growing human population has caused the widespread construction of large reservoirs particularly over the last fifty years (e.g. Syvitski et al. 2005; Vörösmarty et al. 2003).  These large reservoirs may accumulate considerable amounts of sediments and associated nutrients (e.g. carbon, nitrogen and phosphorus), thus reducing their supply to downstream receiving environments (Syvitski et al. 2005; Walling and Fang, 2003). In contrast, urban development, agriculture, deforestation and mining results in the increased supply of sediments and nutrients to the downstream receiving waters.  While catchment erosion has increased, there has been an overall globally reduced flux of sediments and nutrients to the coast largely due to the construction of dams (Syvitski, 2003; Syvitski et al. 2005).  Changes to the supply of sediments and nutrients may have serious implications for coastal estuaries, coral reefs, seagrass communities and coastal fisheries and also result in geomorphological changes to the coastline (e.g. erosion and coastal retreat) (McLaughlin et al. 2003; Restrepo et al. 2006; Syvitski et al. 2005; Vörösmarty et al. 2003)

It is estimated that the total sediment flux to the Great Barrier Reef (GBR) has increased by 4-5 fold since the arrival of Europeans ~150 years ago (Brodie et al. 2003; Furnas, 2003; McCulloch et al. 2003).  Therefore, the management of sediment runoff is a key goal within the Reef Water Quality Protection Plan (State of Queensland and Commonwealth of Australia, 2003).  Of the waterways within the GBR catchment area, the Burdekin River contributes the largest amount of suspended sediment to the marine environment with an average annual export of 3.8 million tonnes or approximately thirty percent of the total sediment supply to the GBR (Furnas, 2003).  In large, above average flow events such as the 2007/08 water year (total discharge of 26.5 million ML), the Burdekin River alone exported a total of 12.3 million tonnes of suspended sediment (Bainbridge et al. 2008).  Therefore, the management of soil erosion in the Burdekin River catchment is a key goal for natural resource managers, although it is currently unclear of where remedial works should be prioritised within this large catchment area of ~130,000 km².

Current SedNet and ANNEX modelling of the Burdekin catchment suggests that the Burdekin Falls Dam (BFD) is a very efficient trap for sediment and particulate matter (Fentie et al. 2006; Prosser et al. 2002; Post et al. 2006).  The latest models estimate that the BFD traps 77-82% of suspended sediment (SS), and 79% of particulate nitrogen and phosphorus, with negligible trapping of dissolved materials (Fentie et al. 2006; Post et al. 2006).  However, field studies using sediment traps, water column/bottom profiling and water sampling within the dam reservoir during flow events do not support this high trapping efficiency (Griffiths and Faithful, 1996; Faithful and Griffiths, 2000). It is critical to have an accurate estimate of trapping within the BFD. If limited sediments are being transported past the dam then remedial works above the dam will have a negligible effect on the amount of sediment and particulate matter being delivered to the mouth of the river and to the GBR lagoon.  As much of the remedial work is targeted at reducing bulk sediment loads to the GBR, works above the dam would not be undertaken for this purpose if the current dam trapping models are accurate.  Moreover, the conflict between the SedNet model predictions and the field studies needs to be resolved so that SedNet and ANNEX can be used with more confidence to identify and quantify the sources of sediment to the GBR from the sub-catchments of the Burdekin.  Here we present suspended sediment load data from a three-year monitoring program in the Burdekin River catchment to quantify the sediment trapping efficiency of the BFD.  The three-year dataset provides insights into the dam trapping efficiency over small (2005/06), average (2006/07) and large (2007/08) flow events. Particle size data of suspended sediments collected during these events also provide insights into the sediment dynamics operating within this system.