Channel and Bank Stability of the Burnett River in the Aftermath of the 2011 and 2013 Floods

7th Australian Stream Management Conference – Short Communication

1 1 2 Simon, A. , Bankhead , N., and Wilson, P.

1. Cardno ENTRIX, PO Box 1236, Oxford, MS 38655, USA. Email:[email protected] 2. Burnett Mary Regional Group, 193 Bourbong St, Bundaberg QLD 4670, Australia

Key Points • Bank erosion is the single largest contributor of sediment in the Burnett River Catchment • Banks contribute approximately 44% of the sediment load in the catchment • Significant load reductions can be obtained with a range of mitigation measures

Keywords Burnett River, bank stability, sediment loading, flood recovery

Introduction The Burnett River experienced severe flooding in early 2011 and 2013, with the latter flood breaking all historical records. As a result of these floods, damage to assets, infrastructure and the loss of agricultural land from bank erosion was considerable. Exacerbated by the floods, is concern about sediment delivered to downstream locations including dams/weirs, the harbour and ultimately to the Coral Sea and the Great Barrier Reef (GBR). The geographic scope of this study extends from the mouth of the Burnett River east of Bundaberg, upstream to Eidsvold, about 300 km. In addition, the lower 30 km of the Kolan River was included. These two reaches represent priority areas of investigation for the Burnett-Mary Regional Group (BMRG) to aid in their flood-recovery efforts. The primary objectives of this work were to: (1) provide BMRG with a means of determining strategies for cost-effective protection of local assets and (2) determine the relative contributions of bank sediment (particularly fine-grained material) to overall sediment loads and implications for sediment export to the GBR.

Methods and Initial Analysis Results of the system-wide Rapid Geomorphic Analysis (RGA) show considerable variation in relative channel stability along the 300 km study reach of the Burnett River main stem, with fluvial deposition and mass failure (instability) of channel banks being the dominant processes recorded. Although the river has been impacted by flows with return periods of approximately 20 and 140 years in just the past two years, the overall impressions of the entire reach are that the Burnett River main stem has not been devastated. The RGA data highlighted that the effects of the impoundments on channel stability can be dramatic, with peaks in channel instability commonly being located just downstream from the impoundments and some other river-crossing structures. The reaches with the most severe instabilities are located just downstream of Walla Weir and Paradise Dam, with the most stable reaches extending upstream from these two impoundments.

Analysis of aerial imagery between 2009 and 2013 revealed that a total of about 27.8 million m3 (47.3 Mt) of land was eroded from the banks of the lower 300 km of the Burnett River main stem. This translates to about 6.1 million m3/y (10.4 Mt/y). Erosion of these bank materials does not equate to an equal volume being delivered to the river mouth and the Coral Sea, as an unknown proportion is deposited on low-bank surfaces, beds, bars and floodplains. It can be assumed, however, that the majority of the fine-grained materials (silts and clays) are transported through the system and out to sea. Approximately 21 million m3 (35.6 million t) or about two thirds of these sediments were eroded from banks downstream of Paradise Dam. One can assume that much of the hydraulically-controlled bank sediment eroded from reaches upstream of rkm 170 was trapped behind Paradise Dam (7.0 million m3).

In addition, long-term simulations (42 years) were conducted using the Bank Stability and Toe Erosion Model (BSTEM), to compare longer term averages with the 2009-2013 rates calculated from aerial imagery. The results estimated that over this longer time period annual bank-erosion rates are in the order of 3.1 million Mt/y, about 18 times greater than the value predicted by SedNet. Bank-erosion rates below Paradise Dam are 2.4 million t/y. Assuming 100 years of simulation

Simon, A, Bankhead, N, and Wilson, P. (2014). Channel and Bank Stability of the Burnett River in the Aftermath of the 2011 and 2013 Floods, in Vietz, G; Rutherfurd, I.D, and Hughes, R. (editors), Proceedings of the 7th Australian Stream Management\ Conference. Townsville, Queensland, Pages 196-197. 196 7ASM Short Communication

Simon et al. – Channel and Bank Stability of the Burnett River and using an empirical relation between the length of BSTEM simulations and calculated erosion rates, a conservative value for the average, annual rate of bank erosion is 2.0 Mt/y, and this does not include tributary contributions. The BSTEM results and aerial imagery analysis, support each other, with similar magnitudes of average annual erosion volumes being calculated by both methods. Bank erosion, instead of being a minor source of sediment representing 8% of the total as reported by others, was found to be the single largest contributor of sediment in the Burnett River Catchment, representing at least 44% 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.

Site-specific stability issues were also addressed at eight locations. At-a-site investigations of unit-erosion rates (expressed in m3/m of channel) were carried out using BSTEM, populated using geotechnical and hydraulic-erodibility data collected in situ at each of the sites. The BSTEM results for various mitigation scenarios showed that at some of the sites protection of the bank toe was the essential component in managing and reducing streambank erosion, and therefore, banktop retreat. In these cases the most successful mitigation measures were protection of the entire toe using rock, with the addition of vegetation to the banks often further reducing bank erosion by protecting the upper bank from both hydraulic and geotechnical erosion. At some sites, however, the before and after geometries output from BSTEM indicated that even where the toe was protected, some erosion could occur above the protected zone. In these cases, mitigation strategies focusing on reducing shear stresses in the entire near-bank zone, rather than just protection of the toe, were found to be more successful in terms of erosion reduction and prevention of bank top retreat.

Conclusions The implications of these findings are considered in the context of erosion sources and rates in Brodie et al., (2003) who report that on average, 2.75 Mt/y are eroded from the catchment. By replacing their 0.175 Mt/y from the banks with the 2.0 Mt/y calculated in this study, a new total of 4.6 Mt/y is obtained. Assuming that the reported values from gully (0.93 Mt/y) and hillslope (1.65 Mt/y) sources are accurate, bank erosion becomes the single largest contributor, delivering 44% of the total sediment load. Given these significant differences in both the relative importance and absolute rates of bank erosion than was earlier reported, sediment management should be re-focused to include this important source. Doing so would not only protect local assets by limiting land loss and bank retreat, but would also help maintain reservoir and harbour capacity, minimize downstream flooding, reduce dredging costs, and protect marine resources

Cost estimates prepared as part of this work served as an approximate guide for the mitigation scenarios presented. Overall, the scenarios involving rock in any form (rock toe or rock weirs) were the most expensive at any given site. Costs vary according to rock size, and height of bank protected, ranging from $1,000 to $3,000 per m of bank). Planting of vegetation was the least expensive (approximately $5/m2 of bank face or top) but not necessarily the most effective without associated battering and toe protection. The balance between sustainability and cost-effectiveness of a given mitigation measure is presented along with the risk associated with the potential for additional erosion, and/or the failure of the implemented mitigation measure.

Acknowledgments We wish to thank Emily Maher and Cathy Mylrea of BMRG for outstanding support throughout the course of this investigation, and to Dwayne Honor of Bundaberg Regional Council for his efforts to get us all together.