Channel bank stability (or lack of), the implications and rehabilitation options for the Mid Brisbane River. (#145)
Channel bank erosion is the dominant source of sediment in southeast Queensland waterways, and can increase with increased hydraulic force or when bank resistance (stability) decreases. Increased sediment loads can have detrimental effects on the downstream environment, and quality and quantity of potable water. Significant erosion of the Mid Brisbane River was observed during the 2011 and 2013 flood events, and became an issue for landholders, Seqwater and other Mid Brisbane stakeholders. This study aimed to identify and quantify the main types of erosion present, the erosion processes and rehabilitation options for the Mid Brisbane River following the flood events. Using on-ground rapid visual assessments and repeat LiDAR analyses, the main types of erosion were identified as a large number (168) of wet flow failures and a small number of large fluvial scour failures, which accounted for 237,000 m3 and 350,000 m3 of sediment volume change, respectively. Historical evidence shows the main catchment disturbances in the Mid Brisbane since European arrival are riparian vegetation clearing, alteration of flow regime and gravel extraction. Modelling indicates that the probability of fluvial erosion and wet-flow failures occurring could have been decreased through vegetation to stabilise the bank toe and increase the cohesive strength of the soil, respectively. Our study shows that for the Mid Brisbane River, more than 90% does not require major engineering works to stabilise the channel banks and re-establishing dense riparian vegetation would decrease the probability of channel bank erosion and therefore, reducing sediment loads and improving water quality.