Progress report: Contemporary erosion sources supplying sediment to Lake Wivenhoe

Abstract

This document reports on progress in identifying contemporary erosion sources supplying sediment to Lake Wivenhoe, a project being conducted on behalf of Seqwater as part of the research agreement with Griffith University. The dominant erosion process generating the sediment delivered to Lake Wivenhoe has been assessed by comparing concentrations of 137Cs on sediment samples collected from Lake Wivenhoe with those on the surface soil and channel source end members. Activity concentrations of 137Cs in 53 of the 55 samples collected from the lake fall within the concentration range of the channel source samples (< 7.21 Bq kg-1) and we have estimated the relative contribution of surface erosion to the group of samples as 0.04 ± 0.04. This result is consistent with channel erosion (stream bank and gully) being the dominant source of sediment to Lake Wivenhoe. The type and extent of channel erosion along the Upper Brisbane River has been assessed using repeat LiDAR surveys captured before and after the 2011 flood. The before-flood LiDAR was commissioned by DERM and was captured in 2001, while the after-flood LiDAR was commissioned by the current project and was captured in 2011. The January 2011 flood event caused substantial erosion which we estimate to be equivalent to 2,640,000 ± 610,000 m3 . Currently there is no way to determine how much of the sediment eroded from the macro channel banks was deposited underneath the water surface in the channel further downstream. However, given that the flows were largely confined to the macro-channel and the relatively short transport distance it is considered likely that a substantial amount of this eroded sediment was transported to Lake Wivenhoe. This preliminary analysis, which will be refined over the coming months, also indicates that erosion was greater in areas with low woody vegetation cover. The section of river surrounding Harlin was particular badly eroded. The extent of gully erosion was mapped for the first time using SPOT5 and Google Earth Imagery. The linear extent of gullies in this catchment is approximately 384 km. The total linear extent of actively eroding gullies in south-east Queensland is approximately 715 km. Analysis of the gully distribution in association with the catchment geology shows that the highest gully extents are associated the Neara Volcanics and the alluvium category (Qa-SEQ) made up of Quaternary, Tertiary and Pleistocene material and includes floodplain, lower and second level river terraces and high level alluvium in the landscape. The Esk Formation and Eskdale Granodiorite geologies also show significant totals of eroding gully length. The management implications of this study are that conservation works aimed at reducing the supply of sediment to Lake Wivenhoe should focus primarily on stabilization and rehabilitation of the gully and channel network.