Cutting back, down, back and out: assessment of channel erosion in a sensitive floodplain wetland. (11614)
Processes of erosion influence channel formation and evolution in rivers and floodplain wetlands. However it is often difficult to determine the role of different types of erosion processes such as knickpoint retreat (cutting back), incision (cutting down) or widening (cutting out) when they co-occur in a changing fluvial system. In floodplain wetlands like the Macquarie Marshes, N.S.W., erosion plays a pivotal role in the (re)connection of channels and in avulsion (the relocation of channels on the floodplain), with potentially adverse impacts on the surrounding wetlands. We assessed the type and rate of erosion processes in a large knickpoint that drains a wetland with high ecological value using a combination of topographic surveys, erosion pin surveys and time-lapse, fixed-point photography. Knickpoint retreat, bed incision and bank widening all occurred to some extent under environmental flow conditions over a three month period. The spatial patterns of erosion were variable, but topographic surveys were the best method for assessment of changes in bank morphology. Channel widening occurred in the upper section of the knickpoint (maximum 7%), while channel incision occurred more variably throughout the knickpoint (maximum 25.33%) and accumulation of sediment also occurred in places on the channel bed (maximum 3.8%). In particular, the upstream ends of the eroding channels cut further into the wetland (maximum extension 2.9%). Erosion of this kind is likely to pose a serious threat to wetland ecological values in thresholds of channel and wetland stability are breached, leading to extensive channelisation and drainage of the wetlands. Knowledge of these processes and the broader spatial patterns of erosion risk in the wetlands can inform water and land management strategies that currently rely on ad hoc and short-term interventions.