Assessment of channel expansion and contraction using cross-section data from repeated LiDAR acquisitions in the Macquarie Marshes, NSW

Floodplain wetlands have alluvial channels that change over time due to both erosion (sediment export) and sedimentation (sediment accumulation). Adjustments in channel capacity resulting from erosion and sedimentation influence the timing and extent of floodplain wetland inundation and alter in channel and other habitats and their ecological processes, and are a key factor in river and floodplain management. Nineteen sites in the southern Macquarie Marshes were studied, some with multiple channels and others with a single channel, yielding 54 channel cross-sections in all. Two Light Detection And Ranging (LiDAR) Digital Elevation Model (DEM) datasets acquired in 2008 and 2014 were used to assess changes in channel size and shape due to erosion and sedimentation. Channel depth measurements could not resolve any changes in depth, and were not accurate when channels contained water. Comparisons of channel width measurements showed that 17% of channels experienced expansion over the 6 year period between LiDAR acquisitions, while 5% had a reduction in channel width, and 78% had no measureable change. The trunk streams of the Macquarie River and Bulgeraga Creek and a return channel of Buckiinguy expanded, suggesting erosion, whereas one reach of Bulgerara Creek and the distributary channel Monkey Creek contracted, suggesting sedimentation. Analysis of a DEM of Difference (DoD) for the whole area covered by the Macquarie Marshes LiDAR data was not able to produce reliable results for vertical level changes (deposition or degradation), particularly where water and dense vegetation were present. It is inferred from the DoD that any sedimentation or erosion in the system during this brief time window was beyond the limits of detection (±15 cm vertical; ±45 cm horizontal) and was not significant.