Oral Presentation 9th Australian Stream Management Conference 2018

Assessment of channel expansion and contraction using cross-section data from repeated LiDAR acquisitions in the Macquarie Marshes, NSW (#16)

Neda Yousefi 1 , Tim Ralph 1 , Hsing-Chung chang 1 , Paul Hesse 1 , William Farebrother 1
  1. Macquarie University, Marsfield, NSW, Australia

Floodplain wetlands have alluvial channels that change over time due to the dual action of 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 for 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 related 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 widening 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 accumulation or degradation, particularly where water and dense vegetation occurred. It is inferred from the DoD that there was no significant sedimentation or erosion in the system during this brief time window, beyond the limits of detection (±15 cm vertical; ±45 cm horizontal).

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