Virtual Poster Presentation - 3 minute narrated video presentation AFSS Conference 2020

Can stream power variations explain channel erosion risk in ‘lazy’ floodplain wetlands? (#125)

Zacchary T Larkin 1 , Timothy J Ralph 1
  1. Macquarie University, NSW, Australia

Multi-channelled rivers and floodplain wetlands in low-gradient settings are sometimes considered ‘lazy’, as they are not particularly energetic or prone to rapid change. Yet previous research has shown that erosion is pronounced in floodplain wetlands where step-changes occur in topography, sometimes leading to channel incision, and that channelization and avulsion (periodic relocation of channels) are important processes driving changes in water routing and inundation patterns in these systems. Where excessive erosion occurs, however, wetlands can become disconnected from their feeder channels.

Stream power is a measure of energy expended by water flowing against a rivers’ bed and banks and can be used to assess the likelihood of erosion or sedimentation. To identify hotspots of relatively high energy flows, stream power was estimated at 100 m intervals along channels in the southern Macquarie Marshes, NSW. Channel capacity and slope metrics were derived from a 1 m resolution LiDAR-derived digital elevation model, and bankfull discharge was modelled using established relationships between channel width and discharge rate.

Spatial patterns of stream power in the southern Marshes largely accords with the current understanding of the hydrology and geomorphology of the system. Most channels have relatively low energy (<5 W m-2), particularly where channels disintegrate into wetlands, however there are key hotspots of relatively high stream power (>10 W m-2). These hotspots typically occur where channel slopes increase as water drains from wetlands into downstream channels, or where water drops over grade-control structures.

Comparison with an independent, field-based assessment of erosion risk indicates that our stream power modelling and mapping is largely successful in identifying important sites that experience, or are susceptible to, erosion. This provides a robust tool with which to assess erosion risk at a high resolution in large, low-gradient floodplain wetlands and can guide detailed investigations to inform management and intervention strategies.