River responses to bushfires and floods: Insights from the Wadbilliga River, NSW (#203)
Carl Helander
1
,
Tim J Ralph
1
- School of Natural Sciences, Macquarie University, North Ryde, NSW, Australia
- Bushfires are landscape-scale disturbances that have a range of hydrological, geomorphic, and biogeochemical impacts. Hillslope erosion rates are known to increase after fire causing water quality issues downstream, but the hydrological and geomorphic response of rivers is relatively poorly understood. Catchment-scale changes to hydrological conditions can increase peak discharge while burning of riparian vegetation can make rivers susceptible to large-scale channel reorganisation and floodplain erosion. Yet, the heterogeneity of river responses to bushfire and subsequent flooding makes it difficult for land and water managers to prioritise riparian management efforts.
- The Wadbilliga River is a major tributary of the Tuross River, on the south coast of NSW. Its catchment was burnt severely by the Badja fire in the ‘Black Summer’ of 2019/20. Analysis of satellite imagery and digital elevation models, combined with field topographic surveys and sediment sampling, allowed investigation of the dramatic response of the Wadbilliga River to severe bushfire and subsequent flooding.
- During floods in 2020 and 2021, the Wadbilliga River channel widened significantly as a result of bank erosion and floodplain stripping and, in places, developed an anabranching planform. The magnitude of channel widening and floodplain stripping that occurred in the Wadbilliga River contrasts with observations of other rivers with burnt catchments that experienced no significant geomorphic response.
- Understanding the potential drivers of the variability of river responses to bushfire will be important for hazard assessment, risk mapping and riparian management prioritisation efforts.