Conventional flood mitigation strategies in eastern Australia have largely relied on engineered infrastructure, including dams, levees and channelisation of waterways. These strategies can have adverse environmental impacts, and the latter approaches, which confine and accelerate flow velocities to reduce flood levels in targeted locations, can increase flood risk downstream. In recent years, a different approach has emerged called Natural Flood Management (NFM). NFM uses “natural” hydrological and biophysical processes to reduce flood risk, whilst delivering a range ecological and societal co-benefits.
Within NFM research and published guidelines to date, the impact of channel enlargement on flood flow conveyance has been largely overlooked. Four case study areas in South East Queensland were used to investigate the impact of erosional channel enlargement on downstream peak discharges. The LiDAR-based hydraulic modelling results indicate that erosional channel enlargement over a ten-year period increased peak discharges in some systems by approximately 10–15%. The findings also highlight that, where downstream hydraulic controls such as valley constrictions create backwater effects, the influence of channel enlargement on flow conveyance becomes less pronounced with increasing flood magnitude.
These geomorphic insights present an opportunity to enhance NFM outcomes in practice, by firstly aligning the pursuit of NFM outcomes with river rehabilitation more generally, and secondly, by considering geomorphic setting when prioritising placement of NFM interventions to maximise the hydraulic benefits they provide. The challenges associated with large-scale implementation of NFM, including secondary consequences of successful NFM, are also explored.