Mapping geomorphic floodplain wetlands as assets for consideration in delivering natural flood management (NFM) across coastal catchments of New South Wales (NSW), Australia

Floodplain wetlands are often under considered as key assets for Natural Flood Management (NFM), a nature-based flood mitigation option that harnesses hydrological and geomorphic processes to slow flood flows and attenuate flood levels. The catastrophic 2021–2022 floods in eastern Australia underscored the urgent need to assess the geomorphic capacity for NFM across the coastal catchments of New South Wales (NSW).

This study establishes a protocol for identifying and mapping four geomorphic floodplain wetland types: cutoffs, palaeochannels, backswamps, and lagoons using readily available remote sensing data. A GIS database of geomorphic floodplain wetlands was created, comprising over 21,000 wetlands delineated across 18 coastal catchments of NSW, covering 129,947 km², using satellite data, Digital Elevation Model-derived terrain metrics, and an integrated approach incorporating the Random Forest method. The distribution of floodplain wetlands allows quantification of total number, area, mean DEM-derived depth, and position on the valley bottom. Overall, floodplain wetlands occupy 594 km². Backswamps, occurring on low-gradient floodplains, dominate in both abundance and area, while lagoons, found in similar positions but containing persistent standing water, are also prevalent. Combined, cutoffs and palaeochannels comprise nearly half of all wetlands but account for only 17% of total wetland area. The distribution and valley-bottom position (distal or proximal) of wetlands vary with valley setting.

These findings provide a geomorphic baseline and database of floodplain wetlands that can now be used to determine their functional role and potential for integration into NFM strategies and flood resilience planning in coastal catchments of NSW.