Trends in post‐1950 riparian vegetation recovery in coastal catchments of NSW Australia: Implications for remote sensing analysis, forecasting and river management

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Natural flood management (NFM), a nature‐based solution to flood mitigation where hydrological and biophysical processes are harnessed to reduce flow velocity, erosive energy and flood risk, is an emerging global theme of water and river management. The catastrophic 2021 and 2022 floods in eastern Australia are used to assess the hydrological properties of discrete events and to start an investigation of whether widespread changes in flood hydrology are occurring. We find that most coastal rivers in New South Wales (NSW) had a noticeable decrease in flood wave celerity (increase in flood travel time) when the 2021 and 2022 floods are contrasted with equivalent floods since the 1970s and that several also exhibit increases in flood peak attenuation. For some rivers, there is a coincident trend between these changing flood properties, riparian vegetation regrowth (regreening) and geomorphic recovery over the last +30 years. These may be the first signals that passive riparian management is counteracting some of the more severe hydrological effects of floods on these rivers and that some degree of NFM is possible. Lessons from this work are that there is an immediate opportunity to implement large‐scale NFM in coastal catchments of NSW, but this will only occur if we re‐examine current flood mitigation and adaption strategies and recognise and prioritise nature‐based solutions that include space‐to‐flood and corridors of river recovery, as essential parts of the modern flood mitigation toolkit.

Section: Recent Work Has Shown That Along Most Rivers Of Coastal Newmentioning

confidence: 99%

Section: Recent Work Has Shown That Along Most Rivers Of Coastal Newmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 97%

mentioning

confidence: 99%

See 3 more Smart Citations

Natural flood management: Lessons and opportunities from the catastrophic 2021–2022 floods in eastern Australia

Fryirs

Zhang

Ralph

et al. 2023

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Detection of decadal time‐series changes in flow hydrology in eastern Australia: Considerations for river recovery and flood management

Arash,

Fryirs,

Ralph

2023

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Since European colonisation, many coastal rivers of New South Wales (NSW) have been highly modified by channelisation, flow regulation, sediment mining, intensive grazing and agriculture, deforestation, and riparian vegetation and wood removal. Since the late 1980s, nearly 55% of these rivers have undergone a significant ‘re‐greening’ and geomorphic recovery with changes to instream and riparian roughness. However, little research has been undertaken to quantify if there have been any coeval changes in flow hydrology. Approximately 7000 flow hydrographs with one‐hour time‐steps from 117 gauges on 45 study rivers (17 of 20 coastal catchments of NSW) were used to assess changes to in‐channel and overbank hydrology over decadal timeframes. Hydrograph shape and attenuation characteristics for three morphologically defined flow stages were analysed: in‐channel fresh, high flow and overbank flood. Time‐series analysis was used to quantify changes in flow hydrology from the early‐20th century to present and identify trends occurring between known degradation phases (1910s–1940s and 1940s–1980s) and a recovery phase (1990s to present) for these rivers. Our findings indicate that, on average, and for some key examples, changes in flow hydrology are occurring. Flows are slowing and attenuating as indicated by decreases in flood wave celerity and hydrograph kurtosis, skewness and rate of rise and increases in peak‐to‐peak travel time, flood peak attenuation and flood wave attenuation index. The most significant changes have occurred since the 1980s with the most noticeable effects on the behaviour of high flows (around the bankfull stage). However, there is not a consistent direction change in these indicators in all places. We use these findings to categorise the flow mitigation signal of the study regions and catchments and discuss the implication for river and flood management.

A hierarchical method and workflow for the semi‐automated mapping of valley bottom geomorphic units using publicly available remote sensing datasets

Zhang,

Fryirs

2024

Geomorphic units (GUs) are the landforms that make up the valley bottom and are produced by fluvial processes that determine river structure and function. Mapping of GUs can be used to interpret river type and behaviour and to analyse river condition and recovery processes. The advancement of remote sensing technologies and big‐data acquisition are enabling the development and operationalisation of tools to semi‐automate the mapping of assemblages of GUs across large spatial areas. In this study, we develop a hierarchical method that combines a landscape classification approach (Geomorphons) with supervised classification using light detection and ranging data (LiDAR) and satellite images to semi‐automate the mapping of GUs across valley bottoms. We have also produced a new method for identifying and mapping pools in the absence of bathymetry data. We applied our method on 78 river sections in coastal catchments of NSW, Australia. We were able to identify 20 refined GU types and four further sub‐types of bank‐attached bars. Our method produced GU maps that are consistent with desktop manual delineation from aerial images and digital elevation models. Our hierarchical method offers GU maps with varying accuracy and resolution, accommodating a user's decisions regarding amount of effort invested relative to map quality and accuracy required. Initial runs produce maps with 12 preliminary GUs with 61%–75% consistency when compared to desktop manual mapping. With additional effort and manual corrections, a higher level of GU identification is possible (i.e. refined GU mapping increases the consistency to 70%–81%). The delineation of more intricate sub‐types of GU or sub‐units on compound GUs, which is essential for interpretation of river behaviour, condition, and recovery, still requires on‐site field verification to achieve the best results.