Flood hydrology and river recovery in eastern Australia

<p dir="ltr">Spatial and temporal dynamics of floods are influenced by a mix of imposed and flux controls in catchments and rivers. These controls govern the travel time, celerity, stage height and rates of rise and fall of various types of flows and floods, thereby influencing the shape of hydrographs produced at any location in a catchment. However, there has been little research undertaken to quantify the mix and dominance of imposed and flux controls operating on behaviour of different types of flows and floods (namely in-channel fresh, high flow/near bankfull, and overbank flood). This thesis addresses this knowledge gap using 7,000 hydrographs from 117 gauges on 45 rivers in 17 rural catchments of coastal New South Wales (NSW), Australia. These rivers experienced considerable hydrological, morphological and vegetative changes following European colonisation in the 19th century.</p><p dir="ltr">For the study catchments and rivers, the thesis provides a systematic analysis and quantification, identification and classification of flow hydrographs and the mix and dominance of imposed catchment morphometric controls on flood behaviour over various spatial and temporal scales. The hydro-morphic classification of catchments is used to analyse the key morphometrics that control the shapes of hydrographs produced and therefore flood behaviour. The topographic characteristics of catchments (i.e. relief and longitudinal slope) are the dominant controls on the shape of all high flow and overbank flood hydrographs. Drainage network structure (i.e. drainage density), catchment size (i.e. gauge position in the network) and catchment shape (i.e. elongation ratio) characteristics are less dominant controls.</p><p dir="ltr">Customisable workflows for extracting and calculating flow hydrograph shape metrics were developed and applied to time-series analysis of changes in at-a-station and upstream-to-downstream hydrograph shape, as measured by kurtosis, skewness, rate-of-rise, flood peak travel time, and flood peak and flood wave attenuation. This analysis was used to determine whether a flood mitigation signal is emerging for these rivers, and if so, where. The results indicate that most rivers are showing signs of flood mitigation expressed as increases in peak-to-peak travel time, flood peak attenuation and flood wave attenuation index, and decreases in flood wave celerity and hydrograph kurtosis, skewness and rate of rise, with more pronounced effects on the behaviour of high flows. To account for these changes in hydrology, the thesis develops a method for calculating stage-dependent riparian vegetation roughness for different flood types using a Terrestrial Laser Scanning (TLS) gap fraction method. This is applied to a case study (Hunter River at Singleton) where significant geomorphic and vegetative recovery has occurred over several decades. This case study is used to consider the extent to which the flux control of riparian vegetation roughness contributes to changes in flood hydrology over time. Finally, the thesis discusses implications for river and flood management (e.g. incorporating on-ground experiences into hydrological studies).</p>