Peter J. Wallbrink

Large-scale patterns of erosion and sediment transport in river networks, with examples from Australia

This paper examines the patterns of sediment transport in rivers in terms of the sources of sediment and its transport and deposition through the river network. The analysis is in the context of dramatic human influences on river sediment transport and how they might influence freshwater ecosystems. The review of Australian work shows that erosion of hillslopes and stream banks has greatly increased in historical times, supplying vast quantities of sediment to rivers, much of which is still stored within the river system. The stored sediment will continue to effect in-stream and estuarine ecosystems for many decades. In most Australian catchments the dominant source of sediment is streambank erosion. An analysis of historical channel widening suggests that a conceptual framework of relative stream power can explain the diversity of behaviour observed in the numerous case studies. Sediment delivery through catchments is considered first in a generic whole network sense, which emphasizes the crucial role played by riverine deposition in determining catchment sediment budgets. A method is then presented for analysing the diverse spatial patterns of sediment storage in any river network. Finally, the paper considers the temporal changes to channel morphology in response to a human-induced pulse of sediment.

Heating effects on water repellency in Australian eucalypt forest soils and their value in estimating wildfire soil temperatures

Wildfires can induce or enhance soil water repellency under a range of vegetation communities. Accord- ing to mainly USA-based laboratory studies, repellency is eliminated at a maximum soil temperature (T )o f 280-400 ◦ C. Knowledge of T reached during a wildfire is important in evaluating post-fire soil physical prop- erties, fertility and seedbed status. T is, however, notoriously difficult to ascertain retrospectively and often based on indicative observations with a large potential error. Soils under fire-prone Australian eucalypt forests tend to be water repellent when dry or moderately moist even if long unburnt. This study aims to quantify the temperature of water repellency destruction for Australian topsoil material sampled under three sites with contrasting eucalypt cover (Eucalyptus sieberi, E. ovata and E. baxteri). Soil water repellency was present prior to heating in all sam- ples, increased during heating, but was abruptly eliminated at a specific T between 260 and 340 ◦ C. Elimination temperature varied somewhat between samples, but was found to be dependent on heating duration, with longest duration resulting in lowest elimination temperature. Results suggest that post-fire water repellency may be used as an aid in hindcasting soil temperature reached during the passage of a fire within repellency-prone environments.

Fire severity, water repellency characteristics and hydrogeomorphological changes following the Christmas 2001 Sydney forest fires

Soil water repellency can enhance overland flow and erosion and may be altered by fire. The Christmas 2001 bushfires near Sydney allowed investigation of the relationship between fire severity, water repellency and hydrogeomorphological changes. For two sub-catchments with differences in fire severities in Nattai National Park, south-west of Sydney, this paper considers: (1) the links between fire severity based on SPOT image analysis and ground observation of fire severity and repellency; (2) the textural and organic/minerogenic characteristics of eroded sediment; and (3) erodibility, erosion and deposition of soils in both catchments. Ground surveys show that image analysis reflects well the degree of vegetation consumption by fire, but cannot adequately predict the degree of ground litter consumption, associated soil heating and repellency effects. Fire had varying effects on repellency, leaving it unchanged, destroying it or enhancing it, depending on the soil temperature reached. The main post-fire hydrogeomorphological changes have been widespread erosion and colluvial and alluvial deposition of topsoil in foot-slope locations and river systems, but only localised redistribution of the highly erodible, repellent sandy subsurface layer. The fire did not trigger major geomorphological change in the study area, but fires probably cause important topsoil and nutrient depletion and may also affect water quality.

Tareena Billabong – a palaeolimnological history of an ever-changing wetland, Chowilla Floodplain, lower Murray–Darling Basin, Australia

A 427-cm sediment core was extracted from Tareena Billabong, a Murray River floodplain wetland in the extreme south-west of New South Wales, Australia. Analysis of fossil diatoms and pollen, sediment 210Pb and 137Cs profiles and radiocarbon and luminescence dating reveal that Tareena Billabong has undergone substantial environmental change in its ~5000-year history. Shortly after its formation, the billabong was a freshwater lagoon with a diatom flora dominated by Synedra ulna and Planothidium lanceolatum. An increase in Aulacoseira granulata, a river plankton dominant today, reflects two phases of increased connectivity with the Murray River in the mid to late Holocene. A shift to lagoonal taxa after ~3000 years BP is attributed to water balance and river-flow changes, possibly associated with regional climate change. Importantly, it appears to have undergone an extended phase of increasing turbidity, and possibly wetland salinity, commencing ~3000 years BP. Sedimentation increased at least 15-fold in the European phase. Billabong salinity increased markedly soon after European settlement, reaching a peak in the late 1800s AD. While regulation then increased the degree of connection between the billabong with the River in the 1920s AD, salinity levels remained high. Increased salinity is revealed by increases in the diatom taxa Amphora spp., Cyclotella meneghiniana, Gyrosigma acuminatum, Planothidium delicatulum and Tryblionella hungarica and by declines in Casuarinaceae, Eucalyptus, Myriophyllum and Cyperaceae pollen. Tareena Billabong was subjected to considerable environmental pressures from the early stages of European settlement in terms of sediment load, hydrological change and salinity.

Towards best practice implementation and application of models for analysis of water resources management scenarios

Water resources management models are widely used to evaluate planning or operational scenarios to support water resource management decision-making. However, the approaches to modelling used in the past have led to problems, such as modellers having difficulty establishing the credibility of their model with stakeholders, and stakeholders having difficulty understanding and trusting model results. A best practice approach to the implementation and application of water resources management models based on a quality assurance procedure is an appropriate means of overcoming these difficulties, and there are a number of guidelines and papers available promoting this approach. However, guidance in these on the use of models to analyse water resource planning scenarios is limited or not provided. This paper therefore provides guidance on the implementation and application of water resources management models with an emphasis on scenario analysis. This guidance is principally intended for practising modellers, and also for peer reviewers and stakeholders such as managers, decision makers, and community-based groups. Adoption strategies and recommendations for future directions are also discussed. We provide guidance on quality assured model application for water resources management planning.This guidance is based on the concept of achieving best practice.The emphasis is on scenario modelling and evaluation of modelling results.Guidance on project administration and decision support is given as well.This guidance is mainly intended for modelling practitioners and also for stakeholders.

Sediment aggregation and water quality in wildfire-affected river basins

Off-site transfer of nutrient-rich burnt soil has implications for downstream water quality. Coarsening of effective particle size (EPS) distributions in burnt material via aggregation of fines into composite particles modifies post-fire sediment and nutrient transport dynamics. Experiments were undertaken to establish temperature controls on wildfire-enhanced soil aggregation. Burnt and unburnt soil from a temperate eucalypt forest were analysed for EPS and settling velocity using a LISST-ST (Laser In Situ Scatter and Transmissometry with Settling Tube) particle size analyser. Next, samples were burnt (250–550°C) before further analysis with the LISST-ST. Settling velocities of naturally burnt soil aggregates were greater than unburnt aggregates of the same EPS. Experimental burning indicated that dense water-stable aggregates form at relatively low temperatures (250°C) probably due to distillation and carbonisation, through pyrolysis, of organic volatiles in surface litter. Under these conditions, the EPS distribution of burnt surface soil coarsens with up to 50% of the <63-μm fraction becoming aggregated. A positive relationship between ‘plant-available’ phosphorus and burn temperature was observed. Given that a large proportion of soil particulate phosphorus is associated with the <63-μm fraction, fire-related aggregation processes have potentially important implications for post-fire fine sediment and nutrient transport and storage dynamics.

Corrigendum to: Large-scale patterns of erosion and sediment transport in river networks, with examples from Australia

This paper examines the patterns of sediment transport in rivers in terms of the sources of sediment and its transport and deposition through the river network. The analysis is in the context of dramatic human influences on river sediment transport and how they might influence freshwater ecosystems. The review of Australian work shows that erosion of hillslopes and stream banks has greatly increased in historical times, supplying vast quantities of sediment to rivers, much of which is still stored within the river system. The stored sediment will continue to effect in-stream and estuarine ecosystems for many decades. In most Australian catchments the dominant source of sediment is streambank erosion. An analysis of historical channel widening suggests that a conceptual framework of relative stream power can explain the diversity of behaviour observed in the numerous case studies. Sediment delivery through catchments is considered first in a generic whole network sense, which emphasizes the crucial role played by riverine deposition in determining catchment sediment budgets. A method is then presented for analysing the diverse spatial patterns of sediment storage in any river network. Finally, the paper considers the temporal changes to channel morphology in response to a human-induced pulse of sediment.