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Hancock G.R.,University of Newcastle | Evans K.G.,University of Newcastle | Evans K.G.,Environmental Research Institute of the Supervising Scientist
Earth Surface Processes and Landforms

Understanding landscape features such as gullying and soil erosion is an important issue in the long-term dynamics and evolution of both natural, agricultural and rehabilitated (i.e. post-mining) landscapes. Considerable research has been undertaken examining the initiation, movement and overall dynamics of such features. This study reports on a series of 34 gully heads and other erosion features, such as scour holes (five in total), located in channels in a catchment largely undisturbed by European activity in the Northern Territory, Australia over a 5 year period (2002-2007). During this period the erosion features were monitored for their headward advance/retreat, enlargement or in-filling. The erosion features ranged in depth from 0.2 m to 1.5 m and widths of 0.3 m to 8 m. Hillslope erosion was also monitored using erosion pins. The catchment was subject to a range of rainfall regimes including extreme rainfall and a Category 5 cyclone and also was burnt every second year so that all grass cover was removed according to traditional management practice. The results of this monitoring show that the erosion features have changed little during this 5 year period. A remote sensing assessment found no relationship between erosion feature morphology and hillslope erosion. The monitored gullies heads and scour holes appear to be resilient landscape features, yet have a morphology that suggests they are ready for rapid headward movement and expansion, leading to a destabilisation of the catchment. Hillslope erosion was found to be related to wetness indices derived from a digital elevation model. Significant linkages were found between hillslope erosion and change in erosion feature depth, indicative of a strong hillslope-channel coupling. Copyright © 2010 John Wiley & Sons, Ltd. Source

Erskine W.D.,University of Newcastle | Erskine W.D.,Environmental Research Institute of the Supervising Scientist
Australian Geographer

Historical planform changes in a 14.7 km reach of the lower Pages River were determined to assess whether they were autogenic (inherent in the river regime) or allogenic (driven by external changes) in nature so as to better focus river management activities and river restoration works. A pattern metamorphosis or complete change in river morphology occurred during the February 1955 flood. The peak discharge of this event exceeded the slope and grain size (intrinsic) threshold for braiding, converting the narrow, slightly sinuous stream to a wide, braided-like river. Five subsequent intrinsic threshold exceeding floods did not cause further bar development because an over-widened channel already existed. Autogenic channel planform changes included sinuosity variations due to lateral migration and pattern metamorphosis due to the exceedance of a discharge-slope-grain size geomorphic threshold. Allogenic channel planform changes included: (1) realignment/channel straightening and artificial cutoffs by river training works; (2) lateral migration by increased bank erodibility due to riparian vegetation clearing; (3) lateral migration by the operation of a transitive geomorphic threshold involving the onset of a flood-dominated regime after 1946 and increased catchment runoff after 1830 due to large-scale clearing of catchment vegetation; and (4) the occurrence of a large flood in February 1955. Multiple forcing factors have clearly caused historical channel planform changes of the lower Pages River, making the design of river management and restoration works a complex matter outside the scope of simple formulaic protocols. © 2011 Geographical Society of New South Wales Inc. Source

Hancock G.R.,University of Newcastle | Murphy D.,University of Western Australia | Evans K.G.,University of Newcastle | Evans K.G.,Environmental Research Institute of the Supervising Scientist

The role of geomorphology in relation to the spatial and temporal distribution of soil carbon is of considerable interest in terms of landscape management and carbon sequestration. Soil carbon plays an important role in soil water holding capacity, soil structure and overall soil health. Soil is also a significant store of terrestrial carbon. This study examines total soil carbon (SC) concentration at the hillslope and catchment scale in the Tin Camp Creek catchment, Arnhem Land, Northern Territory, Australia. The catchment is largely undisturbed by European agriculture or management practices and is located in the monsoonal tropics. Results show that SC concentration along hillslope transects has remained consistent over a number of years and it is strongly related to hillslope position and topographic factors derived from precision surveying and provides a baseline assessment. Poor relationships were found when using a good quality medium resolution digital elevation model to derive topographic factors. This finding demonstrates the need for high resolution survey data for the prediction of total C at the hillslope and catchment scale. There was little difference in SC concentration between years and overall, SC down the hillslope profile varies little temporally suggesting that concentrations are relatively stable in this environment. An assessment of the relationship between SC and soil erosion using 137Cs and erosion pins demonstrates that sediment transport and deposition play little role in the distribution of SC in this environment. Vegetative biomass appears to be the major contributor to SC concentration with vegetative biomass being strongly controlled by topographic factors. While the SC concentration is constant over the study period further sampling is required to assess decadal trends. Crown Copyright © 2009. Source

Fryirs K.,Macquarie University | Brierley G.J.,University of Auckland | Erskine W.D.,Environmental Research Institute of the Supervising Scientist | Erskine W.D.,University of Newcastle
Earth Surface Processes and Landforms

Applications of ergodic reasoning (or location for time substitution) aid efforts at environmental reconstruction and prediction, providing a useful tool to analyse and communicate stages of landscape evolution. Analysis of the historical range of behaviour and change that a river system has experienced can be used to interpret thresholds that have been breached, and underlying controls and/or triggers for adjustment and change. This information can be used to forecast future trajectories of adjustment and provide target conditions for management activities. This paper uses a case study from upper Wollombi Brook, New South Wales, Australia to demonstrate how ergodic reasoning can be used to assess river behaviour, change and responses to natural and human-disturbances. The 'river evolution diagram' developed by Brierley and Fryirs (Geomorphology and River Management: Applications of the River Styles Framework. Blackwell Publishing: Oxford, 2005) is presented as a means for depicting the range of behaviour and evolutionary variability of this river. These approaches can be readily applied in other systems. Implications for approaches to analysis of river evolution and management are outlined. © 2012 John Wiley & Sons, Ltd. Source

Erskine W.D.,Environmental Research Institute of the Supervising Scientist | Erskine W.D.,University of Newcastle | Saynor M.J.,Environmental Research Institute of the Supervising Scientist
Journal of Hydrology

Rainfall, discharge and bedload were measured at three gauging stations (East Tributary, Upper Swift Creek and Swift Creek) in the Ngarradj Creek catchment at Jabiluka, Northern Territory, Australia. Hand-held, pressure difference, Helley-Smith bedload samplers were used to measure bedload fluxes for the 1998/1999, 1999/2000, 2000/2001 and 2001/2002 wet seasons. Rainfall is strongly seasonal over the Ngarradj Creek catchment, being concentrated in the wet season between November and April. Mean annual point rainfall between 1998 and 2007 for the water year, September to August, inclusive varied over the Ngarradj Creek catchment from 1731±98mm (SE) to 1754±116mm. Between 190 and 440mm of rainfall are required before streamflow commences in December in most years. Streamflow persists until at least April. Mean annual runoff, as a percentage of mean annual rainfall, decreases slightly with increasing catchment area. Bedload ratings were calculated for four data sets. Significant bedload ratings were defined as those that were not only statistically significant (ρ0.05) but also explained at least 0.60 of the variance in bedload flux. For the three stations, twenty-three bedload ratings complied with the above criteria. Sixteen equations were accepted for East Tributary, four bedload ratings were accepted for Upper Swift Creek and three bedload ratings were accepted for Swift Creek. Significant bedload ratings were established between bedload flux and discharge, unit bedload flux and discharge, transport rate of unsuspended bedload by immersed weight per unit width and time and both unit and excess unit stream power, and finally, adjusted submersed bedload weight and both unit and excess unit stream power for raw and log10-transformed data. 'Censored data sets' were compiled for Upper Swift Creek and Swift Creek to include only bedload fluxes measured when there was no apparent scour or fill so that there were no changes in sand supply from the catchment (i.e. equilibrium conditions).Bedload sediments are similar at all sites. There is little difference in grain size statistics between wet season bedload and dry season bed material. The differences which were significant suggest that most of the bed material is transported as bedload during the wet season. Size selective transport occurs at all three gauging stations with bedload being better sorted than bed material and the coarsest fraction (Cobble gravel) is mobile only during extreme events. © 2013. Source

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