Jacky Croke

Gully initiation and road-to-stream linkage in a forested catchment, southeastern Australia

This study reports the nature of sediment delivery pathways and road-to-stream linkage in a forested catchment in southeastern Australia, and evaluates the causal factors associated with this linkage. Detailed field surveying of approximately 20 per cent of the 75 km road network reveals that 18 per cent of road drains show complete channel linkage via gully development from a road outlet to a stream. An additional 11 per cent of road drains show evidence of partial channel linkage where the gully does not extend the full hillslope length. Inclusion of the full range of road-to-stream linkage categories, including direct linkage at stream crossings, road bridges and fords, results in a 6 per cent increase in drainage density since initial road construction in 1964. The majority of this linkage is associated with relief culverts draining cut-and-fill roads in mid-valley positions. These drainage structures have contributing road lengths that are on average three times longer than those draining mitre drains on ridgetop roads. Runoff from these roads is also discharged onto hillslopes that are at least twice as steep as those used on ridgetop roads. Contributing road length (m) and the gradient of the discharge hillslope (tan θ) are successfully used here in a linear discriminant analysis to separate channelled and non-channelled flow pathways within the catchment. The successful delineation of these pathways using two easily measured variables suggests that this approach has potential in the planning and rehabilitation of forest roads. Copyright

Macrochannels and their significance for flood-risk minimisation: examples from southeast Queensland and New South Wales, Australia

Understanding the frequency and causes of extreme events is crucial for environmental, social and economic protection and planning. In Australia this was never more apparent than January 2011 when widespread flooding across Queensland, New South Wales (NSW), and Victoria resulted in the loss of human lives and devastating impacts to infrastructure and local economies. However, understanding the interplay between the geomorphology of catchments and their hydrology remains poorly developed in floodplain planning guidelines. This paper seeks to explain spatial patterns of flood inundation in terms of downstream variations in channel morphometry; and to discuss the significance of these findings within the context of improving flood risk avoidance strategies and environmental outcomes for urban streams. A prominent characteristic of streams draining catchments in the Lockyer Valley south east Queensland and the Illawarra region of NSW, for example, are well developed macrochannels that have formed in mid-catchment zones. Detailed hydraulic modeling using HEC-RAS, HEC-GeoRAS and ArcGIS indicates that these macrochannels are scaled to accommodate high magnitude floods by operating as ‘bankfull’ channels during such events. In south east Queensland, locations where macrochannels debouch onto unconfined low gradient floodplains appear especially vulnerable to catastrophic flooding because of the efficient delivery and minimal attenuation of flood peaks generated in headwater catchments. Macrochannels and associated landforms can be clearly distinguished and mapped on fine-scale digital elevation models, offering the opportunity to integrate analyses of fluvial landforms and channel processes into hydraulic modeling studies, and ultimately, flood-risk avoidance strategies. Such an approach has the potential to improve on traditional flood risk avoidance methods that are focused primarily on design-flood heights by enabling the interpretation of hydraulic modeling outputs in the context of fluvial landforms that exert a significant control on flood behaviour.

Determining floodplain sedimentation rates using 137Cs in a low fallout environment dominated by channel- and cultivation-derived sediment inputs, central Queensland, Australia

Fallout (137)Cs has been widely used to determine floodplain sedimentation rates in temperate environments, particularly in the northern hemisphere. Its application in low fallout, tropical environments in the southern hemisphere has been limited. In this study we assess the utility of (137)Cs for determining rates of floodplain sedimentation in a dry-tropical catchment in central Queensland, Australia. Floodplain and reference site cores were analysed in two centimetre increments, depth profiles were produced and total (137)Cs inventories calculated from the detailed profile data. Information on the rates of (137)Cs migration through local soils was obtained from the reference site soil cores. This data was used in an advection-diffusion model to account of (137)Cs mobility in floodplain sediment cores. This allowed sedimentation rates to be determined without the first year of detection for (137)Cs being known and without having to assume that (137)Cs remains immobile following deposition. Caesium-137 depth profiles in this environment are demonstrated to be an effective way of determining floodplain sedimentation rates. The total (137)Cs inventory approach was found to be less successful, with only one of the three sites analysed being in unequivocal agreement with the depth profile results. The input of sediment from catchment sources that have little, or no, (137)Cs attached results in true depositional sites having total inventories that are not significantly different from those of undisturbed reference sites.

Validation of a spatially distributed erosion and sediment yield model (SedNet) with empirically derived data from a catchment adjacent to the Great Barrier Reef Lagoon

The use of spatially distributed erosion and sediment yield models has become a common method to assess the impacts of land-use changes within catchments and determine appropriate management options. Lack of model validation is, however, increasingly recognised as an issue, especially for models applied at the large-catchment or regional scale. The present study applies the spatially distributed erosion and sediment yield model SedNet to a 6000-km2 subcatchment of the Fitzroy River in central Queensland, Australia. Model outputs are compared with the results from sediment-source tracing, measured floodplain deposition rates and available hydrometric station data. Results indicated that significant improvement can be made to model predictions when catchment-specific observations (such as river bank and gully geometry and gully erosion history) are used to refine model-input parameters. It was also shown that the use of generic input parameters used by previous SedNet applications within the Great Barrier Reef catchment area resulted in overestimates of sediment yields. Previous model applications may have overestimated the significance of post-European catchment disturbance on the sediment yields of the dry-tropical catchments draining to the Great Barrier Reef. Our findings illustrated the value of obtaining empirically derived data to validate spatially distributed models applied at large scales.

Reducing uncertainty with flood frequency analysis: the contribution of palaeo- and historical flood information

Using a combination of stream gauge, historical and palaeoflood records to extend extreme flood records has proven to be useful in improving flood frequency analysis (FFA). The approach has typically been applied in localities with long historical records and/or suitable river settings for palaeoflood reconstruction from slackwater deposits (SWDs). However, many regions around the world have neither extensive historical information nor bedrock gorges suitable for SWDs preservation and palaeoflood reconstruction. This study from subtropical Australia demonstrates that confined, semi-alluvial channels such as macrochannels provide relatively stable boundaries over the 1000-2000 year time period and the preserved SWDs enabled palaeoflood reconstruction and their incorporation into FFA. FFA for three sites in subtropical Australia with the integration of historical and palaeoflood data using Bayesian Inference methods showed a significant reduction in uncertainty associated with the estimated discharge of a flood quantile. Uncertainty associated with estimated discharge for the 1% Annual Exceedance Probability (AEP) flood is reduced by over 50%. In addition, sensitivity analysis of possible within-channel boundary changes shows that FFA is not significantly affected by any associated changes in channel capacity. Therefore, a greater range of channel types may be used for reliable palaeoflood reconstruction by evaluating the stability of inset alluvial units, thereby increasing the quantity of temporal data available for FFA. The reduction in uncertainty, particularly in the prediction of the ≤ 1% AEP design flood, will improve flood risk planning and management in regions with limited temporal flood data. This article is protected by copyright. All rights reserved.

Geomorphic effectiveness: a linear concept in a non-linear world

Geomorphic effectiveness has been an influential concept in geomorphology since its introduction by Reds Wolman and John Miller in 1960. It provided a much needed framework to assess the significance of an event by comparing event magnitude to the resultant geomorphic effects. Initially, this concept was applied primarily in river channels, under the linear assumption that geomorphic responses to similarly sized flood events will be consistent. Numerous authors have since attempted to quantify a direct, proportional relationship between event magnitude and different forms of geomorphic response in a variety of geomorphic settings. In doing so, these investigations applied an array of metrics that were difficult to compare across different spatiotemporal scales, and physiographic and geomorphic environments. Critically, the emergence of other geomorphic concepts such as sensitivity, connectivity, thresholds, and recovery has shown that relationships between causes (events) and geomorphic effects (responses) are often complex and non-linear. This paper disentangles the complex historical development of the geomorphic effectiveness concept and reviews the utility of various metrics for quantifying effectiveness. We propose that total energy (joules) is the most appropriate metric to use for quantifying the magnitude of disturbance events (cause) and volumetric sediment flux associated with landform modification is the most appropriate metric for quantifying geomorphic effects. While both metrics are difficult to quantify, they are the only ones which facilitate comparison across a range of spatiotemporal scales (comparability) in a variety of geomorphic environments (flexibility). The geomorphic effectiveness concept can continue to be useful provided that geomorphologists use flexible and comparable metrics. Today, geomorphologists are better prepared to consider the influence of non-linear processes on determinations of geomorphic effectiveness, allowing investigators to not only determine if a disturbance event was effective but also to explain why or why not. Copyright