Search

We propose a Bayesian model which produces probabilistic reconstructions of hydroclimatic variability in Queensland Australia. The model provides a standardized approach to hydroclimate reconstruction using multiple palaeoclimate proxy records derived from natural archives such as speleothems, ice cores and tree rings. The method combines time-series modeling with inverse prediction to quantify the relationships between a given hydroclimate index and relevant proxies over an instrumental period and subsequently reconstruct the hydroclimate back through time. We present case studies for Brisbane and Fitzroy catchments focusing on two hydroclimate indices, the Rainfall Index (RFI) and the Standardized Precipitation-Evapotranspiration Index (SPEI). The probabilistic nature of the reconstructions allows us to estimate the probability that a hydroclimate index in any reconstruction year was lower (higher) than the minimum (maximum) value observed over the instrumental period. In Brisbane, the RFI is unlikely (probabilities < 5%) to have exhibited extremes beyond the minimum/maximum values observed between 1889 and 2019. However, in Fitzroy there are several years during the reconstruction period where the RFI is likely (>50% probability) to have exhibited behavior beyond the minimum/maximum of what has been observed, during the instrumental period. For SPEI, the probability of observing such extremes prior to the beginning of the instrumental period in 1889 doesn't exceed 30% in any reconstruction year in Brisbane, but exceeds 50% in multiple years in Fitzroy.

Understanding of the pre-development, baseline denudation rates that deliver sediment to the Great Barrier Reef (GBR) has been elusive. Cosmogenic 10Be in sediment is a useful integrator of denudation rates and sediment yields averaged over large spatial and temporal scales. This study presents 10Be data from 71 sites across 11 catchments draining to the GBR: representing 80% of the GBR catchment area and provide background sediment yields for the region. Modern, short-term, sediment yields derived from suspended load concentrations are compared to the 10Be data to calculate an Accelerated Erosion Factor (AEF) that highlights denudation "hot-spots" where sediment yields have increased over the long-term background values. The AEF results show that 58% basins have higher modern sediment yields than long-term yields. The AEF is considered a useful approach to help prioritise on-ground investments in remediation and the additional measured empirical data in this paper will help support future predictive models.

The concept of connectivity has found great traction in understanding the movement of fluxes across the surface of the earth through disciplinary perspectives including hydrology, geomorphology and ecology (Bracken and Croke, 2007; Bracken et al 2013;2015). Connectivity-based approaches have also generated new understanding of structural-functional relationships that characterise complex systems, for instance in computational neuroscience, social network science and systems biology (Turnbull et al., 2018). Whilst the concept of hydrological connectivity has been used widely, at all scales and with respect to fluxes of both water and sediment, critique and development of the concept is less frequent in the literature. In this paper we revisit the existing body of work around hydrological connectivity to examine whether the concept has been used to it’s full potential and explore further ways in which the concept of hydrological connectivity could be expanded to continue to drive geomorphological research. One potential avenue for research is to learn from complex systems and use the concept of connectivity to embrace human dynamics (through managing the landscape and guiding policy and regulation) on one hand and climate change (which drives system inputs) on the other. This opportunity is explored here using the water sector as a case study where planning, and managing for, water security under growing population pressures and future climate change are explored through this broader interpretation of connectivity. We see this wider coupling between humans and system inputs playing a significant role in shaping earth surface processes and sediment dynamics and a widening of definition may enable hydrologists and geomorphologists to better integrate socio-ecological systems into our research.

Understanding of the pre-development, baseline denudation rates that deliver sediment to the Great Barrier Reef (GBR) has been elusive. Cosmogenic

Sediment runoff has been cited as a major contributor to the declining health of the Great Barrier Reef (GBR), however, climate and land use drivers have not been jointly evaluated. This study used alluvial archives from fluvial benches in two tributaries of the Upper Burdekin catchment together with the best available land use history and climate proxy records to provide insights into the timing of depositional events in this region over the past 500 years. This study suggests that mining and the increased runoff variability in the latter half of the nineteenth century are the likely sources of the original excess sediment that was used to build the bench features in these catchments. Grazing also contributed to increased bench sedimentation prior to 1900, however, the contribution of grazing was likely more significant in the second half of the 20th century, and continues to be a dominant land use contributor today.

The application of palaeoflood hydrology in Australia has been limited since its initial introduction > 30 years ago. This study adopts a regional, field-based approach to sampling slackwater deposits in a subtropical setting in southeast Queensland beyond the traditional arid setting. We explore the potential and challenges of using sites outside the traditional physiographical setting of bedrock gorges. Over 30 flood units were identified across different physiographical settings using a range of criteria. Evidence of charcoal-rich layers and palaeosol development assisted in the identification and separation of distinct flood units. The OSL-dated flood units are relatively young with two-thirds of the samples being 80% of them classified as ‘extreme event’. This study opens up the renewed possibility of applying palaeoflood hydrology to more populated parts of Australia where the need for improved estimation of flood frequency and magnitude is now urgent in light of several extreme flood events. Preliminary contributions to improve the understanding between high magnitude floods and regional climatic drivers are also discussed. Recognised regional extreme floods generally coincide with La Nina and negative IPO phases, while tropical cyclones appear to be a key weather system in generating such large floods.

The nature and timing of fluvial response and coincident changes in climate as inferred from multiple, independent proxies are examined in the wet tropics of north-eastern Australia. Chronostratigraphic data provide evidence for regionally synchronous fluvial aggradation at 30-13 ka, 8-5 ka and again from 1 ka. Terrace incision and removal occurred at 13-8 ka and 5-1 ka. A new synthesis of regional palaeoenvironmental proxy records spanning the last 30 ka highlights key periods of environmental change. Comparison of the records of fluvial and environmental change reveals the relationship is complex with no clear relationship between river aggradation/incision and wetter/drier conditions. The nature and timing of fluvial response in the wet tropics also shows broad similarities with coastal valleys in south-eastern New South Wales, suggesting continental-scale controls on fluvial response to changing climate during the Late Quaternary in Australia. Copyright (C) 2017 John Wiley & Sons, Ltd.

In perennial stream settings, there is abundant literature confirming that riparian vegetation affects flood hydrology by attenuating the flood wave, enhancing deposition and reducing bank erosion. In contrast, relatively little is known about the effectiveness of riparian vegetation during floods in hydrologically-variable regions. The dominant channel form in these settings is often referred to as a 'macrochannel' or compound channel-in-channel which displays multiple inundation surfaces where it is often difficult to identify the active channel bank and bank top. This study uses the inundation pattern of recent flood events in the Lockyer Valley of South East Queensland (SEQ), Australia to present a framework which specifically considers the interaction between inundation frequency and trapping potential on a range of inundation surfaces. Using hydrological modelling and a consistent definition of floodplains and within-channel features, it outlines five key priority areas for the placement of riparian vegetation to alleviate common flood problems within the catchment. The highest priority for the placement of riparian vegetation to ameliorate the effects of small-moderate floods is on within-channel benches. For out-of-macrochannel flows, riparian vegetation is most effective on genetic floodplains which occupy the largest spatial extent within the valley. In particular, it identifies the need for, and benefits of, revegetation in spill out zones (SOZ) which occur where upstream channel capacity is larger and flow is funnelled at high velocity onto the floodplain downstream. This study highlights the importance of understanding the key geomorphic processes occurring within a catchment and developing effective catchment management plans to suit these conditions.

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.

Along the eastern margin of Australia, hydrological variability reaches a peak in the subtropics of south-east Queensland and many rivers have entrenched characteristics. To address the nature of entrenchment and the relationship with adjacent alluvium, this paper presents the results of detailed chrono-stratigraphic analysis of alluvial units in the partly confined mid-reaches of Lockyer Creek, Australia. Four sites were investigated using topographic, sedimentological and chronological data. Radiocarbon and single grain optically stimulated luminescence dating indicate a large proportion of the valley fill reflects a major phase of aggradation of fine-grained alluvium from ca. 35 ka throughout the Last Glacial Cold Period. Synchronous incision of Pleistocene alluvial fills between 11.5 and 9.3 ka suggests the current entrenched Lockyer Creek formed in response to changes in late Quaternary climate. Holocene floodplains set within the entrenched Pleistocene valley floor have basal ages of ca. 7.5 ka, but whose proximal margins are still actively accreting. This Holocene fill has accreted over the mid- to late Holocene but overlaps with the contemporary hydrological regime. The sedimentary nature of the Holocene fill appears to be related to persistent antecedent controls in the form of bedrock and terrace constriction.

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

We report a comprehensive inventory of Be-10-based basin-wide denudation rates (n=160) and Al-26/Be-10 ratios (n=67) from 48 drainage basins along a 3,000 km stretch of the East Australian passive continental margin. We provide data from both basins draining east of the continental divide (n=37) and discharging into the Tasman and Coral Seas, and from basins draining to the west as part of the larger Murray-Darling and Lake Eyre river systems (n=11). Be-10-derived denudation rates in mainstem samples from east-draining basins range between 7.7 ± 1.9 (±1 sigma; Mary) and 54.6 ± 13.7 mm kyr-1 (North Johnstone). Denudation rates in tributary samples range between 3.0 ± 0.7 (Burdekin) and 70.2 ± 18.9 mm kyr-1 (Liverpool). For west-draining basins, denudation rates are overall lower and with a more restricted range of 4.8 ± 1.2 (Barcoo) to 15.4 ± 3.6 mm kyr-1 (Maranoa) in mainstem samples, and between 4.4 ± 1.0 (Murrumbidgee) and 38.5 ± 7.8 mm kyr-1 (Murray) in tributary samples. East Australian denudation rates (median all = 14.5 mm kyr-1; median east of continental divide = 15.9 mm kyr-1; median west of the continental divide = 9.3 mm kyr-1) are similar to those found in other postorogenic landscapes (global median = 12.4 mm kyr-1) and the medians of the top 10% denudation rates recorded here (46.5 mm kyr-1) and in other passive margin settings are also similar, despite differences in topography and precipitation. These median denudation rate values are close to the 95th percentile denudation rate for all tectonically passive basins (≈53 mm kyr-1) and are very similar to the global silicate weathering speed limit (≈58 mm kyr-1) calculated as the 95th percentile of global soil weathering rates. The above suggests that in post-orogenic terrain, the overall rates of topographic decay have a ‘speed limit’ that is imposed by the rate at which rock is converted to soil by chemical weathering. Denudation rates along the East Australian margin correlate with topographic metrics at both the mainstem basin scale and at the smaller tributary basin scale suggesting that topography exerts the main control on rates of landscape lowering in this setting. An important link between denudation rate and rainfall is also inferred: the highest mainstem Be-10 denudation rates all occur in basins which also have the highest rainfall amounts and there is a strong correlation between the distance knickpoints have travelled upstream from the river mouth and basin area — a proxy for discharge and so to some extent also rainfall. Lastly, we assign basin Al-26/Be-10 ratios to identify non-steady state sediment storage and generation processes. On both sides of the divide, in all but a few of the samples, Al-26/Be-10 ratios are consistently lower than what is expected in a setting where sediments experience a simple and continuous exposure history. East of the continental divide Al-26/Be-10 ratios range between 4.99 ± 0.31 (±1 sigma; Mitchell) and 7.86 ± 0.77 (Don), and between 3.96 ± 0.32 (Barcoo) and 6.39 ± 0.40 (Maranoa), west of the continental divide. East of the continental divide, the lowest Al-26/Be-10 ratios are found in basins that also experience increased flood variability. We posit that there is a causal link between the hydrological variability that characterises the coastal rivers of eastern Australia and the observed non-steady state Al-26/Be-10 ratios: the periodic stripping of vertically accreted floodplains by large floods means that deeper and potentially older material is periodically incorporated into the sediment mix transported by the modern river.

This paper reconstructs past flooding from a range of settings in Lockyer Creek, a key tributary of the mid-Brisbane River, which experienced extreme flood events in AD 2011 and AD 2013. Optically stimulated luminescence samples (n = 110) were collected from alluvial material preserved in within-channel benches and floodplains. Age distributions from material in the bedrock reaches confirm an event ∼ 300 years ago which stripped the valley alluvium to bedrock. In the unconfined reaches floodplain deposits indicate lateral stability over the past 6000 years. Marked differences in the inundation patterns of the AD 2011 event highlighted changes in downstream channel geometry. The age distribution of alluvium in reaches not inundated during AD 2011 was older, ∼12 000 years, with no preserved evidence of deposition during the past 1000 years. A relatively continuous record of floodplain deposition in reaches which were inundated in AD 2011 identifies a major peak in flood activity also around 300 years ago (∼AD 1730) with five additional peaks occurring at approximately AD 1962, AD 1897, AD 1300, AD 550 and 5400 BC. The main climatic driver of changes in flooding over this timescale is oscillations in El Nino Southern Oscillation and although proxy records are scarce for this region, some correlations with high-resolution records of rainfall variability are apparent.

Flood risk management is an essential responsibility of state governments and local councils to ensure the protection of people residing on floodplains. Globally, floodplains are under increasing pressure from growing populations. Typically, the engineering-type solutions that are used to predict local flood magnitude and frequency based on limited gauging data are inadequate, especially in settings which experience high hydrological variability. This study highlights the importance of incorporating geomorphological understanding into flood risk management in southeast Queensland (SEQ), an area badly affected by extreme flood events in 2011 and 2013. The major aim of this study is to outline the hydrological and sedimentological characteristics of various ‘inundation surfaces’ that are typical of catchments in the sub-tropics. It identifies four major inundation surfaces; within-channel bench [Q ~ 2.33 yr average recurrence interval (ARI)]; genetic floodplain (Q = 20 yr ARI); hydraulic floodplain (20 yr 1000 yr ARI). These surfaces are considered typical of inundation areas within, and adjacent to, the large macrochannels common to this region and others of similar hydrological variability. An additional area within genetic floodplains was identified where flood surfaces coalesce and produce an abrupt reduction in channel capacity. This is referred to here as a Spill-out Zone (SOZ). The associated vulnerability and risk of these surfaces is reviewed and recommendations made based on incorporating this geomorphological understanding into flood risk assessments. These recommendations recognize the importance to manage for risks associated with flow inundation and sediment erosion, delivery and deposition. The increasing availability of high resolution topographic data opens up the possibility of more rapid and spatially extensive assessments of key geomorphic processes which can readily be used to predict flood risk. Copyright © 2016 John Wiley & Sons, Ltd.

Extreme flood events have detrimental effects on society, the economy and the environment. Widespread flooding across South East Queensland in 2011 and 2013 resulted in the loss of lives and significant cost to the economy. In this region, flood risk planning and the use of traditional flood frequency analysis (FFA) to estimate both the magnitude and frequency of the 1-in-100 year flood is severely limited by short gauging station records. On average, these records are 42 years in Eastern Australia and many have a poor representation of extreme flood events. The major aim of this study is to test the application of an alternative method to estimate flood frequency in the form of the Probabilistic Regional Envelope Curve (PREC) approach which integrates additional spatial information of extreme flood events. In order to better define and constrain a working definition of an extreme flood, an Australian Envelope Curve is also produced from available gauging station data. Results indicate that the PREC method shows significant changes to the larger recurrence intervals (≥100 years) in gauges with either too few, or too many, extreme flood events. A decision making process is provided to ascertain when this method is preferable for FFA.

A channel evolution model (CEM) represents stages of channel development in response to specific types of disturbance. In recent years, classic incised/disturbed CEMs have provided process-based understanding of channel adjustment and formed the cornerstone for river restoration and rehabilitation. While broadly applicable to alluvial systems in temperate and semi-arid regions, these models cannot be assumed to be universally applicable. Lockyer Creek in South East Queensland, Australia, has notable macrochannel morphology and is subject to high hydrological variability typical of many subtropical climates. The aim of this paper is to present a case study of channel adjustment and evolution in lower Lockyer Creek, to determine if existing CEMs adequately describe processes of channel adjustment and the associated trajectories of change typical of river systems in subtropical settings. Lockyer Creek has recently been subjected to a spate of flooding resulting in significant channel erosion. This offers an ideal opportunity to investigate the nature and rate of channel adjustment processes and place them in context of longer-term geomorphic adjustments in these systems. Specifically we address two questions. Firstly, do the classic incised/disturbed CEMs adequately represent the observed macrochannel adjustment? Secondly, if current CEMs are inadequate, what is the channel evolution model for these systems, of which lower Lockyer Creek is an example? Results show that these are non-incising systems where wet-flow bank mass failures (WBMFs) are the dominant process of channel adjustment. They occur within the channel bank top boundary resulting in no change to overall bank-top width. Furthermore, subsequent floods deposit sediment in the failure scars and failure headwalls generally do not retreat beyond channel bank-top. Channel adjustment has not followed the evolutionary stages for incised/disturbed channels and a new four stage macrochannel CEM is outlined for these subtropical systems. The proposed CEM illustrates a cyclical pattern of erosion by channel bank WBMF followed by re-aggradation, through deposition and oblique processes, contributing to bank rebuilding. This CEM provides sufficient information to determine the stage of macrochannel adjustment, enabling decisions to be made over whether intervention is required or will be successful.

This study aimed to determine the extent of geomorphic change resulting from the catastrophic flood of 2011 in the Lockyer Valley in southeast Queensland and to place these impacts within a history of geomorphic adjustment. Aerial photographs dated from 1933 to 2011 and parish maps and historical on-ground photographs dating from 1865 to 1966 were examined for evidence of geomorphic adjustment since European settlement in the first half of the nineteenth century. Eleven forms of geomorphic adjustment were identified in three categories; erosional, depositional, and reorganisational. Only 26% of the Lockyer Creek channel length has been affected by some form of geomorphic adjustment since European settlement. Most of this adjustment was localised and dominated by reorganisation of geomorphic unit assemblages within the macrochannel and sediment deposition on floodplains. No wholesale river change in the form of lateral migration or avulsion has occurred, and the river's morphology has remained relatively characteristic over time (i.e., morphology remains relatively uniform in a reach-averaged sense). Geomorphic responses to extreme flooding have been minor, and the geomorphic effectiveness of floods in this system (including the 2011 flood) has been limited over the last several hundred years. The system is likely still adjusting to past flooding events that ‘set’ the morphology of the current system (i.e., the macrochannel). A form of event resilience has resulted in this system such that it is less prone to geomorphic adjustment during events than would normally be considered geomorphically effective. As a result, antecedent controls on macrochannel presence and capacity are considered to be first-order controls on contemporary forms and processes in this system. Work is required to test whether the resilience of this system will hold in the future, with more extreme episodes of flooding predicted to occur in this region under future climate change.

The role of extreme events in shaping the Earth's surface is one that has held the interests of Earth scientists for centuries. A catastrophic flood in a tectonically quiescent setting in eastern Australia in 2011 provides valuable insight into how semi-alluvial channels respond to such events. Field survey data (3 reaches) and desktop analyses (10 reaches) with catchment areas ranging from 0.5 to 168 km2 show that the predicted discharge for the 2011 event ranged from 415 to 933 m3 s−1, with unit stream power estimates of up to 1077 W m−2. Estimated entrainment relationships predict the mobility of the entire grain-size population, and field data suggest the localised mobility of boulders up to 4.8 m in diameter. Analysis of repeat lidar data demonstrates that all reaches (field and desktop) were areas of net degradation via extensive scouring of coarse-grained alluvium with a strong positive relationship between catchment area and normalised erosion (R2 = 0.72–0.74). The extensive scouring in the 2011 flood decreased thalweg variance significantly removing previous step pools and other coarse-grained in-channel units, forming lengths of plane-bed (cobble) reach morphology. This was also accompanied by the exposure of planar bedrock surfaces, marginal bedrock straths and bedrock steps. Post-flood field data indicate a slight increase in thalweg variance as a result of the smaller 2013 flood rebuilding the alluvial overprint with pool-riffle formation. However, the current form and distribution of channel morphological units does not conform to previous classifications of bedrock or headwater river systems. This variation in post-flood form indicates that in semi-alluvial systems extreme events are significant for re-setting the morphology of in-channel units and for exposing the underlying lithology to ongoing erosion.

Methods for prioritising catchment remediation are based on understanding the source of sediment over the short-medium timescales (10–10 2 years) using techniques such as sediment finger-printing, sediment flux monitoring, and catchment modelling. Because such approaches do not necessarily quantify the natural variation in sediment flux over the longer timescale, they often represent background or pre-agricultural erosion rates poorly. This study compares long-term (∼100 to >10,000 years) erosion rates derived from terrestrial cosmogenic nuclides ( 10 Be) with contemporary erosion rates obtained by monitoring sediment fluxes over #11×10#10 years. The ratio of these two data sets provides a measure of the accelerated erosion factor (AEF), which can be used to identify erosion hot-spots at the sub-catchment scale. The study area is the Burdekin catchment, the largest source of contemporary sediment to the Great Barrier Reef lagoon. Long-term erosion rates vary from −1 in the Suttor and Belyando sub-catchments to 0.0296 mm yr −1 in the Bowen. The contemporary erosion rates are highest on small hillslopes with patchy ground cover (0.2726 mm yr −1 ) and in the Bowen sub-catchment (0.2207 mm yr −1 ), and lowest in the Belyando sub-catchment (0.0019 mm yr −1 ). All but two of the sub-catchment sites have an AEF > 1.0, indicating higher contemporary erosion rates than estimated long-term averages. Results confirm that the contemporary or agriculturally-induced erosion rates at these sites have increased considerably. Within the context of the Reef Water Quality Protection Plan, these results provide justification for water quality targets to be set at the sub-catchment scale, particularly for large and geomorphically diverse catchments.

The sediment (dis)connectivity concept is the water-mediated transfer of sediment between different compartments of a catchment sediment cascade involving four possible dimensions or linkages (longitudinal, lateral, vertical and temporal). Quantifying the strength of these linkages within and between compartments provides a means to understand the internal sediment flux dynamics of a catchment. The aims of this paper are to examine (1) the dynamics of longitudinal and lateral (dis)connectivity by quantifying patterns of erosion and deposition that occurred during a catastrophic flood, and (2) how the patterns of connectivity can be changed through management actions that better utilise floodplain sediment storages. Multi-temporal LiDAR and air photos are used to quantify volumetric change with respect to geomorphic settings and units. The results show that over the length of the trunk stream, the high-magnitude event was net depositional with high longitudinal sediment disconnectivity. At the reach scale, an alternating pattern of high and low longitudinal connectivity associated with contraction and expansion zones was evident. The efficiency of sediment transfer from the uppermost compartment to the most downstream compartment decreased exponentially, while the strength of lateral connectivity increased for each expansion reach. Modelling results show that increasing channel boundary roughness along expansion reaches with riparian revegetation can increase the frequency of lateral connectivity and floodplain sediment storage, thereby decreasing reach-to-reach connectivity and reducing end-of-catchment sediment delivery. This contrasts with the current trend of building levees along the bank tops of expansion reaches, which decrease lateral connectivity and increase reach-to-reach connectivity.

A growing body of field, theoretical and numerical modelling studies suggests that predicting river response to even major changes in input variables is difficult. Rivers are seen to adjust rapidly and variably through time and space as well as changing independently of major driving variables. Concepts such as Self-Organized Criticality (SOC) are considered to better reflect the complex interactions and adjustments occurring in systems than traditional approaches of cause and effect. This study tests the hypothesis that riverbank mass failures which occurred both prior to, and during, an extreme flood event in southeast Queensland (SEQ) in 2011 are a manifestation of SOC. Each wet-flow failure is somewhat analogous to the 'avalanche' described in the initial sand-pile experiments of Bak et al. (Physical Review Letters, 1987, 59(4), 381-384) and, due to the use of multitemporal LiDAR, the time period of instability can be effectively constrained to that surrounding the flood event. The data is examined with respect to the key factors thought to be significant in evaluating the existence of SOC including; non-linear temporal dynamics in the occurrence of disturbance events within the system; an inverse power-law relation between the magnitude and frequency of the events; the existence of a critical state to which the system readjusts after a disturbance; the existence of a cascading processes mechanism by which the same process can initiate both low-magnitude and high-magnitude events. While there was a significant change in the frequency of mass failures pre- and post-flood, suggesting non-linear temporal dynamics in the occurrence of disturbance events, the data did not fit an inverse power-law within acceptable probability and other models were found to fit the data better. Likewise, determining a single 'critical' state is problematic when a variety of feedbacks and multiple modes of adjustment are likely to have operated throughout this high magnitude event. Overall, the extent to which the data supports a self-organized critical state is variable and highly dependent upon inferential arguments. Investigating the existence of SOC, however, provided results and insights that are useful to the management and future prediction of these features.

Flooding is a persistent natural hazard, and even modest changes in future climate are believed to lead to large increases in flood magnitude. Previous studies of extreme floods have reported a range of geomorphic responses from negligible change to catastrophic channel change. This paper provides an assessment of the geomorphic effects of a rare, high magnitude event that occurred in the Lockyer valley, southeast Queensland in January 2011. The average return interval of the resulting flood was similar to 2000 years in the upper catchment and decreased to similar to 30 years downstream. A multitemporal LiDAR-derived DEM of Difference (DoD) is used to quantify morphological change in two study reaches with contrasting valley settings (confined and unconfined). Differences in geomorphic response between reaches are examined in the context of changes in flood power, channel competence and degree of valley confinement using a combination of one-dimensional (1-D) and two-dimensional (2-D) hydraulic modelling. Flood power peaked at 9800 W m(-2) along the confined reach and was 2-3 times lower along the unconfined reach. Results from the DoD confirm that the confined reach was net erosional, exporting similar to 287,000 m(3) of sediment whilst the unconfined reach was net depositional gaining similar to 209,000 m(3) of sediment, 70% of the amount exported from the upstream, confined reach. The major sources of eroded sediment in the confined reach were within channel benches and macrochannel banks resulting in a significant increase of channel width. In the unconfined reach, the benches and floodplains were the major loci for deposition, whilst the inner channel exhibited minor width increases. The presence of high stream power values, and resultant high erosion rates, within the confined reach is a function of the higher energy gradient of the steeper channel that is associated with knickpoint development. Dramatic differences in geomorphic responses were observed between the two adjacent reaches of contrasting valley configuration. The confined reach experienced large-scale erosion and reorganisation of the channel morphology that resulted in significantly different areal representations of the five geomorphic features classified in this study. (C) 2013 Elsevier B.V. All rights reserved.

Wet-flow river bank failure processes are poorly understood relative to the more commonly studied processes of fluvial entrainment and gravity-induced mass failures. Using high resolution topographic data (LiDAR) and near coincident aerial photography, this study documents the downstream distribution of river bank mass failures which occurred as a result of a catastrophic flood in the Lockyer Valley in January 2011. In addition, this distribution is compared with wet flow mass failure features from previous large floods. The downstream analysis of these two temporal data sets indicated that they occur across a range of river lengths, catchment areas, bank heights and angles and do not appear to be scale-dependent or spatially restricted to certain downstream zones. The downstream trends of each bank failure distribution show limited spatial overlap with only 17% of wet flows common to both distributions. The modification of these features during the catastrophic flood of January 2011 also indicated that such features tend to form at some ‘optimum’ shape and show limited evidence of subsequent enlargement even when flow and energy conditions within the banks and channel were high. Elevation changes indicate that such features show evidence for infilling during subsequent floods. The preservation of these features in the landscape for a period of at least 150 years suggests that the seepage processes dominant in their initial formation appear to have limited role in their continuing enlargement over time. No evidence of gully extension or headwall retreat is evident. It is estimated that at least 12 inundation events would be required to fill these failures based on the average net elevation change recorded for the 2011 event. Existing conceptual models of downstream bank erosion process zones may need to consider a wider array of mass failure processes to accommodate for wet flow failures.

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.

Advances in remote sensing and digital terrain processing now allow for a sophisticated analysis of spatial and temporal changes in erosion and deposition. Digital elevation models (DEMs) can now be constructed and differenced to produce DEMs of Difference (DoD), which are used to assess net landscape change for morphological budgeting. To date this has been most effectively achieved in gravel-bed rivers over relatively small spatial scales. If the full potential of the technology is to be realised, additional studies are required at larger scales and across a wider range of geomorphic features. This study presents an assessment of the basin-scale spatial patterns of erosion, deposition, and net morphological change that resulted from a catastrophic flood event in the Lockyer Creek catchment of SE Queensland (SEQ) in January 2011. Multitemporal Light Detection and Ranging (LiDAR) DEMs were used to construct a DoD that was then combined with a one-dimensional flow hydraulic model HEC-RAS to delineate five major geomorphic landforms, including inner-channel area, within-channel benches, macrochannel banks, and floodplain. The LiDAR uncertainties were quantified and applied together with a probabilistic representation of uncertainty thresholded at a conservative 95% confidence interval. The elevation change distribution (ECD) for the 100-km2 study area indicates a magnitude of elevation change spanning almost 10 m but the mean elevation change of 0.04 m confirms that a large part of the landscape was characterised by relatively low magnitude changes over a large spatial area. Mean elevation changes varied by geomorphic feature and only two, the within-channel benches and macrochannel banks, were net erosional with an estimated combined loss of 1,815,149 m3 of sediment. The floodplain was the zone of major net deposition but mean elevation changes approached the defined critical limit of uncertainty. Areal and volumetric ECDs for this extreme event provide a representative expression of the balance between erosion and deposition, and importantly sediment redistribution, which is extremely difficult to quantify using more traditional channel planform or cross-sectional surveys. The ability of LiDAR to make a rapid and accurate assessment of key geomorphic processes over large spatial scales contributes to our understanding of key processes and, as demonstrated here, to the assessment of major geomorphological hazards such as extreme flood events.

This study reports the nature and timing of Quaternary fluvial activity in the Fitzroy River basin, which drains a diverse 143,000 km(2) area in northeastern Queensland, before discharging into the ...

Many catchment-scale sediment models assume connectivity from the top to the bottom of the catchment. The purpose of this study is to provide evidence that particular topographic features can affect catchment connectivity and the transfer of sediment through the catchment. This study investigates floodplain sediment deposition in a valley bottom constriction and tributary junction site within the Fitzroy River basin, a large semiarid–subtropical catchment in NE Australia. These sites can cause backwater effects during flooding that enhance sediment deposition. Isotope analysis of floodplain sediment cores for cesium-137 and plutonium-239 + 240 is used to determine deposition rates since the 1960s. Results show overall higher deposition rates across the lower tributary floodplain proximal to the confluence and within the valley constriction reach. Deposition rates in the upper constriction reach were less than in the constriction reach but similar to rates reported by other studies in the Fitzroy River basin. However, evidence suggests that the region of influence of the valley constriction on sediment deposition is dependent on flood size with large floods depositing sediment further upstream than the sampling sites of this study. Nevertheless, the study shows evidence that backwater-inducing features (such as confluences of disparate channels) can promote sediment deposition and cause a disconnection in the transfer of sediment from tributary to main channel. The effect of these processes on storing sediment needs to be considered in catchment-scale sediment budget models.

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.

Floodplains comprise geomorphologically important sources and sinks for sediments and associated pollutants, yet the sedimentology of large dryland floodplains is not well understood. Processes occurring on such floodplains are often difficult to observe, and techniques used to investigate smaller perennial floodplains are often not practical in these environments. This study assesses the utility of Cs-137 inventory and depth-profile techniques for determining relative amounts of floodplain sedimentation in the Fitzroy River, northeastern Australia; a 143 000 km(2) semi-arid river system. Caesium-137 inventories were calculated for floodplain and reference location bulk soil cores collected from four sites. Depth profiles of Cs-137 concentration from each floodplain site and a reference location were recorded. The areal density of Cs-137 at reference locations ranged from 13 to 978 Bq m(-2) (0-1367 Bq m(-2) at the 950% confidence interval), and the mean value +/- 2 (standard error of the mean) was 436 264 Bq m(-2), similar to published data from other Southern Hemisphere locations. Floodplain inventories ranged from 68 to 1142 Bq m(-2) (0-1692 Bq m(-2) at the 95% confidence interval), essentially falling within the range of reference inventory values, thus preventing calculation of erosion or deposition. Depth-profiles of Cs-137 concentration indicate erosion at one site and over 66 cm of deposition at another since 1954. Analysis Of Pu239+240 concentrations in a depositional core substantiated the interpretation made from Cs-137 data, and depict a more tightly constrained peak in concentration. Average annual deposition rates range from 0 to 15 mm. The similarity between floodplain and reference bulk inventories does not necessarily indicate a lack of erosion or deposition, due to low Cs-137 fallout in the region and associated high measurement uncertainties, and a likely influence of gully and bank eroded sediments with no or limited adsorbed Cs-137. in this low-fallout environment, detailed depth-profile data are necessary for investigating sedimentation using Cs-137. Copyright (C) 2009 John Wiley & Sons, Ltd.

Rivers draining to the Great Barrier Reef are receiving increased attention with the realisation that European land use changes over the last ∼ 150 years may have increased river sediment yields, and that these may have adversely affected the reef environment. Mitigation of the effects associated with such changes is only possible if information on the spatial provenance and dominant types of erosion is known. To date, very few field-based studies have attempted to provide this information. This study uses fallout radionuclide (137Cs and 210Pbex) and geochemical tracing of river bed and floodplain sediments to examine sources over the last ∼ 250 years for Theresa Creek, a subcatchment of the Fitzroy River basin, central Queensland, Australia. A Monte Carlo style mixing model is used to predict the relative contribution of both the spatial (geological) sources and erosion types. The results indicate that sheetwash and rill erosion from cultivated basaltic land and channel erosion from non-basaltic parts of the catchment are currently contributing most sediment to the river system. Evidence indicates that the dominant form of channel erosion is gully headcut and sidewall erosion. Sheetwash and rill erosion from uncultivated land (i.e., grazed pasture/woodland) is a comparatively minor contributor of sediment to the river network. Analysis of the spatial provenance of floodplain core sediments, in conjunction with optical dating and 137Cs depth profile data, suggests that a phase of channel erosion was initiated in the late nineteenth century. With the development of land underlain by basalt in the mid-twentieth century the dominant source of erosion shifted to cultivated land, although improvements in land management practices have probably resulted in a decrease in sediment yield from cultivated areas in the later half of the twentieth century. On a basin-wide scale, because of the limited spatial extent of cultivation, channel sources are likely to be the largest contributor of sediment to the Fitzroy River. Accordingly, catchment management measures focused on reducing sediment delivery to the Great Barrier Reef should focus primarily on decreasing erosion from channel sources.

Slope–Area relationships have been widely used to identify channel initiation points in the landscape. More recently, the importance of road-related rill and gully erosion has been highlighted. Previous studies indicate that channel initiation at road drain outlets can be predicted from the contributing road area and hillslope gradient at the drain outlet. In this study the validity and usefulness of channel initiation thresholds for road runoff management has been evaluated using field data from three catchments in south-east Australia. None of these catchments show a clear separation between channelised and non-channelised sites based on contributing road area and hillslope gradient. Many drains without channel have similar contributing road area and slope gradients as drain with a channel at the outlet. This demonstrates a relatively large variability within our dataset and limits the identification of a slope–area threshold for these catchments. Logistic regression using road area and slope gradient shows that both variables contribute significantly to channel initiation in the Albert and Tyers River catchments, while for the Sandy Creek catchment only slope gradient is significant at the 0.05 level. Stepwise logistic regression on a wider range of variables shows the significance of cut-batter height in two of the catchments, indicating that flow interception at cut-and-fill roads may be important. Comparison of road-related channel initiation thresholds to previously published topographic thresholds, indicates that channels at road outlets tend to form at relatively small contributing areas and steep slopes. Current forest management guidelines do not consider the risk of channel initiation at road drain outlets in road design and drain spacing. The slope–area thresholds are still considered useful in minimising the risk of gully erosion, especially in forestry areas where it appears most likely that channel initiation occurs during a significant rainfall event immediately following hillslope disturbance or road construction.

The position of mountain streams high in the channel network and their proportional dominance mean that channel modifications and adjustments within these systems will have important implications for downstream processes and linkages. This study develops an analysis framework for examining the catchment-scale distribution of reach morphologies, and the relationship among reach type, catchment lithology and flow competence in southeast Australian mountain streams. The analysis framework is applied to three catchments which have contrasting proportions of the two dominant lithologies of the region, Devonian granites and Ordovician metasediments. The model successfully delineated 68% of reach types, and the resultant spatial maps allowed the effects of stream network position and catchment specific controls on channel morphology to be evaluated. Maximum lengths of the majority of reach morphology types were in second-order streams and the maximum number of morphology types (six) was present in third-order streams, with dramatic reductions in reach type variability as the network expands. The position of catchment lithology within the channel network structure was recognized as more important than the aerial extent of a particular lithology on the distribution and abundance of reach morphologies. The model provides an important tool in the management of channel networks for the protection or restoration of ecological diversity, by identifying river segments and tributaries with high morphological diversity.

Bedload transport data from planebed and step-pool reach types are used to determine grain size transport thresholds for selected upland streams in southeast Australia. Morphological differences between the reach types allow the effects of frictional losses from bedforms, microtopography and bed packing to be incorporated into the dimensionless critical shear stress value. Local sediment transport data are also included in a regime model and applied to mountain streams, to investigate whether empirical data improve the delineation of reach types on the basis of dimensionless discharge per unit width (q*) and dimensionless bedload transport (q*b). Instrumented planebed and step-pool sites are not competent to transport surface median grains (D50s) at bankfull discharge (Qbf). Application of a locally parametrized entrainment equation to the full range of reach types in the study area indicates that the majority of cascades, cascade-pools, step-pools and planebeds are also not competent at Qbf and require a 10 year recurrence interval flood to mobilize their D50s. Consequently, the hydraulic parameters of the regime diagram, which assume equilibrium conditions at bankfull, are ill suited to these streams and provide a poor basis of channel delineation. Modifying the diagram to better reflect the dominant transported bedload size (equivalent to the D16 of surface sediment) made only slight improvements to reach delineation and had greatest effect on the morphologies with smaller surface grain sizes such as forced pool-riffles and planebeds. Likewise, the Corey shape factor was incorporated into the regime diagram as an objective method for adjusting a base dimensionless critical shear stress (τ* c50b) to account for lithologically controlled grain shape on bed packing and entrainment. However, it too provided only minor adjustments to reach type delineation.

The term 'connectivity' is increasingly being applied in hydrological and geomorphological studies. Relevant research encompasses aspects of landscape connectivity, hydrological connectivity and sedimentological connectivity. Unlike other disciplines, notably ecology, published studies show no consensus on a standard definition. This paper provides an overview of how existing research relates to the concept of connectivity in both ecology and hydrology by proposing and evaluating a conceptual model of hydrological connectivity that includes five major components: climate; hillslope runoff potential; landscape position; delivery pathway and lateral connectivity. We also evaluate a proposed measure of connectivity called the volume to breakthrough to quantify changing connectivity between different environments and catchments.

This paper outlines a conceptual and methodological approach to evaluating the risk of road derived runoff delivery, which is based on the principle of hydrological connectivity. Three different types of runoff delivery pathways are identified (stream crossings, gullied pathways and diffuse pathways) and the volume of runoff that may reach the stream through these pathways during a one in 10 year 30 minute event is estimated. The methodology is applied to three catchments of contrasting forest use, both plantation and native. Results show that degree of connectivity of a road depends on catchment characteristics such as the topography, road placement, drain spacing and road and drainage density. Maps outlining the distribution of different delivery pathways within a catchment are used to assess the potential for runoff connectivity. In one of the selected study catchments, the Albert River, greatest potential connectivity can be isolated to a single road. The upper part of this road crosses many tributaries resulting in high connectivity via stream crossings, whereas the lower part of the road is located within the valley bottom, where the majority of drains will contribute runoff during a one in 10 year event through diffuse overland flow. The presented methodology is also used to highlight hot-spots in terms of runoff and sediment delivery through the creation of risk assessment maps, which allows for the evaluation of different procedures for road rehabilitation. Using examples from the Albert River catchment, we demonstrate that minimizing diffuse overland flow can generally be achieved by the placement of additional road drains, whereas at highly connected road segments the relocation of the road might be the only option.

Alluvial terraces provide a record of aggradation and incision and are studied to understand river response to changes in climate, tectonic activity, sea level, and factors internal to the river system. Terraces form in all climatic regions and in a range of geomorphic settings; however, relatively few studies have been undertaken in tectonically stable settings in the tropics. The preservation of alluvial terraces in a valley is driven by lateral channel adjustments, vertical incision, aggradation, and channel stability, processes that can be further understood through examining catchment force–resistance frameworks. This study maps and classifies terraces using soil type, surface elevation, sedimentology, and optically stimulated luminescence dating across five tropical catchments in northeast Queensland, Australia. This allowed for the identification of two terraces across the study catchments (T1, T2). The T1 terrace was abandoned ~ 13.9 ka with its subsequent removal occurring until ~ 7.4 ka. Abandonment of the T2 terrace occurred ~ 4.9 ka with removal occurring until ~ 1.2 ka. Differences in the spatial preservation of these terraces were described using an index of terrace preservation (TPI). Assessments of terrace remnant configuration highlighted three main types of terraces: paired, unpaired, and disconnected, indicating the importance of different processes driving preservation. Regional-scale variability in TPI was not strongly correlated with catchment-scale surrogate variables for drivers of terrace erosion and resistance. However, catchment-specific relationships between TPI and erosion–resistance variables were evident and are used here to explain the dominant processes driving preservation in these tropical settings. This study provides an important insight into terrace preservation in the tectonically stable, humid tropics and provides a framework for future research linking the timing of fluvial response to palaeoclimate change.

The concepts of sediment transport capacity (Q(c)) and sediment supply (Q(s)) have shown promise in broadly differentiating mountain streams. The important role of lithology in determining reach characteristics is also noted but as yet not fully included in existing process domain frameworks. This study uses topographic and grain size surveys undertaken over 42 channel reaches with slopes between 0.01 and 0.20 m/m to describe mountain stream morphology in southeastern Australia. Reaches with two different lithologies are surveyed to specifically address the role of lithology in resultant channel morphology. Spatial autocorrelation analysis based on Moran's I is used to detect the frequency and occurrence of in-stream features, such as bars and pools. These data are combined with the grain size data to describe eight dominant reach morphologies. Most channel morphologies display broad characteristics similar to previous accounts of mountain streams, although a number of intermediate morphologies affecting subtle variations in form and process where evident. There was no significant longitudinal arrangement of the observed morphologies, particularly with respect to catchment area and slope. This is related to the strongly segmented longitudinal profiles characteristic of these streams where increasing catchment area does not result in a commensurate decrease in slope. Five of the eight channel morphologies identified are lithology dependent with respect to grain size and shape. Regime diagrams applied to quantify the physical controls on the different reach morphologies identified different Q(c)-Q(s) domains for most morphologies. However, some lithology-dependent morphologies could not be differentiated supporting the need to incorporate the effects of dominant grain shape (or sphericity) in assigning dimensionless critical shear stress values in determining Q(s). Overall, the differences in mountain stream reach morphologies described in this study reflect the characteristic segmentation of the longitudinal profiles of these streams and contrasts in the supply and breakdown of parent material. (c) 2006 Elsevier B.V. All rights reserved.

Predictive erosion models are useful tools for evaluating the impact of land-use practices on soil and water properties, and as often used by environmental protection authorities, for setting guidelines and standards for regulation purposes. This study examines the application of three erosion models of varying complexity and design for predicting runoff and soil erosion from logged forest compartments in south eastern Australia. These are: the Universal Soil Loss Equation (USLE), the Water Erosion Prediction Project (WEPP), and TOPOG, a physically based hydrologic modelling package. Data on rates of soil loss and redistribution collected during a series of large-scale rainfall simulator experiments were used as model input parameters and validation. The models were evaluated in terms of general ease of use, input data requirements and accuracy of process understanding and prediction. Results suggest that in this application the USLE overestimated soil loss, and have the limitation that it does not predict sediment yield or sediment redistribution for specific storm events. When used at the hillslope scale, WEPP and TOPOG have predicted runoff and soil loss reasonably well, particularly on disturbed surfaces such as skid trails. On less disturbed surfaces such as the general harvesting area, both models performed less accurately, generally under-predicting soil loss and sediment yield, notably on sites with low observed values. The complexity and data requirements of WEPP and TOPOG limit their usability as a general-purpose, erosion hazard predicting tool. In terms of process understanding, none of the existing models accurately depict the nature and extent of sediment redistribution quantified in the rainfall simulator experiments. In order to advance the application and accuracy of modelling tools in forestry environments, this redistribution process should be considered integral to the refinement and redevelopment of future models.

The opening or removal of forest canopies during harvesting or land clearing results in a predictable sequence of responses, the descriptions of which appear remarkably similar around the world. Such activities are now widely acknowledged to have adverse impacts upon water quality and in-stream ecology. Sediment delivery, therefore, encapsulates the dominant process by which water resources are impacted and the process that can be best managed to limit off-site impacts. This paper is a review of current processes, and perceptions, of sediment delivery in managed forests. We outline the major components of sediment and runoff delivery as they relate specifically to timber harvesting activities. Whilst much existing research has focused upon soil loss as the major component of timber harvesting impacts, this review highlights both the need for, and benefits from, a conceptual advance in our thinking of sediment delivery. We advocate that by managing runoff delivery pathways and the resultant pattern of hydrological connectivity, we can limit the potential adverse effects of forest harvesting on in-stream water quality. Specific attention is given here to the interaction of the forest road and track network with both sediment and runoff delivery. The result is a comprehensive account of how best to manage timber harvesting for both on-site sustainability and off-site water resource protection.Key words: timber harvesting, sediment delivery, road network, connectivity, best management practices (BMPs).

This study examines runoff and sediment generation rates within the road prism on unsealed road segments in the Cuttagee Creek catchment near Bermagui in New South Wales, Australia. A large (600 m(2)) rainfall simulator was used to measure runoff and sediment yields from each of the potential sediment and runoff sources and pathways. These included the road surface, table-drain, upslope contributing area and cutslope face, and the entire road segment as measured at the drain outlet. Experiments were conducted on two major types of road (ridge-top and cut-and-fill) of varying traffic usage and maintenance standard for two 30-minute simulations of increasing rainfall intensity. From the range of possible sources within the road prism, the road surface produced the dominant source of excess runoff and sediment at each site with limited contributions from the table-drain, cutslope face or contributing hillslope. Sediment generation varied significantly with road usage and traffic intensity. Road usage was strongly related to the amount of loose available sediment as measured prior to the experiments. Table-drains acted primarily as sediment traps during the low rainfall event but changes in sediment concentration within the drains were observed as runoff volumes increased during the higher rainfall event of 110 mill h(-1), releasing sediment previously stored in litter and organic dams. The experiments demonstrate the potential roles of various features of the road prism in the generation and movement of sediment and water. Copyright (C) 2006 John Wiley & Sons, Ltd.

Limiting connectivity between road runoff sources and stream networks is crucial for preservation of water quality in forested environments. Where flow is non-eroding, the length of hillslope available to accommodate volumes of discharged water is the key to restricting connectivity. Hairsine et al. (2002, Hydrological Processes 16: 2311-2327) proposed a probabilistic model of diffuse overland flow that predicted the hillslope lengths required to infiltrate road discharge, based on the concept of volume to breakthrough (Vbt). This paper extends this analysis to a different forest environment with the aim of testing the portability of the Hairsine et al. (2002) model. The volume of flow required to travel overland to a distance of 5 and 10 m (Vbt5 and Vbt10) from drainage outlets was measured in deep, highly conductive mountain soils in the Upper Tyers catchment, Victoria, Australia. Rainfall, hydraulic conductivity and soil depths contrasted markedly with those in the Hairsine et al. (2002) study, and represent an extreme in Australian forests. Statistical analyses revealed the population of Vbt5 to be indistinguishable from that observed by Hairsine et al. (2002), indicating the model is valid for a range of forest soils. There was no significant correlation of sediment plume length with site characteristics such as slope, width of flow, or existence of incised pathways. It is suggested there are universal properties of pathways draining tracks and roads, with bioturbation acting to restore available pore paces filled by antecedent plumes. Drain discharge design criteria may be developed for local conditions using, the Hairsine et al. (2002) model, providing a robust tool for protection of water quality in the siting of new forest roads, Sand maintenance of exiting roads and tracks. Copyright (c) 2006 John Wiley & Sons, Ltd.

Hydrological connectivity is a term often used to describe the internal linkages between runoff and sediment generation in upper parts of catchments and the receiving waters. In this paper, we identify two types of connectivity: Direct connectivity via new channels or gullies, and diffuse connectivity as surface runoff reaches the stream network via overland flow pathways. Using a forest road network as an example of a landscape element with a high runoff source strength, we demonstrate the spatial distribution of these two types of linkages in a 57 km 2 catchment in southeastern Australia. Field surveys and empirical modelling indicate that direct connectivity occurs primarily due to gully development at road culverts, where the average sediment transport distance is 89 m below the road outlet. The majority of road outlets were characterised by dispersive flow pathways where the maximum potential sediment transport distance is measured as the available hillslope length below the road outlet. This length has a mean value of 120 m for this catchment. Reductions in sediment concentration in runoff plumes from both pathways are modelled using an exponential decay function and data derived from large rainfall simulator experiments in the catchment. The concept of the volume to breakthrough is used to model the potential delivery of runoff from dispersive pathways. Of the surveyed road drains (n=218), only 11 are predicted to deliver runoff to a stream and the greatest contributor of runoff occurs at a stream crossing where a road segment discharges directly into the stream. The methodology described here can be used to assess the spatial distribution and likely impact of dispersive and gullied pathways on in-stream water quality. © 2005 Elsevier B.V. All rights reserved.

This paper reports results from field experiments and hydrological modelling on the dynamics of runoff generation in highly convergent parts of the landscape in a logged and burnt eucalypt forest in south-eastern Victoria, Australia. Large-scale rainfall simulation experiments were conducted to explore runoff generating mechanisms from harvested areas, and to assess the effectiveness of standard water quality protective measures, here a disturbed filter strip, in preventing accession of sediment to near-stream areas. We then examined the likely effects of varying antecedent moisture conditions on surface and subsurface runoff generating mechanisms. Very small volumes of surface runoff were generated only at very high rainfall intensity rates that exceeded a 100 year recurrence interval event during the simulated experiments. There was little or no identifiable impact of either compaction from logging operations or fire-induced hydrophobicity on surface infiltration or generation of surface runoff. Measured soil hydraulic properties and soil depths explained the paucity of surface runoff, and the dominance of subsurface storm flow as the prime runoff generating mechanism. Deep lateral subsurface flow was observed from the cut-face of a fire access track and into a streamhead downslope of the experimental plots. Water balance modelling using Topog-Dynamic indicated the conditions under which saturated overland flow in this environment could be generated are rare, but that care should be taken in siting of roads and tracks in lower parts of convergent landscapes. Copyright (C) 2004 John Wiley Sons, Ltd.

In this paper, we use the combined results from two contrasting methods to investigate the effectiveness of Best Management Practices (BMPs) on sediment erosion and redistribution in managed forestry environments in southeastern New South Wales (NSW), Australia. The first method employed a rainfall simulator at the skid track (snig track)—water bar (cross-bank)—general harvest area (GHA) spatial scale. This showed the majority of production and redistribution occurring from the skid tracks and coinciding with the period after initial disturbance. However, 5 years after harvesting, little redistribution between these elements was occurring. Furthermore, water bar structures were effective in trapping coarse sediment generated from skid tracks but markedly less so for fine

Most land-based forestry systems use extensive networks of unsealed tracks to access the timber resource. These tracks are normally drained by constructing cross-banks, or water bars, across the tracks immediately following logging. Cross-banks serve three functions in controlling sediment movement within forestry compartments: 1. they define the specific catchment area of the snig track (also known as skid trails) so that the overland flow does not develop sufficient energy to cause gullies, and sheet and rill erosion is reduced; 2. they induce some sediment deposition as flow velocity reduces at the cross-bank; 3. they redirect overland flow into the adjacent general harvesting area (GHA) so that further sediment deposition may take place. This paper describes a simple model that predicts the third of these functions in which the rate of runoff from the track is combined with spatial attributes of the track and stream network. Predictions of the extent of the overland flow plumes and the volume of water delivered to streams is probabilistically presented for a range of rainfall-event scenarios with rainfall intensity, time since logging and compartment layout as model inputs. Generic equations guiding the trade-off between intercross-bank length and flow path distance from cross-bank outlet to the stream network needed for infiltration of track runoff are derived. Copyright © 2002 John Wiley & Sons, Ltd. Copyright © 2002 John Wiley & Sons, Ltd.

The effects of timber harvesting activities are often characterised by significant changes in surface soil properties, runoff production and erosion rates. Here we quantify trends in surface soil recovery following timber harvesting activities, specifically snig track (skid trail) construction and logging, based on changes in soil surface compaction, infiltration, and erosion with time since initial disturbance. Sites range in age from 0.5 to 5 years. Surface erosion on the snig tracks and general harvesting areas (GHA), and sediment storage and delivery between the two areas, are measured using a large rainfall simulator. Surface soil compaction shows no significant recovery over this time period, illustrating the persistent effects of machinery compaction on surface soil structure. In contrast, surface infiltration rates on the snig tracks increase by a factor of two and snig track erosion rates decline by almost one-order of magnitude over the 5-year period. These findings have important implications for assessing the cumulative impact of timber harvesting activities.