Sediment concentration changes in runoff pathways from a forest road network and the resultant spatial pattern of catchment connectivity

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 km² 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.     

Improving at-site flood frequency analysis with additional spatial information: a probabilistic regional envelope curve approach

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.     

Evidence of Self‐Organized Criticality in riverbank mass failures: a matter of perspective?

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. Copyright © 2014 John Wiley & Sons, Ltd.     

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 © 2016 John Wiley & Sons, Ltd.     

A genetic classification of floodplains

Floodplains are formed by a complex interaction of fluvial processes but their character and evolution is essentially the product of stream power and sediment character. The relation between a stream's ability to entrain and transport sediment and the erosional resistance of floodplain alluvium that forms the channel boundary provides the basis for a genetic classification of floodplains. Three classes are recognised: (1) high-energy non-cohesive; (2) medium-energy non-cohesive; and (3) low-energy cohesive floodplains. Thirteen derivative orders and suborders, ranging from confined, coarse-grained, non-cohesive floodplains in high-energy environments to unconfined fine-grained cohesive floodplains in low-energy environments, are defined on the basis of nine factors (mostly floodplain forming processes). These factors result in distinctive geomorphological features (such as scroll bars or extensive backswamps) that distinguish each floodplain type in terms of genesis and resulting morphology. Finally, it is proposed that, because floodplains are derivatives of the parent stream system, substantial environmental change will result in the predictable transformation of one floodplain type to another over time.     

Suspected cool R Coronae Borealis stars in the Magellanic Clouds

Six stars out of a sample of ∼2300 carbon stars in the Magellanic Clouds have been identified as having strong C₂ bands but CN bands that are very weak or absent. It is argued that five of these are likely to be R Coronae Borealis (RCB) stars on the basis of their spectral characteristics and peculiar colours. Most are variables and the Large Magellanic Cloud (LMC) members have extreme radial velocities that are more like the planetary nebula population than the carbon stars. This sample consists of four LMC members (only one of them previously recognized as an RCB star), one Small Magellanic Cloud (SMC) member (the first RCB star reported in the SMC) and one foreground Galactic star.     

Quantifying different riverbank erosion processes during an extreme flood event

Riverbank erosion is a major contributor to catchment sediment budgets. At large spatial scales data is often restricted to planform channel change, with little information on process distributions and their sediment contribution. This study demonstrates how multi‐temporal LiDAR and high resolution aerial imagery can be used to determine processes and volumes of riverbank erosion at a catchment scale. Remotely sensed data captured before and after an extreme flood event, enabled a digital elevation model of difference (DoD) to be constructed for the channel and floodplain. This meant that: the spatial area that could be assessed was extensive; three‐dimensional forms of bank failures could be mapped at a resolution that enabled process inference; and the volume and rates of different bank erosion processes over time could be assessed. A classification of riverbank mass failures, integrating form and process, identified a total of 437 mass failure polygons throughout the study area. These were interpreted as wet flow mass failures based on the presence of a well defined scarp wall and the absence of failed blocks on the failure floor. The failures appeared to be the result of: bank exfiltration, antecedent moisture conditions preceding the event, and the historic development of the channel. Using one‐dimensional hydraulic modelling to delineate geomorphic features within the main boundary of the macrochannel, an estimated 1 466 322 m² of erosion was interpreted as fluvial entrainment, occurring across catchment areas from 30 to 1668 km². Only 8% of the whole riverbank planform area was occupied by mass failures, whilst fluvial entrainment covered 33%. A third of the volume of material eroded came from mass failures, even though they occupied 19% of the eroded bank area. The availability of repeat LiDAR surveys, combined with high‐resolution aerial photography, was very effective in erosion process determination and quantification at a large spatial scale. Copyright © 2013 John Wiley & Sons, Ltd.     

Sediment source changes over the last 250 years in a dry-tropical catchment, central Queensland, Australia

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 ²¹⁰Pb_ex) 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.