Ian Rutherfurd

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

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

Where along a river's length will vegetation most effectively stabilise stream banks?

Riparian vegetation has different impacts on stream processes depending upon its position in a catchment. Native riparian vegetation is increasingly becoming the favoured stream management tool but managers need to locate revegetation schemes where they will most effectively achieve ecological, geomorphological, or other, project goals. Using the Latrobe River in SE Australia as an example, this paper illustrates a structured decision-making approach for assessing the role of vegetation in stream bank erosion at different points throughout a catchment. Three bank-erosion process groups are identified: subaerial preparation, fluvial entrainment, and mass failure. Although these processes act on banks throughout the catchment there exists spatial zoning in the dominance of each process group over the others. Bank erosion in upper reaches is dominated by subaerial preparation, in mid-basin reaches by fluvial entrainment, and in the lower reaches by mass failure. We find that in upper reaches, windthrown trees are responsible for most bank sediment transfer to the flow. Where direct fluvial entrainment of bank material is the dominant erosion process, flow resistance due to vegetation becomes crucial. In reaches where bank slumping is the dominant erosion process, increased bank shear strength due to root reinforcement is the major role of vegetation in stabilising banks. Other effects, such as tree surcharge, and altered bank hydrology appear to exert only minor influences on the slumping process. Considering the above variables we are able to define a critical zone in which revegetation will be most effective in reducing bank erosion. On the Latrobe River, this zone occurs in that portion of the river where it first leaves the mountain front and meanders across a broad floodplain. This reach occupies the second quarter of the rivers length. This information, combined with other scale analyses (e.g. ecological, hydrological), will assist river managers to plan physically based riparian revegetation strategies.

The effect of riparian tree roots on the mass-stability of riverbanks

Plants interact with and modify the processes of riverbank erosion by altering bank hydrology, flow hydraulics and bank geotechnical properties. The physically based slope stability model GWEDGEM was used to assess how changes in bank geotechnical properties due to the roots of native Australian riparian trees affected the stability of bank sections surveyed along the Latrobe River. Modelling bank stability against mass failure with and without the reinforcing effects of River Red Gum (Eucalyptus camaldulensis) or Swamp Paperbark (Melaleuca ericifolia) indicates that root reinforcement of the bank substrate provides high levels of bank protection. The model indicates that the addition of root reinforcement to an otherwise unstable bank section can raise the factor of safety (Fs) from Fs = 1·0 up to about Fs = 1·6. The addition of roots to riverbanks improves stability even under worst-case hydrological conditions and is apparent over a range of bank geometries, varying with tree position. Trees growing close to potential failure plane locations, either low on the bank or on the floodplain, realize the greatest bank reinforcement. Copyright

Historical ENSO teleconnections in the eastern hemisphere

Examination of instrumental data collected over the last one hundred years or so shows that rainfall fluctuations in various parts of the eastern hemisphere are associated with the El Niño-Southern Oscillation (ENSO) phenomenon. Using proxy rainfall data-sets from Indonesia, Africa, North China; and a chronology of droughts from India, we investigate the occurrence of ENSO-related floods and droughts over the last five hundred years. The aim of this work is to examine the stability of the pattern of ENSO teleconnections over this longer period, noting any changes in ENSO behaviour which may be relevant in estimating its future behaviour, such as its response to climate change due to the enhanced greenhouse effect.Comparisons of the various data sets with each other and with El Niño chronology from South America, showed statistically significant evidence of teleconnections characteristic of ENSO back to around 1750. Prior to that time, relationships characteristic of ENSO were weak or absent. The disappearance of the ENSO signal in the early period is considered to be most likely due to the poorer quality of the data at that time. From the 18th Century onwards, chronologies of ENSO and anti-ENSO events are given and compared with similar chronologies in the literature.

Bank erosion of an incised upland channel by subaerial processes: Tasmania, Australia.

The headwaters of many rivers are characterized by gullies and incised streams that generate significant volumes of sediment and degrade downstream water quality. These systems are characterized by harsh climates, ephemeral flows that do not reach bank top, and bare cohesive banks of clay and weathered bedrock. We investigated the rates and processes of bank erosion in an incised canal that has such characteristics. Detailed measurements of bank position were made over two years with a purpose-built groundprofiler and photo-electronic erosion pins (PEEPs). Stage height and turbidity were also monitored. The bare banks eroded at 13 ± 2 mm a−1. Erosion is controlled by subaerial processes that loosen bank material. Observations show that needle-ice growth is important in winter and desiccation of clays predominates in summer. Flows are unable to erode firm cohesive clays from the banks, and erosion is generally limited by the availability of loosened material. This produces strong hysteresis in turbidity during events. Peak turbidity is related to the number of days with low flow between events, and not peak stage. Rehabilitation with a moderate cover of grass is able to prevent bank erosion by limiting the subaerial erosion processes. Projections of current erosion suggest that without vegetation cover the banks are unlikely to stabilize for many years. Copyright

An analysis of the influence of riparian vegetation on the propagation of flood waves

Abstract Over the last 200 years the condition of Australias streams has changed dramatically. The removal of massive volumes of woody debris and the impoverishment of native riparian vegetation has resulted in channels where flow is minimally obstructed. Such hydraulically efficient channels are able to carry larger discharges before flooding commences. For the last two decades the major stream rehabilitation activity in Australia has been to revegetate the riparian zone and to reinstate large woody debris (LWD). However, to date little has been done to understand ramifications of riparian revegetation on flood behaviour. This paper describes a modelling study that seeks to quantify the impact of riparian vegetation on both the shape of a flood hydrograph and the speed at which it propagates down a river reach. A one-dimensional flow-routing model (FLDWAV) is used to solve the fully dynamic formulation of the Saint-Venant equations. The hydraulic properties of riparian vegetation are computed using a simple model of vegetation resistance, where Mannings ‘ n ’ is a function of flow depth and the geometry of the cross-section. This study demonstrates that channel roughness, and hence riparian condition, is a significant determinant of wave celerity, hydrograph dispersion and skewness. The impact of roughness is moderated by the magnitude of the hydrograph (peak discharge), showing that smaller floods are more sensitive to vegetation condition than larger floods.

Chapter 4 – The Hydrology of the Mekong River

Publisher Summary The Mekong rises on the Tibetan Plateau at an altitude of about 5200 m and flows 4800 km southeast to the South China Sea, through six developing countries: China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. The Mekong catchment has an unusual shape. Most catchments tend to have a dendritic form, with the width of the catchment gradually decreasing downstream, producing a characteristic teardrop shape. The Mekong catchment, by contrast, progressively widens down valley so that its widest point is immediately upstream of its delta. The hydrology of the Mekong River is characterized by a huge mean annual discharge; concentrated in an extremely regular wet-season peak. The size of the wet-season peak and its highly predictable timing are the defining characteristics of large tropical monsoonal rivers. In the upper part of the Lower Mekong system, at Vientiane, the flow originating from China and Burma, the so-called Yunnan Component, not only provides most of the dry-season flows, but in addition, most of the floodwater during the majority of years. Even though floods can cause major devastation along the Mekong River, the peak discharge of the largest floods tends to be only about double the size of the bankfull discharge.

Progress, problems and prospects in Australian river repair

Effective river restoration requires an integrative approach among researchers, managers and stakeholders, grounded in sound science. Using Australia as a case study, we examined contemporary responses to the following three global challenges for river management: first, to base management practice on ‘best available science’ (BAS); second, to integrate diverse, discipline-bound knowledge within cross-disciplinary and trans-disciplinary approaches; and third, to achieve adaptive management based on monitoring and evaluation. Analysis of 562 papers from the six Australian national stream-management conferences held since 1996 provided insight into the rapidly growing area of management, and the degree to which these three challenges are being met. The review showed that discipline-bound abiotic or biotic science was the focus of 46% of papers. Cross-disciplinary science, defined as the integration of biophysical sciences, was presented in 36% of papers, and trans-disciplinary science, defined as the merging of biophysical science with social and economic perspectives, in 17%. Monitoring and evaluation results were presented in only 12% of papers, whereas applications of adaptive management were reported in a mere 2%. Although river management has been transformed in recent decades, much remains to be done to create a holistic foundation for river restoration that links biophysical science to social science and economics.

Does the weight of riparian trees destabilize riverbanks

In contrast to the generally accepted stabilizing effects of riparian vegetation, the surcharge of trees on riverbanks has been widely implicated as a source of bank instability. Fieldwork conducted along the Latrobe River in Victoria, Australia shows that the bank-destabilizing effects of surcharge, due to silver wattle (Acacia dealbata), are minimal. Field observations indicate that it is unlikely that the weight of silver wattles growing on an otherwise stable bank section will directly cause mass failure. Observations of deep-seated failures and silver wattle stands on the Latrobe River indicate that where average-sized slump-blocks support an average number of average-sized silver wattles, the trees represent only 4.1% of the total saturated slump mass. Infinite slope stability analysis indicates a threshold of around 48° where banks become prone to shallow-planar slide failures as they steepen. Where bank sections are inherently unstable and prone to shallow-planar slide failure, the additional weight of the trees may contribute to overall instability. However, manipulation of other stability parameters within reasonable constraints negates the effect of surcharge so it is not possible to demonstrate conclusively a destabilizing influence of silver wattles. Copyright

Evaluating stream rehabilitation projects: reasons not to, and approaches if you have to

Abstract Over the last thirty years, the work of river engineers has shifted from the protection of physical and economic assets, to include the protection and enhancement of environmental assets. River engineers now work in multi-disciplinary teams that include biologists and social scientists. Environmental flow, and river rehabilitation projects are typical of this work. Unlike the well established engineering designs and practices of the past, there is now less confidence in the effectiveness of interventions, particularly where the outcomes involve animals, ecological functions, or imprecise ideas of ‘stream health’. Quite rightly, there is pressure to ‘evaluate’ the effectiveness of every public project. However, there is confusion about the type of evaluation that is appropriate for rehabilitation projects. In this paper we propose a hierarchy of evaluation types. We begin with measures of outputs, responsiveness, and appropriateness. We also discuss the dangers of moving to the next level of evaluation that is often proposed: measuring biological or water-quality variables in order to identify the outcomes of interventions. We demonstrate that it is rare that this type of evaluation is feasible, given the variability of fluvial systems. Apart from the cost (which usually exceed the physical cost of the project), we argue that such evaluation attempts can do more harm than good by appearing to ‘conclusively’ demonstrate that there has been no effect, when the problem lies with the experimental design. We also describe the levels of ‘confidence’ that each type of evaluation provides, as well as the risks associated with each type. The hierarchy of methods described allows managers to clearly specify the ‘level’ of evaluation that they are proposing to apply to any given project, and will help managers to avoid committing to infeasible evaluations.