Colin R. Thorne

Recent morphological evolution of the Lower Mississippi River

Abstract This study documents slope and stream power changes in the Lower Mississippi River during the pre-cutoff (1880s–1930s), and post-cutoff (1943–1992) periods. The study reach extends from New Madrid, MO, to Natchez, MS, a distance of about 900 km. Analyses for six major reaches and 13 sub-reaches for the pre- and post-cutoff periods indicate that the river presently has a much larger slope and stream power than prior to the cutoffs. The largest increases have occurred between Fulton, TN, and Lake Providence, LA, where slope and stream power increases range from about 27% to 36% and 20% to 38%, respectively. Increases in slope and stream power in reaches upstream and downstream have also occurred, but to a lesser degree. Previous investigations have shown that no coarsening of the bed material has occurred since 1932, and that the bed material may actually be somewhat finer overall. As the Lower Mississippi River is not a sediment-starved system, an increase in stream power with no change in D50 would be expected to be offset by an increase in the bed material load as the river adjusts towards equilibrium. Previous investigators have inferred a reduction in the sediment loads on the Mississippi River this century based on analyses of total measured suspended loads. However, these results should be viewed as primarily representing the changes in wash load and should not be taken to imply that bed material loads have also decreased. Therefore, the bed material loads in the study reach should be greater than in the pre-cutoff period. Excess stream power in the sub-reaches directly affected by cutoffs resulted in scour that increased downstream bed material load. These elevated sediment loads play a key role in driving morphological adjustments towards equilibrium in the post-cutoff channel. The stability status of the channel in the study reach currently ranges from dynamic equilibrium in the farthest upstream reaches through severe degradation to dynamic equilibrium in the middle reaches, and aggradation in the lowest reaches. These evolutionary trends cannot be explained by consideration of changes in slope and stream power alone. Changes in the incoming bed material load to each reach generated by upstream channel evolution must also be considered.

Quantified scenarios analysis of drivers and impacts of changing flood risk in England and Wales: 2030–2100

Abstract Flood risk to the economy, society and the environment reflects the cumulative effects of environmental and socio-economic change over decades. Long-term scenarios are therefore required in order to develop robust and sustainable flood risk management policies. Quantified national-scale flood risk analysis and expert appraisal of the mechanisms causing change in flood risk have been used to assess flood risk in England and Wales over the period 2030–2100. The assessment involved the use of socio-economic and climate change scenarios. The analysis predicts increasing flood risk unless current flood management policies, practices and investment levels are changed—up to 20-fold increase in economic risk by the 2080s in the scenario with highest economic growth. The increase is attributable to a combination of climate change (in particular increasing precipitation and relative sea level rise in parts of the UK) and increasing socio-economic vulnerability, particularly in terms of household/industrial contents and infrastructure vulnerability. The policy implications of these findings are discussed.

CHANNEL ADJUSTMENT OF AN UNSTABLE COARSE‐GRAINED STREAM: OPPOSING TRENDS OF BOUNDARY AND CRITICAL SHEAR STRESS, AND THE APPLICABILITY OF EXTREMAL HYPOTHESES

Channel adjustments in the North Fork Toutle River and the Toutle River main stem were initiated by deposition of a 2.5 km3 debris avalanche and associated lahars that accompanied the catastrophic eruption of Mount St. Helens, Washington on 18 May 1980. Channel widening was the dominant process. In combination, adjustments caused average boundary shear stress to decrease non-linearly with time and critical shear stress to increase non-linearly with time. At the discharge that is equalled or exceeded 1 per cent of the time, these trends converged by 1991–1992 so that excess shear stress approached minimum values. Extremal hypotheses, such as minimization of unit stream power and minimization of the rate of energy dissipation (minimum stream power), are shown to be applicable to dynamic adjustments of the Toutle River system. Maximization of the Darcy–Weisbach friction factor did not occur, but increases in relative bed roughness, caused by the concomitant reduction in hydraulic depths and bed-material coarsening, were documented. Predictions of stable channel geometries using the minimum stream power approach were unsuccessful when compared to the 1991–1992 geometries and bed-material characteristics measured in the field. It is concluded that the predictions are not applicable because the study reaches are not truly stable and cannot become so until a new floodplain has been formed by renewed channel incision, retreat of stream-side hummocks, and establishment of riparian vegetation to limit the destabilizing effects of large floods. Further, prediction of energy slope (and consequently stream power) by the sediment transport equations is inaccurate because of the inability of the equations to account for significant contributions of finer grained (sand and gravel) bank materials (relative to the coarsened channel bed) from bank retreat and from upstream terrace erosion.

Multiple thread flow and channel bifurcation in a braided river: Brahmaputra–Jamuna River, Bangladesh

Abstract Considerable progress has been made recently in characterising the patterns displayed by the anabranches of braided rivers. However, the physical processes of sediment scour, transfer and deposition that govern the generation and evolution of anabranch channels remain largely unexplained. Direct measurement of three-dimensional flow fields and morphological evolution of the anabranches in the braided Brahmaputra–Jamuna River, Bangladesh, were undertaken to investigate the interactions between fluvial processes and anabranch morphology. These data were used to elucidate the circumstances leading to the bifurcation of a single channel, which is a topic of fundamental importance to understanding the physical processes responsible for braiding. Results indicate that division of the velocity field into multiple threads within a single channel precedes a division in the cross-sectional morphology of the channel and appears to be a necessary prerequisite for development of a bifurcation. An empirical relationship was established to discriminate between channels with single and multi-thread velocity fields, based on the depth-to-width ratio and specific energy of the flow at a representative channel cross-section. This function requires further validation, but could be used to predict the conditions under which a single channel is likely to bifurcate to produce two anabranches.

Influence of large woody debris on morphological evolution of incised, sand-bed channels

This paper documents the influence of Large Woody Debris (LWD) on the morphological evolution of unstable, degrading, sand-bed rivers in the Yazoo Basin, North Mississippi, USA. The study was performed as part of the Demonstration Erosion Control (DEC) project. Twenty-three river reaches were studied, with the aim of determining whether the presence of LWD was beneficial or detrimental to the recovery of stability in degrading, sand-bed river systems and to provide the geomorphic understanding necessary to underpin enhanced LWD management strategies. The results demonstrate that locations of LWD inputs, volumes of LWD stored in different reaches and number of jams per unit channel length are causally related to the morphological processes occurring during different stages of adjustment in these unstable, incised fluvial systems and may be explained using a Channel Evolution Model (CEM). The net impact of LWD jams on reach-scale sediment budgets was found, in general, to be positive: that is, jams trap more sediment than they mobilise. This suggests that LWD probably accelerates rather than retards recovery of a stable longitudinal profile and channel configuration following incision. Field typing of LWD jams, based on their impacts on the flow pattern, reveals that jam type is a function of the size of large, key elements in the jam in relation to the channel width. A Debris Jam Classification Scheme is proposed on this basis, with the spatial relationship between jam type and drainage basin area expressed using a dimensionless function of the ratio between channel width and average riparian tree height. The scheme features four jam types, Underflow, Dam, Deflector and Flow Parallel/Bar Head, each of which has a different morphological impact on local channel geometry. These jam types may be used to classify LWD jams as an aid in determining appropriate management strategies, according to their location within the drainage basin.

Geomorphological River Channel Reconnaissance for River Analysis, Engineering and Management

Geomorphological studies involving river reconnaissance, analysis (both qualitative and quantitative) and assessment can supply information on the form and physical processes operating in a fluvial system. This information is of value to river engineers and managers wishing to work with, rather than against, nature when undertaking engineering and maintenance operations or devising management policies. Qualitative analysis rests on the interpretation of process from form using careful observation across the whole system, together with the application of well-established geomorphological concepts. Quantitative analyses of specific study reaches identified as critical in the broader qualitative study are based on regime theory and hydraulic geometry, whereby observed channel dimensions and features are compared with the same hydrological and sedimentary controls. The utility of geomorphological river reconnaissance is illustrated using a case study from the River Blackwater, southeast England. A geomorphological assessment is used to suggest alternative modifications to the operational maintenance regime that would allow the channel to recover more of its natural form and, thereby, promote environmental restoration of aquatic and riparian habitats.

Computer program for stability analysis of steep, cohesive riverbanks

The ability to predict the stability of eroding riverbanks is a prerequisite for modelling alluvial channel width adjustment and a requirement for predicting bank erosion rates and sediment yield associated with bank erosion. Mass-wasting of bank materials under gravity occurs through a variety of specific mechanisms, with a separate analysis required for each type of failure. This paper presents a computer program for the analysis of the stability of steep, cohesive riverbanks with respect to planar-type failures. Planar-type failures are common along stream channels destabilized by severe bed degradation. Existing stability analyses for planar-type failures have a number of limitations that affect their physical basis and predictive ability. The computer program presented here is based on an analysis developed by Darby and Thorne. The software takes account of the geotechnical characteristics of the bank materials, the shape of the bank profile, and the relative elevations of the groundwater and surface water to estimate stability with respect to mass failure along a planar-type failure surface. Results can be displayed either in terms of a factor of safety (ratio of resisting to driving forces), or probability of failure. The computer analysis is able to determine the relative amounts of bed degradation and bank-toe erosion required to destabilize an initially stable bank. Data for the analysis are supplied in the form of either HEC-2 hydrographic survey data files or user-supplied bank profile data, in conjunction with user-supplied geotechnical parameter values. Some examples, using data from the Upper Missouri River in Montana, are used to demonstrate potential applications of the software. Copyright

Geomorphic analysis of large alluvial rivers

Geomorphic analysis of a large river presents particular challenges and requires a systematic and organised approach because of the spatial scale and system complexity involved. This paper presents a framework and blueprint for geomorphic studies of large rivers developed in the course of basic, strategic and project-related investigations of a number of large rivers. The framework demonstrates the need to begin geomorphic studies early in the pre-feasibility stage of a river project and carry them through to implementation and post-project appraisal. The blueprint breaks down the multi-layered and multi-scaled complexity of a comprehensive geomorphic study into a number of well-defined and semi-independent topics, each of which can be performed separately to produce a clearly defined, deliverable product. Geomorphology increasingly plays a central role in multi-disciplinary river research and the importance of effective quality assurance makes it essential that audit trails and quality checks are hard-wired into study design. The structured approach presented here provides output products and production trails that can be rigorously audited, ensuring that the results of a geomorphic study can stand up to the closest scrutiny.

A Geomorphological Justification of River Channel Reconnaissance Surveys

Geomorphologists involved in river management in many countries have shown an increasing interest in developing schemes for evaluating river channel geomorphology based on a single reconnaissance survey which uses skills of observation and interpretation. However, many questions surrounding this approach have yet to be resolved. This paper presents arguments to demonstrate first that reconnaissance surveys are the only viable source of relevant geomorphological data for many projects, secondly, that reconnaissance complements other components of integrated and multidisciplinary approaches to river management and, thirdly, that, although experience suggests that reconnaissance survey data can never be a substitute for detailed, repeat data collection, this limitation is more than offset by the external visibility and demystification afforded to the discipline of geomorphology by the surveys. Finally, comments are made on the potential for developing standardized approaches.

A general framework for using the rate law to simulate morphological response to disturbance in the fluvial system

A general framework for modelling morphological responses to perturbation is proposed, based on the underpinning principle that the rates of morphological response in alluvial channels are initially high and then decrease through time as the system relaxes following disturbance. The framework includes three morphological response models, each developed from the fundamental rate law, which has the form of an exponential decay function. These models consider the possibility that characteristic behaviours of the fluvial system, such as delayed response and/or cumulative effects, may affect morphological responses, making them capable of representing relaxation paths and times for a range of morphological response variables, whatever their initial states. To test their utility, the models in the framework were applied to simulate the sequence of geomorphological responses to disruption observed in selected rivers with well-documented histories of morphological perturbation, adjustment and recovery. The results demonstrate that the models in the general framework can successfully simulate temporal and spatial patterns of morphological response in the fluvial system under a range of different circumstances, while also indicating how their reliability could be further improved.