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Philip Ashworth

Bio: Philip Ashworth is an academic researcher from University of Brighton. The author has contributed to research in topics: Fluvial & Bedform. The author has an hindex of 43, co-authored 82 publications receiving 5222 citations. Previous affiliations of Philip Ashworth include University of London & University of Stirling.
Topics: Fluvial, Bedform, Sediment transport, Bed load, Facies


Papers
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Journal ArticleDOI
TL;DR: In this paper, the entrainment of mixed-size gravel bed material was studied in nine reaches of three high-power streams in Scotland and Norway, and measurements of at-a-point shear stress and bed load transport (by hand-held sampler) were made.
Abstract: Entrainment of mixed size gravel bed material was studied in nine reaches of three high-power streams in Scotland and Norway. Paired measurements of at-a-point shear stress (estimated from velocity profiles) and bed load transport (by hand-held sampler) were made. They extended to very high values (400 N m -2, 3.5 kg m-x s-x). Analyses of maximum bed load diameter, mean bed load diameter, transport rates of individual size fractions, and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size, despite strong relative size effects. Precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates. Size-selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant.

349 citations

Journal ArticleDOI
TL;DR: The initiation and evolution of a kilometre-scale, sand braid-bar was monitored during a 28-month survey period from 1993 to 1996 in one of the world's largest braided rivers, the Jamuna River, Bangladesh as mentioned in this paper.
Abstract: The initiation and evolution of a kilometre-scale, sand braid-bar was monitored during a 28-month survey period from 1993 to 1996 in one of the world's largest braided rivers, the Jamuna River, Bangladesh. Repeated bathymetric surveys through two monsoon flood seasons, combined with bar-top surveys during exposure of the bar at low flow, provide the most detailed chronology of braid-bar growth yet compiled for a large sand-bed river. During rising and peak flow of the 1994 monsoon flood, a 1.5-km-long, 0.5-km-wide, 12-m-high, symmetrical mid-channel bar was deposited in the centre of a major channel downstream of a zone of flow convergence and significant bank erosion. Initial deposition and growth of the bar core were probably caused by amalgamation of dunes that are present in the Jamuna channels at all flow stages. Bar-top aggradation continued through downstream migration of an `accretionary dune front', a 3-m-high, angle-of-repose slipface that was composed of amalgamated, 0.5- to 1-m-high dunes. At waning and low flow, the mid-channel bar widened by up to 1 km through the lateral accretion of dunes onto the margins of the initial bar core. A low-velocity zone in the sheltered wake region of the bar-tail led to the accumulation of substantial volumes of silts and clays. During the rising and peak flows of the next monsoon flood, the mid-channel bar extended its bar-tail by up to 1.5 km, as one of the anabranches became dominant, and flow was deflected across the bar-tail. Accretion at the bar-tail generated a lobate, transverse bar-front with a 10-m-high, angle-of-repose avalanche face. Emergence of several smaller bars along this depositional front produced an overall reach morphology that more closely resembled an alternate bar rather than several mid-channel bars. The conversion of a mid-channel bar to an alternate bar is contrary to many previous descriptions of the braiding process.

245 citations

Journal ArticleDOI
TL;DR: In this article, a 3 km long, 1 km wide, 12 m high, mid-channel sand braid bar in the Jamuna River, Bangladesh is described and its depositional characteristics are assessed from a unique combination of ground-penetrating radar surveys, vibracoring, and trenching that are allied to a series of bathymetric surveys taken during growth of the bar over a 29-month period.
Abstract: The three-dimensional subsurface alluvial architecture of a large (approximately 3 km long, 1 km wide, 12 m high), mid-channel sand braid bar in the Jamuna River, Bangladesh is described. Evolution of the bar and its depositional characteristics are assessed from a unique combination of ground-penetrating radar surveys, vibracoring, and trenching that are allied to a series of bathymetric surveys taken during growth of the bar over a 29-month period. This methodology permits identification of the formative processes of different packages of braid-bar sedimentation and provides a facies model for deposition within the entire bar. Mid-channel bar growth occurred in a region of flow expansion and was probably initiated by the stalling and amalgamation of large dunes. These dunes created a bar-core that grew by (i) propagation of a downstream-accreting slipface, (ii) vertical accretion through stacking of dunes on both bar stoss and top, and (iii) lateral accretion on the bar margins during recession of the flood hydrograph. Braid-bar sedimentation is dominated by four radar facies: (1) large-scale, predominantly planar, dipping reflections interpreted as cross-stratification, up to 8 m in height and greater than 100 m in width, that is produced by the cross-channel migration of bar margins, (2) medium-scale, trough-shaped and planar discontinuous reflections interpreted as cross-stratification up to 4 m in height and 300 m wide, that is deposited from large, sinuous-crested sand dunes, (3) discontinuous reflections, up to 2 m high and 30 m wide, interpreted as small-scale trough cross-stratification, that is the product of smaller sinuous-crested dunes, and (4) high-amplitude, undulating reflections interpreted as mud drapes, deposited in regions of slow flow, often in the bar-tail region at low stage. Dune size decreases vertically within the bar, in response to the progressively shallower flows on the emerging bar top. Later evolution of the bar, as one anabranch channel became dominant, created a 1.5 km extension to the bar tail with an 8 m high, angle-of-repose, bar-margin slipface, formed by flow transverse to the long axis of the bar. Seven styles of deposition can be defined that constitute the alluvial architecture: bar-margin slipface, vertical accretion in channel, bar-top vertical accretion, upstream accretion, lateral accretion, downstream accretion, and low-stage mud drapes. A model of braid-bar sedimentation is presented that shares many similarities with previous studies of smaller sand-bed braid bars with the dominance of dune-scale cross-stratification, the presence of large-scale, bar-margin cross-stratification, and the occurrence of lateral, vertical, upstream, and downstream accretion. However, the contribution of the bar-margin facies to the preserved stratigraphy highlighted herein may have been underestimated in previous models of braided rivers in which the braid bars were migrating slowly. This study suggests a scale invariance in several aspects of mid-channel bar sedimentation in sand-bed rivers and proposes a model of braid-bar sedimentation that may be applied widely within studies of braided alluvial architecture.

236 citations

Book
30 Oct 1996
TL;DR: In this article, the authors used Particle Image Velocimetry (PIV) to study the structure of coherent flow structures over smooth and rough boundary layers in Gravel-bed rivers.
Abstract: List of Contributors. Acknowledgements. Coherent Flow Structures in Smooth-wall Turbulent Boundaary Layers: Facts, Mechnaisms and Speculations (C. Smith). Generalized Scaling of Coherent Bursting Structures in the Near-wall Region of Turbulent Flow over Smooth and Rough Boundaries (A. Grass and M. Mansour-Tehrani). Laboratory Observations of Particlr Entrainment into Suspension by Turbulent Bursting (M. Garcia, et al.). Transverse Spacing of Low-speed Streaks in a Channel Flow over a Rough Bed (A. Defina). A Model of an Impinging Jet on a Granular Bed, with Application to Turbulent, Event-driven Bedload Transport (A. Hogg, et al.). Ratio of Lift and Shear Forces Over Rough Surfaces (A. Dittrich, et al.). Scales of Turbulent Coherent Flow Structures in Gravel-bed Rivers (A. Roy, et al.). Markov Analysis of Velocity Fluctuations in Gravel-bed Rivers (R. Ferguson, et al.). The Application of Particle Image Velocimetry to the Study of Coherent Flow Structures over a Stabilizing Sediment Bed (S. Tait, et al.). Flow Sediment Interactions in Separating Flows over Befforms (S. McLean, et al.). The role of Coherent Structures in Developing Beforms During Sediment Transport (A. Gyr and A. M?ller). Geometrical Analysis of the Feedback Between Flow, Bedforms and Sediment Transport (A. M?ller and A. Gyr). Genesis Morphology of Erosional Shapes Associated with Turbulent Flow Over a Forward-facing Step (A. Pollard, et al.). Coherent Flow Structures in Open-channel SLot Flow (D. Keogh and P. Addison). Mean Flow and Turbulence Structure over Fixed Ripples and the Ripple-dube Transition (S. Bennett and J. Best). Turbulent Sand Suspension Events: Fraser River, Canada (R. Kostaschuk and P. Villard). Coherent Flow Structures in Squamish River Estuary, British Columbia, Canada (C. Babakaiff and E. Hickin). Observations of Near-bed Suspended Sediment Turbulence Structures using Multifrequency Acoustic Backscattering (P. Thorne, et al). Interaction Between Cellular Secondary Currents and Lateral Alternate Sorting (T. Tsujimoto and T. Kitamura). Coherent Fluctuations in a Vegetated Zone of Open-channel Flow: Causes of Bedload Lateral Transport and Sorting (T. Tsujimotto). The Structure of River Bed Relief (A. Sidorchuk). Some Speculations on the Relation Between Channel Morphology and Channel-scale Flow Structures (G. Parker). The Origin and Downstream Development of Coherent Flow Structures at Channel Junctions (S. McLelland, et al.). Mean Structure of Transport-effective Flows at an Asymmetrical Confluence when the Main Stream is Dominant (B. Rhoads). Secondary Flow and Channel Changes around a Bar in the Brahmaputra River, Bangladesh (W. Richardson, et al.). Morphology and Stage-dependent Flow Structure in a Gravel-bed River (N. Clifford). A Computer Model of Bank Erosion Based on Secondary Flow Simularion (A. Alabyan). Floodplain Secondary Circulation as a Mechanism for Flow and Shear Stress Redistribution in Straight Compound Channels (P. Wormleaton). Meandering Overbank Flow Structures (B. Willetts and P. Rameshwaran). Flow Structure in a Large-scale Model of a Doubly Meandeering Compound River Channel (C. Naish and R. Sellin). Linking Hydraulics, Bedload Transport and River Bed Adjustment with the Conceptual FAST Model (P. Ergenzinger and C. de Jong). On the Origin and Effects of Large-scale Longitudinal Flow Structures in the Outer Humber Estuary (J. Hardisty, et al.). Incoherent Structure: Turbulence as a Metaphor for Stream Braiding (C. Paola). Index.

218 citations

Journal ArticleDOI
15 May 1997-Nature
TL;DR: In this paper, a study of the Jamuna river, one of the world's largest modern braided rivers, showed that bed scour associated with channel confluences and bends alone can be substantial, as much as five times greater than the mean channel depth.
Abstract: Alluvial scour into shallow marine sediments may be caused by the incision of a river adjusting to a new base level1–4 following a fall in sea level. The identification of such erosion surfaces1–3 has therefore been pivotal in the reconstruction of past sea-level changes from ancient sedimentary sequences1–14. Here we report data from a study of the Jamuna river, Bangladesh, one of the world's largest modern braided rivers15, which illustrate that bed scour associated with channel confluences and bends alone can be substantial—as much as five times greater than the mean channel depth. Indeed, the basal erosion surfaces produced by such deep scours have characteristics similar to those of boundaries in some ancient sedimentary sequences that have been assumed to result from sea-level fall1–14, potentially leading to radically different interpretations of past variation in base level and climate. We suggest that, to discount unambiguously the influence of fluvial scour in ancient sediments, the erosive boundary should be greater than five times the mean channel depth and extend for distances greater than the floodplain width. Ideally, it should be traceable between different basins.

206 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, a review of the development of ideas in the fields of geomorphology/Quaternary geology vs. sedimentary geologies is provided, and key processes that operate to produce alluvial stratigraphic records over time-scales of 103−106 years.
Abstract: Summary Fluvial landforms and deposits provide one of the most readily studied Quaternary continental records, and alluvial strata represent an important component in most ancient continental interior and continental margin successions. Moreover, studies of the long-term dynamics of fluvial systems and their responses to external or ‘allogenic' controls, can play important roles in research concerning both global change and sequence-stratigraphy, as well as in studies of the dynamic interactions between tectonic activity and surface processes. These themes were energized in the final decades of the twentieth century, and may become increasingly important in the first decades of this millennium. This review paper provides a historical perspective on the development of ideas in the fields of geomorphology/Quaternary geology vs. sedimentary geology, and then summarizes key processes that operate to produce alluvial stratigraphic records over time-scales of 103−106 years. Of particular interest are changes in discharge regimes, sediment supply and sediment storage en route from source terrains to sedimentary basins, as well as changes in sea-level and the concept of accommodation. Late Quaternary stratigraphic records from the Loire (France), Mississippi (USA), Colorado (Texas, USA) and Rhine–Meuse (The Netherlands) Rivers are used to illustrate the influences of climate change on continental interior rivers, as well as the influence of interacting climate and sea-level change on continental margin systems. The paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea-level change. At present, empirical field-based research on fluvial response to climate and sea-level change lags behind: (a) the global change community's understanding of the magnitude and frequency of climate and sea-level change; (b) the sequence-stratigraphic community's desire to interpret climate and, especially, sea-level change as forcing mechanisms; and (c) the modelling community's ability to generate numerical and physical models of surface processes and their stratigraphic results. A major challenge for the future is to catch up, which will require the development of more detailed and sophisticated Quaternary stratigraphic, sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings. There is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter (102−103 years) and longer (104−106 years) time-scales than has commonly been the case to date, as well as in larger river systems, from source to sink. Studies of Quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre-Quaternary rock record.

1,125 citations

Journal ArticleDOI
TL;DR: In this article, a broad synthesis of riverine landscape diversity is presented, beginning with an account of the variety of landscape elements contained within river corridors and concluding with the role of hydrological connectivity.
Abstract: 1. This review is presented as a broad synthesis of riverine landscape diversity, beginning with an account of the variety of landscape elements contained within river corridors. Landscape dynamics within river corridors are then examined in the context of landscape evolution, ecological succession and turnover rates of landscape elements. This is followed by an overview of the role of connectivity and ends with a riverine landscape perspective of biodiversity. 2. River corridors in the natural state are characterised by a diverse array of landscape elements, including surface waters (a gradient of lotic and lentic waterbodies), the fluvial stygoscape (alluvial aquifers), riparian systems (alluvial forests, marshes, meadows) and geomorphic features (bars and islands, ridges and swales, levees and terraces, fans and deltas, fringing floodplains, wood debris deposits and channel networks). 3. Fluvial action (erosion, transport, deposition) is the predominant agent of landscape evolution and also constitutes the natural disturbance regime primarily responsible for sustaining a high level of landscape diversity in river corridors. Although individual landscape features may exhibit high turnover, largely as a function of the interactions between fluvial dynamics and successional phenomena, their relative abundance in the river corridor tends to remain constant over ecological time. 4. Hydrological connectivity, the exchange of matter, energy and biota via the aqueous medium, plays a major though poorly understood role in sustaining riverine landscape diversity. Rigorous investigations of connectivity in diverse river systems should provide considerable insight into landscape-level functional processes. 5. The species pool in riverine landscapes is derived from terrestrial and aquatic communities inhabiting diverse lotic, lentic, riparian and groundwater habitats arrayed across spatio-temporal gradients. Natural disturbance regimes are responsible for both expanding the resource gradient in riverine landscapes as well as for constraining competitive exclusion. 6. Riverine landscapes provide an ideal setting for investigating how complex interactions between disturbance and productivity structure species diversity patterns.

1,016 citations

Journal ArticleDOI
TL;DR: This test shows that SfM and low-altitude platforms can produce point clouds with point densities comparable with airborne LiDAR, with horizontal and vertical precision in the centimeter range, and with very low capital and labor costs and low expertise levels.
Abstract: The production of topographic datasets is of increasing interest and application throughout the geomorphic sciences, and river science is no exception. Consequently, a wide range of topographic measurement methods have evolved. Despite the range of available methods, the production of high resolution, high quality digital elevation models (DEMs) requires a significant investment in personnel time, hardware and/or software. However, image-based methods such as digital photogrammetry have been decreasing in costs. Developed for the purpose of rapid, inexpensive and easy three-dimensional surveys of buildings or small objects, the ‘structure from motion’ photogrammetric approach (SfM) is an image-based method which could deliver a methodological leap if transferred to geomorphic applications, requires little training and is extremely inexpensive. Using an online SfM program, we created high-resolution digital elevation models of a river environment from ordinary photographs produced from a workflow that takes advantage of free and open source software. This process reconstructs real world scenes from SfM algorithms based on the derived positions of the photographs in three-dimensional space. The basic product of the SfM process is a point cloud of identifiable features present in the input photographs. This point cloud can be georeferenced from a small number of ground control points collected in the field or from measurements of camera positions at the time of image acquisition. The georeferenced point cloud can then be used to create a variety of digital elevation products. We examine the applicability of SfM in the Pedernales River in Texas (USA), where several hundred images taken from a hand-held helikite are used to produce DEMs of the fluvial topographic environment. This test shows that SfM and low-altitude platforms can produce point clouds with point densities comparable with airborne LiDAR, with horizontal and vertical precision in the centimeter range, and with very low capital and labor costs and low expertise levels. Copyright © 2012 John Wiley & Sons, Ltd.

980 citations

Journal ArticleDOI
TL;DR: A ground-penetrating radar (GPR) is a noninvasive geophysical technique that detects electrical discontinuities in the shallow subsurface as mentioned in this paper, which can be used to detect electrical faults.

963 citations

Journal ArticleDOI
TL;DR: In this paper, 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.

952 citations