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James M. Coleman

Bio: James M. Coleman is an academic researcher from Louisiana State University. The author has contributed to research in topics: River delta & Continental shelf. The author has an hindex of 42, co-authored 114 publications receiving 6891 citations.


Papers
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Journal ArticleDOI
TL;DR: The Ganges and Brahmaputra rivers combined have formed one of the largest deltas in the world, comprising some 23,000 sq. miles as discussed by the authors, and the large discharge and heavy sediment load cause the rivers to be extremely unstable, and the channels are constantly migrating laterally.

1,030 citations

01 Jan 1975
TL;DR: In this article, the authors compared 400 similar process and form parameters to investigate the differences among the deltas and to generate distinctive deltaic frameworks, and found that no one delta model could be formulated to use as a basis for predicting vertical sequences in all of the major deltaic vertical sequences, and that sand body distribution, geometry, and characteristics are primarily a function of wave-energy distributions, river-mouth dynamics, subsidence, and alongshore currents.
Abstract: Depositional facies in deltaic sediments result from interacting dynamic processes (climate, hydrologic characteristics, wave energy, tidal action, etc.) which modify and disperse transported riverine sediment. These processes, which vary in both intensity and frequency, control the eventual sedimentary framework of a delta. Approximately 400 similar process and form parameters were compared in 50 significant deltas to investigate the differences among the deltas and to generate distinctive deltaic frameworks. In addition, field studies were conducted in 16 of the deltas to field check the data and to gather additional subsurface and spatial data. The results of this study indicated that no one delta model could be formulated to use as a basis for predicting vertical sequences in all of the deltas. The study also indicated that sand-body distribution, geometry, and characteristics are primarily a function of wave-energy distributions, river-mouth dynamics, subsidence, and alongshore currents. The following deltas are used as examples in illustrating the common deltaic vertical sequences: Mississippi, Klang, Ord, Burdekin, Sao Francisco, and Senegal.

391 citations

Journal Article
TL;DR: In the case of the Mississippi delta, the most significant impacts have resulted from a decrease in sediment input to the river from its tributaries and the alteration of the river's natural sediment dispersal processes through the construction of levees.
Abstract: Over the last century, the river-dominated Mississippi delta has received increasing attention from geoscientists, biologists, engineers, and environmental planners because of the importance of the river and its deltaic environments to the economic well-being of the state of Louisiana and the nation. Population growth, subsurface re source extraction, and increased land-water use have placed demands on the delta's natural geologic, biologic, and chemical systems, therefore modifying the time and spatial scales of natural processes within the delta and its lower alluvial valley. As a result, the combined effects of natural and human- induced processes, such as subsidence, eustatic sea level rise, salt water intrusion, and wet land loss, have produced a dynamically changing landscape and socioeconomic framework for this complex delta. Under natural conditions, the fundament al changes that result in land-building and land loss in the Holocene Mississippi River delta plain are rooted in the systematic diversion of water and sediment associated with major shifts in the river 's course-the process of delta switching. Research over the last half century has shown that major relocations of the Mississippi's course have resulted in five Holocene delta complexes and a sixth one in an early stage of development as a product of the latest Atchafalaya River diversion. Collectively, these Holocene deltas have produced a delta plain that covers an area of ~30,000 km 2 and accounts for 41% of the coastal wetlands in the United States. After a river diversion takes place, the resulting delta evolves through a systematic and semipredictable set of stages generally characterized by: (a) rapid progradation with increasing-to-stable discharge, (b) relative stability during initial stages of waning discharge, (c) abandonment by the river in favor of a higher gradient course to the receiving basin , and (d) marine reworking of a sediment-starved delta as it under goes progressive submergence by the combined processes of subsidence. Delta switching has taken place every 1000 to 2000 years during Holocene times, and resulting deltas have an average thickness of approximately 35 m. Within a single delta there are subdeltas, bayfills, and crevasse-splays that have higher frequency delta cycles ranging from several hundred years to a few decades. These depositional features are usually less than 10 m thick, and some have produced marshland areas of over 300 km 2 . The net result of these delta-building events is a low-lying landscape with components that are changing (building and deteriorating) at different rates. Geologically, these depositional cycles produce a thick accumulation of coarsening, upward deltaic deposits that have various thicknesses in response to development on a variety of temporal and spatial scales. In this river-dominated delta system, distributaries can prograde seaward at rates of over 100 m/year. The cumulative effect of the Holocene depository has been to depress the underlying Pleistocene surface. In a local setting, e.g., the modern Balize Lobe, differential loading causes the vertical displacement of underlying clay-rich facies (shale diapirs-mudlumps). The delta front of this lobe, which has prograded into deep water of the outer continental she lf, is characterized by rapid deposition of silt - and clay- rich sediments and slope instability, which results in sea ward displacement of sediments by a variety of mass-movement processes. Superimposed on the natural processes and forms of the Mississippi deltaic plain and its associated estuarine environments, are human impacts, most of which have been imposed in this century. The most significant impacts have resulted from a decrease in sediment input to the river from its tributaries and the alteration of the river's natural sediment dispersal processes through the construction of levees. Measures are now being taken to reinstate some of the delta's natural processes, thereby mitigating landloss so that decline in animal and plant productivity can be mitigated.

333 citations

Journal ArticleDOI
TL;DR: In the modern delta of the Mississippi River, sediment instabilities operate nearly continuously, mass-moving large quantities of shallow-water deposits to deeper-water environments via arcuate rotational slides and mudflow gullies and depositional lobes as discussed by the authors.
Abstract: Research in the modern delta of the Mississippi River has revealed short-term changes and processes that are of significant magnitude. Deltaic lobes, each lobe covering an area of 30,000 sq km and having an average thickness of 35 km, switch sites of deposition on an average of every 1,500 yr. Through short periods of geologic time, this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas. Within a single delta lobe, and operating on an even higher frequency, are bay fills and overbank splays. Bay fills, having areas of 250 sq km and thickness of 15 m, require only 150 yr to accumulate. Four major events have taken place in the modern Balize delta since 1838. Overbank splays are much smaller, covering areas of less than 2 sq km and having thicknesses of 3 m, but are associated with high floods on the river. At the river mouth, continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long, 8 km wide, and a thickness of 80 m in a period of only 200 yr. Differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr. On the subaqueous delta platform, sediment instabilities operate nearly continuously, mass-moving large quantities of shallow-water deposits to deeper-water environments via arcuate rotational slides and mudflow gullies and depositional lobes. All of these changes and processes operate at differing spatial and temporal scales, but all result in deposition of large volumes of sediment over extremely short periods of time.

308 citations


Cited by
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TL;DR: The authors showed that rivers with large sediment loads (annual discharges greater than about $15 \times 10^{6}$ tons) contribute about $7 −times 10 −9$ tons of suspended sediment to the ocean yearly.
Abstract: New data and new estimates from old data show that rivers with large sediment loads (annual discharges greater than about $15 \times 10^{6}$ tons) contribute about $7 \times 10^{9}$ tons of suspended sediment to the ocean yearly. Extrapolating available data for all drainage basins, the total suspended sediment delivered by all rivers to the oceans is about $13.5 \times 10^{9}$ tons annually; bedload and flood discharges may account for an additional $1-2 \times 10^{9}$ tons. About 70% of this total is derived from southern Asia and the larger islands in the Pacific and Indian Oceans, where sediment yields are much greater than for other drainage basins.

3,409 citations

Journal ArticleDOI
TL;DR: In this article, the authors proposed a new method of analysis for fluvial facies, which subdivides fluvic deposits into local suites consisting of one or more of a set of eight basic three-dimensional architectural elements.

1,804 citations

Journal ArticleDOI
TL;DR: A review of more than sixty recent papers on modern and ancient braided-stream deposits can be found in this article, where several sedimentation models have been developed from a review of recent work.

1,755 citations

Journal ArticleDOI
TL;DR: A simple classification of sedimentary density flows, based on physical flow properties and grain-support mechanisms, and briefly discusses the likely characteristics of the deposited sediments is presented in this paper.
Abstract: The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post-depositional consolidation and soft-sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain-support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non-cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle-support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain-to-grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle-support mechanism depends upon flow conditions, particle concentration, grain-size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse-grain tail (or dense-grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large-scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensuBagnold, 1962), in which fluid turbulence is the main particle-support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge-like flows and quasi-steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge-like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi-steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening-up units capped by fining-up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near-steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co-exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits.

1,454 citations

Journal ArticleDOI
05 Dec 2013-Nature
TL;DR: Whether wetlands continue to survive sea-level rise depends largely on how human impacts interact with rapid sea- level rise, and socio-economic factors that influence transgression into adjacent uplands.
Abstract: Coastal populations and wetlands have been intertwined for centuries, whereby humans both influence and depend on the extensive ecosystem services that wetlands provide. Although coastal wetlands have long been considered vulnerable to sea-level rise, recent work has identified fascinating feedbacks between plant growth and geomorphology that allow wetlands to actively resist the deleterious effects of sea-level rise. Humans alter the strength of these feedbacks by changing the climate, nutrient inputs, sediment delivery and subsidence rates. Whether wetlands continue to survive sea-level rise depends largely on how human impacts interact with rapid sea-level rise, and socio-economic factors that influence transgression into adjacent uplands.

1,303 citations