Channel Inception Research Articles

Stacking pattern transition caused by submarine channel inception and internal levée development

ABSTRACTThe shift from highly amalgamated and laterally offset to aggradational and vertically stacked submarine channels is a pattern that has been widely recognized in seismic reflection datasets as well as in outcropping successions worldwide. However, the sedimentary and stratigraphic details of such an important part of channel evolution and its implications have not been discussed extensively. That gap is addressed here by characterizing a previously undocumented, seismic‐scale outcrop of the Tres Pasos Formation (Magallanes Basin, Chile) that records the transition from a laterally offset/low‐aggradational channel complex to a vertically stacked aggradational complex associated with the development of an internal levée that enhanced channel aggradation. The boundary between the two complexes is defined by a composite erosional surface (up to ca 35 m relief) with two adjacent element‐scale channelized incisions with contrasting sedimentary infills that provide unprecedented insight into this key phase of submarine channel evolution. Observations indicate that, in between these two depositional architecture styles, there is a significant phase of deep incision and bypass. The relief achieved via this deep incision of one or multiple simultaneously active conduits is interpreted to be necessary to set up the conditions for flow stripping and, subsequently, overbank deposition. Finally, the siltstone‐rich intra‐channel lithofacies association observed directly overlying part of the incision is interpreted to represent the stratigraphic expression of the earliest deposits of an active submarine channel preserved due to the abrupt abandonment of this pathway and the development of an adjacent conduit, which resulted in the former being covered with fine‐grained overbank deposits.

Coarse-grained submarine channels: from confined to unconfined flows in the Colombian Caribbean (late Eocene)

Submarine channel mouth settings are hardly preserved in the stratigraphic record. Although they are still poorly known with respect to other segments of turbidite systems, conceptual models are being refined in the light of new discoveries in modern and ancient examples. Still, some questions such as the transition between expansion zones and the traditional Channel-Lobe Transition Zone (CLTZ) remains open in ancient systems. Upper Eocene deposits of the Colombian Caribbean (San Jacinto Fold Belt) are interpreted here as a fan-delta-fed, submarine, coarse-grained channel-lobe system. It displays a well-preserved channel inception stage in the shelf break represented by sigmoidal to lens-shaped gravels, and planar cross-stratified pebbly sandstones (foreset and backset) interpreted as cyclic steps in an expansion zone. In a later stage, a classical channel-levee complex was developed, represented by channel fill elements showing sharp- and erosional-based, fining-upward sequences that are meters thick, having basal massive matrix-supported pebble conglomerates (hard—extrabasinal—clasts, rip-up clasts, coastal bioclasts), vertically evolving to liquefied massive to planar-laminated coarse-grained sandstones with phytodetrital carbonaceous laminae. They are interpreted as concentrated flow deposits (high-density turbidites) coming from continental areas or from coastal systems (i.e., delta reworking). Undifferentiated channel belt thin-bedded turbidites associated with levees and terraces deposits are related to these confined systems. The channel-lobe transition zone is characterized by debrites from cohesionless debris flow in a channel-mouth bar setting, representing bypass processes that developed distally into low-angle, planar cross- and sigmoidally-stratified (upstream antidune) pebble-size to coarse-grained sandstones that fill low-angle scours (cut-and-fill structures) in an antidune field setting with supercritical conditions. When the currents lose channel confinement, the setting is characterized by changes from Froude supercritical to subcritical flow conditions in an inner lobe to lobe off-axis environment. Large seasonal fluctuations in precipitation favor high sediment concentrations, promoting the formation of volumetrically significant fan deltas and coarse-grained submarine channels with high erosive capacity; therefore, their record helps refine interpretations of depositional processes, providing criteria for recognizing areas of the turbiditic systems that are hardly preserved. The particular aggradational conditions for the preservation and stratigraphic characterization of the rare exhumed submarine channel mouth systems make it possible to decipher sediment dispersal patterns and thus connect the models proposed here, from supercritical systems to the traditional models of turbiditic systems.

Conditions of submarine levéed channel inception: Examination by flume experiments

ABSTRACTSubmarine levéed channels are often observed in submarine fans, although submarine fans without continuous levéed channels are also common depending on the composition of supplied sediments, particularly the proportion of muddy/sandy materials. However, parameters governing the inception of levéed channels have not yet been studied. Furthermore, depositional levéed channel topography has not been simulated in experimental flumes in any previous study conducted with dilute flows (flow concentration <10%). Herein, four experimental series were conducted (Series A, B, C and D) to simulate depositional submarine channels and to study their formative conditions. A mixture of sediment and saline water was used in the experiments, and both the width of an outlet diffuser and flow discharge rate were varied in the experimental series. A topography composed of a channel with two ridges resembling natural depositional submarine channels with levées was formed in all experiments. Comparisons between the experimental topographies and natural submarine fans revealed that two of the four experimental series produced channel levées with length to width ratios similar to those produced by natural systems. An experiment with half the outlet size produced a channel two times deeper (4.5 cm against 2.0 cm and 2.2 cm) as compared to other experiments. The flow discharge rate and outlet width were half of those observed in Series A and B. Furthermore, salt was removed from the initial mixture for one of the experiments, resulting in high natural levées with a short channel. This study demonstrated that dilute flows could form purely depositional channel levées without precursor or resultant erosive features. In addition, results revealed that the formation of submarine channels is related to two factors: channel width and muddy suspended sediment.

Channel inception through bottom‐current erosion of pockmarks revealed by numerical simulation

AbstractIn deep‐marine environments, the inception of channels can be induced by the interaction between bottom currents and rough topography. However, it is still unclear under which conditions such features can form and what happens in the earliest phase of channel development. In this study, based on the morphological, sedimentary and oceanographic settings of a pockmark field in the NW South China Sea, we reveal the process of channel inception through the erosion of pockmarks by bottom currents. Using numerical simulations, we show that an appropriate current velocity can induce the erosion of pockmark trains in cohesive sediments, leading to the coalescence of discrete pockmarks and the formation of a channel with a rough thalweg. The interaction of bottom currents with the pockmarks induces a significant erosion along the pockmarks axis. Bottom‐current erosion is strongest at the downstream edges of pockmarks, where the horizontal velocity reaches a maximum and an upwelling forms. Erosion increases as the distance between pockmarks reduces. In our simulation results, a channel is only formed by the coalescence of pockmarks if the distance between pockmarks is <6 times the diameter of the pockmark. This study provides evidence of the formation of channels by bottom currents, which helps reconstruct palaeoceanographic conditions based on sediment architecture. It also shows the complex hydrodynamics at these structures that strongly control sedimentary processes and may affect distribution of benthic ecosystems in marine environments.

River sinuosity describes a continuum between randomness and ordered growth

AbstractRiver channels are among the most common landscape features on Earth. An essential characteristic of channels is sinuosity: their tendency to take a circuitous path, which is quantified as along-stream length divided by straight-line length. River sinuosity is interpreted as a characteristic that either forms randomly at channel inception or develops over time as meander bends migrate. Studies tend to assume the latter and thus have used river sinuosity as a proxy for both modern and ancient environmental factors including climate, tectonics, vegetation, and geologic structure. But no quantitative criterion for planform expression has distinguished between random, initial sinuosity and that developed by ordered growth through channel migration. This ambiguity calls into question the utility of river sinuosity for understanding Earth's history. We propose a quantitative framework to reconcile these competing explanations for river sinuosity. Using a coupled analysis of modeled and natural channels, we show that while a majority of observed sinuosity is consistent with randomness and limited channel migration, rivers with sinuosity ≥1.5 likely formed their geometry through sustained, ordered growth due to channel migration. This criterion frames a null hypothesis for river sinuosity that can be applied to evaluate the significance of environmental interpretations in landscapes shaped by rivers. The quantitative link between sinuosity and channel migration further informs strategies for preservation and restoration of riparian habitat and guides predictions of fluvial deposits in the rock record and in remotely sensed environments from the seafloor to planetary surfaces.

New models for submarine channel deposits on structurally complex slopes: Examples from the Niger delta system

Submarine channel complexes are often described as having a two-phase stratigraphic evolution where an initial phase of migration is followed by aggradation, generating a ‘hockey-stick shaped’ channel trajectory. However, the role of tectonic forcing in modifying time-integrated sedimentary architectures remains poorly understood. Here, we evaluate how tectonically driven changes in slope modify the evolution—both in terms of morphology and stratigraphic architecture—of submarine channels across a range of spatial scales from the fundamental architectural unit, a channel element, to the scale of a channel complex set, using examples from the Niger Delta system. From a 3D, time-migrated seismic reflection volume, we use amplitude extractions, frequency decomposition and RGB blending to determine channel stratigraphic architectures. These observations are used systematically to evaluate the development of cross-sectional and planform architectures as the channel systems interact with a range of active and pre-existing structural bathymetry. Our results indicate that while a channel complex's stratigraphic architecture may be captured by a two-phase evolution on unstructured slopes, this model fails on structurally complex slopes. Unstructured slope channel complexes display a repeated arrangement of migration dominating the early stratigraphic record and subsequent aggradation. The late aggradational phase signals a decrease in the rate of growth in channel complex width and the rate of change in sinuosity relative to aggradation throughout the complex's development. However, tectonically driven changes in sinuosity and the relative rates of channel migration and aggradation modify complex development significantly. We identify three end-member styles of channel-structure interaction, determined by the timing of bathymetry development and its associated style: (1) pre-channel structural bathymetry; (2) coeval positive relief, and (3) coeval negative relief. Where structural relief pre-dates channel inception, a principal adjustment is in the initial channel course with early channel elements being forced around positive relief of the structure, generating long-wavelength bends in the complex's course. Where structure continues to modify slope creating positive and negative bathymetry during complex development, migration and bend development continue with complex width and channel element sinuosity increasing until abandonment. These observations demonstrate that submarine channel architecture and planform are highly sensitive to tectonic perturbation and we use these results to generate graphical models that show predicted architectural evolution of submarine channels on structurally complex slopes in general.

Linking submarine channel–levee facies and architecture to flow structure of turbidity currents: insights from flume tank experiments

AbstractSubmarine leveed channels are sculpted by turbidity currents that are commonly highly stratified. Both the concentration and the grain size decrease upward in the flow, and this is a fundamental factor that affects the location and grain size of deposits around a channel. This study presents laboratory experiments that link the morphological evolution of a progressively developing leveed channel to the suspended sediment structure of the turbidity currents. Previously, it was difficult to link turbidity current structure to channel–levee development because observations from natural systems were limited to the depositional products while experiments did not show realistic morphodynamics due to scaling issues related to the sediment transport. This study uses a novel experimental approach to overcome scaling issues, which results in channel inception and evolution on an initially featureless slope. Depth of the channel increased continuously as a result of levee aggradation combined with varying rates of channel floor aggradation and degradation. The resulting levees are fining upward and the grain‐size trend in the levee matches the upward decrease in grain size in the flow. It is shown that such deposit trends can result from internal channel dynamics and do not have to reflect upstream forcing. The suspended sediment structure can also be linked to the lateral transition from sediment bypass in the channel thalweg to sediment deposition on the levees. The transition occurs because the sediment concentration is below the flow capacity in the channel thalweg, while higher up on the channel walls the concentration exceeds capacity resulting in deposition of the inner levee. Thus, a framework is provided to predict the growth pattern and facies of a levee from the suspended sediment structure in a turbidity current.

Physical Simulation of High-Pressure Water Inrush Through the Floor of a Deep Mine

When mining at great depth, high-pressure karstic water inrush can pose a serious threat. We used physical simulations to investigate channel inception, evolution, and catastrophic flooding. Using newly developed coupled solid–liquid materials and physical simulations of high-pressure water intrusion, a laboratory test was carried out that reproduced the spatial and temporal characteristics of channel formation and crack generation, extension, connection, and failure. Floor failure features caused by the effects of mining and confined water were analyzed, and the formation and evolution of catastrophic flooding were investigated. The results showed that the evolution of a water-inrush channel can be divided into three stages: floor crack development, water channel initiation, and channel evolution. Extrapolating the simulation results to a real-world example, a floor fracture formed when the working face advanced to 45 m, and a water-inrush channel formed when the push distance of the working surface was about 70 m. Thereafter, the point of water eruption widened, water flow increased, and the main cracks in the floor rock mass showed obvious changes. The crack in the lower interface was angled 56° upward, and finally a water eruption point formed in the middle of the mined cavity. These results provide theoretical support for the prediction of water inrush into deep coal mines.

Morphodynamics of submarine channel inception revealed by new experimental approach.

Submarine channels are ubiquitous on the seafloor and their inception and evolution is a result of dynamic interaction between turbidity currents and the evolving seafloor. However, the morphodynamic links between channel inception and flow dynamics have not yet been monitored in experiments and only in one instance on the modern seafloor. Previous experimental flows did not show channel inception, because flow conditions were not appropriately scaled to sustain suspended sediment transport. Here we introduce and apply new scaling constraints for similarity between natural and experimental turbidity currents. The scaled currents initiate a leveed channel from an initially featureless slope. Channelization commences with deposition of levees in some slope segments and erosion of a conduit in other segments. Channel relief and flow confinement increase progressively during subsequent flows. This morphodynamic evolution determines the architecture of submarine channel deposits in the stratigraphic record and efficiency of sediment bypass to the basin floor.

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska

Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼ 2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8-1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼ 100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2-0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50-80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

Supraglacial channel inception: Modeling and processes

Supraglacial drainage systems play a key role in glacial hydrology. Nevertheless, physical processes leading to spatial organization in supraglacial networks are still an open issue. In the present work we thus address from a quantitative point of view the question of what is the physics leading to widely observed patterns made up of evenly spaced channels. To this aim, we set up a novel mathematical model describing a condition antecedent channel formation, i.e., the down-glacier flow of a distributed meltwater film. We then perform a linear stability analysis to assess whether the ice-water interface undergoes a morphological instability compatible with observed patterns. The instability is detected, its features depending on glacier surface slope, ice friction factor, and water as well as ice thermal conditions. By contrast, in our model channel spacing is solely hydrodynamically driven and relies on the interplay between pressure perturbations, flow depth response, and Reynolds stresses. Geometrical features of the predicted pattern are quantitatively consistent with available field data. The hydrodynamic origin of supraglacial channel morphogenesis suggests that alluvial patterns might share the same physical controls.

The Four Heads of Film4

There have been four key figures responsible for Channel 4's film production activities since the Channel's inception in 1982. David Rose, David Aukin, Paul Webster and Tessa Ross have each ensured that Channel 4 retains its key commitment to promoting British film culture, as envisioned by its first chief executive Jeremy Isaacs. They have presided over film and drama through changes within the corporate structure and financing of the Channel, through deregulation of the television industry and the expansion of the independent production sector, through the growth of the digital economy and multi-channel television, and through changes in the funding landscape and the fortunes of the UK film industry. As vital contributors to the film culture they have each expressed their own individual passion for and commitment to British film, have championed establish film-makers and nurtured new talent, and have worked tirelessly to advance British film on the world stage and to broaden its domestic audience base. Occupying a unique position at the interface of the television and film industries, and between the cultural patronage of public service broadcasting and international co-production finance, they share a rare skill above all: an eye for a good script.

Submarine channel initiation, filling and maintenance from sea‐floor geomorphology and morphodynamic modelling of cyclic steps

AbstractAdvances in acoustic imaging of submarine canyons and channels have provided accurate renderings of sea‐floor geomorphology. Still, a fundamental understanding of channel inception, evolution, sediment transport and the nature of the currents traversing these channels remains elusive. Herein, Autonomous Underwater Vehicle technology developed by the Monterey Bay Aquarium Research Institute provides high‐resolution perspectives of the geomorphology and shallow stratigraphy of the San Mateo canyon‐channel system, which is located on a tectonically active slope offshore of southern California. The channel comprises a series of crescent‐shaped bedforms in its thalweg. Numerical modelling is combined with interpretations of sea‐floor and shallow subsurface stratigraphic imagery to demonstrate that these bedforms are likely to be cyclic steps. Submarine cyclic steps compose a morphodynamic feature characterized by a cyclic series of long‐wave, upstream‐migrating bedforms. The bedforms are cyclic steps if each bedform in the series is bounded by a hydraulic jump in an overriding turbidity current, which is Froude‐supercritical over the lee side of the bedform and Froude‐subcritical over the stoss side. Numerical modelling and seismic‐reflection imagery support an interpretation of weakly asymmetrical to near‐symmetrical aggradation of predominantly fine‐grained net‐depositional cyclic steps. The dominant mode of San Mateo channel maintenance during the Holocene is interpreted to be thalweg reworking into aggrading cyclic steps by dilute turbidity currents. Numerical modelling also suggests that an incipient, proto‐San Mateo channel comprises a series of relatively coarse‐grained net‐erosional cyclic steps, which nucleated out of sea‐floor perturbations across the tectonically active lower slope. Thus, the interaction between turbidity‐current processes and sea‐floor perturbations appears to be fundamentally important to channel initiation, particularly in high‐gradient systems. Offshore of southern California, and in analogous deep‐water basins, channel inception, filling and maintenance are hypothesized to be strongly linked to the development of morphodynamic instability manifested as cyclic steps.

Deep‐sea channel evolution and stratigraphic architecture from inception to abandonment from high‐resolution Autonomous Underwater Vehicle surveys offshore central California

AbstractThe Lucia Chica channel system is an avulsion belt with four adjacent channels that progressively avulsed to the north‐east from a single, upslope feeder channel. Avulsion occurred from underfilled channels, leaving open channels that were reactivated by flows stripped from younger, adjacent channels. Differences in relief (height from channel thalweg to levée crest), sinuosity and levée stratigraphy between adjacent channels correspond to relative channel age, and indicate a change in channel morphology and architecture with time. Potential triggers for the change over time include differences in gradient, flow behaviour and characteristics, and channel evolution. Gradient does not appear to be a major control on channel formation and avulsion because adjacent channels formed on the same gradient. Based on available ultra‐high‐resolution remote imaging obtained with an Autonomous Underwater Vehicle, differences in adjacent channel morphology are interpreted to be primarily a result of differences in channel maturity. The interpreted sequence of channel maturity involves erosional channel inception through scouring and incipient channels (defined by linear trains of scours) prior to development of continuous thalwegs. Channel narrowing, formation and growth of levées, increasing channel relief and development of sinuosity occurred as channels evolved. The evolutionary sequence interpreted from the high‐resolution Lucia Chica dataset provides a unique perspective on intrinsic controls of architecture for single channel elements. In addition to helping bridge the gap between outcrop and industry‐standard reflection‐seismic data resolutions and scopes, interpretations in this study also expose potential problems with hierarchical classifications in three‐dimensional imaging of distributary systems, and provide potentially important analogues for evolutionary morphologies not resolved in other deep‐water channel systems.

Erosion at inception of deep-sea channels

We present a general model for channel inception and evolution in the deep sea by integrating observations from two complementary datasets: (1) high-resolution multibeam bathymetry and chirp sub-bottom profiles of the Lucia Chica channel system on the seafloor offshore central California, and (2) the well-exposed channelized strata of the Tres Pasos Formation in southern Chile. The Lucia Chica channel system shows laterally offset, sub-parallel channels that evolved across a similar gradient, but display different architecture, reflecting the influence of channel maturity and intrinsic cyclicity of channel formation. The stratigraphically oldest channel is narrower with well-developed levees while the younger channelized features are broader and bounded by low-relief levees or no levees at all.The high-resolution Lucia Chica dataset is integrated with detailed field observations of channel axis-to-margin sedimentary facies relationships and the stratigraphic context afforded from depositional-dip continuity in outcrops of the Tres Pasos Formation. Numerous channels from the outcrop belt are characterized by initial erosional stages.By combining these two datasets with numerical analysis, experimental work, and previous interpretations of additional outcropping strata and seafloor examples, we hypothesize that an initial erosional template extending into a basin is a pre-requisite for creation of channels in deep-sea environments.

Transient two-layer thin-film flow inside a channel

The flow of two superimposed newtonian layers in a channel is investigated numerically in this study. The two-layer film flows inside a long channel due to a pressure gradient. The scaled conservation equations for two-layer incompressible newtonian film flow are first introduced. The weighted residual approach first proposed by Amaouche et al. [Phys. Fluids 19, 084106 (2007)] is used for finding the suitable weight functions before depth averaging. Subsequently, a linear stability analysis of thin-film equations for two-layer Poiseuille flow is carried out. The formulas which give the asymptotic stability with respect to long-wave perturbations obtained with Navier-Stokes equations [J. Non-Newtonian Fluid Mech. 71, 1 (1997)] are recovered with our averaging equations. In order to mimic the disturbance effect on the coextrusion flow, the steady flow, which is simply the uniform flow of two layers of fluid inside a channel, is then perturbed at inception. Following a finite difference based scheme, two types of boundary conditions are considered at the channel inception, namely a Dirac-type pulse and periodic forcing. The perturbation takes the form of a wave packet which may or may not be amplified as it moves downstream, depending on the values of the parameters involved in the problem. Furthermore, Gaster's relation is used to calculate the spatial growth rate of perturbation. The values obtained from this relation are in good quantitative agreement with those coming from the numerical simulations of thin-film equations. Then, our averaging equations also describe the nonlinear behavior of the interfacial instabilities occurring for the Poiseuille flow of two thin layers.

PERIODIC CONFIGURATION OF CHANNELS FORMED ON STEEP SLOPES

In this study, we perform a theoretical study on ch annel inception in the case of steep slopes. The co nceptual model for the channelization process under steep sl ope is formulated by considering uniform steady sha llow water flowing in a typically smooth straight ending with upward concave steep slope gradually changing to a very mild slope. The flow on the steep slope is ass umed to be supercritical whereas the flow on mild s lope is assumed to be subcritical in the Froude sense. In t he formulation process, we used two-dimensional depthaveraged shallow water equations. A linear stabilit y analysis is used to study the problem. The proble m is an eigenvalue boundary problem, solved using the spect ral collocation method with the Chebyshev polynomials. Finally, we found a relation between growth rate of perturbation and characteristic wave number for th e case of steep slopes. The dominant wave length is found to be 20 to 29 times the Froude critical depth.

Channel inception in cohesionless sediment by seepage erosion

This study presents a series of systematic experiments on channelization by seepage erosion with various sediment layer depths and chamber slopes. When the depth of the sediment layer increases, the threshold discharge for channel initiation decreases, but the channel width increases. This is due to the fact that an increase in the weight of sediment induces an increase in a gravitational driving force along the chamber bed. When the chamber slope increases, we also found a similar effect as an increase in the sediment layer depth that the threshold discharge decreases, but the width increases. While channel bifurcation was never found in the previous study using plastic pellets as sediment, it was observed in many experiments in the present study, in which very coarse sand was used. We hypothesized that the characteristics of groundwater flow field and the resistibility of sediment material to slope failure play important roles on the channel bifurcation.

Meandering channel dynamics in highly cohesive sediment on an intertidal mud flat in the Westerschelde estuary, the Netherlands

Small meandering channels of about 1 m wide on an intertidal mudflat in the Westerschelde estuary the Netherlands) were studied with the aim to improve understanding of the effect of highly cohesive bed and bank sediment on channel inception and meander geometry and dynamics. The study is supported by experiments and modelling. The estuarine meandering channels are less dynamical than alluvial meandering rivers, and the dynamics are more localised. Moreover, the high thresholds for bed sediment erosion and for bank failure lead to two processes, uncommon in larger rivers, that cause most of the morphological change. First, the beds of the channels are eroded by backward migrating steps under hydraulic jumps, while the remainder of the bed surface along the channel is hardly eroded. Second, channel banks erode i) where eroding steps locally cause undercutting of otherwise stable channel banks and ii) in very sharp bends where the flow separates from the inner-bend channel boundary and impinges directly on the bank on the opposite side of the channel. Further morphological change is probably induced by rainfall splash erosion and by storm waves that weaken the mud, and by large mud fluxes from the estuary. The steps were successfully reproduced in laboratory flume experiments. An existing model for step migration predicted celerities consistent with field and laboratory observations and demonstrated a strong dependence on the threshold for erosion. Bank stability models confirm that banks and steps only fail when undercut and weakened by waves, rain or excess pore pressure in agreement with observations. The effects of a high threshold for bank erosion was implemented in an existing meander simulation model that reproduced the observed locations of bank erosion somewhat better than without the threshold, but flow separation and its effect on meander bends remains poorly understood.

Channelization on plateaus composed of weakly cohesive fine sediment

In this paper, we present a mathematical model of the formation of gullies at the downstream end of plateaus composed of weakly cohesive fine material such as freshly deposited volcanic ash as an extension of the theory of channel inception on purely erosional slopes proposed by Izumi and Parker (2000). We perform a linear stability analysis using the momentum equations of flow, the transport equation of suspended sediment, and the Exner equation for beds subject to both erosion and deposition of bed material. We find from the analysis that deposition leads to uniform growth of perturbations at small wave numbers (or large wavelengths) and suppresses the dominant wavelength associated with the maximum growth rate. When the effect of deposition increases further, channel initiation itself tends to be inhibited. As long as the effect of deposition is not large, slopes become unstable to transverse perturbations, and the dominant wavelength appears in the range of 6–15 times the Froude critical depth of the sheet flow on slopes, divided by the bottom friction coefficient. Assuming that the bottom friction coefficient is of the order of 0.01, we find that the dominant gully spacing is of the order of 1000 times the Froude critical depth, which is similar to the result obtained in the case without deposition. We find that deposition affects channel initiation itself rather than channel spacing.