Bedform Migration Research Articles

MORPHODYNAMIC UPSCALING WITH THE MORFAC APPROACH

The Morphological Acceleration Factor (MORFAC) approach for morphodynamic upscaling enables the simulation of long term coastal evolution. However the general validity of the MORFAC concept for coastal applications has not yet been comprehensively investigated. Furthermore, a robust and objective method for the a priori determination of the highest MORFAC that is suitable for a given simulation (i.e. critical MORFAC) does not currently exist. This paper presents some initial results of an ongoing, long-term study that attempts to rigorously and methodically investigate the limitations and strengths of the MORFAC approach. Based on the results of a numerical modelling exercise using the morphodynamic model Delft3D, the main dependencies and sensitivities of the MORFAC approach are investigated. A criterion (based on the CFL condition for bed form migration) for the a priori determination of the critical MORFAC is also proposed.

A study of particle splash on developing ripple forms for two bed materials

A custom-designed Particle Tracking Velocimetry (PTV) system was developed to measure particle splash dynamics in two materials: mixed quartz sand and ground acrylic particles approximating the density of snow. The strengths and limitations of the PTV system are described in the context of five experiments carried out at friction velocities slightly greater than the threshold friction velocity for particle entrainment and saltation. Analysis of particle collisions on the bed surface (splash) demonstrates that the simulated snow particles impact the surface at a smaller angle than quartz sand. They also eject a greater number of particles, though at lower velocities and angles than observed for the sand particles. These relations appear to be highly sensitive to the porosity of the bed surface, while particle shape and elasticity play a secondary role. With regard to PTV measurements carried out along the wind-aligned axis of migrating aeolian ripples, particles ejected from the lee slope tend to be slightly larger than those on the stoss slope, and in general are ejected with a lower speed and greater angle to the surface. Irrespective of the type of granular material, when the average impact angle is close to or greater than the negative slope of the evolving bedform, the particle ejection rate from the stoss slope was measured to be approximately twice that for the lee slope. This empirical observation confirms the assumption made in models of ripple development that the ejection flux governs the bedform migration rate. However, systematic under-prediction of this rate by an early model suggests that additional processes, inclusive of surface creep, must also be accounted for.

1-D numerical simulation of sediment transport in alluvial channel beds

ABSTRACT Quantitative estimate of sediment transport processes is important especially in river-control engineering and for water management projects. In this paper the sediment routing of a 1-D model is presented and applied to some study cases. The procedure simulates the variations of bed roughness conditions due to natural sorting and to generation and migration of bed forms. The suspended-load and the bed-load are treated separately.

A simple model of bedform migration

AbstractA model linking subaqueous dune migration to the effective (grain related) shear stress is calibrated by means of flume data for bedform dimensions and migration rates. The effective shear stress is calculated on the basis of a new method assuming a near‐bed layer above the mean bed level in which the current velocity accelerates towards the bedform crest. As a consequence, the effective bed shear stress corresponds to the shear stress acting directly on top of the bedform. The model operates with the critical Shields stress as a function of grain size, and predicts the deposition (volume per unit time and width) of naturally‐packed bed material on the bedform lee side, qbcrest. The model is simple, built on a rational description of simplified sediment mechanics, and its calibration constant can be explained in accordance with estimated values of the physical constants on which it is based. Predicted values of qbcrest correlate with measured values of bedform height multiplied by bedform migration rate with R2 = 0·83. Copyright © 2010 John Wiley & Sons, Ltd.

Experimental Observations of Flow and Bed Processes in Large-Amplitude Meandering Flume

Meanders of large amplitude often exhibit asymmetric planform shape or subsidiary bends. The present work is aimed at improving on understanding of the morphodynamic phenomena affecting the bed evolution of large amplitude meandering channels. Attention is focused on the development of the steady point bar-pool configuration and of the superimposed large-scale migrating bed forms; of particular interest is the role of the changing channel curvature and bed topography variation on flow pattern. A series of experiments was carried out in a sine-generated large-amplitude meandering flume, for two values of width-to-depth ratio. Maps documenting the bed topography and the flow pattern along the meandering bends are reported. Two point bars per bend were observed and seem to be part of a series of damped oscillations developing in response to the changing channel curvature. In response to the bed deformation, the maximum flow velocity moves at the outer bank at the entrance of the bend.

Experiments in a high‐amplitude Kinoshita meandering channel: 2. Implications of bend orientation on bed morphodynamics

Experiments carried out under flat smooth bed condition in a periodic, asymmetric, Kinoshita meandering channel showed that bend skewness controls the three‐dimensional mean flow (e.g., primary and secondary flow) and turbulence structure (e.g., location of shear layers and turbulent shear stresses). In this paper, similar hydraulic conditions are used to perform movable‐bed experiments with uniform sediment size in order to study the effect of bend orientation on bed morphodynamics. Two main differences between the upstream‐ and downstream‐skewed meander configurations are found: (1) for the case of bends oriented upstream valley, the bed forms are produced just upstream of the bend apex, whereas for the case of bends oriented downstream valley they are observed around the upstream inflection point, and (2) the downstream‐skewed condition produces the deepest scour region, which is located downstream of the bend apex. A comparison with observations at two bends on the Wabash River, Illinois, shows some remarkable similarities with the bed morphology observed in the Kinoshita channel. Further analysis of the bed form celerity, bed load transport during evolution, and superimposition of migrating bed forms and their implications on bend migration are presented.

Bedforms and bed material transport pathways in the Changjiang (Yangtze) Estuary

Bed features were mapped using a thermal depth recorder and a side-scan sonar in the Changjiang (Yangtze) Estuary, and flows were observed and bed surface materials were sampled to investigate bed material transport pathways. Distinct scales and dimensions of dunes were observed in terms of the classification scheme of Ashley [Ashley, G.M., 1990. Classification of large-scale sub-aqueous bedforms: A new look at an old problem. Journal of Sedimentary Research, 61, 160–172], forming a dune set in the flood season. Large dunes occurred predominately in the surveyed area of the estuarine zone, and two-dimensional dunes predominated in the dry season while three-dimensional dunes prevailed in the flood season. Sharp lee sides of dunes have low angles, indicating a significant suspended-load transport for the bedform migration, and the majority of them face down-estuary reflecting the controlling process by the ebb current. The bed material migrations in terms of relative maximum gross transport show a satisfactory agreement with the sonar mapping in the dry season. The relative maximum gross transport as an independent estimator of migration direction, can be used to effectively determine the bed material transport directions in the Changjiang Estuary.

Dynamics of a Transgressive Prodeltaic System: Implications for Geography and Climate Within a Pennsylvanian Intracratonic Basin, Colorado, U.S.A.

Detailed sedimentological analysis of a prodeltaic turbidite unit within the Pennsylvanian Minturn Formation provides insight into the nature of fan-delta deposition, and allows reconstruction of paleogeographic and paleoclimatic conditions within a tectonically active cratonic basin. The formation, a thick succession of fan-delta deposits, exhibits a complex stratigraphic architecture of alluvial, fluvial, and deltaic fithofacies that were deposited in the Central Colorado Basin (CCB), U.S.A., a fault-bounded depression within the Ancestral Rockies. An ~ 20 to 35-m-thick, regionally extensive, unconformity-bounded prodelta unit was deposited during a large eustatic sea-level rise that temporarily interrupted coarsegrained fluvial-deltaic deposition. The unit consists of dark-green shale and sandstone event beds with tool marks produced by abundant plant debris. The sandstone event beds were previously interpreted as turbidites because they contain current ripples, parallel lamination, and sole marks consistent with deposition from turbidity currents in the absence of storm-generated surface gravity waves. We have found, however, that proximal prodeltaic deposits contain evidence for combined oscillatory and unidirectional flow (i.e., asymmetric hummocky cross stratification (HCS), quasi-planar lamination, and combined-flow ripple stratification), indicating deposition under large waves in relatively shallow water. Paleohydraulic calculations indicate that formation of the HCS beds requires a fetch of at least 75 km to generate the necessary wave periods. Given the position of the CCB in the lee of the ancestral Front Range relative to the prevailing trade winds, and the narrow seaway to the south, storms either tracked across the Cordilleran seaway from the west or formed in situ within the CCB. The former hypothesis is consistent with paleoclimate models that purport the development of summer monsoonal circulation patterns over western equatorial Pangea during the late Paleozoic, and regional observations of southeastward eolian bedform migration directions.

Architectural analysis of a volcaniclastic jökulhlaup deposit, southern Iceland: sedimentary evidence for supercritical flow

AbstractThe 1918 eruption of the glacially capped Katla volcano, southern Iceland, generated a violent jökulhlaup, or glacial outburst flood, inundating a large area of Mýrdalssandur, the proglacial outwash plain, where it deposited ca 1 km3 of volcaniclastic sediment. The character of the 1918 jökulhlaup is contentious, having been variously categorized as a turbulent water flow, a hyperconcentrated flow or as a debris flow, based on localized outcrop analysis. In this study, outcrop‐based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi‐stationary antidunes and down‐current migrating antidunes, and from regular based bedsets, associated with transient chute‐and‐pool bedforms, all of which are associated with turbulent, transcritical to supercritical water flow conditions. Antidune wavelengths range from 24 to 96 m, corresponding to flow velocities of 6 to 12 m sec−1 and average flow depths of 5 to 19 m. This range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts. Architectural analysis of the 1918 jökulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jökulhlaup that could not have been achieved through localized outcrop analysis. The observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions. The physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes; sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations, regarding modes of deposition and formative flow hydraulics, are to be made.

The potential impact of bedform migration on seagrass communities in Torres Strait, northern Australia

Seagrass communities in the northwest of Torres Strait are known to disappear episodically over broad areas. Sediment mobility surveys were undertaken within two study areas during the monsoon and trade wind seasons, in the vicinity of Turnagain Island, to find out if the migration of bedforms could explain this disappearance. The two study areas covered sand bank and sand dune environments to compare and contrast their migration characteristics. Repeat multibeam sonar surveys were used to measure dune-crest migration during each season. Our results show that seagrass beds occur in the troughs of sediment-starved dunes, but no seagrass occurs in association with full-bedded dunes that are superimposed on large sand bank features. The coincidence of seagrass beds with the sediment-starved dunes is in spite of the fact that they migrate faster (0.59 m day −1) than full-bedded dunes (0.13 m day −1), which indicates that some other factor (other than dune migration rate) limits seagrass growth within Torres Strait. We suggest that seagrasses are unable to colonise full-bedded dunes because of the semi-continuously transported sand that characterises this environment. In contrast, the troughs of sediment-starved dunes experience only limited bedload transport and are less hostile for seagrasses. A conceptual model is presented to explain the occurrence of seagrass beds in relation to their proximity to migrating sand dunes. Based on our analysis, we conclude that the widespread dieback of seagrasses documented for the Turnagain Island region was not caused by dune migration.

Surf zone megaripple orientation: Measurements and models

Two existing models for bed form orientation are tested against measurements of megaripple crest line orientation from Scripps Beach. Optical time‐averaged images of the surf zone sand bed at 10 min intervals are processed with an automatic crest line tracing algorithm. Flow measurements concurrent with images of the sand bed collected during 3–7 April and 17–23 July 1999 are used as input to the gross bed form normal transport model (bed forms align to maximize gross transport across crests) predicting steady state crest orientation and the defect model (bed forms reorient at a rate proportional to the differential velocity of crest line ends, or defects) predicting a time‐dependent orientation. Generally poor agreement was found between predictions and measurements of crest line orientation. The gross bed form normal transport model predicted crest line normal orientation within 20° of measured orientation for 38% of records, with increased agreement coinciding with longitudinal and oblique bed forms. The defect model predicted crest line normal orientation within 20° of measurements for 22% of records when the ratio between defect and transport event timescales is assumed ≫1 and 14% of the records for the ratio ≪1. Although migration and mean sediment transport direction are assumed to coincide in both models, RMS deviation in bed form migration direction from the calculated 30 min mean sediment transport direction was found to be 38°. Additionally, RMS deviation of megaripple migration direction from crest line normal orientation was 39°.

Field Assessment of Alternative Bed-Load Transport Estimators

Measurement of near-bed sediment velocities with acoustic Doppler current profilers (ADCPs) is an emerging approach for quantifying bed-load sediment fluxes in rivers. Previous investigations of the technique have relied on conventional physical bed-load sampling to provide reference transport information with which to validate the ADCP measurements. However, physical samples are subject to substantial errors, especially under field conditions in which surrogate methods are most needed. Comparisons between ADCP bed velocity measurements with bed-load transport rates estimated from bed-form migration rates in the lower Missouri River show a strong correlation between the two surrogate measures over a wide range of mild to moderately intense sediment transporting conditions. The correlation between the ADCP measurements and physical bed-load samples is comparatively poor, suggesting that physical bed-load sampling is ineffective for ground-truthing alternative techniques in large sand-bed rivers. Bed velocities measured in this study became more variable with increasing bed-form wavelength at higher shear stresses. Under these conditions, bed-form dimensions greatly exceed the region of the bed ensonified by the ADCP, and the magnitude of the acoustic measurements depends on instrument location with respect to bed-form crests and troughs. Alternative algorithms for estimating bed-load transport from paired longitudinal profiles of bed topography were evaluated. An algorithm based on the routing of local erosion and deposition volumes that eliminates the need to identify individual bed forms was found to give results similar to those of more conventional dune-tracking methods. This method is particularly useful in cases where complex bed-form morphology makes delineation of individual bed forms difficult.

The effect of bedform dynamics on computing suspended sediment fluxes using optical backscatter sensors and current meters

The sensitivity of the suspended sediment flux is tested with respect to rapid changes in bed-level across the surf zone of a sandy beach. The suspended flux was computed using a fixed instrument array, but bed-level changes due to ripple migration caused the instrument elevations to be significantly changed during the course of the experiment. The nominal elevations of the instruments were adjusted during data processing (using the MOBS array) to maintain a fixed elevation with respect to bed-level changes. The resultant suspended sediment concentrations and fluxes were significantly different from the unadjusted data, and for the present data set O(35%) less when averaged over the tide. The maximum difference between adjusted and unadjusted fluxes may be O(260%). The results indicate that changes in bed-level, particularly those due to bedform migration, must be accounted for when processing OBS data if reliable estimates of suspended sediment transport are to be obtained in the field.

Microbial mat mediated structures in the Ediacaran Sonia Sandstone, Rajasthan, India, and their implications for proterozoic sedimentation

A strong influence of microbial mats on the physics of Neoproterozoic sedimentation is explored within a 60 m-thick stratigraphic interval within the Sonia Sandstone, Jodhpur Group, western India. This marine interval is bounded by two terrestrial units, its base being marked by a transgressive lag and the top by an unconformity. Progradation from upper neritic, above fair-weather wave base to supralittoral settings was later terminated by a transgression; deposits of lower shoreface-upper shoreface transition thus overlie supralittoral aeolian sandstones, the basal contact of the former deposits being marked by another transgressive lag. A wide spectrum and abundant examples of microbial mat or mat-derived structures supports unusual cohesiveness within granular sand deposited in a high-energy marine environment. The cohesiveness is manifested in abundant preservation of several delicate primary structures and also in their replication in overlying beds, even after they were subjected to high-energy currents. A low rate of sedimentation and severely restricted sediment reworking resulted from prolific mat growth, and consequent depletion in the sediment budget was manifested in bedform migration and evolution. These factors could also have influenced the sequence-building pattern in a distinctive way, as is already reported from several Meso- and Neoproterozoic successions. Ubiquitous mat growth thus had the potential to impart significant distinctions to Proterozoic clastic sedimentary successions.

Dynamics of intertidal drainage channels on a multi‐barred macrotidal beach

AbstractThe intertidal drainage channels on a macrotidal bar–trough (ridge‐and‐runnel) beach were monitored during a 17‐day survey. Type 1 channels were persistent, dominantly longshore systems essentially limited to the wide intertidal zone between mean high and low water neap tidal levels. The cumulative length of this channel type fluctuated as a function of topographically controlled through‐flow or flow impedance in troughs, and showed no correlation with the semi‐lunar tidal cycle. Smaller, ephemeral type 2 channels appeared as dominantly cross‐shore systems incising bars on the narrower upper and lower beach zones during spring tides. They disappeared during neap tides through infill by waves and aeolian activity. The only significant phase of type 1 channel mobility occurred during a brief moderate‐energy storm at the start of the survey. The effect of this mobility on beach morphology was inextricably linked to that of waves and currents. Meander bend migration, forced by wave‐ and longshore‐current‐induced migration of a bar during the storm, resulted in important but highly localized morphological change that was only a minor part of an irregular saw‐tooth pattern of change that affected the entire beach profile, and that was largely controlled by wave processes and longshore currents. The flow velocities in channels on this beach are too weak to generate the formation and longshore migration of high‐energy bedforms. Channel mobility and impact on beach morphology are expected to increase under storm conditions. Copyright © 2007 John Wiley & Sons, Ltd.

Multibeam bathymetric investigations of the morphology of sand ridges and associated bedforms and their relation to storm processes, Sable Island Bank, Scotian Shelf

Storm-formed sand ridges are important morphologic features occurring on many continental shelves of the world. Our understanding of the morphology, migration, and the controlling processes over this type of sand ridges under different environmental settings is still limited. In the present study, an integrated approach of combining multibeam bathymetry survey, seabed sampling, sidescan/seismic surveys and seabed instrumentation measurements was used to characterize the surficial geology and morphology of offshore sand ridges on Sable Island Bank, Scotian Shelf, and to evaluate the relationship between sand ridge morphology, migration and the storm processes on this storm-dominated outer shelf bank. Multibeam data obtained at selected sites demonstrated the series of NE–SW oriented sand ridges with an average height of 4.3 m and wavelength of 1.5 km. Sand ridges attain maximum spacing, height and steepness in the intermediate depths. Sand ridge size and steepness also decrease from west to east and this decrease is correlated with decreasing dynamic condition, sediment grain size, and depth. Sand ridge profiles show weak easterly asymmetry. This morphological asymmetry augments the sediment texture and grain size asymmetry to suggest eastward long-term sand ridge migration and net sediment transport in the study area. Sand ridge asymmetry also shows strong local variation and the number of sand ridges asymmetric to the west is nearly equal to that asymmetric to the east. The likely cause is the nearly equal number of major storms passing to the south versus to the north of the Island so that the westerly and easterly peak storm-driven currents associated with these storm path scenarios affect equally the sand ridges on Sable Island Bank. A suite of smaller bedforms were found superimposed on the sand ridges: sand waves widely superimposed on both flanks of sand ridges, mixed 2D and 3D megaripples commonly occurring in the sand ridge troughs and on the lower western flanks, and large-wave ripples ubiquitously found in the troughs of all megaripples. Surficial sediment is coarsest (coarse sand with shell fragments) in the sand ridge trough and becomes progressively finer eastward toward the sand ridge crest and down the eastern flank, while best sorting occurs near the sand ridge crest. Box cores collected across sand ridges reveal characteristic storm event bed structure that reflects the development and migration of bedforms with the evolution of storms.

Seismic Attributes from Ultra-Thin Layered Reservoir

Seismic Attributes from Ultra-Thin Layered Reservoir — We propose the method of com- putation seismic AVO attributes (intercept and gradient) from ultra-thin geological model based on the SBED modelling software. The SBED software is based on manipulating sine-functions, creating sur- faces representing incremental sedimentation. Displacement of the surfaces creates a three dimensional image mimicking bedform migration, and depositional environments as diverse as tidal channels and mass flows can be accurately recreated. The resulting modelled deposit volume may be populated with petrophysical information, creating intrinsic properties such as porosity and permeability (both vertical and horizontal). The Backus averaging technique is used for up-scaling within the centimetre scale (the intrinsic net-to-gross value controls the acoustic properties of the ultra-thin layers). It results in pseudo- log data including the intrinsic anisotropy parameters. The synthetic seismic modelling is given by the matrix propagator method allows us to take into account all pure mode multiples, and resulting AVO attributes become frequency dependent. Within this ultra-thin model we can test different fluid saturation scenarios and quantify the likelihood of possible composite analogues. This modelling can also be used for inversion of real seismic data into net-to-gross and fluid saturation for ultra-thin reservoirs.

Self-Burial of Short Cylinders Under Oscillatory Flows and Combined Waves Plus Currents

Self-burial processes of finite-length cylinders under oscillatory flows and waves plus currents were examined with the following two different experimental facilities: a large oscillating water-sediment tunnel (LOWST) and a large wave-current tank. More than 130 experiments, with different model cylinders, were conducted within both facilities. The burial mechanisms studied include burial due to local scour and bedform migration. Burial due to fluidization in the tunnel was also explored, but only in a qualitative way. In the case of experiments with LOWST, the equilibrium burial depth was found to be a power function of the shields parameter (thetas). In the wave-current tank, the equilibrium burial depth was also found to be a function of the Shields parameter, albeit with larger scatter. The experimental observations made in both facilities have similar trends but different magnitudes. For equivalent values of the Shields parameter, smaller equilibrium burial depths were observed in the wave flume when compared to the ones in LOWST. After burial induced by local scour takes place, bedform (ripples and sandwaves) formation and evolution play a strong and, in some cases, dominant role on the equilibrium burial depth of the cylinders. Depending on how the vertical dimensions of bedforms compare to the specimen's diameter, cyclical covering and uncovering of the object may take place due to the passage of the migrating sandwaves. In such case, burial depth Bd no longer coincides with the vertical displacement (Vd) of the object as in the case when the burial process is dominated by local scour.

Evolution of the Nearshore Bed Envelope

The temporal growth of the envelope of bed motion owing to the migration of bedforms, which can be considered a proxy for maximum object burial depth, is examined using five different data sets. These data sets support the hypothesis that the envelope of bed motion will grow as an exponential taper, quickly at first, tapering off and approaching an asymptotic value. This growth is largest and fastest in the surf zone where wave and current flows are strong. Within the surf zone, envelopes owing solely to the migration of megaripples (bedforms with heights from 20 to 40 cm and lengths from 1 to 5 m) grow for about 8 d and reach an asymptote of about 40 cm. When wave energy becomes larger ( 1 m), bed envelopes are dominated by migrating sand bars and approach an asymptote of 3-4 m, but only after 2-12 years (depending on the beach). In addition, the frequency of object burial (the percentage of time that an object would be buried by the crests of migrating bedforms) is highest in the surf zone and grows rapidly with time.

A mapping technique for numerical computations of bed evolutions

Free surface flow is one of the most difficult problems in engineering to be solved, since velocity and pressure fields depend on the free surface. On the other hand, the position of the free surface is unknown previously. Furthermore, the boundary condition on the free surface is expressed by a complicated equation. In an alluvial stream, where the boundaries of the domain are not fixed, addition of free surface at the bed will increase this difficulty. A domain mapping technique is developed in this paper to study the bed evolutions. The flow is considered 2D, choosing two coordinates in streamwise and upward directions. With a proper transformation, the hydrodynamics and sediment transport governing equations in irregular domain will be mapped into a simple rectangular one. The new domain can be discretize by finite elements. The transformed governing equations are solved to obtain desired variables in the mapped domain. With a proper transformation, there is no need of inverse mapping to obtain the free water surface profile and bedform evolution and migration in the actual domain. The model has been applied to streams with movable bed and the results show a good agreement with the experimental experiences.