Sand Transport Mechanisms Research Articles

Machine-learning-assisted seismic prediction of massive (up to 500 m) sands and their geologic meaning: Lower wilcox intraslope basins, south-central Texas, USA

We studied deep (up to 6000 m) Lower Wilcox strata (Guadalupe A) immediately above the Midway shale using a 1000-km2 3D seismic volume and sparse six deep-well, wireline-log data in south-central Texas. Although some of the wells encounter sandstones collectively as thick as 500 m, traditional seismic attributes and seismic inversion are unable to reveal the sign and extent of the massive sand distribution in the intraslope minibasins. A new workflow that incorporates machine learning into seismic lithology and geomorphology was tested for quantitative mapping of sand-content sandstone thickness. The procedure significantly updates current workflows for better geologic and reservoir prediction with higher resolution and accuracy. In addition to good fit to impedance in training wells, results tied gamma-ray and sand-content curves at a blind-test well, ensuring high confidence in stratigraphic and depositional interpretation. The study recognized multiple thick, sandy intraslope minibasins in a 30-km-wide upper slope. Clinoforms on the shelf, erosional channels at the shelf edge, sandy channels, and fans in intraslope minibasins were found to support the interpretation of a unique sediment-disposal system triggered by hyperpycnal flows. Existence of massive sand accumulations in the upper slope zone in the Gulf Coast suggest the existence of another major mechanism of large sand transport during the early Wilcox in a high sea-level stand and possibly a future deep gas play in the area.

Field investigation of the fetch effect and essential conditions for saturated sand flow

AbstractSeveral models of classical aeolian sand transport mechanism have been developed to predict the rate of sand transport. However, the values predicted through modeling almost always display large and significant differences from the measured values. The reason for such discrepancies is not fully understood. By studying the effects of wind velocities and fetch lengths on the C values in the model proposed by Lettau and Lettau in 1978, we found that the saturation level of the measured sand flow can significantly affect the accuracy of the predicted value. To solve this problem, we introduced an index (δ) to represent the saturation level. The closer the δ value is to 1, the greater the chance that the sand flow is saturated. This new approach only requires measurement of the saturated sand flow data to verify the robustness of the aeolian sand transport model, a step that has been neglected by previous researchers. Our results suggest that the value of C = 6.7 proposed in the Lettau and Lettau model is sound and reliable if the sand flow is saturated. We proposed the conditions required for the sand flow to reach saturation and generated an equation to predict the sand transport saturation length (Lsat). Accordingly, we estimated Lsat to be approximately 92 m when = 0.47 m s−1 for the studied sand surface, which is also the minimum wind shear velocity at which the sand flow can potentially reach saturation.

Observations of near-bed orbital velocities and small-scale bedforms on the Dutch lower shoreface

The lower shoreface, with water depths between about 8 and 20 m, forms the transition between the inner shelf and upper shoreface. Knowledge of the lower shoreface is essential, as it is – in many cases – the sediment source for the upper shoreface and beach. This paper presents new data of near-bed orbital velocities and small-scale bedforms at various depths and locations on the Dutch lower shoreface. Near-bed orbital velocities were beyond 1 m/s during high-energetic wave conditions. They increase with wave height and decrease with water depth, and can be reasonably well described by linear wave theory. Ripple heights range between 0.01−0.03 m and ripple lengths between 0.08−0.20 m. Ripple dimensions are controlled by wave mobility, with lower and shorter ripples for higher waves, and not so much by the currents. The Van Rijn (2007) formula generally overpredicts the ripple heights, and the variation with tidal currents in time. The measurements clearly indicate significant sediment mobility at the lower shoreface under higher wave events. It is yet unclear what this means for the net sand transport. This will depend on the subtle timing of sediment suspension, wave-mean currents and near-bed orbital velocities. It requires detailed modeling to determine lower shoreface net transport rates, and to unravel the controlling sand transport mechanisms.

Aeolian sand transport influenced by tide and beachface morphology

Beach aeolian sand transport is one of the most complicated but significant processes in beach-dune interactions. Focusing on tidal and morphological effects, field observations over several tidal cycles were carried out successively on two macrotidal and gently sloped beaches to clarify the characteristics, dominant factors and possible mechanism of aeolian sand transport in the intertidal zone. The results show that there is a clear “time window” for aeolian sand transport in the intertidal zone under the joint effect of tidal action and ridge morphology. During ebb tide, the factors controlling the initialization of intertidal aeolian sand transport shifted sequentially from ridge inundation (controlled by tide) to seepage, capillary and drying processes. During the time window, the fluctuations of dew point appear to be a good indicator of the variations in aeolian transport rates. During flooding, the dominant factor for the cessation of intertidal aeolian sand transport is inundation (controlled by the tidal elevation). Further exploration shows that for the wide and gently sloped beachface, the presence of intertidal ridges may be necessary for the aeolian process to entrain and transport sand from the moist intertidal zone. Finally, a conceptual model for the intertidal aeolian sand process under the joint effect of tide and ridge morphology is proposed.

Particle-Transport Mechanism in Liquid/Liquid/Solid Multiphase Pipeline Flow of High-Viscosity Oil/Water/Sand

Summary In this study, an investigation of sand transport in heavy-oil/water multiphase flow is performed. The study is conducted in three multiphase-flow pipeline-test facilities with internal diameters (IDs) of 1, 1, and 3 in. The pipeline orientations relative to the horizontal in the facilities are 0, +30, and 0°, respectively. Oil viscosity of 3.5 and 10.0 Pa·s with sand volume fractions from 0.010 to 0.100 vol% were used in the study. The effects of oil viscosity, upward inclination, sand volume fraction, pipe ID, and water cut on the sand-transport mechanism in pipelines are investigated. In the horizontal test section, flow patterns—namely, dispersed flow (DF), plug flow (PF), plug flow with moving sand bed (PFM), and plug flow with stationary sand bed (PFS)—were identified through flow visualization. In addition to the aforementioned, two flow patterns—stratified wavy flow with moving sand bed (SWM) and stratified wavy flow with dunes (SWD)—were observed in the inclined pipeline orientation. The pressure gradient measured decreased with a decrease in water cut until a minimum value was reached. Beyond the minimum pressure gradient, further reduction in water cut led to an increase in pressure gradient. The sand minimum transport condition (MTC) in the oil/water/sand test was largely the same for the 1-in. 30° upward inclined and the 1-in. horizontal test section. In contrast, that of the 3-in. horizontal test section was considerably higher. An improved MTC predictive correlation is proposed for multiphase heavy-oil/water/sand flow. The proposed correlation outperforms the existing models when tested on the heavy-oil/water/sand data set.

Monitoring spatiotemporal variation in beach surface moisture using a long-range terrestrial laser scanner

The measurement of surface moisture on beaches is vital for studying aeolian sand transport mechanisms, but existing techniques are not adequate for monitoring the surface moisture dynamics over a substantial beach section. In this study, we investigated the suitability of a new remote sensing method to monitor the spatiotemporal variation in surface moisture on a sandy beach using a long-range static terrestrial laser scanner (TLS). The TLS was permanently deployed on top of a 42 m high building overlooking the study site at Ostend-Mariakerke, Belgium. Considering the effect of target surface roughness on the intensity and the laboratory’s length limitation, a new intensity correction method is proposed which only uses the field point cloud data measured on a homogenous beach surface (without time-consuming indoor experiments). Based on the corrected intensity data, the relation of the beach surface moisture to the corrected intensity was modeled by an exponential model with a correlation-coefficient squared of 0.92. A moisture estimation model was developed which can directly derive the beach surface moisture from the original intensity data of the TLS with a standard error of 2.27%. The hourly surface moisture dynamics across two tidal cycles on the beach were investigated as a case study, in which the point clouds derived from corresponding unmanned aerial vehicle (UAV) imagery are utilized to improve the calculation accuracy of the incidence angles of TLS point clouds at long distances. Results reveal that, after the intensity correction, the long-range static TLS is an extremely suitable technique to monitor the surface moisture dynamics (daytime and nighttime) over a substantial beach section (hundreds of meters) at a high scanning frequency (minutes to hours).

Artificial simulation of complex unsteady wind in an ABL wind tunnel with dual axial fans

The wind tunnel test plays an important role in studying the mechanism of aeolian sand transport. An atmospheric boundary layer (ABL) wind tunnel with dual axial flow fans to simulate unsteady wind is described in this paper. Air flow is generated using two fans arranged at each end of the wind tunnel. During the experiment, the rotation rates of the fans are changed by varying the frequency of the converters, which can be controlled by PLC. The speed of each fan varies according to a sinusoidal function. As a result of changing the parameters of the sinusoidal function, the two fans produce different forms of sinusoidal wind, and finally compose the controllable unsteady wind, which manifests as typical unsteady wind characteristics in the test section of the wind tunnel. The experimental results show that the wind speed profile of unsteady wind generated in the wind tunnel conforms to the logarithmic profile of the ABL. The proposed method is very useful for experimental research on the unsteady sand transport mechanism.

SHEETFLOW SEDIMENT TRANSPORT UNDER SKEWED - ASYMMETRIC WAVES AND CURRENTS

Prototype scale laboratory experiments have been conducted to investigate the sheetflow sediment transport of uniform sands under different skewed-asymmetric oscillatory flows. Experimental results reveal that in most of the case with fine sand, the "cancelling effect†, which balances the on-/off-shore net transport under pure asymmetric/skewed flows and results a moderate net transport, was developed for combined skewed-asymmetric flow. However, under some certain conditions (T > 5s) with coarse sands, the onshore sediment transport was enhanced by 50% under combined skewed-asymmetric flows. Sand transport mechanism under oscillatory sheetflow conditions is also studied by comparing the maximum bed shear stress and the phase lag parameter at each half cycle. A comparison of measurements including the new experimental data with a number of practical sand transport formulations shows that the Dong et al. (2013) formulation performs the best in predicting the measured net transport rates over a wide range of experimental conditions

Experimental study of pore water pressure and bed profile change under regular breaking waves

There lies a close relationship between the seabed destruction and the distribution of pore water pressure under the action of breaking wave. The experiments were carried out in a wave flume with a 1:30 sloping sandy seabed to study regular breaking wave induced pore water pressure. A wide range of measurements from the regular wave runs were reported, including time series of wave heights, pore pressures. The video records were analysed to measure the time development of the seabed form and the characteristics of the orbital motion of the sand in the wave breaking region. The pore water pressure in the breaker zone showed the time variation depending on the wave phases including wave breaking and bore propagation. The time-averaged pore water pressure was higher near the seabed surface. The peak values of pore water pressure increase significantly at the breaking point. The direction of pore water pressure difference forces in the breaker zone is of fundamental importance for a correct description of the sediment dynamics. The upwardsdirected pressure differences may increase sand transport by reducing the effective weight of the sediment, thereby increasing the bed form evolution. The seabed configuration changed greatly at the wave breaking zone and a sand bar was generated remarkably. The amplitude of the pore water pressure changed with the seabed surface. The results are to improve the understanding of sand transport mechanisms and seabed responses due to breaking regular waves over a sloping sandy bed.

The influence of hydrodynamic boundary conditions on characteristics, migration, and associated sand transport of sand dunes in a tidal environment

Large parts of the tidal estuary of river Elbe (Germany) are characterized by regular patterns of sand dunes. They evolve due to complex sand transport mechanisms and show multi-faceted migration patterns, which are influenced by hydrodynamic boundary conditions such as runoff and tidal forces. This study aims at increasing the understanding of the way hydrodynamic boundary conditions influence dune behavior. This is the basis of an effective sediment management as well as an important requirement for planning offshore structures. From a unique data set of up to six annual bathymetrical multibeam soundings between 1995 and 2010, bedform characteristics and migration rates were processed and analyzed with a set of automated methods. The influence of river runoff, water levels, current velocities, tidal range, and river depth on characteristics and migration were tested statistically. The results show that migration is mainly dominated by the incoming flood tide while rates and directions depend on the amount of runoff originating from the inland catchment.

Geomorphological evolution revealed by aeolian sedimentary structure in Badain Jaran Desert on Alxa Plateau, Northwest China

The Badain Jaran Desert, located in the Alxa Plateau, Northwest China, features mega-dunes and a unique dune-lake alternation landscape. This paper presented the aeolian sediment structures of three representative dunes in the Badain Jaran Desert using ground-penetrating radar (GPR). We processed and analyzed the GPR data and investigated the feasibility of using integrated GPR and sedimentological data to reconstruct dunes structure, sedimentary environment and geomorphological evolution. The results show that the internal structures of star dune and transverse dune represent various stages of mega-dune evolution: the main deposition processes of mega-dune are similar to those of transverse dunes but have a more complicated mechanism of sand transport and deposition because of the superimposition of dunes; the upper section of the mega-dune has a structure similar to that of star dune, with vertical aggradations on top. Diffraction hyperbolae in the GPR profile indicates that the presence of ancient dunes characterized by calcareous cementation layers is involved in the maintenance of mega-dunes, and water levels, shown by continuous, sub-horizontal GPR reflections, are supposed to be closely related to mega-dunes and the interdune lakes. Outcrop of wet sand and horizontal stratifications on the GPR image indicate moisture potentials with different levels inside mega-dunes. The multiplex geomorphology in the Badain Jaran Desert is the result of global climatic undulation, the unique geographical location, the geological structural features, etc.

MECHANISM OF OFFSHORE SAND DISCHARGE INTO SUBMARINE CANYON TRIGGERED BY CONSTRUCTING DETACHED BREAKWATER CLOSE TO SHORELINE

Several submarine canyons have developed offshore of the Aramata region of the Shimoni-ikawa coast in Toyama
 Bay, which is one of the three deep bays in Japan. The Kurobe River with a steep bed slope flows into the sea
 immediately north of this region, and a large amount of sediment has been supplied to this area, which has been
 transported by southward longshore sand transport. However, beach erosion has occurred owing to the decrease in sediment supply from the Kurobe River. Furthermore, since the construction of detached breakwaters as a measure
 against beach erosion, offshore sand transport has accelerated and beach erosion has become more severe. In this
 study, aerial photographs and bathymetric survey data were analyzed, then the mechanism of offshore sand transport
 was investigated using the BG model proposed by Serizawa et al. (2007).

DUNE MIGRATION AND SAND TRANSPORT RATES IN TIDAL ESTUARIES: THE EXAMPLE OF THE RIVER ELBE

Large parts of the tidal estuary of river Elbe (Germany) are characterized by regular patterns of sand dunes. They are presumed to evolve due to complex sand transport mechanisms and show multi-faceted migration patterns. Direction and magnitude of their migration are influenced by hydrodynamic boundary conditions such as river runoff and tides. Dune Migration can lead to residual sand transport rates, depending on its direction and magnitude and the dune's characteristics. The understanding of dune migration patterns and associated sand transport is the basis of an effective sediment management as well as an important requirement for planning offshore structures. This study focuses on methods for computing migration and sand transport rates in automated ways. In a comparison and validation of different approaches, a cross-correlation technique was found to produce best results. From a unique data set of up to six annual bathymetrical multi-beam soundings between 1995 and 2010, dune characteristics and migration rates were processed and analysed autonomously. The findings show that in a long perspective, average dune migration and sand transport rates in the present study reach are directed upstream.

砂丘斜面上の飛砂に関する現地観測

This study conducted field observations in terms of the number of blown sand impacts and wind velocity for two hours on 21 February 2009 at an open ocean beach in Japan in order to investigate characteristics of the aeolian sand transport flux over a sand dune slope. The Charnock constant related to wind shear on the sand bed surface calculated from the wind velocity data on the slope had a larger value over a flat. Furthermore the aeolian flux on the slope tended to be larger than that on the flat. Knowledge of the Charnock constant and the aeolian flux on the flat and slope was obtained from the field measurements which will become a great help for further investigation of the aeolian sand transport mechanisms.

Holocene drainage systems of the English Fenland: roddons and their environmental significance

The roddons of the English Fenlands are fossilised silt and sand-filled tidal creek systems of mid- to late-Holocene age, incised into contemporaneous clay deposits. However, anthropogenic change (drainage and agriculture) has caused the former channels to become positive topographical features. Three stratigraphically discrete generations of roddon have been discriminated. They all show well-developed dendritic meander patterns, but there is little or no evidence of sand/silt infill during meandering; thus, unlike modern tidal creeks and rivers they typically lack laterally stacked point bar deposits, suggesting rapid infill. Major “trunk” roddons are rich in fine sands and there is little change in grain size from roddon mouth to the upper reaches, suggesting highly effective sand transport mechanisms and uniform conditions of deposition. Tributaries are silt-rich, while minor tributaries also have a significant clay component. During infill, active drainage networks appear to have been choked by sediment, converting mudflat/salt-marsh environments into widespread peat-forming freshwater reed swamps.

Forms and sand transport in shallow hydraulic fractures in residual soil

Four sand-filled hydraulic fractures were created at a depth of 1.5 m, and the vicinities of the fractures were excavated and mapped in detail. All the fractures were shaped like slightly asymmetric saucers between 4.5 and 7.0 m across that were roughly flat-lying in their centers and curved upward to dip between 10° and 15° along their peripheries. Three different colors of sand were injected in sequence to trace the relative ages of the sand in the fracture. The first sand to be injected remained in the vicinity of the injection casing, whereas the last sand moved rapidly to the leading edge. Sand transport occurred through localized, channel-like pathways that extended from the injection casing and then branched into multiple paths as they approached the leading edge. At least four branching pathways of different ages were identified in one fracture, suggesting that this represents a fundamental mechanism of sand transport in these shallow fractures. A theoretical model was developed by adapting Franc2d, a code well-known in structural mechanics, to predict the propagation paths of curved hydraulic fractures at shallow depths. The model predicts fracture forms that are remarkably similar to those in field exposures when properties typical of field conditions are used.Key words: hydraulic fracture, field test, mechanics, modeling.

Shear stress and sediment transport calculations for sheet flow under waves

A simple method is provided for calculating transport rates of not too fine ( d 50≥0.20 mm) sand under sheet flow conditions. The method consists of a Meyer-Peter-type transport formula operating on a time-varying Shields parameter, which accounts for both acceleration-asymmetry and boundary layer streaming. While velocity moment formulae, e.g.., Q s = Constant× u ∞ 3 , calibrated against U-tube measurements, fail spectacularly under some real waves (Ribberink, J.S., Dohmen-Janssen, C.M., Hanes, D.M., McLean, S.R., Vincent, C., 2000. Near-bed sand transport mechanisms under waves. Proc. 27th Int. Conf. Coastal Engineering, Sydney, ASCE, New York, pp. 3263–3276, Fig. 12), the new method predicts the real wave observations equally well. The reason that the velocity moment formulae fail under these waves is partly the presence of boundary layer streaming and partly the saw-tooth asymmetry, i.e., the front of the waves being steeper than the back. Waves with saw-tooth asymmetry may generate a net landward sediment transport even if u ∞ 3 =0 , because of the more abrupt acceleration under the steep front. More abrupt accelerations are associated with thinner boundary layers and greater pressure gradients for a given velocity magnitude. The two “real wave effects” are incorporated in a model of the form Q s( t)= Q s[ θ( t)] rather than Q s( t)= Q s[ u ∞( t)], i.e., by expressing the transport rate in terms of an instantaneous Shields parameter rather than in terms of the free stream velocity, and accounting for both streaming and accelerations in the θ( t) calculations. The instantaneous friction velocities u *( t) and subsequently θ( t) are calculated as follows. Firstly, a linear filter incorporating the grain roughness friction factor f 2.5 and a phase angle ϕ τ is applied to u ∞( t). This delivers u *( t) which is used to calculate an instantaneous grain roughness Shields parameter θ 2.5( t). Secondly, a constant bed shear stress is added which corresponds to the “streaming related bed shear stress” −ρ( u ̃ w ̃ ) ∞ . The method can be applied to any u ∞( t) time series, but further experimental validation is recommended before application to conditions that differ strongly from the ones considered below. The method is not recommended for rippled beds or for sheet flow with typical prototype wave periods and d 50<0.20 mm. In such scenarios, time lags related to vertical sediment movement become important, and these are not considered by the present model.

SAND RIPPLE GEOMETRY AND SAND TRANSPORT MECHANISM DUE TO IRREGULAR OSCILLATORY FLOWS

This paper describes characteristics of sand ripples and sand transport mechanism in regular and irregular oscillatory flows on the basis of detailed laboratory measurements. A set of empirical relations were proposed to evaluate the sand ripple geometry as well as the onset of the sheet flow transport. The applicability of the proposed relationships to the irregular wave conditions with prototype scales was confirmed with existing field data.

SEDIMENTOLOGICAL INFLUENCES OF DETACHED BREAKWATERS

Wave diffraction and refraction in the surroundings of a new detached breakwater induce currents strong enough to cause substantial local sediment transport and consequently morphologic changes, the main feature of which are sand spits or tombolos. As shown by field studies the morphologic changes cause an equilibrium state with some minor fluctuations due to changing wave climate. Model studies have also proved the existance of such an equilibrium state for any given geometry and sea state The present paper treats mainly this equilibrium state, but also attempts to explain the mechanism of sand transport characterizing the initial and transitional states. According to observations by the authors in small scale models, the transporting mechanism in the transitional state involves sea bottom erosion, especially near the breakwater heads outside the protected area, and transport of sand towards the shore of the sheltered area in the form of small migrating sand bars, which finally join the shore line and widen it. Results of experimental investigations conducted by the authors, for the particular case of high impervious breakwaters attacked by waves of normal incidence, as well as results of field and model studies given by others, are used to define relationships among the factors determining the equilibrium state and to base a new hypothesis regarding the equilibrium state. This hypothesis states that a morphologic and sedimentologic equilibrium is reached behind a detached breakwater, when the shape of the contour lines becomes such, that along the sheltered beach the diffracted waves have components of momentum opposed to the gradients of the mean sea level induced by radiation stress due to non uniform wave heights along the wave fronts approaching from both sides of the breakwater. The significant parameters characterizing the dimensions of the spit or tombolo in the equilibrium state are shown to be the relative length of the breakwater (compared to its distance from the original shore line), the relative distance from the original shore line (compared to the position of the breakers' line) and the relative height of the breakwater crest (above M.S.L, compared to the incident wave height).

SECONDARY SAND TRANSPORT MECHANISMS

The basic concept of coastal littoral drift consisting of a "river" of sand driven by the direct alongshore component of oblique waves has long been considered as a basis for littoral transport equations. This concept however is highly simplified since the actual littoral drift on real beaches represents only the final result of the inter-action of dozens or perhaps hundreds of secondary hydraulic processes which continuously occur in Nature. On many Coasts littoral transport is taken to represent the primary mechanism of beach recession or build-up but many of the secondary mechanisms themselves are only partially understood and their individual contributions to the primary process largely unexplored. This paper therefore discusses some of these secondary processes and since offshore- onshore sediment transport is probably the most important of these a mechanism for this transport mode is suggested based upon the interaction between wave energy and a work capacity parameter for the beach sediments.