Sand Waves Research Articles

Coastline dynamics in the extreme south of Brazil and their socio-environmental impacts

A coastal inflection of ∼45 km in the extreme south of Brazil demonstrates differences in coastline behavior with areas of deposition towards north and areas of severe erosion towards south, threatening coastal occupation. These areas are believed to be controlled by the sand wave behavior of the coastline with adjacent erosion and progradation areas that have relevant alongshore sediment transport imposed by S quadrant waves. In this study, we quantified the rates of change of coastline and described the erosion and deposition controls behind it, using ultra-high-resolution orbital images and UAV (unmanned aerial vehicle) orthomosaics obtained between 2005 and 2020 for three urbanized beach sectors within the area, i.e., the São Simão, Mostardense, and Lighthouse Beaches. The rates of change of coastline were calculated using the change polygon method. The results indicated total recession values > 40 m at the Mostardense and Lighthouse beaches corresponding to a rate of ∼3 m/y. Such values characterized the coastal erosive hotspots that have not been previously identified in the area. Future coastlines were projected from the estimated annual variation rates. Buildings previously destroyed or threatened by marine/wind processes were also mapped. A continuous high-resolution and high-frequency coastal monitoring approach for the area is essential for planning the future development of these beaches to minimize and possibly avoid environmental degradation.

Engineering Feasibility Assessment of Cage Aquaculture in Offshore Wind Power Generation Areas in Taiwan

This paper investigates the engineering feasibility of cage aquaculture in the offshore wind farm area of Changhua, Taiwan. Two types of net cages commonly used in Taiwan are compared under typhoon wave and monsoon wave conditions to determine the suitable one. The submersible function of the selected cages to reduce the typhoon wave effect is analyzed. The yearly probability of the remaining volumes of the selected cages (deformed volume/undeformed volume) under a wave scatter diagram combined with a tidal current of the study area are analyzed and discussed. The influences of water depth on the selected cages for the site selection are investigated. Regarding the seabed sand waves in the study area, two potential disasters of mooring line failure and anchor sinking on the selected cages are considered and discussed. Both issues are a concern for the wind farm developers, fishery groups, and government authorities. According to the results, the selected cage aquaculture is feasible from an engineering standpoint in the case study area.

The role of idealized storms on the initial stages in sand wave formation: A numerical modeling study

Sand waves are rhythmic bedforms observed in many shallow shelf seas. By adopting a process-based numerical morphodynamic model, Delft3D, the effect of the storm-related processes on the growth and migration of sand waves was studied systematically in this paper. In our model, the storm effect was included by adopting the shear stresses due to the nonlinear wave effect, wave-current interaction, and wind-driven current. A series of numerical experiments were performed, and it was found that accounting for the wave nonlinearity is important in predicting the growth rate. Moreover, the effect of wave-current interaction on sand wave growth and migration is significant. The asymmetric velocity of water particles induced by wave-current interaction leads to net sediment transport rates which resulted in the migration of sand waves. The wind-driven current effect can breach the symmetry of the tidal current and cause the migration of sand waves. In our specified cases, the effect of wind-driven current or wave action on the LFGM was not significant, while the combination of these two effects causes a larger wavelength of sand waves. The model results show that the storms influence sand wave dynamics in the formation stage significantly and are therefore important to account for in designing offshore engineering operations in sand wave fields.

Do tidal sand waves always regenerate after dredging?

Tidal sand waves are rhythmic bedforms found on sandy continental shelves that pose a threat to offshore activities. While emphasis is placed on studying their natural morphodynamic evolution, little is known about if and how fast sand waves recover after dredging. This work presents an analysis of multibeam echosounder data collected at three former sand extraction sites on the Belgian continental shelf. At one of the sites, sand waves seemed to reappear approximately 5 years after dredging had stopped, which did not happen at the other two sites during the measurement period (5 and 9 years). The lack of recovery in those sites is likely the result of larger depths and smaller local sediment availability compared with the site where recovery occurred. Furthermore, these data reveal that in the latter site sand wave recovery was established mainly through local sediment redistribution.•Tidal sand waves are isolated from bathymetric data of the Belgian continental shelf.•At only one of the three sites, sand waves seemed to regenerate after dredging.•Possible explanations are differences in water depth and local sediment availability.•The regenerating tidal sand waves do so as a result of local redistribution of sand.

Roughness Effects of Subaquaeous Ripples and Dunes

Numerous questions and problems on Earth and questions with respect to other planets arise from morphodynamic processes caused by sediment movements driven by flows of fluids, such as water, air and other gases. A sediment surface opposes the current with a resistance that is determined by its skin or grain roughness. As soon as sand waves, such as ripples and/or dunes, are formed, these bedforms cause a further resistance to the flow, the so-called form roughness. Dependent on the dimensions of the ripples and dunes, the form roughness can be much more pronounced than the skin roughness. The relevant literature provides a large number of solution approaches based on different basic ideas and different result quality. The aim of this paper is a comparative analysis of solution approaches from the literature. For this purpose, 14 approaches to bedform-related friction in the subaqueous case are evaluated using 637 measurements from laboratory and natural settings. We found that all approaches were significantly more accurate for ripples than for dunes. Since this was equally the case for all approaches tested, it is reasonable to assume that this is caused by measurement inaccuracies for dunes in the natural case rather than due to the approaches themselves. The approach of Engelund 1977 proved to be most accurate among all approaches investigated here. It is based on the Borda–Carnot formulation and an additional empirical term. An analytical derivation and justification is provided for this additional term.

Reassessing evidence of Moon–Earth dynamics from tidal bundles at 3.2 Ga (Moodies Group, Barberton Greenstone Belt, South Africa)

AbstractPast orbital parameters of the Moon are difficult to reconstruct from geological records because relevant data sets of tidal strata are scarce or incomplete. The sole Archean data point is from the Moodies Group (ca 3.22 Ga) of the Barberton Greenstone Belt, South Africa. From the time‐series analysis of tidal bundles from a well‐exposed subaqueous sand wave of this unit, Eriksson and Simpson (Geology, 28, 831) suggested that the Moon’s anomalistic month at 3.2 Ga was closer to 20 days than the present 27.5 days. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although this study’s detailed geological mapping and section measuring of the site confirmed that the sandstone bed in question is likely a migrating dune, the presence of angular mud clasts, channel‐margin slumps, laterally aggrading channel fills and bidirectional paleocurrents in overlying and underlying beds suggests that this bedform was likely located in a nearshore channel near lower‐intertidal flats and subtidal estuarine bars; it thus carries risk of incomplete preservation. Repeated measurements of foreset thicknesses along the published traverse, measured perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along traverses measured parallel to bedding along the 20.5 m wide exposure are affected by minor faulting, uneven outcrop weathering, changing illumination, weather, observer bias and show a low reproducibility. The most robust measurements herein confirm the periodicity peak of approximately 14 in the original data of Eriksson and Simpson (Geology, 28, 831). Because laminae may have been eroded, the measurements may represent a lower bound of about 28 lunar days per synodic month. This estimate agrees well with Earth–Moon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a lower orbit around a faster‐spinning Earth.

Modelling the influence of Tidal Energy Converters on sediment dynamics in Banks Strait, Tasmania

As the tidal energy industry grows, developers are interested in the influence that large scale arrays might have on their surrounding environment including hydrodynamic and sediment. Understanding of local and regional sediment dynamics is an important element for environment impact and site assessments, as changes in sediment processes could alter bed ecosystems. In Australia, the Banks Strait located in the Northeast of Tasmania has been identified as a promising site for tidal energy. Very large sand waves have been observed near the area suitable for tidal turbines installation and they may interfere with human activities. Building from previous work, a 2D regional scale model investigated the influence of different tidal farms on the morphology of Banks Strait and especially the sand waves. Results showed that tidal farm of more than 49 MW rated power will alter the circulation of residual currents in Banks Strait leading to significant changes in the sediment dynamics, and farms less than 24.5 MW will have negligible influence on the migration of sand waves.

Upslope migrating sand waves on sediment-starved shelves: An example from the southeastern continental margin of the Korean Peninsula

An uncharted field of sand waves was discovered in a low-relief submarine canyon incised in the outer shelf on the southeastern continental margin of the Korean Peninsula in water depths of 180–190 m. We characterize the nature and origin of the waves and the sand forming them using sub-bottom chirp profiles, eXpendable bathythermograph (XBT) profile, multibeam echosounder (MBES) data, and sediment samples from four piston cores. Two types of sand waves characterized by distinct height versus wavelength relationships were found in the study area. The sand waves in the upper, narrower part of the shelf-incised canyon are sinuous-crested, with amplitudes of 0.3–2.1 m (mean: ~1 m) and wavelengths of 10–45 m (mean: ~24 m). Their asymmetry indicates migration upslope in a southwesterly direction, opposite to the surface currents. In contrast, the lower part of the canyon that is wider and closer to the margin of the continental shelf hosts nine long (ca. 1 km) curvilinear-crested sand waves with symmetrical crests; these waves likely reflect transient bedforms forming under fluctuating current conditions.The sediment of the sand waves consists of a variable mixture of siliciclastic and carbonate materials. The carbonate fraction (~22–55%; mean: ~34%) is derived mainly from the remains of bryozoans, bivalves, echinoderms, foraminifers, gastropods, and serpulids. Six bioclasts were dated by the radiocarbon method between ca. 41.3 and 11.8 ka BP. These relatively old ages and palaeontological data supports reworking from a shallow-marine environment during the last glacial transgression and limited sedimentation/sediment supply in the study area. The siliciclastic fraction (~44–79%; mean: ~37%) is composed of rounded to subrounded quartz and feldspar of moderate to good sorting and a mean grain size of ~1.3 phi (medium sand). The uppermost ~30 cm of all the sand wave cores reveals a decrease in the grain size of the siliciclastic fraction coupled with an increase in the carbonate/siliciclastic ratio, suggesting episodic sediment reworking and migration of the sand waves in response to fluctuating bottom currents. The coarser sediment that forms the core of the sand waves records bedload transport during periods of stronger currents. Finer carbonate-rich pelagic sediment (i.e., plankton) accumulated at the top of the sand waves during periods of weaker bottom currents. Significantly, our results show that the grain size and mineralogy of the sediment composing the sand waves are controlled by changes in hydrodynamic conditions. Our study provides novel geomorphological evidence for the influence of SW-flowing cold-water incursions (Korean Strait Bottom Cold Water) on the seafloor sediments.

A new formula for predicting movable bed roughness coefficient in the Middle Yangtze River

Computing movable bed roughness plays an important role in the modeling of flood routing and bed deformation, and the magnitude of movable bed roughness is closely associated with complex bedform configurations that change with the sand wave motion. The motion of sand wave is dependent on the incoming flow and sediment conditions and channel boundary. After the operation of the Three Gorges Project, the flow and sediment regime changed remarkably in the Middle Yangtze River (MYR), followed by significant channel adjustments. A dramatic decrease in sediment concentration caused continuous channel degradation and significant variations in cross-sectional profiles of the MYR. These adjustments in the channel boundary influence the motion of sand wave, which can further affect the magnitude of movable bed roughness. A new formula for predicting the movable bed roughness coefficient is developed, which can be expressed by a power function of both Froude number and relative water depth. The proposed formula was first calibrated using 1266 datasets of measurements at five hydrometric stations in the MYR during 2001–2012. A back-calculation process shows that the roughness coefficients calculated by the proposed formula agree well with the observations, with the determination coefficient being equal to 0.88. The proposed formula was further verified using 651 datasets of measurements at these hydrometric stations during 2013–2017. Furthermore, four common roughness formulas selected from the literature were tested for comparison. The results indicate that the calculation accuracy of the proposed formula is significantly higher than that of the previous formulas, and the Manning roughness coefficients predicted by the proposed formula have the errors less than ±30% for 96% of the datasets. Therefore, the new bed roughness predictor proposed in this study can accurately calculate the roughness coefficients straightforwardly without iterative solution and graphical interpolation, and the parameters required in the roughness predictor are easily obtained from the hydrometric observations.

The sedimentary response of mixed lithoclastic-bioclastic Lower- Pleistocene shallow-marine systems to tides and waves in the south Apennine foredeep (Basilicata, southern Italy)Tidalites Field Trips Special Volume - Tidalites, Matera, Field Trip T3

This two-day-long field trip is associated with the 10th International Congress of Tidal Sedimentology (Tidalites), Matera, Italy. The foreland-basin system of southern Italy preserves mixed lithoclastic-bioclastic deposits mostly accumulated in shallow-marine environments, reproducing types of mixing and degrees of segregation (sensu Chiarella and Longhitano, 2012; Chiarella et al., 2017) that are thought to be indicative of a variety of geological processes, including surficial waves and tides, acting at different time scales.During the Late Pliocene and Early Pleistocene, small open piggyback basins reproduced marginal-marine settings with various hydrodynamic conditions in the wedge-top depozone. Siliciclastic-bioclastic sediments accumulated under the influence of geomorphological elements, such as coastal sheltering, promontories, presence of tectonic highs and local inlets, hosting shoreface to offshore-transition zones, whose cross-stratified facies indicate how sensitive shallow-water mixed systems are to recording surficial waves and weak tidal currents.Along the outer-foredeep depozone, carbonate-bioclastic coastal wedges back-stepped over the gently sloping rocky flanks of a structural high, leading to the retrogradational stacking of seismic-scale prograding bodies. Their size and cross-sectional outcrop views apparently resemble those typical of large tidal sand waves developed in wide tide-dominated oceans. In contrast, internal facies features reveal the dominance of surficial waves and gravity-driven avalanches along clinoform depositional slopes, and a very negligible to no tidal influence.

Mixed‐diurnal tidal record from a Palaeoproterozoic embayment in the Par Formation, Gwalior Group, Central India

The Palaeoproterozoic Par Formation, Gwalior Group, central India contains a mixed, predominantly diurnal, tidal record in an embayment‐like estuarine setting where flood energy exceeded the ebb energy. From two order differences in set thickness between flood‐ and ebb‐generated bedforms an asymmetry in peak tidal current velocity of flood and ebb is inferred with domination of flood current. From the presence of crossover in successive neap records and 27.75 neap‐spring cycles per year, an unambiguous clue for tidal forcing by lunar declination rather than lunar phase is inferred. Harmonic analysis of foreset‐lamina thickness from tidal sand waves suggests that a tropical month of minimum 26 lunar days was in effect at the Gwalior Sea during the late Palaeoproterozoic (2,000–1,791 Ma).

Deepwater seismic facies and architectural element interpretation aided with unsupervised machine learning techniques: Taranaki basin, New Zealand

The use of unsupervised machine learning (ML) methods such as self-organizing maps (SOMs), has gained a significant foothold within the seismic interpretation community to enhance results and help identify similar patterns in the data. Workflows for each geological setting become, however, necessary. We analyzed a series of geometrical, instantaneous, spectral, and textural seismic attributes to provide multi-attribute input combinations that, with SOMs, allow for a proper interpretation of architectural elements in deepwater settings. By studying the Miocene section within the Pipeline 3D seismic dataset in southern Taranaki Basin, we show that GLCM entropy, RMS amplitude, sobel filter similarity, and frequencies seismic attributes used as inputs into the SOM enhance the interpretation of the features of interest, initially overlooked by single attributes analysis. This method had not been applied in the area yet. Results allow understanding and delineating architectural elements of deepwater systems, such as mud- and sand-filled channels, mass transport deposits (MTDs), and marine shales in the basin floor. Additionally, more minor features such as levees, overbank deposits, and sand waves are better mapped. The use of the Pukeko well helped to understand the system's evolution, which we interpret as a fan system incised by younger, more sinuous channels. Overall, the system turns more mud-prone as it becomes younger (upwards). Our study contributes to understanding the paleogeomorphology of the area, which can help optimize drilling plans, whereas our method offers three input attribute combinations for SOMs that can be applied in similar geological settings to aid in the interpretation optimization process.

Effect of topographic background on sand wave migration on the eastern Taiwan Banks

Observational analysis has revealed an apparent immobility of giant sand waves on the Taiwan Banks, while sand waves on the eastern Taiwan Banks show distinct dynamics influenced by the underlying larger-scale sand ridge bedforms. In this study, the effects of these sand ridge bedforms on sand wave migration and the local hydrodynamic environment were investigated using bathymetric and hydrodynamic datasets from 2016 and 2018. Upon separating the NNW-SSE sand ridge crest, the opposing sand wave migration and complementary sediment loss/gain on either side of the sand ridge are interpreted as convergence processes that serve to maintain the shape of the sand ridge. Analysis of the hydrodynamics revealed that, during the flood phase, the northward directed flow on the western side of the ridge crest was enhanced due to rapid shoaling. This caused the development of a flow shadow zone on the eastern side of the ridge crest, where the flow was rotated and weakened. During a tidal cycle, the dominance of the flood current on the western side of the ridge resulted in northward migration of sand waves. Conversely, due to the weakened flood current on the eastern side of the ridge, sand waves migrated southward in response to the stronger ebb current. This study thus provides insights into bedform dynamics in complex morphological settings and the maintenance mechanism of morphodynamic bedform systems.

Development of Physics-Based Morphology Model with an Emphasis on the Interaction of Incoming Waves with Transient Bed Profile due to Scouring and Accretion using Dynamic Mesh

A physics-based morphology model [Seoul Foam] was developed using the dynamic mesh technique to explain the interaction between the sea bed, which undergoes deformation due to siltation and scouring, and the incoming waves. In doing so, OlaFlow, an Open Foam-based toolbox, was used as a hydrodynamic model. To verify the proposed physically-based morphology [Seoul Foam] in this study, numerical simulations of the shoaling process over the beach of the uniform slope were implemented. The numerical result shows that the formation process of a sand bar over the foreshore was successfully simulated. As can be easily anticipated, the size of the sand bar was closely linked to the nature of incoming waves, and in the case of a rough sea, the foreshore slope was rapidly deformed due to scouring. In mild seas, several sand waves were formed near the shoreline, and when the exposure time was the same, the size of the sand waves was not as large as in rough seas.

A physically-modified spectral model for prediction of sand wave migration

Sand waves are rhythmic bedforms existing in shallow seas and typically induced by the interaction of currents and topography. In this study, a physically-modified spectral model for simulating small-scale sand wave migration is proposed by integrating non-linear spectral analysis, typhoon wind field and sediment transport theory. We introduce a power-average velocity to modify the spectral analysis, which gives explicit physical meanings to the morphological method and improves its stability and reliability. Then, the effects of typical and extreme weather are decoupled and the short-term migration caused by typhoons is removed from the total migration. Results show that a single typhoon will cause large sand wave migration, but the migration caused by multiple typhoons in a long period of time will cancel each other out in the measured data. Moreover, the bilateral reverse migration trend of one-group sand waves in a small scale (∼3 km) is captured, which is consistent with field data and previous researches. Compared with existing researches, the proposed model is able to identify the influences of extreme conditions on sand wave migration and provide a feasible and high efficiency way to predict sand wave migration when the available hydrodynamic data is limited.

Modelling the Past and Future Evolution of Tidal Sand Waves

This study focuses on the hindcasting and forecasting of observed offshore tidal sand waves by using a state-of-the-art numerical morphodynamic model. The sand waves, having heights of several meters, evolve on timescales of years. Following earlier work, the model has a 2DV configuration (one horizontal and one vertical direction). First, the skill of the model is assessed by performing hindcasts at four transects in the North Sea where sand wave data are available of multiple surveys that are at least 10 years apart. The first transect is used for calibration and this calibrated model is applied to the other three transects. It is found that the calibrated model performs well: the Brier Skill Score is ‘excellent’ at the first two transects and ‘good’ at the last two. The root mean square error of calculated bed levels is smaller than the uncertainty in the measurements, except at the last transect, where the M2 is more elliptical than at the other three transects. The calibrated model is subsequently used to make forecasts of the sand waves along the two transects with the best skill scores.

Linear stability of sand waves sheared by a turbulent flow

In this paper, the linear stability of sand waves sheared by a turbulent flow is analyzed. The velocity distribution in the streamwise direction is considered to follow the logarithmic law. The modified pressure distribution owing to the streamline curvature induced by the sand waves is employed in the formulation. The pressure distribution is derived using the Boussinesq approximation for the variation of streamline curvature over the flow depth. The flow model is coupled with the sediment transport model to study the stability of sand waves. Both the modes of sediment transport as bedload and suspended load are considered. The linear stability analysis reveals the favorable region for the formation of dunes and antidunes on the plane formed by the Froude number and the dimensionless wavenumber. The region of instability increases with an increase in particle parameter, while it decreases with an increase in relative roughness. The analysis reveals that the sand waves form beyond a limiting Froude number corresponding to the threshold Shields number. The theoretical results compare well with the available experimental data.

Numerical Analysis of Modified Seabed Topography Due to the Presence of Breakwaters of Varying Reflection Characteristics using Physics-based Morphology Model [SeoulFoam

Numerical simulations were implemented to look into the modified seabed topography due to the presence of breakwaters of varying reflection characteristics. The numerical model was composed of OlaFlow, an OpenFoam-based tool box, and a physics-based morphology model [Seoul Foam]. In doing so, the interaction between the seabed, which undergoes deformation due to siltation and scouring, and the incoming waves was described using Dynamic Mesh. The rubble-mound, vertical, and curved slit caisson breakwaters with varying reflection characteristics resulted in standing waves that differ from each other, shown to have a significant influence on the seabed topography. These results are in line with Nielsen’s study (1993) that sands saltated under the surface nodes of standing waves, where the near-bed velocities are most substantial, convected toward the surface antinodes by boundary-layer drift. Moreover, the crest of sand waves was formed under the surface antinodes of standing waves, and the trough of sand waves was formed under the surface antinodes. In addition, sand wave amplitude reaches its peak in the curved slit caisson with a significant reflection coefficient, and the saltation of many grains of sand would cause this phenomenon due to the increased near-bed velocity under the nodes when the reflection coefficient is getting large.

Predicting Dominance of Sand Transport by Waves, Tides, and Their Interactions on Sandy Continental Shelves

AbstractWaves and tidal currents resuspend and transport shelf sediments, influencing sediment distributions and bedform morphology with implications for various disciplines including benthic habitats, marine operations, and marine spatial planning. Shelf‐scale assessments of wave‐tide‐dominance of sand transport tend not to fully include wave‐tide interactions, which nonlinearly enhance bed shear stress and apparent roughness, change the current profile, modulate wave forcing, and can dominate net sand transport. Assessment of the contribution of wave‐tide interactions to net sand transport requires computationally/labor intensive coupled numerical modeling, making comparison between regions or climate conditions challenging. Using the Northwest European Shelf, we show the dominant forcing mode and potential magnitude of net sand transport is predictable from readily available, uncoupled wave, tide, and morphological data in a computationally efficient manner using a k‐Nearest Neighbor algorithm. Shelf areas exhibit different dominant forcing modes for similar wave exceedance conditions, related to differences in depth, grain size, tide range, and wave exposure. Wave‐tide interactions dominate across most areas in energetic combined conditions. Meso‐macrotidal areas exhibit tide‐dominance while shallow, fine‐grained, microtidal regions show wave‐dominance over a statistically representative year, with wave‐tide interactions dominating extensively >30 m depth. Sediment transport mode strongly affects seabed morphology. Sand wave geometry varies significantly between predicted dominance classes with increased wave length and asymmetry, and decreased height, for increasing wave‐dominance. This approach efficiently indicates where simple noninteractive wave and tide processes may be sufficient for modeling sediment transport, and enables efficient interregional comparisons and sensitivity testing to changing climate conditions with applications globally.

LSTM-Based Remote Sensing Inversion of Largescale Sand Wave Topography of the Taiwan Banks

Shallow underwater topography has important practical applications in fisheries, navigation, and pipeline laying. Traditional multibeam bathymetry is limited by the high cost of largescale topographic surveys in large, shallow sand wave areas. Remote sensing inversion methods to detect shallow sand wave topography in Taiwan rely heavily on measured water depth data. To address these problems, this study proposes a largescale remote sensing inversion model of sand wave topography based on long short-term memory network machine learning. Using multi-angle sun glitter remote sensing to obtain sea surface roughness (SSR) information and by learning and training SSR and its corresponding water depth information, the sand wave topography of a largescale shallow sea sand wave region is extracted. The accuracy of the model is validated through its application to a 774 km2 area in the sand wave topography of the Taiwan Banks. The model obtains a root mean square error of 3.31–3.67 m, indicating that the method has good generalization capability and can achieve a largescale topographic understanding of shallow sand waves with some training on measured bathymetry data. Sand wave topography is widely present in tidal environments; our method has low requirements for ground data, with high application value.