Sand Wave Height Research Articles

Consequences of limited sediment supply for long-term evolution of offshore tidal sand waves, a 3D model perspective

Field data show that offshore tidal sand waves in areas where sediment supply is limited have different characteristics (shape and dimensions) compared with their counterparts in areas with sufficient sediment supply. So far, only the initial formation of tidal sand waves on a sediment-starved shelf has been studied with a 2DV model that ignores variations along the crests. In this study, a 3D non-linear morphodynamic model is used to investigate the effects of sediment availability on the long-term evolution of offshore tidal sand waves. Overall, the simulated sand waves have characteristics that resemble those of observed sand waves. The mature sand waves that develop in the case of limited sediment supply (i.e., thickness of erodible sediment layer is smaller than the height of sand waves) are more three-dimensional, i.e., having isolated and more irregular crestlines compared with those in the case of sufficient supply. With decreasing sediment supply, sand waves have larger spacings between successive crests, smaller heights and they migrate faster. These differences in the characteristics of the sand waves start to occur once the hard bed underneath the erodible sediment layer is exposed.

Bed forms under the combined action of waves and wind-driven currents in the remote foreshore of the non-tidal sea

The paper presents a prediction of bed form parameters (length and height) in the remote foreshore of the non-tidal sea. The study site is located in the south Baltic region, near the Coastal Research Station (CRS) in Lubiatowo, Poland (ca. 1–2 Nm off the shoreline at depths of around 16–20 m). The study site is an area with hydrodynamics and lithodynamics typical of the south Baltic coast, built of fine sands. Predictions are based on numerical modelling with an input of measured data. Numerical modelling uses assumptions of specific relations of sand wave height and length with shear stress, grain size, kinematic viscosity, or storm event frequency. To establish these relations for the study area, wave, sea temperature and wind data were collected near CRS Lubiatowo. To verify the predictions made in this research, bathymetric data from a field survey in the study area was used. The results show that the flow is mostly current-dominated. Sand waves formed by hydrodynamic conditions in this area have a steepness of 0.01 to 0.02, a length of 40 to 70 m and a height of 0.6 to 2.3 m.

Calibration and Validation of Two Tidal Sand Wave Models: A Case Study of The Netherlands Continental Shelf

Tidal sand waves form a dynamic bed pattern, widely occurring in shallow shelf seas such as the North Sea. Their importance to coastal engineering has inspired many advances in process-based sand wave modelling, aimed at explaining physical mechanisms in the formation stage (‘linear regime’) and capturing the finite amplitude evolution to equilibrium states (‘nonlinear regime’). However, systematic validation of particularly the nonlinear sand wave models is still lacking. Here, we perform a two-step calibration and validation study of a sand wave model (specifically, their linear and nonlinear model versions) against field data from the North Sea. In the first step, the linear model is calibrated by seeking overall values of two uncertain input parameters (slip parameter, wave period) for which the modeled and observed wavelengths show the best agreement. In the second step, using the calibrated input parameters and preferred wavelengths from the linear model, equilibrium heights from the nonlinear sand wave model are validated against the observed sand wave heights. Our results show satisfactory agreement between observed and modeled sand wave lengths (from the linear sand wave model) and a systematic overprediction of sand wave heights (using the nonlinear model). Regression analysis can be used to rescale the nonlinear model results to obtain realistic predictions of sand wave heights.

Giant sand waves on the Taiwan Banks, southern Taiwan Strait: Distribution, morphometric relationships, and hydrologic influence factors in a tide-dominated environment

Giant sand waves (heights >10 m) are widely developed on the Taiwan Banks, the distribution pattern, morphotypes and morphometric relationships of which have not yet been fully clarified. Based on multi-beam bathymetric swath data covering 780 line km of survey track, giant sand waves with heights up to 22.5 m and wavelengths up to 2115 m have been recorded. The relationship between height (H) and wavelength (L) of the complete data set follows the regression trend of Hmean = 0.0625 L0.8152 (r = 0.94), which is in good general agreement with a previously determined average global trend of Hmean = 0.0677 L0.8098. This global trend did not include any data from the China seas. The giant sand waves have been grouped into four morphotypes, namely double-crested, symmetric, symmetric-asymmetric and asymmetric forms. The individual bedform morphotypes and their spatial distribution were investigated in relation to water depth, textural characteristics of the sediment, and the hydrodynamic conditions recorded by several 25-hour repeated ship-based current observations. The relationship between mean water depth (D) and mean sand wave height (H) and mean wavelength (L) on the Taiwan Banks are Hmean = 0.29D (r = 0.35) and Lmean = 36D (r = 0.42), respectively. With increasing water depth, the shape of the sand wave becomes gentler. It is found that the coarser sediment (median grain size of 0.51–0.54 mm) and stronger flow in the present study may contribute to the larger H/D and L/D ratios at the given water depths than other sand wave fields. And flows of varying from equal to unequal northerly and southerly components correspond with sand waves from symmetrical to asymmetrical forms. A decrease in velocity and a veering in direction of flows have been observed over the giant sand waves, indicating the interaction between large bedforms and the near-bed water layer in the study area.

Pressure Sensing Technique for Observing Seabed Deformation Caused by Submarine Sand Wave Migration

Long-term, continuous in-situ observation of seabed deformation plays an important role in studying the mechanisms of sand wave migration and engineering early warning methods. Research on pressure sensing techniques has examined the possibility of using the temporal characteristics of the vertical deformation of the seafloor to identify important factors (e.g., wave height and migration rate) of submarine sand wave migration. Two pressure sensing tools were developed in this study to observe the seabed deformation caused by submarine sand wave migration (a fixed-depth total pressure recorder (TPRFD) and a surface synchronous bottom pressure recorder (BPRSS)), based on the principle that as a sand wave migrates under hydrodynamic forcing, the near-bottom water pressure, bottom pressure and total fixed pressure synchronously change with time. Laboratory flume experiments were performed, using natural sandy sediments taken from the beach of Qingdao, China, to better present and discuss the feasibility and limitations of using these two pressure sensing methods to acquire continuous observations of seabed deformation. The results illustrate that the proposed pressure sensor techniques can be effectively applied in reflecting elevation caused by submarine sand wave migration (the accuracy of the two methods in observing the experimental bed morphology was more than 90%). However, an unexpected step-like process of the change in sand wave height observed by BPRSS is presented to show that the sensor states can be easily disturbed by submarine environments, and thus throw the validity of BPRSS into question. Therefore, the TPRFD technique is more worthy of further study for observing submarine sand wave migration continuously and in real-time.

Morphodynamic response of sand waves in the Taiwan Shoal to a passing tropical storm

Data from three repeated multi-beam echo-sounder surveys of pre-storm, post-storm, and one year after the storm were used to study the response of sand waves in the Taiwan Shoal to the tropical storm, Talim. The measurements showed significant decreases in wave height of giant sand waves between post-storm and pre-storm, and moderate increases in one year period after the storm. The maximum decrease in wave height is more than 2 m and the averaged decrease is 1.2 m along the measured profile. The linear regression analysis shows an increasing trend of wave height changes from northwest to southeast along the profile. The uneven evolution pattern was possibly caused by the asymmetry of the currents induced by the cyclone winds. The analysis based on the Morlet wavelet function showed that the storm smeared out small-scale sand waves and eroded the giant sand wave crests, and recovery of bedforms after the storm. Two-dimensional features of storm effects were analyzed using the imagery of a Digital Terrain Model processed in a 2D survey area for pre-storm, post-storm and one year after the storm. The analysis revealed sand wave migration processes during the storm and the growing-back feature of the eroded wave crests after the storm.

Experimental study on the formation and migration of submarine sandwaves under wave-only and combined wave-current conditions

Based on the assumption that the crest lines of sandwaves are perpendicular to the main flow direction, the formation and migration of submarine sandwaves are simulated in a 2 D flume under wave-only and combined wave-current conditions. The effects of various flow parameters, including the wave height and current velocity, on the characteristic dimensions and migration rates of sandwaves are discussed. Measured morphological data indicate that the seabed is composed of both large-scale sandwaves and small-scale sand ripples, and sandwaves are dominant in shaping the seabed form. Starting from a flat seabed, the height of sandwaves increases gradually and tends to become dynamically stable, while the wave length of sandwaves reaches a constant value at the initial stage. Under wave-only conditions, the characteristic wave heights and wave lengths of sandwaves increase with increasing incident wave height. The relative locations of the sandwave peak and trough are almost constant, indicating that sandwaves scarcely migrate under wave-only conditions. Under combined wave-current conditions, the characteristic heights and lengths of sandwaves also increase with increasing current velocity. The migration direction of sandwaves coincides with the direction of the residual current, and the migration rate increases with increasing current velocity. Finally, the characteristic dimensions of sand ripples are also analyzed, revealing that the Reynolds number (based on the particle size) is dominant in determining the size of a sand ripple. Both the characteristic wave lengths and the characteristic wave heights of sand ripples decrease with increasing Reynolds numbers.

The Influence of Storms on Sand Wave Evolution: A Nonlinear Idealized Modeling Approach

We present a new 2DV nonlinear process‐based morphodynamic model to investigate the effects of storms, specifically wind‐driven flow and wind waves, on finite amplitude tidal sand wave evolution. Simulations are performed on periodic domains of two lengths: (i) on a 350‐m domain, comparable to the wavelength of observed sand waves, we study the evolution toward equilibrium shapes, and (ii) on a 4‐km domain, we study the evolution from a randomly perturbed seabed. Our model results demonstrate that both wind‐driven flow and wind waves reduce sand wave height and tend to increase wavelength. Wind‐driven flow breaks the tidal symmetry, resulting in horizontal sand wave asymmetry and migration. Waves alone do not induce migration but can enhance migration induced by, for example, tidal asymmetry and wind‐driven flow. On the 350‐m domain, we further find that migration rates decrease with increasing sand wave height. However, in an irregular sand wave field, large sand waves tend to overtake the smaller ones, suggesting a complicated interaction among neighboring bed forms. The above results concern steady state storm conditions. However, since storms occur on an intermittent basis, we also simulated a synthetic storm climate consisting of alternating short periods of storm conditions and long periods of fair‐weather conditions. Simulations reveal a dynamic equilibrium with sand wave heights significantly below those obtained for tide‐only conditions, also for relatively short storm duration. Our work identifies mechanisms that explain why sand wave heights are generally overpredicted by numerical models that do not include storm processes.

Experiment Research on Geometry and Evolution Characteristics of Sand Wave Bedforms Generated by Waves and Currents

Sand waves generated under waves and wave-current have been investigated experimentally in a wave flume. The evolution of bedforms were measured by a Trasonic Terrain automatic Measurement and analysis System (TTMS). With the data measured, the configuration of the bed was analysed. With long time wave action, a large-scale bedforms as sand waves can be formed and many ripples superimposed upon them. Ripples with 2D or 3D patterns can be formed at different locations. The growth rate of sand waves was increased at the beginning and then decreased with the increase of the time. Moreover, it has been found that the sand wave features as sand wave length and height have relationship with hydrodynamics. In general, the sand wave height increases with the increasing of wave height and residual current respectively. The sand wave length increases with the increase of residual current and decrease of wave height. The sand wave growth rate increases with the increase of the hydrodynamic conditions.

Modelling the effect of suspended load transport and tidal asymmetry on the equilibrium tidal sand wave height

Tidal sand waves are rhythmic bed forms found in shallow sandy coastal seas, reaching heights up to ten meters and migration rates of several meters per year. Because of their dynamic behaviour, unravelling the physical processes behind the growth of these bed forms is of particular interest to science and offshore industries. Various modelling efforts have given a good description of the initial stages of sand wave formation by adopting a linear stability analysis on the coupled system of water movement and the sandy seabed. However, the physical processes causing sand waves to grow towards equilibrium are far from understood. We adopt a numerical shallow water model (Delft3D) to study the growth of sand waves towards a stable equilibrium.It is shown that both suspended load transport and tidal asymmetry reduce the equilibrium sand wave height. A residual current results in asymmetrical bed forms that migrate in the direction of the residual current. The combination of suspended load transport and tidal asymmetry results in predicted equilibrium wave heights comparable to wave heights found in the field.

Spatially Varying Environmental Properties Controlling Observed Sand Wave Morphology

Sand wave morphology and dynamics on continental shelves vary substantially, and we hypothesize that these spatial variations depend on local bed properties and hydrodynamic characteristics. To date, process‐based modeling studies have not been able to simulate realistic equilibrium sand wave heights and empirical studies are mostly limited to case studies. In order to explain the spatial variation in the morphology of equilibrium sand waves on continental shelves with processes and local bed conditions, a large‐scale investigation is required. In this paper, we use high‐resolution multibeam echo soundings, hydrodynamic models, and databases and sedimentary data for the analysis of, respectively, sand wave shape characteristics and the comparison to hydrodynamic and sedimentary characteristics for the Netherlands Continental Shelf. The results are quantified lengths, heights, and asymmetry of all sand waves in the Dutch part of the North Sea. Furthermore, we show that the mode of sediment transport (bed load or suspended) is a dominant factor in explaining sand wavelength, height, and asymmetry. Full results of shape characteristics of all sand waves on the Netherlands Continental Shelf together with the tidal velocity, water depth, surface wave height, and median grain size are provided in a repository with this paper (http://doi.org/10.4121/uuid:0d7e016d‐2182‐46ea‐bc19‐cdfda5c20308). These results are highly valuable for applied offshore engineering projects and to modelers for validating their morphodynamic model results.

Sand wave deposition in the Taiwan Shoal of China

The Taiwan Shoal is the convex terrain in the southern Taiwan Strait, the largest strait in China. In 2006 and 2007, 21 samples and more than 200-km sub-bottom data as well as 80-km near shore side-scan sonar data were gotten, which gave an initial image of the boundaries of the Taiwan Shoal and revealed the internal structure of the sand waves in this area. The results showed that the major component of the sediment samples was sand, and sand waves occurred everywhere in this area, which closely followed the range of the Taiwan Shoal as we know. The western boundary of the Taiwan Shoal thus reaches the 30 m isobaths near the shore, and as a result, its area potentially covers approximately 12 800–14 770 km2. The sand waves have different shapes under the complex ocean dynamics, and the height of sand waves in the near shore is usually smaller than that in the Taiwan Shoal. The number of sand waves ranged from1–5 per kilometer, with more waves in the isobath-intensive area, suggesting the importance of topography for the formation of sand waves. The stratigraphic structure under the seabed has parallel bedding or cross bedding, and large dipping groove bedding can be seen locally in different parts, which may be the result of terrestrial deposition since the Late Pleistocene.

Band gaps and localization of surface water waves over large-scale sand waves with random fluctuations

Band structure and wave localization are investigated for sea surface water waves over large-scale sand wave topography. Sand wave height, sand wave width, water depth, and water width between adjacent sand waves have significant impact on band gaps. Random fluctuations of sand wave height, sand wave width, and water depth induce water wave localization. However, random water width produces a perfect transmission tunnel of water waves at a certain frequency so that localization does not occur no matter how large a disorder level is applied. Together with theoretical results, the field experimental observations in the Taiwan Bank suggest band gap and wave localization as the physical mechanism of sea surface water wave propagating over natural large-scale sand waves.

個別要素法を用いた河床変動計算

In this paper, we try to simulate a bed-deformation using numerical computation. For calculation of sand particles, we employed Distinct Element Method called DEM. This scheme calculates the sand particle collisions by mechanical elements, such as springs, dashpots, and sliders. DEM enable us to calculate multi-particle collision. For flow calculation, the Unsteady Reynolds Averaged Navier Stokes model called URANS model was employed. In this model, we selected 2nd ordered non-liner k-e model for turbulence scheme. To coupling between the particle calculation and flow calculation, we apply the relationship between the saltation layer and the Shields stress which proposed by M. Colombini and A. Stocchino. In our previous results, we cannot simulate dune characteristic, such as sand wave movement, wave length, and wave height which correspond particular fluid condition. From this point, this paper proposes a new scheme which evaluate saltation layer with Shields layer. We conduct a numerical computation follow a well-studied fluid condition, and evaluate its effect on the bed-formations.

Predictive distribution modeling of the sandy-beach supralittoral amphipod Atlantorchestoidea brasiliensis along a macroscale estuarine gradient

Exposed marine beaches are physically rigorous habitats in which macrofaunal patterns have been well correlated with physical factors. In this context, the habitat safety hypothesis (HSH) predicts an increase in abundance of supralittoral species from dissipative to reflective conditions in microtidal oceanic beaches. However, the HSH has not been adequately tested in estuarine sandy beaches. Here, we build a predictive model based on the supralittoral talitrid amphipod Atlantorchestoidea brasiliensis that allowed us to test the HSH along a macroscale estuarine gradient. Fifteen Uruguayan sandy beaches along a salinity gradient (from 0.1 to 34.3) generated by the Río de la Plata estuary (SW Atlantic Ocean) were sampled over a two-year period. A conditional two-step procedure using a General Additive Model (GAM) was performed in order to model A. brasiliensis occurrence (1st-step) and abundance (2nd-step), based on a comprehensive set of environmental variables [salinity, water temperature, beach face slope (BFS), mean grain size, wave height and sand compaction, moisture and organic matter]. Each GAM was parameterized using generalized linear models (GLMs). An external validation procedure was used. Data were divided randomly into training (75%) and test (25%) sets. The 1st-step GAM/GLM retained 5 physical descriptors in the model (decreasing order of importance): wave height, salinity, BFS, organic matter and sand compaction. The 2nd-step GAM/GLM retained 6 physical descriptors: sand compaction, water content, salinity, wave height, water temperature and organic matter. The predictive ability of these models and the resulted combined model, as well as their external validity, was highly significant and supported the predictions of the HSH. However, the highest and lowest abundances were observed, respectively, on reflective and dissipative beaches in the outer estuary; i.e., the predictions based on the HSH did not account for the high levels of abundance observed on these beaches compared with oceanic ones. Therefore, the HSH was expanded to include microtidal estuarine beaches.

Long-term evolution of sand waves in the Marsdiep inlet. I: High-resolution observations

A seven-year long data set from 1998 to 2005 of sand waves in the Marsdiep tidal inlet in the Netherlands is presented. The sand waves are visible on digital terrain models (DTMs) that were compiled from water depths obtained with a ferry-mounted acoustic Doppler current profiler. This multi-year data set allows to study the spatial and temporal evolution of sand-wave migration, height, length, asymmetry, and orientation. Horizontal migration rates were successfully obtained with a simple cross-correlation technique similar to techniques used in particle tracking. The study area can be divided in roughly two halves. In the southern half of the inlet, the sand waves are of the progressive type with area-mean heights of about 3 m and lengths of about 190 m. In the northern half, the sand waves are asymmetrical-trochoidal with area-mean heights of about 2 m and lengths of about 165 m. Across the inlet, the sand waves migrate in the flood direction with rates between 0 and 90 m y - 1 . Only the progressive waves in the southern half migrate in directions perpendicular to their crests. A striking observation is the seasonal variability in height and migration of the sand waves in the northern half of the inlet. The sand-wave heights are about 0.5 m higher in fall than in spring and the migration rates are about 30 m y - 1 higher in winter than in summer. This paper only presents observations and analysis techniques. In a subsequent paper, the ADCP-current data will be applied to explain the spatial and seasonal variability of the sand waves.

Long-term evolution of sand waves in the Marsdiep inlet. II: Relation to hydrodynamics

A discussion is presented about the mechanisms that govern the spatial and seasonal variability in sand-wave height and migration speed in the 4 km wide Marsdiep tidal inlet, the Netherlands. Since 1998, current velocities and water depths have been recorded with an ADCP that is mounted under the ferry ‘Schulpengat’. In this paper, the current measurements were used to explain the sand-wave observations presented in Buijsman and Ridderinkhof [this issue. Long-term evolution of sand waves in the Marsdiep inlet. I: high-resolution observations. Continental Shelf Research, doi: 10.1016/j.csr.2007.10.011]. Across nearly the entire inlet, the sand waves migrate in the flood direction. In the flood-dominated southern part of the inlet, the ‘measured’ (i.e. based on sand-wave shape and migration speed) and predicted bedload transport agree in direction, magnitude, and trends, whereas in the ebb-dominated northern part the predicted bedload and suspended load transport is opposite to the sand-wave migration. In the southern part, 55% of the bedload transport is due to tidal asymmetries and 45% due to residual currents. In addition to the well-known tidal asymmetries, asymmetries that arise from the interaction of M 2 and its overtides with S 2 and its compound tides are also important. It is hypothesised that in the northern part of the inlet the advection of suspended sand and lag effects govern the sand-wave migration. The relative importance of suspended load transport also explains why the sand waves have smaller lee-slope angles, are smaller, more rounded, and more three-dimensional in the northern half of the inlet. The sand waves in this part of the inlet feature the largest seasonal variability in height and migration speed. This seasonal variability may be attributed to the tides or a seasonal fluctuation in fall velocity. In both cases sediment transport is enhanced in winter, increasing sand-wave migration and decreasing sand-wave height. The influence of storms and estuarine circulation on the sand-wave variability is negligible.

Modelling offshore sand wave evolution

We present a two-dimensional vertical (2DV) flow and morphological numerical model describing the behaviour of offshore sand waves. The model contains the 2DV shallow water equations, with a free water surface and a general bed load formula. The water movement is coupled to the sediment transport equation by a seabed evolution equation. Using this model, we investigate the evolution of sand waves in a marine environment. As a result, we find sand wave saturation for heights of 10–30% of the average water depth on a timescale of decades. The stabilization mechanism, causing sand waves to saturate, is found to be based on the balance between the shear stress at the seabed and the principle that sediment is transported more easily downhill than uphill. The migration rate of the sand waves decreases slightly during their evolution. For a unidirectional steady flow the sand waves become asymmetrical in the horizontal direction and for a unidirectional block current asymmetrical in the vertical. A sensitivity analysis showed the slope effect of the sediment transport plays an important role herein. Furthermore, the magnitude of the resistance at the seabed and the eddy viscosity influence both the timescale and height of sand waves. The order of magnitudes found of the time and spatial scales coincide with observations made in the southern bight of the North Sea, Japan and Spain.

Morphodynamique d'une dune sous-marine du détroit du Pas de Calais. Morphodynamics of an undersea sandwave of the Dover Straits

This paper investigates the dynamics of sandwaves in relationship with tidal currents and weather conditions. The studied sandwaves are located in the Dover Straits and are covered by megaripples. These megaripples have migration speeds of and slopes of 34°, suggesting the potential for avalanches to occur along the flanks of the sandwave. Tidal cycles without reversing currents were observed during stormy weather. Wind-induced currents lead to a unidirectional migration of megaripples and sandwaves. Well-defined areas without megaripples were observed and correlated with an increase in sandwave height. We propose interpretations for understanding sandwave saturation and migration

Morphodynamique d'une dune sous-marine du détroit du pas de Calais

This paper investigates the dynamics of sandwaves in relationship with tidal currents and weather conditions. The studied sandwaves are located in the Dover Straits and are covered by megaripples. These megaripples have migration speeds of and slopes of 34°, suggesting the potential for avalanches to occur along the flanks of the sandwave. Tidal cycles without reversing currents were observed during stormy weather. Wind-induced currents lead to a unidirectional migration of megaripples and sandwaves. Well-defined areas without megaripples were observed and correlated with an increase in sandwave height. We propose interpretations for understanding sandwave saturation and migration. To cite this article: D. Idier et al., C. R. Geoscience 334 (2002) 1079–1085.