Submarine Sand Waves Research Articles

Research on the fluid dynamics interaction between submarine sand waves and seawater by seismic oceanography

IntroductionThe seafloor is an important interface between the lithosphere and the hydrosphere, where processes related to circulation and energy exchange happen along with various marine processes widely developed in the water column near the seafloor. These processes are still not yet completely understood as there are constraints of submarine detection technology and the interdisciplinary nature of these complex environments. Seismic reflection data have been a preferable tool to study and image these processes due to their characteristics in terms of spatial resolution. In seismic reflection data, submarine sand waves show hair-like reflection configurations with low continuity and wearing-hair style, appearing with an angle with the seafloor. Investigation of the relationship between the characteristics of submarine sand waves induced hair-like reflection configuration and hydrodynamics is crucial for understanding hair-like reflection configuration generation and spatiotemporal evolution. MethodsThis study combines fluid dynamics numerical simulation and seismic oceanography to discuss the seismic response characteristics and formation mechanisms of the hair-like reflection configuration. First, we create a seawater time-variant fluid-dynamical model followed by the numerical simulation of seismic oceanography data. This procedure results in seismic oceanography numerical simulation sections with hair-like reflection configurations for different constant flow conditions forced on the boundary. Optimal matching method is then applied to interpret field seismic reflection sections given the results obtained with the numerical experiments. Results and discussionAs consequence, the fluid dynamic explanation for the formation mechanism of the hair-like reflection configuration due to differences in seawater thermohaline is proposed. The study provides additional comprehension and further insights into the dynamic process of submarine sand waves induced hair-like reflection configuration using the seismic oceanography method.

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.

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.

Numerical research on evolvement of submarine sand waves in the Northern South China Sea

Submarine sand waves, vital to seabed stability, are an important consideration for oceanic engineering projects such as oil pipe lines and submarine cables. The properties of surface sediment and the evolvement of submarine sand waves in a specified area in the South China Sea are studied using both a hydrological model and field observational data. The bottom flow field data between 2010 and 2011 in the study area are simulated by the Regional Ocean Model System (ROMS). The migration of submarine sand waves is calculated using Rubin’s formula along with typhoon data and bottom flow field data, which allows for the analysis of sand wave response under the influence of typhoons. The migration direction calculated by Rubin’s formula and bottom flow are very similar to collected data. The migration distance of different positions is between 0.0 m and 21.8 m, which reciprocates cumulatively. This shows that Rubin’s formula can predict the progress of submarine sand waves with the bottom flow simulated by ROMS. The migration distances of 2 sites in the study area are 2.0 m and 2.9 m during the typhoon “Fanapi”. The proportion of the calculated migration distance by the typhoon is 9.17% and 26.36% of the annual migration distance, respectively, which proves that the typhoon can make a significant impact on submarine sand waves.

Sediment mathematical model for sand ridges and sand waves

A new theoretical model is formulated to describe internal movement mechanisms of the sand ridges and sand waves based on the momentum equation of a solid-liquid two-phase flow under a shear flow. Coupling this equation with two-dimensional shallow water equations and wave reflection-diffraction equation of mild slope, a two-dimensional coupling model is established and a validation is carried out by observed hydrogeology, tides, waves and sediment. The numerical results are compared with available observations. Satisfactory agreements are achieved. This coupling model is then applied to the Dongfang 1-1 Gas Field area to quantitatively predict the movement and evolution of submarine sand ridges and sand waves. As a result, it is found that the sand ridges and sand waves movement distance increases year by year, but the development trend is stable.

Bathymetric mapping of submarine sand waves using multiangle sun glitter imagery: a case of the Taiwan Banks with ASTER stereo imagery

Submarine sand waves are visible in optical sun glitter remote sensing images and multiangle observations can provide valuable information. We present a method for bathymetric mapping of submarine sand waves using multiangle sun glitter information from Advanced Spaceborne Thermal Emission and Reflection Radiometer stereo imagery. Based on a multiangle image geometry model and a sun glitter radiance transfer model, sea surface roughness is derived using multiangle sun glitter images. These results are then used for water depth inversions based on the Alpers–Hennings model, supported by a few true depth data points (sounding data). Case study results show that the inversion and true depths match well, with high-correlation coefficients and root-mean-square errors from 1.45 to 2.46 m, and relative errors from 5.48% to 8.12%. The proposed method has some advantages over previous methods in that it requires fewer true depth data points, it does not require environmental parameters or knowledge of sand-wave morphology, and it is relatively simple to operate. On this basis, we conclude that this method is effective in mapping submarine sand waves and we anticipate that it will also be applicable to other similar topography types.

Observation of submarine sand waves using ASTER stereo sun glitter imagery

Signatures of sun glitter images strongly depend on the viewing angle. Stereo observation can provide more valuable information than observation from one angle. In this work, the sun glitter patterns caused by submarine sand waves were studied using stereo images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), which operates a high spatial resolution (15 m) optical and near-infrared sensor on board the Terra satellite. The previous imaging geometrical model has been improved, and the difference in the viewing angle among pixels is considered. We found that brightness reversal occurs in some stereo imagery of submarine sand waves. Based on the imaging geometry model, an interaction model of current topography, and a sun glitter radiance transfer model, a simulation model was developed for sun glitter from submarine sand waves at multiple viewing angles. The cases of nadir-looking and backward-looking were simulated with the model. The results show the following differences between the two viewing angles, which have also been observed in the ASTER images. Both tendency and extent of the simulated radiance are in good agreement with those in actual images. In the nadir-looking view, obvious differences have been observed between the normalized significant radiance from smooth and rough facets. This difference increases with increasing viewing angle, to a peak, and then decreases. However, in most cases, the difference in the back view is smaller and shows an opposite tendency regarding the viewing angle from the nadir-looking view. Thus, the sand-wave characteristics in the nadir-looking images seem to be more enhanced than that in the backward-looking images.

Whitecap features induced by submarine sand waves in stereo optical imagery

AbstractUnder high wind speed conditions, waves break on the sea surface producing whitecaps, which can be recorded in optical satellite images. However, the breaking of waves is also influenced by submarine topography. In this study, we investigate the whitecap features induced by submarine sand waves using optical stereo images. We determine the whitecap coverage in images, and estimate the probabilities of wave breaking. The observed and estimated results are in agreement with some previous investigation results. A physical model is used to discuss the mechanisms of wave breaking along sand wave crests. The physical model is able to qualitatively explain the differences between the observed whitecap coverage and the estimated probability of wave breaking between the sand wave area (SWA) and the background area (BGA). Further analysis shows that the periodic sand wave system may play the role of a frequency‐control device, and may result in more broken waves in the SWA than in the BGA. This study demonstrates that it is possible to observe submarine topography under high‐wind conditions by using whitecap features.

The brightness reversal of submarine sand waves in “HJ-1A/B” CCD sun glitter images

The brightness reversal of submarine sand waves appearing in the small satellite constellation for environment and disaster monitoring and forecasting (“HJ-1A/B”) CCD sun glitter images can affect the observation and depth inversion of sand wave topography. The simulations of the normalized sun glitter radiance on the submarine sand waves confirm that the reversal would happen at a specific sensor viewing angle, defined as the critical angle. The difference between the calculated critical angle position and the reversal position in the image is about 1′, which is excellent in agreement. Both the simulation and actual image show that sand wave crests would be indistinct at the reversal position, which may cause problems when using these sun glitter images to analyze spatial characteristics and migration of sand waves. When using the sun glitter image to obtain the depth inversion, one should take the advantage of image properties of sand waves and choose the location in between the reversal position and the brightest position. It is also necessary to pay attention to the brightness reversal when using “HJ-1A/B” CCD images to analyze other oceanic features, such as internal waves, oil slicks, eddies, and ship wakes.

Study of Erosion Mechanism by Modelling Abandoned Yellow River Delta

Three dimensional physical model designed to study erosion mechanism of estuary subaqueous delta is one of the most important approaches for investigating erosion mechanism of delta; In this study, effects of pore water pressure on seabed and subsequent alterations of seabed topography during wave action were examined by using a physical model. This model provides a good tool to understand impacts of silt liquefaction on the seabed, the whole process of erosion of estuary subaqueous delta, formation and changes of sandbars, and the forces and changes of soft strata through interaction with wave action. We have demonstrated that the characteristic features of a soft stratum and the erosive forces existing in its aquatic environment tend to induce rheological diversification and creep. This often leads to ultimate diapir causing significant damages to underwater and offshore engineering facilities. Moreover, submarine sand wave is a common geomorphological feature of seabed. Impacts of waves and seabed topography on the formation and characteristics of sand waves are analyzed particularly in this study.

Sun glitter imaging of submarine sand waves on the Taiwan Banks: Determination of the relaxation rate of short waves

[1] Above sand waves on the seafloor, surface short waves, which are responsible for the radiance distribution in remote sensing imagery, are modulated gradually by the submarine topography. The relaxation rate μr characterizes the rate at which the short waves reach their saturation range after being disturbed. It is a key parameter in the weak hydrodynamic interaction theory and is also a most important parameter in the imaging mechanism used for mapping submarine bottom topography. In this study, a robust expression containing intensity and phase (advection effect) modulations of the perturbed action spectrum of short waves was deduced, by using the first-order weak hydrodynamic interaction theory. On the basis of the phase modulation, a method was developed to determine the relaxation rate in the Sun glitter imaging mechanism. The relaxation rates were estimated using in situ data measured on a cruise over the sand waves of the Taiwan Banks, a sea area between the East China Sea and the South China Sea, on 28–29 August 2006. Results showed that, under a wind speed of 5.0 m s−1, the relaxation rate of short waves was about 0.055 s−1 in response to current variations and about 0.025 s−1 equivalently in response to sea bottom topographic variations. The former value could be applied to interpret the amplitude of submarine topography by using satellite imagery, while the latter one (equivalent relaxation rate μ′r) could help to more accurately calibrate the spatial position of the retrieved sea bottom topography.

The influence of quasi resonant internal waves on the radar imaging mechanism of shallow sea bottom topography

During previous field experiments in the North Sea it was often assumed that the water column in such shallow coastal tidal waters is vertically well mixed and stratification was neglected when discussing the Normalized Radar Cross Section modulation caused by the sea floor. In this paper the influence of quasi resonant internal waves with the sea bed on the radar imaging mechanism of submarine sand waves itself is investigated. In situ data of the tidal current velocity and several water quality parameters such as sea surface temperature, fluorescence, and beam transmittance were measured in the Southern Bight of the North Sea in April 1991. Simulations of the total NRCS modulation caused by sand waves and internal waves as a function of the current gradient or strain rate induced by the internal wave current field at the sea surface have been carried out using the quasi-steady approximation and linear internal wave theory. As a first approximation the strain rate depending on stratification was calculated using the two-layer model. These simulations demonstrate that at least a density difference between the two layers of the order of Δρ ≈ 1 kg m –3 is necessary for a sinusoidal thermocline to effect the total NRCS modulation considerably. The NRCS modulation as a function of wind friction velocity has been calculated independently and is discussed with regard to the strain rate of the surface current field caused by the superimposed imaging mechanisms of sand waves and internal waves. It turned out that the existence of a surface roughness-wind stress feedback mechanism cannot be excluded.

On the behaviour of tidal current directions due to the presence of submarine sand waves

Abstract Side-scan sonar records reveal that the crestlines of megaripples in the troughs as well as on both sides of the slopes of sand waves form an angle with the crestline of the sand waves itself. The existence of such an angle between the crestline of megaripples and sand waves indicates that changes of the (tidal) current direction across sand waves can be expected. Direct evidence that such (tidal) current direction changes occur above sand waves is derived from measurements of the Air–Sea Interaction Drift Buoy (ASIB) system at the sea surface and from side-scan sonar records of the sea bed. Both systems were operated from on board research vessels during two C-STAR field experiments in the Hoek van Holland study area off the Dutch coast in April 1996 and in April 1997. Measurements performed by electrical resistance wires on board the ASIB system indicate that variations of the short and moderate wave direction and the wave directional spread are associated with changes of the (tidal) current direction at the sea surface. According to the results of the measurements derived from the ASIB system and the side-scan sonar records it can be summarised that the variation of the direction of short and moderate period water waves as well as changes of the (tidal) current speed and direction across large sand waves must be considered for correct modelling of the wave-current interaction mechanism.

Mapping submarine sand waves with multiband imaging radar: 2. Experimental results and model comparison

On August 16, 1989, and on July 12, 1991, experiments were performed to study the mapping of submarine sand waves with the airborne imaging radar, a polarimetric (and, in 1991, interferometric) airborne P, L, and C band synthetic aperture radar system. The experiments took place in an area 30 km off the coast of the Netherlands, where the bottom topography is dominated by sand waves with a height between 2 and 6 m and a crest‐to‐crest distance of about 400 m at an average depth of 22 m. Ground measurements were recorded on a nearby platform and on a ship in the test area, which also acted as a position fix. On August 16, 1989, the wind was 5 m/s directed toward the northeast, while the surface current velocity was around 0.5 m/s directed toward the southwest. One overflight was made, with the flight direction parallel to the sand wave crests and the radar looking upwind. At P band, the sand waves are clearly visible as dark bands, as predicted by theory, while at L band the sand waves show up as sawtooth‐shaped modulations. On July 12, 1991, wind and surface current had the same (opposite)‐directions as in 1989, though the wind was much higher (10 m/s). Three flights were made, with the radar pointing upwind, cross wind, and downwind. The upwind and downwind images are very similar. Despite the high wind speed, the sand waves are clearly visible as sawtooth‐shaped modulations at P band and vaguely visible at L band. At C band, only wind streaks can be seen. All cross wind images show the sand waves as dark bands, now with the highest modulations at C band. The wind streaks that dominated the upwind and downwind images at C band are much less pronounced in the cross wind images. The images are compared with predictions from a new model of the imaging mechanism which includes contribution to the radar cross section of waves moving both from and to the radar. Wave blocking or wave reflection is treated in an approximate manner. For the radar looking upwind or downwind, the predicted modulations at P and L band agree well with the observations, while those at C band are too high. For the radar looking cross wind, the model severely underestimates the modulations. It is questioned whether a local relaxation source term can describe such a situation. The interferogram shows some structure caused by bottom‐induced surface current variations.

Mapping submarine sand waves with multiband imaging radar: 1. Model development and sensitivity analysis

In 1989 and 1991, unexpected modulations caused by submarine sand waves were observed with the airborne imaging radar (AIR), a P, L, and C band synthetic aperture radar. The measurements were performed in a sand wave area off the Dutch coast. The sand waves have an asymmetric, sawtooth‐shaped profile with the steep slope oriented toward the northeast. During the experiments, wind and current had opposite directions: the wind was directed toward the northeast and the current toward the southwest. Under these conditions, existing models for the imaging mechanism predict that the modulation depth decreases with increasing radar frequency and that the steep slopes show up as dark lines in radar images. The AIR images show a relation between modulation depth and radar frequency as predicted. However, only the P band image of 1989 shows the steep slopes of the sand waves as dark lines. The P band image of 1991 and the L band images of 1989 and 1991 show a sawtooth‐shaped modulation which cannot be explained by existing theory. A new model is developed that is one dimensional in position space and fully two dimensional in wavenumber space. The radar cross section is calculated using both first‐order Bragg and an integral equation scattering model in which the cross section originates from a spectral band rather than a single value. The new model contains two improvements over previous ones. First, wave blocking or, better, wave reflection is included, though in an approximate manner. It is shown to be of importance at P band only. Second, the contribution to the radar cross section from waves moving both to and from the radar is included, whereas some previous models consider only one of these contributions. This explains the observed modulations, at least qualitatively, under the assumptions that the angular distribution of short waves is almost constant and that waves moving against the wind have small relaxation rates.

A simple analytical model for brightness modulations caused by submarine sand waves in radar imagery

Presented here is a simple analytical model based on established physics of the magnitude of the hydrodynamic modulations caused by sand waves. The model describes the modulations of the radar backscatter when first‐order Bragg scattering is assumed. The major difference between this model and existing analytical models is that the specific shape of sand waves is incorporated. An assessment is made of the key parameters (shape, oceanographic, meteorological and radar) that influence the radar backscatter modulation. It is shown that depth, steepness of the slope, and height of the sand wave are the shape parameters that determine the radar backscatter modulation. The maximum backscatter modulation that can be found for sand waves in nature is approximately 3 dB. It is shown that sand waves in the North Sea near the Dutch coast have a linear relation between their heights and slopes. Implementation of this relation simplifies the model further. Furthermore, backscatter modulations calculated with two radar backscatter models are compared and discussed. The correlation between predictions and measurements with the airborne imaging radar of NASA's Jet Propulsion Laboratory is considered encouraging. Measurements from the images indicate a relation between sand wave height and brightness modulation similar to that predicted by the model.

Radar imaging of submarine sand waves in tidal channels

The simple theoretical model of Alpers and Hennings describing the radar imaging of submarine bottom topography in coastal waters with strong unidirectional tidal currents is analytically extended to show the influence of advection. The theory applies for L band radar, where second‐order terms in the hydrodynamic interaction can be neglected as a first approximation. If future imaging radars from satellites and space platforms as the ERS‐I (First European Remote Sensing Satellite), the JERS‐I (First Japanese Earth Remote Sensing Satellite), and the EOS (Earth Observing System) are to be used for cartographic applications, it is necessary to include the effect of advection to improve accuracy. This extension of the model simulates the position of the radar cross‐section modulation relative to coastal geomorphological bedforms. By application of that theory it is possible to map features such as the crests of sandbanks and sand waves.

Radar imaging of submarine sand waves in tidal channels: Hennings, Ingo, 1990 J. geophys. Res., 95(C6):9713–9721

Radar imaging of submarine sand waves in tidal channels: Hennings, Ingo, 1990 J. geophys. Res., 95(C6):9713–9721

Sedimentology of Late Jurassic Barrier-Island Complex--Lower Sundance Formation of Black Hills

The lower Sundance Formation (Upper Jurassic, Callovian) in the Black Hills region of South Dakota and Wyoming contains a sequence of sediments deposited during progradation of a regressive, northeast-trending barrier-island complex. The regressive complex, which comprises the Lak, Hulett, and upper part of the Stockade Beaver Members, overlies the transgressive Canyon Springs sandstone that was deposited as large, submarine sand waves. The regressive sequence begins with the Stockade Beaver shale, an offshore, marine-shale facies that grades vertically into the Hulett sandstone through a transition facies of flaser- and lenticular-bedded sandstone and shale. In the northern Black Hills, the transition unit and a bioturbated, small-scale cross-bedded, and low-angle laminated facies of the Hulett sandstone comprise an upward-coarsening sequence that formed in the inferred beach environments of lower, middle, and upper shoreface and foreshore. An ichnofauna of vertical, tubelike Monocraterion and Diplocraterion implies very shallow water depths. A channel facies of cross-bedded sandstones locally replaces the barrier-island profile and represents probable tidal-channel deposits. The barrier sequence (up to 30 m thick) s overlain by a greenish silty and sandy lagoonal facies containing sandstones interpreted as washover fans. The Hulett sequence in the southern Black Hills is thin (5 to 6 m), and consists of a flaser- and lenticular-bedded transition facies overlain by the silty and sandy lagoonal facies. In this region, the transition facies is characterized by indicators of intermittent emergence such as flattened ripples and wrinkle marks, and is interpreted as having formed low on an intertidal flat behind the barrier. The barrier sequence passes on the west into a sequence of silty cross-bedded sandstones and shales interpreted as an offshore-bar deposit. The Lak Member overlies the Hulett in most of the region, and consists of well-sorted silty red beds. Well-developed stratification is absent, and the rock has a peculiar, faint wispy fabric. The red color, the apparent gradational position above a barrier complex in an overall regressive sequence, and the total lack of fauna suggest a nonmarine origin for the Lak Member. An eolian origin seems plausible for this enigmatic unit.

Field Investigation of Large Submarine Sand Waves

ABSTRACTThere are many submarine sand waves on the bottom of the Bisan Strait at the Seto Inland Sea and they obstruct navigation. This is a report of the investigation which has been conducted to study the mechanism of sand wave formation at Inosakinotsugai in the Bisan Strait. Soundings, collection of bottom materials and observation of tidal currents constitute this investigation. The following items have been made clear through this investigation:1) Formation of sand waves is greatly influenced by sand bank.2) Submarine sand waves are dunes. Those scales are approximately equal to the values calculated by Yalin's formula.3) When tidal currents flow aground onto the sand bank, intensity of turbulence increases and two prominent eddies appear. After they pass over the sand bank, intensity of turbulence decreases and those eddies disappear.