Sand Wave Crests Research Articles

Background Topography Affects the Degree of Three‐Dimensionality of Tidal Sand Waves

AbstractOffshore tidal sand waves on the sandy bed of shallow continental shelf seas are more three‐dimensional (3D) in some places than others, where 3D refers to a pattern that shows variations in three spatial directions. Such sand waves have crestlines that meander, split or merge. The degree of three‐dimensionality seems to vary especially when large‐scale bedforms, such as tidal sand banks, are present underneath the sand waves. Understanding this behavior is important for offshore activities, such as offshore windfarm construction or the maintenance of navigation channels. In this study, the degree of three‐dimensionality of sand waves at five sites in the North Sea is quantified with a new measure. Results show that tidal sand waves on top of tidal sand banks are more two‐dimensional (2D) than those on bank slopes or in open areas. These differences in sand wave pattern are supported by numerical simulations performed with a new long‐term sand wave model. The primary cause of these differences is attributed to the deflection of tidal flow over a sand bank, which causes sand wave crests to be more aligned with the bank at its top than at its slopes. It is subsequently made plausible that the different patterns result from the competition between two known mechanisms. These mechanisms are nonlinear interactions between sand waves themselves (SW‐SW interactions) and nonlinear interactions between sand banks and sand waves (SB‐SW interactions). On bank tops, SB‐SW interactions favor a 2D pattern, while SW‐SW interactions, which elsewhere produce a 3D pattern, are less effective.

Modeling tidal sand wave recovery after dredging: effect of different types of dredging strategies

Maintenance of navigation channels in shelf seas with tidal sand waves usually requires repeated dredging operations. Optimizing these interventions is a difficult task, particularly complicated by the nonlinear morphodynamics of sand wave recovery after dredging. Here we present a process-based model study, incorporating different strategies of dredging in an existing nonlinear sand wave model. We consider ‘topping’ (removing sand from crest) and ‘swiping’ (same, but now placing it in the troughs), for a range of dredging depths. Starting point is a fully developed sand wave or sand wave field, as simulated in the model. Results indicate that sand wave recovery is slowest after swiping. Also, larger dredging volumes imply longer recovery times. Next, we study the maintenance intervals and (cumulative) dredging volumes resulting from adopting a typical temporal strategy: swipe to a prescribed dredging depth, as soon as the recovering sand wave crests have reached a critical depth. Maintenance intervals are found to depend on the dredging depth and, importantly, to vary over time, as well. This last result shows that sand wave recovery depends not only on height, but also on its shape, emphasizing the limitations of existing, empirical (Landau-type of) models based on amplitude (or height) only.

Horizontal and Vertical Sediment Sorting in Tidal Sand Waves: Modeling the Finite‐Amplitude Stage

The bed of coastal seas displays a large number of rhythmic bed features, of which sand waves are relevant to study from an engineering perspective. Sediments tend to be well sorted over these bed forms, which is so far poorly understood in terms of modeling of finite‐amplitude sand waves. Using Delft3D, we employ bed stratigraphy and consider four different grain‐size classes, which are normally distributed on the phi scale. The standard deviation (sortedness) is then varied, whereas the geometric mean grain size is kept constant. The results show that, typically, the crests of sand waves are coarser than the troughs. Residual flow causes net sedimentation on the leeside of the crest, and, consequently, the general sorting pattern is distorted. Since larger grains experience a larger settling velocity, they are deposited on the upper lee slope, whereas the smaller grains are found on the lower lee slope. Due to sand wave migration, also, the internal structure of the sand wave is revealed, which follows the same sedimentation pattern as the lee slope surface. These results qualitatively agree with sorting patterns observed offshore. The sorting processes lead to longer wavelengths and lower wave heights, as a function of standard deviation. This relates to the dampening effect of suspended sediment transport for fine grains. Finally, it appears that the modeled wave heights fall in the same range as observations in the North Sea. These results are valuable for, for example, predicting the morphological response after engineering activities and determining suitable aggregates for sand extraction.

Sediment Characteristics over Asymmetrical Tidal Sand Waves in the Dutch North Sea

The behavior of asymmetrical bedforms, which include many tidal sand waves, is challenging to understand. They are of particular interest since they are mostly located within areas prone to offshore engineering activities. Most experimental investigations regarding asymmetrical bedforms consider the riverine environment, are limited to a single sand wave or a few scattered ones, and focus only on differences between crest and trough. Hardly any information is available on sediment compositional changes along asymmetrical tidal sand waves, despite their abundance offshore. An asymmetrical sand wave field located off the coast of Texel Island in the North Sea was studied in June and October 2017. A total of 102 sediment samples were collected over two seasons along a single transect that covered five complete sand waves to measure the grain size composition, organic carbon concentration, chlorophyll-a (chl-a) concentration, and sediment permeability. We found significant variations in these sediment parameters between the sand wave trough, crest, and gentle and steep slopes, including a difference in permeability of more than 2-fold, as well as a difference in median grain size exceeding 65 µm. Based on these characteristics, a sand wave can be divided into two discrete halves: gentle slope + crest and steep slope + trough. Our results indicate a distinct sediment-sorting process along the Texel sand waves, with a significant difference between the two halves of each sand wave. These data could serve as input for process-based modeling of the link between sediment-sorting processes and seabed morphodynamics, necessary to design offshore engineering projects.

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.

SAND WAVE EVOLUTION MODEL FOR EFFICIENT CHANNEL DEPTH MANAGEMENT

Sand wave is a consecutive wavy bedform on seabed with several meters in wave height and several tens-hundreds of meters in wave length. If the crests of sand waves rise above the required channel depth, they often prevent vessels from safe navigation. In this study, a prediction model of sand wave evolution is applied to predict depth change at the crest of sand waves in Kanmon route, Japan. Kanmon route is a long, narrow and winding channel where strong tidal currents take place due to large tidal difference between Japan sea and Seto-island sea. In these area, a lot of bathymetric survey data have been collected to monitor the water depth. Also, channel deepening project up to -14 m is underway, and therefore reliable prediction model of sand wave evolution is needed for channel depth management in the future.

Video Transects Reveal That Tidal Sand Waves Affect the Spatial Distribution of Benthic Organisms and Sand Ripples

AbstractThe sandy seabed of shallow coastal shelf seas displays morphological patterns of various dimensions. The seabed also harbors a rich ecosystem. Increasing pressure from human‐induced disturbances necessitates further study on drivers of benthic community distributions over morphological patterns. Moreover, a greater understanding of the sand ripple distribution over tidal sand waves may improve morphological model predictions. Here we analyzed the biotic abundance and ripple morphology in sand wave troughs and crests using video transects. We found that both the epibenthos and endobenthos are significantly more abundant in sand wave troughs, where ripples are less abundant and more irregularly shaped. Finally, we show that camera systems are relatively quick and effective tools to study biotic spatial patterns in relation to seabed morphology.

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.

Priori knowledge based a bathymetry assessment method using the sun glitter imagery: a case study of sand waves on the Taiwan Banks

High resolution optical satellite imageries containing the sun glitter, similar to synthetic aperture radar (SAR) imageries, are useful in identifying and mapping of bottom topography in shallow waters. The errors in the previous studies are corrected, and a method for mapping submarine bottom topography is developed using the sun glitter satellite imagery. The method is established on the basis of empirical description of a sand wave using an equation with two unknowns named r and k. In order to determine r and k, a “trial and error” approach is introduced and testified by a case study on the Taiwan Banks using an ASTER imagery. The results show that the inversed water depths match well with the sounding water depths. The agreement between the inversed results and the in situ measurements is about 78% by comparing 371 points. Moreover, this method has the advantage in keeping the original appearance of a sand wave, especially in positions around the sand wave crest. The fine agreement indicates that the imaging model is flexible and the approach developed is feasible.

The genesis of an inter‐field marine sandwave and the associated anti‐asymmetry migration of neighboring crests

A series of high resolution multibeam echo sounding surveys near San Francisco, CA suggests the occasional migration of marine sandwave crests in the direction opposite to that indicated by the shape asymmetry of the individual sandwaves, in contrast to all previous observations of the relationship between bedform shape asymmetry and migration. The anti‐asymmetry migration occurs over approximately the same time period that a new sand wave crest is formed within the large and relatively stable field. The new sandwave crest appears approximately midway between two larger crests, in the vicinity of structural defects in a nearly two‐dimensional portion of the field. The sandwaves in the vicinity of the new crest are found to have migrated away from the new crest regardless of their shape asymmetry. Later a large section of the new crest disappears, and the neighboring sandwaves migrated back toward the vacated crest location. This migration occurs for sandwaves up to at least ten wavelengths away from the new crest.

Hydrodynamic Processes and Bedload Transport Associated with Large-Scale Sandwaves in the Taiwan Strait

Hydrodynamic processes and bedload transport over a large-scale sandwave field were analyzed along three transects in Taiwan Strait based on measurements of water depth, current velocities within the water column, and the physical properties of the sediment at the seabed. The sandwaves in consideration have steep crests and flat troughs, with a width of 14.4–20.3 m and a length of 325–820 m. Generally, the mean H/L ratio falls in the range of large-scale sandwaves, but the ratio is larger than the average, deviating from the global trend. The H/d ratio is much higher than the global average. The current-velocity vectors at the sandwave crests, troughs, stoss sides, and lee sides turned in a clockwise manner within a tidal cycle; the current speed above the crest was relatively larger than above the trough. During the flood tide, the current speed reached a maximum in the layer 9–11 m below the water surface. During the measurement period, there was net accumulation of sediment over the study area. The migration of the sandwaves caused by bedload transport varied widely at different transects: for the three transects in consideration, migration in the flood direction occurred at two transects, with different migrating speeds; and ebb-dominated migration occurred at the third transect. The migration rate predicted by the bedload transport calculations, based on tidal current measurements, is lower than long-term observations by remote sensing, which may be due to the fact that this region is frequently influenced by storm events, and the long-term migration patterns are related to the enhanced currents and waves during storms. Further, time-velocity asymmetry of the tidal currents, as observed in the present study, may also favor the enhanced migration rate during storm events.

Radar imaging mechanism of marine sand waves at very low grazing angle illumination caused by unique hydrodynamic interactions

The investigations carried out between 2002 and 2004 during six field experiments within the Operational Radar and Optical Mapping in monitoring hydrodynamic, morphodynamic and environmental parameters for coastal management (OROMA) project aimed to improve the effectiveness of new remote sensing monitoring technologies such as shipborne imaging radars in coastal waters. The coastal monitoring radar of the GKSS Research Center, Geesthacht, Germany, is based on a Kelvin Hughes RSR 1000 X band (9.42 GHz) vertical (VV) polarized river radar and was mounted on board the research vessel Ludwig Prandtl during the experiments in the Lister Tief, a tidal inlet of the German Bight in the North Sea. The important progress realized in this investigation is the availability of calibrated X band radar data. Another central point of the study is to demonstrate the applicability of the quasi‐specular scattering theory in combination with the weak hydrodynamic interaction theory for the radar imaging mechanism of the seabed. Radar data have been taken at very low grazing angles ≤2.6° of flood and ebb tide–oriented sand wave signatures at the sea surface during ebb tidal current phases. Current speeds perpendicular to the sand wave crest ≤0.6 m s−1 have been measured at wind speeds ≤4.5 m s−1 and water depths ≤25 m. The difference between the maximum measured and simulated normalized radar cross section (NRCS) modulation of the ebb tide–oriented sand wave is 27%. For the flood tide–oriented sand wave, a difference of 21% has been calculated. The difference between the minimum measured and simulated NRCS modulation of the ebb tide–oriented sand wave is 10%, and for the flood tide–oriented sand wave, a value of 43% has been derived. Phases of measured and simulated NRCS modulations correspond to asymmetric sand wave slopes. The results of the simulated NRCS modulation show the qualitative trend but do not always quantitatively match the measured NRCS modulation profiles because the quasi‐specular scattering theory at very low grazing angle is a first‐order theory.

On the formation of sand waves and sand banks

A fully three-dimensional model is proposed for the generation of tidal sand waves and sand banks from small bottom perturbations of a flat seabed subject to tidal currents. The model predicts the conditions leading to the appearance of both tidal sand waves and sand banks and determines their main geometrical characteristics. A finite wavelength of both sand waves and sand banks is found around the critical conditions, thus opening the possibility of performing a weakly nonlinear stability analysis able to predict the equilibrium amplitude of the bottom forms. As shown by previous works on the subject, the sand wave crests turn out to be orthogonal to the direction of the main tidal current. The present results also show that in the Northern Hemisphere sand bank crests are clockwise or counter-clockwise rotated with respect to the main tidal current depending on the counter-clockwise or clockwise rotation of the velocity vector induced by the tide. Only for unidirectional or quasi-unidirectional tidal currents are sand banks always counter-clockwise rotated. The predictions of the model are supported by comparisons with field data. Finally, the mechanisms leading to the appearance of sand waves and sand banks are discussed in the light of the model findings. In particular, it is shown that the growth of sand banks is not only induced by the depth-averaged residual circulation which is present around the bedforms and is parallel to the crests of the bottom forms: a steady drift of the sediment from the troughs towards the crests is also driven by the steady velocity component orthogonal to the crests which is present close to the bottom and can be quantified only by a three-dimensional model. While the former mechanism appears to trigger the formation of counter-clockwise sand banks only, the latter mechanism can give rise to both counter-clockwise and clockwise rotated sand banks.

Modelling sand wave migration in shallow shelf seas

Sand waves form a prominent regular pattern in the offshore seabed of sandy shallow seas. The positions of sand-wave crests and troughs slowly change in time. Sand waves are usually assumed to migrate in the direction of the residual current. This paper considers the physical mechanisms that may cause sand waves to migrate and methods to quantify the associated migration rates. We carried out a theoretical study based on the assumption that sand waves evolve as free instabilities of the system. A linear stability analysis was then performed on a 2DV morphological model describing the interaction between the vertically varying water motion and an erodible bed in a shallow sea. Here, we disrupted the basic tidal symmetry by choosing a combination of a steady current ( M 0) and a sinusoidal tidal motion ( M 2) as the basic flow. We allowed for two different physical mechanisms to generate the steady current: a sea surface wind stress and a pressure gradient. The results show that similar sand waves develop for both flow conditions and that these sand waves migrate slowly in the direction of the residual flow. The rates of migration and wavelengths found in this work agree with theoretical and empirical values reported in the literature.

Radar imaging of sand waves on the continental shelf east of Cape Hatteras, NC, U.S.A.

Imaging radars, under certain environmental conditions, can provide an extensive description of shallow submarine topography. In this investigation, sand waves were observed in shallow water and under light winds, weak flow, and highly stratified conditions with an L band synthetic aperture radar and X band real aperture radar. An analysis of the radar data reveals that regularly spaced modulations seen in the imagery are a result of bathymetric forcing. These modulations appear as a group of bright linear east-west trending features approximately 5 km in length and spaced 230 m apart with observed peak modulations exceeding predicted modulations by 7 dB. Bathymetric measurements extracted from shipboard ADCP data confirm the existence of sand waves in this region. Results from the ADCP data reveal an east-west orientation of the sand wave crest with lee slopes facing north. Mean wavelengths are 230 m and the heights are roughly 2.5 m. The radar modulations lead the sand wave crest, by approximately 135 m suggesting a possible upstream hydrodynamic effect, which is consistent with an observed Froude number less than one. This study shows that bathymetric effects are observed in radar imagery at low current speed, light winds, and strong stratification, demonstrating the critical nature that topographic and stratified hydrodynamic effects have on radar image interpretation in the littoral environment.

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.

Measurements of the semi-diurnal drag coefficient over sand waves

Simultaneous measurements of tidal flow and pressure gradient over a 10 km distance have been used to estimate the frictional drag coefficient over sand waves in the southern North Sea. The measurements were made over a 27-day period in October–November 1988 at 52°10′N, 3°46′E, within a field of essentially two-dimensional sand waves approximately 3 m high and of wavelength 250 m. The M 2 drag coefficient for depth-averaged flow normal to sand wave crests is found to be 2.95 × 10 −3, in good agreement with values used in numerical models of the region. The uncertainty in this value is estimated to be of the order of 10%, primarily due to possible errors in the phase of the flow relative to the pressure gradient. The time series of daily-averaged semi-diurnal ( Z 2 ) drag coefficients over the 27-day period shows surprisingly little correlation with nearbed wave orbital velocities. The time of highest waves is associated with a decrease in drag coefficient rather than the increase predicted by most wave-current interaction theories and some previous observations. It is suggested that this behaviour is caused by stratification near the bed due to sediment resuspension under high waves. Predictions using the Glenn and Grant (1987, Journal of Geophysical Research, 92, 8244–8264) theory provide qualitative support for this hypothesis, though the magnitude of the predicted effect is smaller than observed. The relative insensitivity of the drag coefficient to wave conditions suggests that incorporating simple wave-current algorithms into numerical models may be misleading.

Vertical Disturbance of Sediments by Horseshoe Crabs (Limulus polyphemus) during Their Spawning Season

Biological and physical reworking of sediments can profoundly affect the structure and functioning of benthic communities. The depth of the disturbance is an important factor that controls the types of organisms that can exist within the sediments. Large numbers of horseshoe crabs,Limulus polyphemus, spawn each spring on the sandy shores of Delaware Bay beaches. We have used this abundance peak to provide an estimate of the depth of sediment disruption caused by this species on an intertidal flat adjoining a major spawning beach. Vertical columns of marked sediment were placed in three locations of an intertidal flat. Some columns were protected with cages while others remained unprotected. Analysis of variance of the depth of disruption of the marked sediment indicated that different areas of the intertidal zone were disrupted to different depths. Caged marked sediment was disturbed to an average depth of 3.2 cm while unprotected sediment was disturbed to a mean depth of 11.1 cm. Deepest mixing occurred in a trough between sandwave crests and averaged 17.7 cm deep. These mixing depths are greater than the 1.2 cm deep disturbance produced by nonstorm wave action in the study area.

Microhabitat Utilization by the Megafaunal Assemblage at a Low Relief Outer Continental Shelf Site - Middle Atlantic Bight, USA

Direct underwater observations, using a manned submersible (May 1987) and remote operated vehicle (July and November 1988), were made of the small-scale distribution and microhabitat relationships of the megafaunal assemblage at a 55 m low relief outer continental shelf site (40 50'N, 70 Four microhabitat types were defined: flat sand with amphipod tubes, sand wave crests, shell (single valves and valve aggregates) and biogenic depressions. Microhabitat heterogeneity occurred on the scale of meters. Significant species-specific microhabitat relationships were found. Temperature mediated mesoscale shifts in megafaunal-microhabitat associations were found for several species.