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

2020 ◽  
Vol 8 (6) ◽  
pp. 409 ◽  
Author(s):  
Chiu Hwa Cheng ◽  
Karline Soetaert ◽  
Bas Wijnand Borsje
Keyword(s):  
Grain Size ◽  
North Sea ◽  
Sand Wave ◽  
Experimental Investigations ◽  
Sand Waves ◽  
Offshore Engineering ◽  
Organic Carbon Concentration ◽  
Significant Difference ◽  
Sediment Sorting ◽  
Tidal Sand

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.

Observations of sediment sorting over rapidly developed marine bedforms, using multibeam backscatter

2020 ◽  
Author(s):  
Timo C. Gaida ◽  
Thaiënne A.G.P. Van Dijk ◽  
Mirjam Snellen ◽  
Dick G. Simons
Keyword(s):  
Time Series ◽  
Grain Size ◽  
Marine Ecology ◽  
Sand Wave ◽  
Data Set ◽  
Seabed Sediment ◽  
Sand Waves ◽  
Spatial Sorting ◽  
Sediment Sorting ◽  
Size Sorting

<p>Grain-size sorting in bedforms is well known in river dunes. On continental shelves, however, datasets aimed at grain-size sorting over bedforms, are limited. More extensive observations of sediment sorting over bedforms may help to understand their morphodynamic processes, and are key in habitat mapping, since grain-size is a main control on the composition of benthic fauna. A time series of seven multibeam (MBES) bathymetry and backscatter measurements and box cores were collected for the monitoring of a coastal nourishment in a tidal inlet at Ameland, Netherlands. Prior to the nourishment (April 2017), 10-15 m long and 1.5 m high megaripples occurred. The time series shows the rapid development of high and steep megaripples in the newly replenished sediment, with a wavelength of 40 m and height of 2.5 m within three months (during-nourishment; October 2017), which then grew into 120 m long and 3 m high sand waves in relatively shallow water (10 - 14 m) within 5 months (post-nourishment; March 2018). <br>Relative backscatter (BS) strengths, which are corrected for, among others, transmission losses and bed morphology, represent seabed sediment characteristics. Bed classification of BS strengths, using an unsupervised Bayesian method, resulted in a high-resolution map of 5 acoustic classes (ACs), to which sediment types were assigned using the box cores as ground truthing. These box cores, however, were not taken at the detailed level of sand wave crests and troughs. <br>The acoustic sediment classes (ASCs) exhibit a repetitive pattern, indicating horizontal sediment sorting over bedforms, that shifted and intensified during the growth of the megaripples into sand waves. The ASC megaripple pattern is less consistent, but generally comprises finer sediments (ASC2-3: sand) on the stoss sides and coarser sediments on the lee sides (ASC3-4: sand to slightly gravelly sand). The sand wave pattern is very consistent and comprises coarse sediments on the stoss sides (ASC5: gravel- and shell-containing sands), finer sediments towards the crests (ASC2-3: sand) and even finer sediments (ASC1: sandy mud) in the troughs. In the course of one year, both the morphological and sorting patterns seem to repeat itself. A similar sorting evolution was observed during the growth of megaripples just farther offshore. <br>In a different data set, farther offshore on the Netherlands Continental Shelf and built up over several years, grab samples were collected in transects, specifically at crests and troughs of sand waves and long bed waves, and were analysed for grain size, organic matter and CaCO3 contents. Median grain sizes in the troughs of bedforms are consistently finer than at the crests, and reveal significant signatures between sand wave fields, with crest-trough differences among sites ranging between 10 and 85 micrometer. Unfortunately, MBES-BS data are not available for establishing large-scaled, spatial sorting patterns.<br>This evolution of horizontal sediment-sorting patterns during the growth of marine bedforms may support modelling studies of hydrodynamic responses of flow over undulating beds and may explain the morphodynamic evolution of marine bedforms, as relevant in marine ecology. However, coherent empirical datasets are required. </p>

Sediment sorting in tidal sand waves fields: the internal structure revealed?

2020 ◽  
Author(s):  
Johan Damveld ◽  
Bas Borsje ◽  
Pieter Roos ◽  
Suzanne Hulscher
Keyword(s):  
Grain Size ◽  
Internal Structure ◽  
Nonlinear Effects ◽  
Wave Formation ◽  
Sand Wave ◽  
Small Scale ◽  
Suspended Sediment Transport ◽  
Geophysical Research ◽  
Sand Waves ◽  
Tidal Sand

<p>Tidal sand waves are rhythmic bed forms found on coastal shelves all around the world. An important property of sand waves is their mobility, as they display migration rates of several meters per year. Insight in these dynamics is of practical relevance, as this behaviour may interfere with offshore engineering activities. State-of-the-art morphodynamic models are used to predict sand wave dynamics, but they still overestimate dimensions such as their height (Van Gerwen et al, 2018). Moreover, these models often assume a uniform grain size distribution, whereas field observations indicate a clear sorting of sediments along sand waves. Previous modelling studies found that a combination of sediment mobility effects and tidal current strength may explain these sorting patterns (e.g. van Oyen and Blondeaux, 2009). However, as these models were limited to the early stage of sand wave formation, they did not account for the nonlinear effects of increasing sand wave amplitudes. Our goal is to include these nonlinear effects in order to further unravel sorting processes, in particular the internal sand wave structure.</p><p>Hereto we extend the work by van Gerwen et al (2018), allowing for an arbitrary number of sediment fractions, and we adopt the active layer approach of Hirano (1971) to account for bed stratigraphy. To investigate the role of asymmetry in hydrodynamic forcing, we include a residual current superimposed on the dominant tidal component.</p><p>Results show that in general the crests of sand waves are coarser than the troughs. In the case of an asymmetrical forcing, larger sediments are found on the upper lee slope, whereas the smaller grains are deposited on the lower lee slope. Due to migration, also the internal structure of the sand wave is revealed over time, showing the same pattern as found on the lee slope surface. Many field studies have shown that these model results qualitatively agree with observations on surficial sorting patterns (e.g. Cheng et al, 2018). However, as field data on the internal sediment structure is scarce, it is difficult to validate this model output.</p><p>Hence, the question remains whether the results on the internal sorting are a true representation of the substrate of sand waves. Nonetheless, the model results give insight in the processes governing grain size sorting over and in sand waves, which could be a valuable element in developing future coastal management strategies, such as sand extraction.</p><p><em>Cheng, C.H., Soetaert, K., & Borsje, B.W. (2018). Small-scale variations in sediment characteristics over the different morphological units of tidal sand waves offshore of Texel. NCK Days 2018.<br></em><em>Hirano, M. (1971). River bed degradation with armouring. Trans. Jpn. Soc. Civ. Eng, 3, 194-195.<br></em><em>Van Gerwen, W., Borsje, B.W., Damveld, J.H., & Hulscher, S.J.M.H. (2018). Modelling the effect of suspended load transport and tidal asymmetry on the equilibrium tidal sand wave height. Coastal Engineering, 136, 56-64.<br></em><em>Van Oyen, T., & Blondeaux, P. (2009). Tidal sand wave formation: Influence of graded suspended sediment transport. Journal of Geophysical Research: Oceans, 114(C7).</em></p>

Grain sorting effects on the formation of tidal sand waves

2009 ◽  
Vol 629 ◽  
pp. 311-342 ◽  
Author(s):  
TOMAS VAN OYEN ◽  
PAOLO BLONDEAUX
Keyword(s):  
Grain Size ◽  
Size Distribution ◽  
Grain Size Distribution ◽  
Fine Fraction ◽  
Sand Wave ◽  
Flat Bottom ◽  
Sand Waves ◽  
Sand Mixture ◽  
Sea Bed ◽  
The Stability

A model is developed to investigate the process which leads to the formation of sand waves in shallow tidal seas characterized by a heterogeneous sea bed composition. The main goal of the analysis is the evaluation of the effects that a graded sediment has on the formation of the bottom forms and the investigation of the sorting process induced by the growth of the bottom forms. The analysis is based on the study of the stability of the flat bed configuration, i.e. small amplitude perturbations are added to the flat bottom and a linear analysis of their time development is made. For an oscillatory tidal current dominated by one tidal constituent, the results show that the graded sediment can stabilize or destabilize the flat bottom configuration with respect to the uniform sediment case, depending on the standard deviation σ* of the grain size distribution and on the ratio between the horizontal tidal excursion and the water depth. For moderate values of , i.e. values just larger than the critical value for which the sediment is moved and sand waves appear, the presence of a sand mixture stabilizes the flat bed. On the other hand, for large values of , the mixture has a destabilizing effect. In both cases the effect that a sand mixture has on the stability of the flat bed configuration is relatively small. Moreover, for moderate values of , the fine fraction of the mixture tends to pile up at the crests of the bottom forms while the coarse fraction moves towards the troughs. For large values of , the grain size distribution depends on the value of σ*. The results are physically interpreted and provide a possible explanation of the apparently conflicting field observations of the grain size distribution along the sand wave profile, carried out in the North Sea.

Macrobenthos richness and biomass preferentially geared towards one half of asymmetrical sand waves

2020 ◽  
Author(s):  
Chiu Cheng ◽  
Bas Borsje ◽  
Sarah O'Flynn ◽  
Olivier Beauchard ◽  
Tom Ysebaert ◽  
...  
Keyword(s):  
Community Composition ◽  
North Sea ◽  
Spatial Scales ◽  
Environmental Data ◽  
Sand Wave ◽  
Small Scale ◽  
Valuable Insight ◽  
Sandy Sediment ◽  
Sediment Grain Size ◽  
Sand Waves

<p>Sand waves are dynamic, sinusoidal bedforms that have been thoroughly studied in the context of the physical and hydrodynamical processes dominating these environments. However, information about the ecological and biogeochemical characteristics within these bedform habitats have been far fewer in comparison. To address this knowledge gap, a field campaign was undertaken in the summer of 2017 to investigate the biogeomorphology of asymmetrical sand waves in the Dutch North Sea, near island Texel. The goal was specifically to address both the macrofaunal community composition and the associated biogeochemistry along the different sections of these sand waves.  Using a combination of several field sampling techniques and lab incubations on board the NIOZ RV-Pelagia, we collected a comprehensive dataset covering the macrofauna assemblage, nutrient flux, oxygen consumption, sediment grain size and permeability, as well as physical and environmental data, within a transect line (< 1 km) that covered several sand waves. Here, we show considerable variability in the species abundance, composition and biomass, which were all significantly higher on the steeper sides of the sand waves; the multivariate statistical analyses on the datasets showed a significant influence of the sand wave position on benthic composition. Correspondingly, measurements from the steep slopes also exhibited a higher concentration of chl-a and organic matter, higher O<sub>2</sub> consumption, more fine particles and lower sediment permeability. Despite the overall homogeneity (e.g., sandy sediment) of a well-developed bedform environment such as a sand wave field, it is clearly possible to find significant variations in the benthic community composition and biogeochemical activity on a small spatial scale.  Oftentimes, studies look at larger spatial scales to maximize the characterization of an entire region. However, given the diverse environmental gradients within the North Sea, our observations may not be sufficiently captured or even missed altogether when superimposed upon such large spatial scales.  Thus, a close examination of the interrelated parameters such as biology, biogeochemistry, sedimentology and morphology should also be considered, at a high resolution, over a small local scale for such seemingly uniform habitats.  We hope our results will contribute valuable insight into small-scale patterns of variability in dynamic bedform environments. </p>

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

2020 ◽  
Vol 125 (10) ◽  
Author(s):  
J. H. Damveld ◽  
B. W. Borsje ◽  
P. C. Roos ◽  
S. J. M. H. Hulscher
Keyword(s):  
Finite Amplitude ◽  
Sand Waves ◽  
Sediment Sorting ◽  
Tidal Sand

Interaction Between Offshore Pipelines and Migrating Sand Waves

2012 ◽  
Author(s):  
Jordan Matthieu ◽  
Tim Raaijmakers
Keyword(s):  
Wave Crest ◽  
Sand Wave ◽  
Offshore Pipelines ◽  
Sand Waves ◽  
Long Term Stability ◽  
Wave Trough ◽  
Migration Direction ◽  
Operational Life ◽  
Tidal Sand ◽  
Wave Migration

Large areas of shallow, sandy seas are covered by migrating tidal sand waves. Sand wave migration rates are on the order of 10s of meters per year, with heights between 10 and 30% of the water depth. If such regions are traversed by pipelines, the dynamic interaction between the rock-berm protection of the pipelines and the migratory sand waves must be accounted for to assure the long term stability of both the rock-berms and pipelines. This study employs a 2DV model to demonstrate the hydrodynamic and morphodynamic interaction between migrating sand waves and a rock-berm constructed perpendicular to the migration direction. The timescale of sand waves and the design life of rock-berm are similar, consequently, rock-berms in sand wave regions experience a change in bed level approximately equal to that sand wave height. Due to the large difference in temporal scales between local erosive processes and sand wave migration, the passing of a sand wave is manifest as a general rising or falling of the ambient seabed, while a rock-berm is fixed at its construction elevation. Consequently, the critical design case is for a rock-berm constructed at a sand wave crest since the surrounding bed level decreases throughout the operational life of the pipeline. A conservative design approach is to construct rock berm protection in a sand wave trough, resulting in rising ambient seabed levels throughout the operational lifetime of the underlying pipeline or electrical cable.

scholarly journals A CLASSIFICATION METHOD FOR THE PRESENCE OF TIDAL SAND WAVES AND MAINTENANCE DREDGING DESIGN

2018 ◽  
pp. 48
Author(s):  
Rick De Koning ◽  
Jaap van Thiel De Vries ◽  
Bas Borsje
Keyword(s):  
Large Scale ◽  
Tidal Flow ◽  
Narrow Channel ◽  
Sand Wave ◽  
Maintenance Strategies ◽  
Sand Waves ◽  
Recirculating Flow ◽  
The North ◽  
Tidal Sand ◽  
And Migration

The study into sand wave dynamics in South Channel commenced after large dune forms were observed in monitoring campaigns following the channel deepening project of the Port of Melbourne. The project involved deepening of the harbor berths and channels, but more importantly, it involved the deepening of South Channel in Port Phillip Bay. South Channel, the main shipping channel, crosses the bay over ≈20km. The growth of bedforms at various locations in South Channel now threatens to impede marine traffic. The dimensions and migration rate of the bedforms in the channel are remarkable, especially in the harsh flow conditions in the narrow channel. Therefore, the bedforms in South Channel cannot be given an obvious classification. In this paper it is shown that the bedforms in South Channel can be classified as a tidal sand wave type with a method that requires only insight in water depth, tidal flow velocity and grain size. Tidal sand waves are large-scale bedforms generated by recirculating flow cells that drive sediment to the top of a crest and are commonly observed on shallow coastal seas such as the North Sea. The bedform concern in the channel illustrates the necessity of an evaluation of the present, and alternative, channel maintenance strategies. A numerical model in Delft3D software is applied, along with a probabilistic calculation that combines insights from the simulations and survey data, to assess different maintenance strategies.

scholarly journals OFFSHORE SAND WAVES

1986 ◽  
Vol 1 (20) ◽  
pp. 78 ◽  
Author(s):  
Rolf Deigaard ◽  
Jorgen Fredsow
Keyword(s):  
Grain Size ◽  
Theoretical Model ◽  
Wave Height ◽  
Marine Environment ◽  
Water Depth ◽  
Wave Climate ◽  
Sand Wave ◽  
Sand Waves ◽  
Wave Heights ◽  
Mean Current

A theoretical model for the equilibrium dimensions of offshore sand waves is presented. The model is an adaptation of the dune model by Fredsoe (1982) to the marine environment, making use of the physical analogies between offshore sand waves and river dunes. The predicted sand wave heights and lengths conform well with the observations of sand wave formation reported in the literature. One of the results from the model is that for a given wave climate sand waves will only be formed under a certain range of mean current velocities. This range becomes narrower for increasing wave height or decreasing water depth and grain size.

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