Subaqueous sand dunes and sediment starvation

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Starved versus alluvial river bedforms: an experimental investigation

Laboratory experiments were conducted to investigate the formation of river bedforms under sediment supply-limited conditions, i.e. when a motionless substratum is bared by the dynamics of the mobile sediments. Three series of experiments were organized in a laboratory flume by fixing all the hydrodynamic and morphodynamic parameters but varying the thickness Delta of the initial layer of mobile sediments which covers the rigid bottom of the flume. At the end of all the experiments, which lasted for the same amount of time, the formation of transverse sand dunes was observed. For decreasing , the rigid bottom of the flume was bared progressively earlier during the experiment and the measurements showed a clear tendency of the bedforms to lengthen, i.e. to increase F their crest-to-crest distance. Moreover, under strong supply lim- itation, the two-dimensional transverse dunes turned into three-dimensional barchanoid forms and into isolated barchan dunes characterised by an abrupt reduction in bedform heights. two-dimensional Fourier analysis r of the bottom profile was performed, providing the amplitude of the main streamwise and spanwise harmonic components of the bottom morphology as a function of Delta

Serum amino acid profiles in normal subjects and in patients with or at risk of Alzheimer dementia

Background/Aims: Abnormalities in the plasma amino acid profile have been reported in Alzheimer disease (AD), but no data exist for the prodromal phase characterized by subjective memory complaint (SMC). It was our aim to understand if serum amino acid levels change along the continuum from normal to AD, and to identify possible diagnostic biomarkers. Methods: Serum levels of 15 amino acids and 2 organic acids were determined in 4 groups of participants – 29 with probable AD, 18 with mild cognitive impairment (MCI), 24 with SMC, and 46 cognitively healthy subjects (HS) – by electrospray tandem mass spectrometry. Results: Glutamate, aspartate, and phenylalanine progressively decreased, while citrulline, argi­ninosuccinate, and homocitrulline progressively increased, from HS over SMC and MCI to AD. The panel including these 6 amino acids and 4 ratios (glutamate/citrulline, citrulline/phenylalanine, leucine plus isoleucine/phenylalanine, and arginine/phenylalanine) discriminated AD from HS with about 96% accuracy. Other panels including 20 biomarkers discriminated SMC or MCI from AD or HS with an accuracy ranging from 88 to 75%. Conclusion: Amino acids contribute to a characteristic metabotype during the progression of AD along the continuum from health to frank dementia, and their monitoring in elderly individuals might help to detect at-risk subjects

On the formation of periodic sandy mounds

Le Bot and Trentesaux (Marine Geology 211, 2004) surveyed the periodic morphological patterns which are present in the English Channel close to the strait of Calais-Dover, where the shortage of sand does not allow the formation of typical sand waves (tidal dunes). The field observations show that, for similar hydrodynamic and morphodynamic conditions, the crest-to-crest distance of the observed sandy mounds is larger than the wavelength of the sand waves which form where sand is abundant. The present contribution describes an idealized model able to predict the hydrodynamics and the morphodynamics of the interaction of tidal currents with large scale bedforms such as sand waves and sandy mounds in sand-starved environments. Indeed, when the availability of sand is limited, classical morphodynamic stability analyses cannot be applied for two main reasons. First, part of the rigid substratum becomes bared when bedforms appear and the bed profile is no longer sinusoidal. Second, the formulae commonly used to quantify sand transport are no longer valid when sandy mounds alternate with a rigid substratum. In accordance with the field observations, the analysis shows that the bedforms which appear when the rigid substratum is bared (sandy mounds) are longer than those which form in a rich sand environment (sand waves)

Modeling the turbulent boundary layer at the bottom of sea wave

We investigate the turbulent oscillatory flow generated by propagating surface waves close to the sea bottom focusing our attention on moderate values of the Reynolds number R\u3b4of the bottom boundary layer. For such moderate values of R\u3b4, turbulent fluctuations appear only during parts of the oscillation cycle and the flow recovers a laminar-like behaviour in the remaining parts. Different roughness sizes are considered and both the smooth and the rough flow regimes are analysed. The aim of the present investigation is to test the performance of different two-equation turbulence models to compute the flow field over both smooth and rough walls and for moderate values of the Reynolds number. The considered models are the e 12\u3c9 model by Saffman and Wilcox (1974), two k 12\u3c9 models (Wilcox (1988) and a model derived from Wilcox (1992)), a low-Reynolds number k 12\u3b5 model (Foti and Scandura, 2004) and the model by Menter et al. (2003). To evaluate the performance of the models, the numerical predictions of the bottom shear stress are compared both with experimental measurements and with results of direct numerical simulations (DNS). All the models are found to provide fair results for high values of R\u3b4and for a smooth wall. For moderate values of R\u3b4, when turbulence is observed only during parts of the oscillating cycle, only one of the low-Reynolds number k 12\u3c9 models is able to describe the rapid growth of the wall shear stress due to turbulence appearance. On the other hand, if a rough wall is considered, the performance of the models greatly depends on the size of the roughness

Nonlinear dynamics of sand waves in sediment scarce environments

Field observations in the Dover Strait (Le Bot and Trentesaux, 2004) show sandy bed patterns in an environment where sand is scarce. Their morphological features closely resemble tidal sand waves, however, these type of bed forms are characterized by a crest-to-crest spacing which is larger than the wavelength of sand waves in the same surveyed area where sand is abundant. Based on stability theory, Porcile et al (2017) developed a morphodynamic model that was able to explain these features. They found that where the motionless substratum is exposed due to the growth of dunes, the lack of sand affects sediment transport, and consequently the morphology of the bed patterns. Their results also showed that the continuous growth leads to a lengthening of the dunes, and an increasing irregularity of the spacing. The found that their results were supported by the field observations. Since the model by Porcile et al (2017) is partly based on the perturbation principle, the results are only valid for small amplitude patterns. To further understand the nonlinear behaviour of these sand starved dunes (e.g. shape, height), we here apply the fully numerical sand wave model by Damveld et al (2020). We extend this model by accounting for the presence of a hard substrate just below a thin layer of sand. Moreover, we start with a randomly perturbed bed pattern to give the morphodynamic system the freedom of self-organization. Preliminary results show that the numerical model is able to reproduce the results found by Porcile et al (2017). In situations where sand is less abundant, wavelengths increase, and so does the spacing irregularity. Moreover, it is found that the average height of the sandy dunes is becoming increasingly smaller with decreasing sand availability. Further analysis should reveal dependencies to different environmental parameters, such as grain size, depth and tidal current strength

Typhoon-induced megarips as triggers of turbidity currents offshore tropical river deltas

Intense rip currents caused by tropical cyclones can drive sediment-laden turbidity flows down submarine canyons, according to numerical simulations. Shoreline shape, bathymetry and incoming wave direction are key factors controlling this phenomenon

Modelling the air-sea-land interactions responsible for the direct trigger of turbidity currents by tropical cyclones

Tropical cyclones directly trigger turbidity currents in submarine canyons as a consequence of storm surges, high waves, onshore blowing winds and extreme currents. The resultant supply of sediment at the heads of the canyons plays a crucial role in the genesis of turbidity currents and thus is key in understanding frequency and duration of their flows. Here we present a single numerical framework capable of modelling turbidity currents driven by cyclone-induced winds and waves through resolved, quasi-3D hydrostatic equations. Our simulations predict the occurrence of a canyon-confined turbid underflow induced by a tropical cyclone that caused a seafloor pipeline to shift. Turbidity current occurrence is shown to be related not just to tropical cyclone intensities but also to their tracks with respect to the canyon head. The modelled underflow is well approximated by similarity profiles from laboratory and field observations which demonstrates the reliability of the model in capturing the structure of turbidity currents. The proposed triggering of turbidity currents off the centre of coastal embayments is likely to occur when the abrupt rotation of incoming winds induced by a passing cyclone remains always coastal-bound all across the cyclone's waxing and waning stages. Our results show that these conditions can give rise to simultaneous, opposite alongshore currents and eventually result in offshore-bound rip currents. Conversely, it is unlikely that turbidity currents will be triggered by cyclone -induced rip currents when the cyclonic rotation results in peak offshore winds (coming from the land), as no fetch is available for generating large breaking waves to induce simultaneous, opposite alongshore currents. Nevertheless, the sole presence of strong alongshore currents deflected at the headlands of a coastal embayment (or delta) is likely to trigger sediment-gravity flows and eventually result in turbidity currents offshore the edge of the embayment without the aid of rip currents

Impact of sand dune morphology on macro-tidal current dynamics: insights from PIV measurements

International audienceHere we present an experimental campaign delving into the complex interplay between sand dune morphology and tidal current dynamics. Employing particle image velocimetry (PIV) measurements, we conducted diverse series of laboratory flume experiments to scrutinize the hydrodynamic influence of dunes on steady currents, both aligned and opposed to the dune asymmetry. Our experiments sought to replicate the hydrodynamics occurring over the tidal dune field of the Goulet de Brest, located in Brittany, France. The Goulet hosts well-formed and highly dynamic dunes shaped by intense tidal currents. Notably, these currents exhibit a distinctive asymmetry, with ebb flows prevailing towards the southwest in contrast to the opposite flood flows. The resulting tidal asymmetry is vividly mirrored in the dune profiles, showcasing an accentuated asymmetry in the southeast part of the field, where dunes grow taller in shallower waters. Conducted in a bidirectional current-permitting flume, each experimental series encompassed varied hydrodynamic conditions, such as ebb and flood, aligned or opposed to the dune asymmetry, and different bottom roughnesses. We utilized sediment mixtures that were progressively coarser, transitioning from a smooth to a fully rough flow regime. The PIV flow measurements provided unprecedented spatial resolution, slightly larger than 1 mm, and temporal resolution sampling data at a frequency of 200 Hz. These measurements were taken at the central dune crest and along the neighboring trough. At the inlet, lower-frequency point measurements were conducted using a Vectrino-II ADV Nortek, ensuring the stability of inlet currents during different PIV acquisitions. Analysis of the collected data unveiled a rich tapestry of turbulent processes and boundary layer dynamics. The identified detachment and reattachment points of the boundary layer, along with their spatio-temporal oscillations, revealed an intermittent advection of coherent turbulent structures downstream from the dune crest proceeding further downward along the lee side in the boundary layer and upward in the water column up to the free surface. These deepened our understanding of the intricate interactions between boundary layer dynamics and turbulence. Additionally, by scrutinizing the vertical current profile along the dune, we estimated the equivalent bottom roughness and local shear stresses, providing crucial insights into the drag imposed by the dunes, with potential implications for sediment transport processes. In essence, this dataset stands as a cornerstone in unraveling the intricate relationship between tidal dune morphology and coastal current dynamics. It underscores the potential of PIV measurements for investigating such large-scale phenomena under controlled conditions. Furthermore, the collected database can inform the development, validation, and sensitivity analysis of new numerical models aiming at the investigation of those fine-scale processes taking place at the interface of sand dunes and tidal current which control sediment transport and thus shape coastal areas