Intitulé du sujet: From bed heterogeneities and aeolian transport regimes to dune morphodynamics

Sujet

Codirection: Philippe Claudin (PMMH)

Nombre de mois: 48 mois

Ecole Doctorale: ED 560 - Sciences de la Terre et de l'environnement et Physique de l'Univers

Unité de recherche et équipe:

Geological Fluid Dynamics Laboratory,
Université Paris Cité, Institut de Physique du Globe de Paris, CNRS,
75238 Paris cedex 05,
France

Coordonnées de l’équipe:

1 rue jussieu,
75238 Paris cedex 05,
France

Phone: 33 (0) 1 83 95 23 74
email: narteau@ipgp.fr

Secteur: Sciences Physiques et Ingénierie / Physical sciences and Engineering

Langue attendue: Anglais

Niveau de langue attendu: B2

Description

Description du sujet:

Since the basic mechanisms of aeolian transport have been elucidated [1], the saltation process has been found to be sensitive to wind speed and the nature of the bed, in particular whether it is erodible or consolidated. These dependencies have been quantitatively documented in the last decade [2-7], but their impact on aeolian landforms remains to be elucidated. Based on field data, laboratory experiments and numerical simulations, the aim of this PhD thesis is to examine the potential influence of bed heterogeneities and high wind speeds on the emergence of sand patches and on dune morphodynamics.

Context

Above a threshold wind shear velocity, individual grain can be lifted from a sandy erodible bed and propel along a ballistic trajectory, the shape of which is primarily determined by the density ratio between the granular material and the fluid. These saltating grains eventually impact the sand bed, potentially ejecting new grains via these collisions. A regime of equilibrium is established when the number of ejections offsets the number of impacts, finally producing a constant rate of transport commonly referred to as the saturated sand flux. In this saturation regime, the number of grains depends on wind shear stress, which is proportional to the square of wind speed (τ~u²). However, the volume fraction of moving grains in this saltation layer exerts a negative feedback on the wind, resulting in all grains flying at the threshold shear velocity. Consequently, the saturated aeolian sand flux on an erodible bed varies as the square of the wind speed, whereas the thickness of the saltation layer is constant and mainly controlled by the maximum height of the ballistic trajectories. This mode of transport and the corresponding transport laws have been demonstrated to be effective in a variety of aeolian environments, comprising different types of erodible sandy beds and a range of reasonable wind speeds.

The situation becomes more complex when aeolian transport occurs on a consolidated bed or under extreme wind conditions, predominantly due to the nature and frequency of grain-grain collisions. Indeed, the frictional dissipation that occur between the saltating grains and those that compose the surface is considerably reduce over a consolidated bed, whether stony, rocky, or cohesive. The lower dissipation of energy associated with these impacts enables higher rebounds, which in turn dilute the transport layer and permit individual particles to attain wind speed. It is therefore commonly assumed that the saturated aeolian sand flux on a consolidated bed varies with the third power of the wind speed. In the context of high wind speeds over an erodible bed, the number of collisions between saltating particles within the saltation layer increases significantly, resulting also in the expansion of this transport layer and its eventual thickness. In this instance, a quadratic correction has been put forth for high wind speeds, whereby the saturated aeolian sand flux is proposed to vary asymptotically with the fourth power of the wind speed. The relevance of these transport regimes remains to be validated in the field, and their impact on the emergence and dynamics of bedforms is yet to be determined.

Methods and Objectives

The objective of this PhD is to investigate the impact of different transport laws on the formation and evolution of isolated or periodic sand patches, as well as on dune morphodynamics. In order to achieve this, different numerical approaches will be employed, the results of which will be compared with field data collected on beaches and in arid deserts, in conjunction with laboratory experiment data.

Based on recent results [7], the beginning of the thesis will be devoted to a systematic exploration of the effect of bed heterogeneities on the formation of meter-scale sand deposits. The candidate will develop and use a numerical model based on a systematic resolution of a system of differential equations that integrates the fundamental principles of dune physics. As this well-established approach has demonstrated that there is a minimum size of approximately 10 m for the formation of dunes, the new challenge is to identify the conditions under which the presence of bed heterogeneities can permit the development of periodic bedforms at a smaller wavelength. If this stage is successful, it will then be possible to determine the duration and the strength of wind events that explain dune formation from the growth of individual sand patch.

The second part of the PhD will be dedicated to mature dunes, which are large enough to accommodate the wind regime under which they form. Using a discrete dune model [8] and the wide range of dune types it has produced in the past [9], the objective is to investigate how these patterns are affected by changes in transport regimes. In zones of high sand availability, the changes in scaling at high wind speed is likely to increase the transport gradient on the stoss slope of dunes, which in turn will modify the dune aspect ratio or the formation and the development of superimposed bedforms. In areas with a high availability of sand, the changes in scaling at high wind speeds are likely to increase the transport gradient on the dune stoss slopes, which in turn will modify the dune aspect ratio or the genesis of superimposed bedforms.

Outcomes

• Aeolian land forms dynamics

• Evolution and stability of sandy beaches, foredune formation.

• Dune dynamics under climate change conditions.

• Comparative planetology, aeolian bedforms in exotic environments.

Compétences requises:

• University education/engineering school in Physics or Geosciences with an honour degree or an outstanding track record
• Experimental and numerical modelling skills. Knowledge of fluid mechanics is recommended.
• Autonomous, conscientious and rigorous.
• Good level of spoken, read and written English.

Références bibliographiques:

The thesis will be co-directed by Clément Narteau (UP Cité, IPGP) and Philippe Claudin (UP Cité, PMMH) within the STEP'UP doctoral school. The thesis will be carried out at the IPGP and the PMMH, which has an instrumented wind tunnel. The field work will be carried out in collaboration with colleagues from the Institut de Physique de Rennes and researcher in Great Britain, poland and China.

1. Bagnold RA, The transport of sand by wind. The Geogr. J. 89, 409–438 (1937).

2. Creyssels M et al., Saltating particles in a turbulent boundary layer: experiment and theory. J. Fluid Mech. 625, 47–74 (2009).

3. Ho TD, P Dupont, A Ould El Moctar, A Valance, Particle velocity distribution in saltation transport. Phys. Rev. E 85, 052301 (2012).

4. Martin RL, JF Kok, Wind-invariant saltation heights imply linear scaling of aeolian saltation flux with shear stress. Sci. advances 3, e1602569 (2017).

5. Pähtz T, O. Durán, Unification of Aeolian and Fluvial Sediment Transport Rate from Granular Physics. Phys. review letters 124, 168001 (2020).

6. Ralaiarisoa JL, et al., Transition from saltation to collisional regime in windblown sand. Phys. Rev. Lett. 124, 198501 (2020).

7. Delorme P, et al., Field evidence for the initiation of isolated aeolian sand patches. Geophys. Res. Lett. 50 (2023).

8. Narteau et al., Setting the length and time scales of a cellular automaton dune model from the analysis of superimposed bedforms. J. Geophys. Res. 114, F03006 (2009).

9. Gao et al., Phase diagrams of dune shape and orientation depending on sand availability.
   Scienttific Rep., 5, 14677 (2015).