Description du sujet de thèse

Retrospective approaches to the hydro-gravel hazard in the Southern Alps

o Main activities
The main activities of the person under contract will be as follows:
1/Caracterise and quantify the occurrence of all hydro-gravel deposits (torrential floods, debris flows, wet avalanches): this will include carrying out sedimentological and geochemical analyses, counting deposits under a digital microscope of thin slides and possibly making thin slides in the laboratory).
2/Identify the activation mechanisms and triggering threshold effects by means of comparative analysis, distinguishing between the timeframes of the forcings (decadal, centennial, millennial) for different periods of the Holocene (deglaciation, forest optimum, anthropogenic destabilisation, etc.) on the basis of all the multi-proxy analyses (e.g. bio-indicators (diurnal, episodic, etc.) and the timeframes of the forcings (decadal, centennial, millennial) for different periods of the Holocene (deglaciation, forest optimum, anthropogenic destabilisation, etc.). g. bio-indicators (diatoms, ostracodes, pollens) studied over the same sequence as part of the RETRO-HYDRO project (ongoing and linked to the thesis project).
3/Reconstruct a chronicle of the occurrence and intensity of the hydro-gravity hazard in the Southern Alps based on statistical analyses of time series and combine this with a reflection on the possibilities of integrating these results into a risk management approach in mountain areas.


o Expected skills:
 Knowledge
A Master's degree in Physical Geography and/or Geosciences, with a strong knowledge of geomorphology.laeoenvironments, human-environment interactions and archaeology.


Operational skills
Training in geography tools (GIS, remote sensing, statistics, laboratory techniques and field measurements), mastery of sedimentology analysis techniques, microscopy, observation of deposits on a micrometric scale and recognition of bio-indicators.
Have experience (several months) of working on sedimentary archives (sampling, observation, making thin sections), be trained in laboratory and field work with a command of the usual techniques for taking sedimentary/soil samples.
Ability to produce geomorphological maps
Fluency in written and spoken English

Contexte de travail

Under the action of hydro-climatic forcings and favourable geological characteristics, mountain geosystems are likely to record significant upstream-downstream sediment transfers over short timeframes and can be the cause of significant human and economic damage (CRED, 2015). There are many examples of this in the southern French Alps (e.g. torrential flooding of the Vésubie in 2020; landslide in the Vallée du Guil in 2014). The recurrence and intensity of hydro-gravitational phenomena are modulated by very specific local factors (regional climate, topography, size of catchment areas, land use) and could be amplified by the phenomenon of extreme precipitation (Giorgi et al., 2016) with major consequences for erosive processes in mountain valleys. Although in situ instrumental monitoring enables detailed characterisation of hydro-gravel dynamics at catchment scale, it covers only a small number of sites and a short timeframe.
Mountain lake sediments are valuable records of past hydro-gravity events (Glur et al., 2013; Brisset et al. 2017; Wilhelm et al., 2022b). These sequences can cover several millennia, in the form of “instantaneous” deposits (intercalated in continuous sedimentation) with very specific characteristics (micrometric scale, terrigenous detrital materials), and mobilised during intense rainfall from the catchment area to the lake bottom. This type of sedimentary sequence can also be used to analyse other proxies (pollens, lake bio-indicators, soil weathering index, etc.) that document the regional environmental context, such as the close interactions between mountain vegetation, agro-pastoral uses, erosion, and the impact on hydrosystems, including in terms of ecological trajectories (Cartier et al., 2018; Bajard et al., 2020; Giguet-Covex et al., 2023).
The lake sediment archives collected upstream and downstream of catchments record extreme hydro-gravity events in their deposits and make it possible to reconstruct the trajectories of these events (in terms of frequency and intensity) in parallel with the evolution of the palaeoenvironments (vegetation, soils, anthropisation) over time windows that go beyond the instrumental period (multi-millennia)
The aim of this thesis project is to reconstruct (with high temporal resolution) the past trajectory of the hydro-gravity hazard, based on the sedimentary sequence taken from the Lac du Lauzet in the lower Ubaye Valley (September 2021, 890 m alt.), which has been continuously recorded for 14,000 years (validated by preliminary radiocarbon dating analyses).
However, the robustness of these reconstructions depends on the strategic use of a range of analysis methods requiring a variety of disciplinary expertise. The hydro-sedimentary (so-called instantaneous) and/or laminated deposits have thicknesses ranging from several centimetres to several micrometres. To characterise the entire depositional sequence on an event, seasonal and/or annual scale, it is crucial to resolve this set of observation scales. It is this type of approach that will enable reliable quantification of the evolution of these phenomena over time. On the other hand, while the study of the detrital fraction of the sediment can be approached through geochemical and sedimentological analyses, the environmental conditions of deposition require palaeoecological analysis skills (e.g. certain remains of lacustrine algae can provide information on the temporality of lacustrine deposition, since their repeated presence will be the mark of an annual deposition modality). In addition, pollen analyses will make it possible to retrace in detail the dynamics of landscapes throughout the catchment area. Obtaining a precise and well-constrained chronological framework for the millennial and/or centennial chronicles of erosive activity will also be an essential prerequisite for the detailed comparison of our data (on floods and changes in landscape/land use) with other palaeoflood archives, such as the reference sequence.
In addition, pollen analyses will make it possible to trace landscape dynamics in detail throughout the catchment area. Obtaining a precise and well-constrained chronological framework for the millennial and/or centennial chronicles of erosive activity will also be an essential prerequisite for the detailed comparison of our data (on floods and changes in landscape/land use) with other palaeoflood archives, such as the reference sequence for Lac d'Allos.

Contraintes et risques

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