Lit majeur | EPFL Graph Search

Le lit majeur, lit d'inondation, plaine d'inondation ou champ d'inondation du lit d'un cours d'eau désigne la partie qui n'est inondée qu'en cas de crue. Il est situé de part et d'autre du lit mineur du fleuve ou de la rivière et est souvent vaste. Ses bordures extérieures correspondent au niveau de la plus grande inondation historique enregistrée. Il n'est donc qu'assez rarement occupé par l'eau et il constitue une capacité de stockage temporaire de cette eau d'inondation. La hiérarchisation entre lit majeur et lit mineur n'est possible que pour les cours d'eau dont le débit est à la fois régulier et rythmé. Rugosité et sédimentation Dans le lit mineur, où la végétation est rare et l'eau plus profonde, le courant peut être rapide et la compétence est élevée. Le lit majeur est caractérisé par un étalement des eaux sur une profondeur beaucoup plus faible, et est souvent encombré par de la végétation arbu

Modelling the potential of Natural Flood Management practices in an upland peatland catchment with the distributed TOPMODEL

Hélène Amélie Madeleine Boisgontier

In order to reduce flood risk, there is a growing interest in more nature-based solutions, such as Natural Flood Management (NFM), rather than hard-engineering ones. To help build the evidence towards these solutions, we analysed the influence of different land cover change and land management in an upland peatland catchment in the Yorkshire Dales, UK. Using the distributed TOPMODEL from Gao et al., we first modelled the impact of revegetation, grazing and prescribed burning on the flood hydrograph. Then, tree planting as woodland and hedgerow, was implemented in the model and its influence on the flood peak was assessed. Revegetation of the whole catchment, from bare peat to \textit{Eriophorum} grasses, reduced the flood peak by 47% and delayed it by an hour and twenty minutes for an 18mm/hour rainfall. Taking a baseline \textit{Eriophorum} cover of the catchment, prescribed burning did not have a significant impact and grazing gave a 45-minute earlier and 31% higher flood peak. Woodland and hedgerow planting on grazed areas both had a positive influence on the flood peak. Planting hedgerows on every field edges in the catchment reduced the flood peak by 14% and delayed it by 15 minutes for the same above rainfall. Different scenarios of woodland planting were analysed: existing, floodplain, riparian and wider catchment woodlands. The most efficient scenarios were floodplain and riparian planting which gave a similar 3-hour late and 59% lower flood peak. The existing woodland scenario was the least efficient (12.5% reduction and 6-minute delay) and the wider catchment scenario was in between (40% reduction and 1-hour delay). Location of planting in the catchment was found to be very important and planting is near-stream zones is more efficient than in the headwaters or midslope of the catchment. Tree type and age at the catchment scale did not have a strong influence. Efficiency of tree planting on flood peak reduction and delay decreases with increasing rainfall intensity and storm duration. Floodplain woodland was found to be the most interesting scenario as flood risk was significantly reduced while only changing the land cover of a small portion of the catchment area (3%).

2018

Coupling X-ray computed tomography and freeze-coring for the analysis of fine-grained low-cohesive soils

Claire Guenat, Renée-Claire Le Bayon, Adrien Francis Olivier Liernur, Andreas Cedric Schomburg, Pascal Turberg

This paper presents the coupling of freeze-core sampling with X-ray CT scanning for the analysis of the soil structure of fine-grained, low-cohesive soils. We used a medical scanner to image the 3D soil structure of the frozen soil cores, providing X-ray CT data at a millimetric resolution over freeze-cores that are up to 62.5 cm long and 25 cm wide. The obtained data and the changes in gray level values could be successfully used to identify and characterize different soil units with distinctly different physical properties. Traditional measurements of soil bulk density, carbon and particle size analyses were conducted within each of the identified soil units. These observations were used to develop a 3D model of soil bulk density and organic matter distribution for five freeze-cores obtained at a restored floodplain in Switzerland. The millimetric X-ray CT scanning was applied to detect the impact of freeze-coring on the soil structural integrity. This allows identifying undisturbed zones, a critical precondition for any subsequent assessment of soil structure. The proposed coupling is thought to be applicable to a wide range of other low-cohesive soil types and has a large potential for applications in hydrogeology, biology or soil science.

Elsevier Science Bv2017

Composition and superposition of alluvial deposits drive macro-biological soil engineering and organic matter dynamics in floodplains

Claire Guenat, Renée-Claire Le Bayon, Rodolphe Schlaepfer, Andreas Cedric Schomburg, Pascal Turberg

Soil structure formation in alluvial soils is a fundamental process in near-natural floodplains. A stable soil structure is essential for many ecosystem services and helps to prevent river bank erosion. Plants and earthworms are successful soil engineering organisms that improve the soil structural stability through the incorporation of mineral and organic matter into soil aggregates. However, the heterogeneous succession of different textured mineral and buried organic matter layers could impede the development of a stable soil structure. Our study aims at improving the current understanding of soil structure formation and organic matter dynamics in near natural alluvial soils. We investigate the effects of soil engineering organisms, the composition, and the superimposition of different alluvial deposits on the structuration patterns, the aggregate stability, and organic matter dynamics in in vitro soil columns, representing sediment deposition processes in alluvial soils. Two successions of three different deposits, silt-buried litter-sand, and the inverse, were set up in mesocosms and allocated to four different treatments, i.e. plants, earthworms, plants + earthworms, and a control. X-ray computed tomography was used to identify structuration patterns generated by ecosystem engineers, i.e. plant root galleries and earthworm tunnels. Organic matter dynamics in macro-aggregates were investigated by Rock-Eval pyrolysis. Plant roots only extended in the top layers, whereas earthworms preferentially selected the buried litter and the silt layers. Soil structural stability measured via water stable aggregates (%WSA) increased in the presence of plants and in aggregates recovered from the buried litter layer. Organic matter dynamics were controlled by a complex interplay between the type of engineer, the composition (silt, sand, buried litter) and the succession of the deposits in the mesocosm. Our results indicate that the progress and efficiency of soil structure formation in alluvial soils strongly depends on the textural sequences of alluvial deposits.