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See detailStream incision and sediment wave consecutive to three centuries of timber floating in the Morvan Massif (Central France)
Gob, Frédéric ULg; Jacob-Rousseau, Nicolas; Le Drezen, Yann et al

Conference (2013, August 30)

The upper catchment of the Yonne River is nowadays usually considered as having been scarcely impacted by human activity. At the Seine basin scale, the gravel bed streams flowing down the small forested ... [more ▼]

The upper catchment of the Yonne River is nowadays usually considered as having been scarcely impacted by human activity. At the Seine basin scale, the gravel bed streams flowing down the small forested massif of the Morvan are even considered as of high ecological quality. However, this perception of the river changes completely once one looks back to the recent past. Indeed, these streams have been subject to intensive industrial activity for more than 300 years, between the 16th and early 20th centuries, as Paris’ need for timber fuel amplified. The Yonne River and all of its tributaries were heavily modified to facilitate the transportation of timber logs towards Paris through floating on the Morvan’s dense network of streams and the Yonne’s and Seine’s main channel. This activity has led to intense modifications of sediment flux still easily observable on the present river morphology, 90 years after the floating activity ended. Every single stream of the upper Yonne catchment was equipped with small ponds allowing the generation of water releases, flushing the logs downstream. Historical archives allowed the discharges and the frequency of these flushes to be calculated. Artificial floods developing specific stream powers of over 250 W/m² were generated several times per week during wintertime in steep-sided streams that were 4 to 5 m wide. Such energy generated a drastic increase in sediment transport and led to erosion and massive incision of the beds. A few kilometers downstream, when the small tributaries joined the main valley, the Yonne River had a larger bed and gentler slope. Artificial floods were thus less powerful there (specific stream powers lower than 80 W/m²) and sediment transport conformed more to natural conditions. Considering the huge amount of sediment supply from upstream and the slowing down of the sediment flux, the Yonne river bed aggrades. A perched riverbed today lies up to 1.5m higher than the floodplain and may be seen on more than 25 km of the course. [less ▲]

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See detailArtificially generating sediment incipient motion in natural conditions
Gob, Frédéric ULg; Houbrechts, Geoffrey ULg; Linares Carreté, Alba et al

Poster (2010)

Incipient motion thresholds for gravel bed rivers are studied in flumes and natural rivers. Flume studies allow variables such as channel slope, water velocity, water depth, sediment size and sediment ... [more ▼]

Incipient motion thresholds for gravel bed rivers are studied in flumes and natural rivers. Flume studies allow variables such as channel slope, water velocity, water depth, sediment size and sediment composition to be controlled. Meanwhile, in the field, the incipient motion of particles is studied in natural conditions allowing the structure of the bed and the flood characteristics to be considered. Though much less developed, an intermediate possibility also exists. By artificially accelerating near bottom velocity of the water flow in a small portion of the bed, it is possible to initiate sediment transport. This allows sediment incipient motion to be observed in natural conditions while controlling the water velocities. The Cemagref (HHLY) has developed a device which confines water flow in a small tunnel on the bottom of the riverbed. It was developed in order to create a boundary layer similar to the one generated by natural flow. Water is injected into a filter which smoothes the flow before it enters a Plexiglas tunnel where sediment motion is observed. The flow is accelerated by two large pumps that allow flow velocities of up to 2.5 m/s in a small area 40 cm long, 20 cm wide and 12 cm high. As the water flow is confined, large scale turbulence similar to that occurring in natural rivers cannot be reproduced using the device. The velocity profile in the tunnel is stable and in equilibrium with the riverbed. Sediment motion was observed using this device on three Belgian gravel bed rivers (3.5 cm <D50<8 cm). The incipient critical velocities measured were coherent in the three rivers sampled, ranging between 1.3 m/s and 1.7 m/s. A progressive increase in velocity in the tunnel permitted size selective entrainment to be observed. Analysis of the particle entrainment schemes and associated velocities allowed the role played by the armoured layer and the Di/D50 ratio to be more fully understood. Critical velocities measured using the device were also compared to critical velocities observed in natural conditions, determined during several pebble tracing campaigns. This revealed that incipient motion velocities of the largest particles recorded in artificial conditions were systematically larger than those observed in natural conditions. This demonstrates the roles played by large scale turbulence and the vertical component of the flow velocity in single particle entrainment. Due to its inherent characteristics, both of these flow components are weak in the artificial conditions generated by the device. In conclusion, our study demonstrates that this type of device is a useful tool in observing and studying incipient motion processes and, in particular, the role played by the bed structure (armouring, protrusion, etc.). However, some improvements are still required in order to more accurately measure critical velocities, allowing critical discharges to be determined. [less ▲]

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See detailCritical specific stream power in gravel-bed rivers
Petit, François ULg; Gob, Frédéric ULg; Houbrechts, Geoffrey ULg et al

in Geomorphology (2005), 69(1-4), 92-101

Experiments with marked pebbles were carried out on different sized rivers of the Belgian Ardenne (catchment areas varying from less than I km(2) to 2700 km(2)). Specific stream power required to cause ... [more ▼]

Experiments with marked pebbles were carried out on different sized rivers of the Belgian Ardenne (catchment areas varying from less than I km(2) to 2700 km(2)). Specific stream power required to cause bedload movement was evaluated and critical values were obtained. Three types of relationship between critical specific stream power (omega(0)) and grain size (D) were established. The values for coo in the largest river (the Ourthe) were the lowest and were close to the values obtained for mountainous rivers carrying large boulders. In medium sized rivers (catchment area between 40 and 500 km(2)), the critical unit stream power was higher. It is likely that it is due to the bedform's greater resistance. This resistance would use up some of the energy that can cause movement and transport of bedload. The amount of resistance of the bedform can be expressed as bedform shear stress (tau ''), determined by the relationship between grain shear stress (tau'-that determines movement and transport of the bedload) and the total shear stress (tau). This ratio varies between 0.4 and 0.5 in the medium sized rivers, compared to 0.7 in the Ourthe. In headwater streams (less than 20 km(2)), there is greater loss of energy due to bedform resistance (tau'/tau < 0.3). Critical specific stream power is higher in this third type of river than in the other two. (c) 2005 Elsevier B.V. All rights reserved. [less ▲]

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See detailSedimentary dynamics and bedload transport in the Semois River (Ardenne, Belgium)
Gob, Frédéric ULg; Houbrechts, Geoffrey ULg; Petit, François ULg

in River Research & Applications (2005), 21(7), 791-804

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See detailEtude des impacts hydrauliques, sédimentologiques et écologiques liés aux travaux hydrauliques sur la Semois
Gob, Frédéric ULg; Houbrechts, Geoffrey ULg; Mols, Julien et al

in Schmitz, Serge (Ed.) Les Journées des Géographes Belges: Evaluer la capacité du milieu (2003)

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See detailLichenometric application to recent dynamics and sediment transport of a Corsican stream (Figarella River– France).
Gob, Frédéric ULg; Petit, François ULg; Bravard, J. P. et al

in Quaternary Science Reviews (2003)

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