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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 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 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)

Detailed reference viewed: 26 (13 ULg)