Utilisation des scories métallurgiques en dynamique fluviale : détermination de la compétence effective des rivières et estimation des vitesses de progression de la charge de fond.

peer reviewedLes compétences effectives de trois rivières du nord de l’Ardenne (Belgique) ont pu être estimées grâce à la présence de nombreuses macroscories dans leur charge de fond. En effet, ces scories, rejetées par les anciens fondeurs dans les rivières, ont été soumises, pendant plusieurs siècles, à de nombreuses crues mobilisatrices. Par conséquent, elles peuvent être considérées comme un marqueur représentatif de la compétence effective des rivières sur une très longue période et sur une grande distance. Ainsi, en comparant la taille des scories en amont et en aval d’un secteur de pertes karstiques (plusieurs kilomètres), nous avons montré que ces pertes n’entraînent pas de baisse sensible de la compétence de la rivière. Une relation entre la taille des scories prélevées et les puissances spécifiques développées dans les secteurs de prélèvement a également été établie. Cette relation montre que le maximum de puissance spécifique des rivières ardennaises se produit dans les rivières dont le bassin versant est voisin de 150 km². Finalement, des scories retrouvées dans l’Ourthe, environ 10 km en aval du site de production le plus proche, ont été utilisées pour évaluer la vitesse de progression de la « vague sédimentaire ». En effet, ces scories sont soumises aux mêmes conditions hydrologiques et de piégeage que les éléments naturels de la charge de fond. Après avoir daté le début du fonctionnement du site de production le plus proche (1450), nous avons calculé la vitesse de progression des scories qui est comprise entre 1,8 et 2,3 km/siècle

Residual doses in recent alluvial sediments from the ardenne (S Belgium)

peer reviewedWe report on our first investigations into the potential of optical dating for determining the rate of river flood sedimentation in the Ardenne region (S Belgium). Two samples collected from a recent alluvial deposit were used to investigate the extent of resetting in different particle size fractions of quartz (4-11 mu m, 63-90 mu m, 90-125 mu m, 125-180 mu m, 180-212 mu m and 212-250 mu m) as well as in polymineral fine (4-11 mu m) grains. Both samples show satisfactory OSL and IRSL characteristics. The IRSL signals from the polymineral fine grains yield an equivalent dose (D-e) of 3-4 Gy, while a D-e of 0.3-0.6 Gy was measured using large aliquots of quartz. Small aliquot analyses of 63-90 mu m and 212-250 mu m quartz grains confirm that the coarser fraction contains more grains with lower D-e's. Furthermore, for a modern sample (< 3 years old), similar to 60% of the aliquots yields a D-e consistent with zero, indicating that these contain only well-bleached grains. These findings suggest that it might be possible to extract the true burial dose from dose distributions measured using small aliquots of coarse-grained (e.g. 212-250 mu m) quartz

Artificially generating sediment incipient motion in natural conditions

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

Réhabilitation de cours d'eau : complémentarité de la morphologie et de la qualité de l'eau

peer reviewe

Introduction to the special issue in honor of Prof. François Petit

peer reviewe

From the primitive Ourthe to the primitive Meuse in the Lower Meuse of Liège - Part 1: Generalities and data

editorial reviewedThe gravel that covers the sub-flat surfaces above 180 m a.s.l. around Liège are the subject of a sedimentological study, in relation to the state of knowledge in terms of Oligocene stratigraphy taking into account the uplift/tilting of the NW flank of the Ardenne. This article reports the generalities and the field and laboratory data that have led to the construction of a new model of the morpho-sedimentary evolution of the hydrographic network of the Ourthe and the Meuse in the area of their current confluence. The interpretation of these data and the resulting model are published in the article that follows in this issue of the journa

From the primitive Ourthe to the primitive Meuse in the Lower Meuse of Liège - Part 2: Model and discussion

editorial reviewedThe gravel that covers the sub-flat surfaces above 180 m a.s.l. around Liège are the subject of a sedimentological study, in relation to the state of knowledge in terms of Oligocene stratigraphy taking into account the uplift/tilting of the NW flank of the Ardenne. General as well as field and laboratory data are set out in the preceding article published in the same volume of the Journal. This leads to a new model of morpho-sedimentary evolution of the hydrographic network linked to an indeterminate Oligocene marine regression in a sub-coastal environment. Two successive routes of the Ourthe through the Hesbaye (north of Liège) join the regression oligocene coast northwards. The first terrace of the Meuse upstream of Liège attests a flow also directed towards that shore. The entry into the scene of the Meuse downstream from Liège and therefore in the Netherlands requires captures in order to successively divert the Vesdre, the Ourthe and then only the Meuse. The validity of the earlier models which connect the Traînée mosane s.l. to terraces in Dutch Limburg is therefore disputed