Publications of Geoffrey Houbrechts
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See detailTime-history of the gravel sheet in Ardennian rivers over the last 100,000 years
Houbrechts, Geoffrey ULiege; Denis, Anne-Cécile ULiege; Juvigné, Etienne ULiege et al

Poster (2013, August 27)

It is generally held that, in north-western Europe, the main part of the gravel sheets under river beds were deposited during the Weichselian period in a periglacial environment. However, other parameters ... [more ▼]

It is generally held that, in north-western Europe, the main part of the gravel sheets under river beds were deposited during the Weichselian period in a periglacial environment. However, other parameters such as propagation of knickpoints in fluvial networks may also influence incision or aggradation. However, only few studies have dated the periods of formation of the gravel sheets and have described their properties. The first aim of this research was to determine the thickness of the gravel sheets still remaining under the river beds and to estimate the potential incision of these rivers before reaching the bedrock. Then we tried to answer a number of other questions: When did these thick gravel deposits fill the valley bottom? When were the lowest terraces abandoned? When did the rivers incise the bedrock? What is the morphology of the bedrock under the gravel layer? Numerous boreholes were made by percussion drilling in different floodplains of the Ardenne Massif and core samples were taken, down to the bedrock. Afterwards, different volcanic tephra from the Late Pleistocene were used as stratigraphic markers to date the relative periods of terrace formation and to reconstruct the past evolution of the gravel sheets. Pollen and metallurgic slag were also used to date the periods of bed level evolution. In the Ardennian massif, the thickness of the gravel sheet beneath the river beds is very variable (from 10 m in the downstream part of the Ourthe River to less than 1 m in the upper catchments). In some valleys, weathered bedrock has been observed under the gravel sheet to a thickness of several meters. Different phases of accumulation and incision over the last 100,000 years have been dated. Some evolutions can be clearly linked to climate changes but some modifications of bed levels also occurred during the Weichselian period and could be a response to the propagation of knickpoints in the fluvial networks. [less ▲]

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See detailBedload dynamics in gravel bed rivers
Houbrechts, Geoffrey ULiege; Hallot, Eric ULiege; Peeters, Alexandre ULiege et al

Conference (2013, August 27)

The aim of this research was threefold: 1) to improve the values of bedload mobilisation discharge in gravel bed rivers, 2) to determine the bedload velocity over the short and long term and 3) to ... [more ▼]

The aim of this research was threefold: 1) to improve the values of bedload mobilisation discharge in gravel bed rivers, 2) to determine the bedload velocity over the short and long term and 3) to quantify bedload transport and to examine bedload discharge in relation to the stream power and to the excess stream power. Since 2006, we used the PIT-tag technique in order to mark pebbles from Ardennian rivers. Ten rivers of different geomorphological properties were fitted out with PIT-tags (in 18 different sites) and 67 surveys were carried out after hydrological events. These data allow a valuable relationship between the stream power during floods and the size of the biggest elements mobilised to be proposed. This relation is obtained for medium sized rivers (50-200 km²) with bed material composed of gravel (D50 between 2 and 10 cm). Furthermore, we also obtained good relationships between the average transport distance and the excess of stream power reached during the hydrological events. In order to estimate the bedload virtual velocity over a longer time scale, we applied these relationships to the hydrological events recorded by gauging stations over more than 30 years. We obtained values of virtual velocity which are between 2 km/century for a low energy river with a sinuous bed and well developed riffle-pool sequences (Rulles River – 20 W/m² at Qb) and 11 km/century in a more powerful river with a straight channel (Aisne River – 100 W/m² at Qb) and a sub-flat bed. The propagation velocity of bedload is therefore influenced by the energy available. However, it is also necessary to take into account the river pattern, the characteristics of the armoured layer and the presence of potential trapping sites. [less ▲]

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See detailLe Ru de Pôleur ancien et sa capture par le Hoëgne
Juvigné, Etienne ULiege; Demoulin, Alain ULiege; Houbrechts, Geoffrey ULiege

in Hautes Fagnes (2013), (1), 18-21

A un moment indéterminé de la période continentale tertiaire qui a suivi le retrait de la mer oligocène des Hautes Fagnes, le Ru de Pôleur (actuellement Hoëgne supérieure) poursuivait son cours vers l'WSW ... [more ▼]

A un moment indéterminé de la période continentale tertiaire qui a suivi le retrait de la mer oligocène des Hautes Fagnes, le Ru de Pôleur (actuellement Hoëgne supérieure) poursuivait son cours vers l'WSW par l’ensellement de Hockai. Au-delà, il empruntait soit la vallée du Ru de Hockai, soit celle du Roannai. Dans les deux cas, il a probablement rencontré la Warche à la bordure occidentale du replat de Cronchamps. Ensuite, il a été capturé en conséquence de l’érosion régressive de la Hoëgne moyenne qui dévalait le flanc NO de la crête de la Vecquée. Ce processus a eu lieu pendant la seconde moitié du Tertiaire, et il a été facilité par l’altération des roches due à la présence de failles transversales perpendiculaires à la crête de la Vecquée à l’endroit du coude que décrit maintenant la vallée. [less ▲]

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See detailLa pente longitudinale de la terrasse principale de la Meuse revisitée de Seilles à Huy
Juvigné, Etienne ULiege; Di Modika, Kevin; Houbrechts, Geoffrey ULiege et al

in Bulletin de la Société Géographique de Liège (2013), 61

Des relevés altimétriques géoréférencés ont été appliqués à des contacts cailloutis/substratum de lambeaux de la terrasse principale T4 de la Meuse entre Namur et Liège, à Seilles/Andenne et à Wanze/Huy ... [more ▼]

Des relevés altimétriques géoréférencés ont été appliqués à des contacts cailloutis/substratum de lambeaux de la terrasse principale T4 de la Meuse entre Namur et Liège, à Seilles/Andenne et à Wanze/Huy. Il en résulte que la pente longitudinale du tronçon étudié est identique à celle de la plaine alluviale actuelle, tandis que des travaux antérieurs basés sur l’interprétation de cartes topographiques y voient une contre-pente et impliquent donc inutilement une déformation tectonique pendant le Quaternaire dans le secteur étudié. [less ▲]

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See detailImages CM de Passega des rivières ardennaises
Houbrechts, Geoffrey ULiege; Hallot, Eric ULiege; Levecq, Yannick ULiege et al

in Bulletin de la Société Géographique de Liège (2013), 61

A method of grain-size distribution data interpretation was proposed by Passega (1957; 1964) to differentiate the deposits of various modes of fluvial and marine transport (uniform suspension, graded ... [more ▼]

A method of grain-size distribution data interpretation was proposed by Passega (1957; 1964) to differentiate the deposits of various modes of fluvial and marine transport (uniform suspension, graded suspension, rolling). This method consists of using two characteristic percentiles (D99 and D50) of a grain-size distribution sediment curve, that are applied to a chart with a double logarithmic scale. The position of the point is compared with a model (CM diagram) determined on the basis of samples collected from deposits for which the conditions of establishment are known. Several studies have also shown that this method allows sedimentary bedforms to be differentiated based on the grain-size characteristics of the sediments that have been deposited, thus permitting the conditions of sedimentation and the transport of old alluvium to be retrospectively determined. However, a number of parameters specific to each water course influence the characteristics of the CM diagram and there is not one universal diagram that may be transposed to all fluvial environments (Bravard and Peiry, 1999). This article presents tens of CM diagrams of Ardennian rivers, obtained on the basis of samples taken from deposits that were laid down in known hydrodynamic conditions and for which the mode of implementation can be reconstituted. The purpose of this article is therefore to provide a reliable tool to provide solid interpretations of the conditions of sedimentation of ancient fluvial deposits of the Ardennian rivers and, more generally, of gravel bed rivers (Eifel, Vosges, Morvan, etc.) in low mountain areas (ω = 15 to 150 W/m²). [less ▲]

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See detailFlux des sédiments en suspension dans les rivières du bassin de la Meuse : proposition d’une typologie régionale basée sur la dénudation spécifique des bassins versants
Van Campenhout, Jean ULiege; Denis, Anne-Cécile ULiege; Hallot, Eric ULiege et al

in Bulletin de la Société Géographique de Liège (2013), 61

Soil erosion may be apprehended at the watershed scale by quantifying the transport of suspended sediment at its outlet. Approximately 2000 measurements of suspended sediment concentration were used to ... [more ▼]

Soil erosion may be apprehended at the watershed scale by quantifying the transport of suspended sediment at its outlet. Approximately 2000 measurements of suspended sediment concentration were used to estimate the annual sediment discharge at 80 stations located in the Walloon region (Belgium), at the outlet of watershed from 16 to 2900 km². Some regional differences have been highlighted depending on the substrate and the land use in the watershed. At the region scale, the mean annual sediment yield reaches 20 t.km-2.yr-1 in Lorraine, 34 t.km-2.yr-1 in Ardenne and 69 t.km-2.yr-1 in Entre-Vesdre-et-Meuse. The size of suspended particles and the proportion of organic matter also show differences between each agro-geographical area. Data from other academic or administrative works were incorporated into the database to identify the best sites for continuous measuring of the suspended load concentration in Wallonia. [less ▲]

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See detailReconstitution de la dynamique d'une rivière ardennaise (La Lienne) depuis le Tardiglaciaire grâce à l'analyse géomorphologique et chronostratigraphique d'une tourbière de fond de vallée
Denis, Anne-Cécile ULiege; Wastiaux, Cécile ULiege; Petit, François ULiege et al

in Géomorphologie: Relief, Processus, Environnement (2013), 2

The geomorphological dynamics (incision, lateral mobility, sedimentation rate) of a river typical of the Ardenne region (the Lienne River) were studied at the level of a valley floor peat deposit. This ... [more ▼]

The geomorphological dynamics (incision, lateral mobility, sedimentation rate) of a river typical of the Ardenne region (the Lienne River) were studied at the level of a valley floor peat deposit. This type of site is of major interest as the morphology of ancient fluvial deposits is preserved underneath peat deposits and pollen conserved in the peat allows different phases of the evolution of the river to be dated. The presence of pollen from the Younger Dryas above a pebble sheet perched 1.4 m above the present-day bed, as well as peat deposits from the Preboreal at the level of the present-day bed indicate that a phase of incision occurred during the Younger Dryas-Preboreal transition. Reworked Laacher See tephra was found in the upper part of the perched pebble sheet, which confirms that it had been laid down during the Younger Dryas, in a periglacial context. The Lienne River had probably multiple channels during the Preboreal. After the abandonment of one of the channels, it moved laterally, allowing the peat to spread until it occupied more than two-thirds of valley from the Atlantic phase. The use of slag from steel working as a stratigraphic marker shows low lateral mobility in the Lienne River as well as a low rate of aggradation of the alluvial plain over the last centuries. [less ▲]

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See detailHolocene floodplain deposition and scale effects in a typical European upland catchment : A case study from the Amblève catchment, Ardennes (Belgium)
Notebaert, Bastiaan; Verstraeten, Gert; Houbrechts, Geoffrey ULiege et al

in Holocene (2013), 23(8), 1184-1197

This study quantifies Holocene alluvial sediment deposition in the Belgian Amblève catchment (1080 km2), situated in the Ardennes uplands. An extended coring data set is used for a quantitative ... [more ▼]

This study quantifies Holocene alluvial sediment deposition in the Belgian Amblève catchment (1080 km2), situated in the Ardennes uplands. An extended coring data set is used for a quantitative description and a quantification of the alluvial depositions. The floodplains fall into three main types: the upper and lower floodplains and the steep reaches. Total Holocene alluvial sediment deposition amounts to 32 Tg, or 0.029 Tg/km2. Dating of sediments using iron slag as tracer shows that on average 42% of contemporary storage was accumulated during the last 600 years. Radiocarbon dating of fluvial deposits at eight sites in the catchment confirms that the majority of the sediment is relatively recent. The increased sedimentation rates for this period are related to anthropogenic land use, possibly enhanced by climatic variations. A fluvial sediment budget was constructed for this 600 yr time period and shows that export from the catchment has about the same importance as storage in the floodplains, while lateral reworking of existing floodplain deposits only affects half the quantity of sediments. Overall, floodplain sediment storage in the Amblève catchment, comparable with other upland catchments, is of lesser importance compared with catchments dominated by loess. This can mainly be explained by lower sediment supply resulting from less intense anthropogenic land use, despite the higher sediment transport efficiency through the fluvial system. Floodplain sediment storage is scale-dependent, with initially a sharp increase in catchment area-specific sediment deposition, followed by a steady decrease with increasing catchment area. [less ▲]

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See detailL'effet des barrages de castors sur le système hydrographique
Petit, François ULiege; Denis, Anne-Cécile ULiege; Levecq, Yannick ULiege et al

Conference given outside the academic context (2012)

Estimation de la sédimentation dans les retenues provoquées par les barrages de castor. Impact en aval des barrages sur l'incision des rivières. Localisarion des barrages de castor en relation avec la ... [more ▼]

Estimation de la sédimentation dans les retenues provoquées par les barrages de castor. Impact en aval des barrages sur l'incision des rivières. Localisarion des barrages de castor en relation avec la tailled es rivières (ordination des cours d'eau). Discussion sur les conséquences éventuelles des retenues sur les débits d'étiage. [less ▲]

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See detailCharacterisation of sand transport in gravel-bed rivers using iron slag dated by historical studies
Houbrechts, Geoffrey ULiege; Levecq, Yannick; Petit, François ULiege

Poster (2012, April 25)

Considerable quantities of iron-smelting slag are present in the bed of the Ardennian rivers. These waste products come from hundreds of ironworks (mainly blast furnaces and finery forges) built close to ... [more ▼]

Considerable quantities of iron-smelting slag are present in the bed of the Ardennian rivers. These waste products come from hundreds of ironworks (mainly blast furnaces and finery forges) built close to different-sized rivers between the 14th and the 19th centuries. In general, slag was crushed by hammers, sorted and piled up in heaps around the furnaces, generally onto the floodplains. Furthermore, some archives mention that they were sometimes thrown out directly into the rivers. This means that for centuries, slag elements have been swept away by floods, mixed with the sediment and spread out along river courses. Due to their distinctive appearance, slag particles are easily recognizable among the natural elements. Thanks to many historical studies conducted on the early iron industry, we are able to date quite precisely the inception and the periods of activity of the different sites established in the catchments. These data are indispensable in order to use slag as a tracer to quantify the particles’ velocity in rivers. Downstream of ironworks, samples of sand have been collected in the surface layer of many gravel-bed rivers. Then, the slag concentration of each sample has been measured in the coarse sand fraction. The representation of the longitudinal evolution of slag concentration in these rivers permits the dispersion of slag to be analysed, the relative bed-material discharges at confluences to be quantified and the velocity of coarse sand to be determined. A survey of the bedload discharge in the Ardennian rivers established that more than 90 % of the bedload transport consists of coarse sand grains that are transported on the bottom of the bed. However, in the literature, this grain-size fraction is generally not considered in bedload discharge estimations because the sandy particles are very difficult to tag and to recover. Consequently, the huge amounts of slag injected in rivers several centuries ago can be considered as a very useful opportunity to characterise the sand transport in gravel-bed rivers. [less ▲]

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See detailComparison of methods for quantifying active layer dynamics and bedload discharge in armoured gravel-bed rivers
Houbrechts, Geoffrey ULiege; Van Campenhout, Jean ULiege; Levecq, Yannick ULiege et al

in Earth Surface Processes and Landforms (2012), 37

Several methods were employed in the Ardennian rivers (Belgium) to determine the depth of the active layer mobilized during floods and to evaluate the bedload discharge associated with these events. The ... [more ▼]

Several methods were employed in the Ardennian rivers (Belgium) to determine the depth of the active layer mobilized during floods and to evaluate the bedload discharge associated with these events. The use of scour chains has shown that the depth of the active layer is systematically less than the b-axis of the average particle size (D50) of the elements which compose the surface layer of the riffles. This indicates that only a partial transport exists during low magnitude floods. The bedload discharge has been evaluated by combining data obtained using the scour chains technique and the distance covered by tracers. Quantities of sediment transported during frequent floods are relatively low (0 02 t km–2) due to the armour layer which protects the subsurface material. These low values are also related to the fact that the distance calculated for mobilized bedload only applies to tracers fitted with PIT (passive integrated transponder)-tags (diameter>20 mm), whereas part of the bedload discharge is composed of sand and fine gravel transported over greater distances than the pebbles. The break-up of the armour layer was observed only once, for a decennial discharge. During this event, the bedload discharge increased considerably (2 t km–2). The use of sediment traps, data from dredging and a Helley–Smith sampler confirm the low bedload transport in Ardennian rivers in comparison to the bedload transport in other geomorphological contexts. This difference is explained by the presence of an armoured layer but also by the imbricated structures of flat bed elements which increase the resistance to the flow. Finally, the use of the old iron industry wastes allowed to quantify the thickness of the bed reworked over the past centuries. In the Lembrée River, the river-bed contains slag elements up to a depth of about 50 cm, indicating that exceptional floods may rework the bed to a considerable depth. [less ▲]

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See detailAnalyse des débits en période de tarissement : Essai d'une typologie régionale appliquée à des rivières du bassin de la Meuse (Belgique)
Gischer, Loïc; Hallot, Eric ULiege; Houbrechts, Geoffrey ULiege et al

in Bulletin de la Société Géographique de Liège (2012), 59

Low flow discharge was analysed in twenty-five rivers of the midlands and highlands of Belgium, particularly during periods when base flow was supplied by aquifers. Recession curves were elaborated: these ... [more ▼]

Low flow discharge was analysed in twenty-five rivers of the midlands and highlands of Belgium, particularly during periods when base flow was supplied by aquifers. Recession curves were elaborated: these were “sequence curves” and not “tangent curves”. In general, two recession coefficients may be identified. In addition, we determined the percentage of “rapid runoff flood” within a regional perspective. It is apparent that this flow varies with the level of the water table. We also tried to estimate the groundwater recharge used in Thornthwaite’s simplified model. This was undertaken by estimating the amount of rain needed before observing a significant increase in discharge, following pronounced drought episodes such as those of 1976 and 2003. Finally, working on discharges during periods of low flow, we looked at the role of beaver dams as a factor contributing to low flow, as has been suggested recently in the literature. From our observations, it appears beaver dams do not influence the characteristic low flow discharge. Furthermore, an analysis of the evolution of the characteristic low flow dicsharge in rivers where there are no beaver dams shows some variability. This leads us, first of all, to qualify the influence of beaver dams, at least on the hydrological regime of these water courses (which are always located at the top of the watershed) but also to question the relevance of characteristic low flow discharge as a descriptor of low flow. [less ▲]

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See detailMicrohabitats survey used in river restoration
Peeters, Alexandre ULiege; Hallot, Eric ULiege; Houbrechts, Geoffrey ULiege et al

Conference (2011, October 14)

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See detailLong-term bedload mobility in gravel-bed rivers using iron slag as a tracer
Houbrechts, Geoffrey ULiege; Levecq, Yannick ULiege; Vanderheyden, Vincent ULiege et al

in Geomorphology (2011), 126

Bedload dispersion is evaluated in gravel-bed rivers using slag elements from ironworks established along rivers in the Ardenne region, between the fourteenth and the nineteenth centuries. Large ... [more ▼]

Bedload dispersion is evaluated in gravel-bed rivers using slag elements from ironworks established along rivers in the Ardenne region, between the fourteenth and the nineteenth centuries. Large quantities of slag were dumped close to these rivers or even directly into the channels. For centuries, slag elements were dispersed in the bedload and transported by floods of varying importance. Consequently, slag may be considered as a reliable tracer to analyze bedload dispersion over several centuries. The size of slag elements was studied along 16 Ardenne rivers. The longitudinal size trend of the largest slag particles allows the effective competence of these rivers to be determined (between 19 and 129 mm for rivers where specific stream power for the bankfull discharge ranges between 20 and 134 W/m²). A direct relationship doesn’t exist between these two parameters as the size of slag elements must be considered with regard to the D50 of the bed. Selective transport was analyzed directly downstream of the input sites. The sorting distance varies from river to river and depends on the velocity of the coarse elements introduced into the river since the inception of the iron industry. Downstream of two metallurgic sites, the slag propagation fronts were located. As the periods of activity at these sites are known from historical studies, the virtual velocity of bedload movement in these rivers was estimated to be 2-4 km/century. [less ▲]

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See detailImpact de la glace de ségrégation sur l'érosion des berges d'un ruisseau ardennais (la Chavanne, Belgique)
Wallemacq, Véronique; Petit, François ULiege; Van Campenhout, Jean ULiege et al

in Géomorphologie: Relief, Processus, Environnement (2011), 1

L’impact érosif de la glace de ségrégation a été analysé dans les berges d’un ruisseau ardennais (la Chavanne, Belgique), dans une région soumise à un climat tempéré. Sur une période de 19 mois (février ... [more ▼]

L’impact érosif de la glace de ségrégation a été analysé dans les berges d’un ruisseau ardennais (la Chavanne, Belgique), dans une région soumise à un climat tempéré. Sur une période de 19 mois (février 2008-septembre 2009), des mesures de recul de berges à l’aide de barres d’érosion ont montré un recul moyen d’une dizaine de centimètres. Cette érosion s’est essentiellement produite durant le mois de janvier 2009 à la suite d’une vague de froid. Par la suite, l’érosion par entraînement hydraulique des matériaux désagrégés par la glace de ségrégation a été plus lente. D’autre part, la glace de ségrégation joue un rôle majeur dans l’accélération de l’incision des chenaux de recoupement de méandres. Enfin, l’érosion latérale moyenne mesurée durant la période d’étude (10,3 cm en 19 mois) a été comparée au recul moyen des berges déterminé sur la base de comparaison diachronique du tracé de la rivière sur photographies aériennes à grande échelle (1952-1982, 1982-2007). Le recul annuel moyen des berges de la rivière s’élève à 0,24 m/a pour la première période et à 0,18 m/a pour la seconde, ce qui est supérieur à la valeur observée entre 2008 et 2009, bien que cette période ait été caractérisée par un hiver rigoureux. Ce ralentissement de l’érosion pourrait s’expliquer par une érosion fluviatile plus faible suite au développement des méandres qui semblent avoir atteint un stade de stabilité et qui tendent maintenant au recoupement. [less ▲]

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See detailFluvial architecture of Belgian river systems in contrasting environments:implications for reconstructing the sedimentation history
Notebaert, Bastiaan; Houbrechts, Geoffrey ULiege; Verstraeten, G. et al

in Netherlands Journal of Geosciences - Geologie en Mijnbouw (2011), 90(1), 31-50

Accurate dating is necessary to get insight in the temporal variations in sediment deposition in floodplains. The interpretation of such dates is however dependent on the fluvial architecture of the ... [more ▼]

Accurate dating is necessary to get insight in the temporal variations in sediment deposition in floodplains. The interpretation of such dates is however dependent on the fluvial architecture of the floodplain. In this study we discuss the fluvial architecture of three contrasting Belgian catchments (Dijle, Geul and Amblève catchment) and how this influences the dating possibilities of net floodplain sediment storage. Although vertical aggradation occurred in all three floodplains during the last part of the Holocene, they differ in the importance of lateral accretion and vertical aggradation during the entire Holocene. Holocene floodplain aggradation is the dominant process in the Dijle catchment. Lateral reworking of the floodplain sediments by river meandering was limited to a part of the floodplain, resulting in stacked point bar deposits. The fluvial architecture allows identifying vertical aggradation without erosional hiatuses. Results show that trends in vertical floodplain aggradation in the Dijle catchment are mainly related to land use changes. In the other two catchments, lateral reworking was the dominant process, and channel lag and point bar deposits occur over the entire floodplain width. Here, tracers were used to date the sediment dynamics: lead from metal mining in the Geul and iron slag from ironworks in the Amblève catchment. These methods allow the identification of two or three discrete periods, but their spatial extent and variations is identified in a continuous way. The fluvial architecture and the limitation in dating with tracers hampered the identification of dominant environmental changes for sediment dynamics in both catchments. Dating methods which provide only discrete point information, like radiocarbon or OSL dating, are best suited for fluvial systems which contain continuous aggradation profiles. Spatially more continuous dating methods, e.g. through the use of tracers, allow to reconstruct past surfaces and allow to reconstruct reworked parts of the floodplain. As such they allow a better reconstruction of past sedimentation rates in systems with important lateral reworking. [less ▲]

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See detailUtilisation des scories sidérurgiques comme marqueur stratigraphique pour l'étude de l'évolution géomorphologique des plaines alluviales
Houbrechts, Geoffrey ULiege; Petit, François ULiege

Conference (2010, November 22)

Afin d’analyser la dynamique récente des plaines alluviales des rivières du massif ardennais, nous avons utilisé les scories, résidus de la transformation du minerai de fer en métal, comme marqueur ... [more ▼]

Afin d’analyser la dynamique récente des plaines alluviales des rivières du massif ardennais, nous avons utilisé les scories, résidus de la transformation du minerai de fer en métal, comme marqueur stratigraphique. Les scories constituent un traceur particulièrement exceptionnel ; elles sont présentes en grande quantité dans les alluvions des rivières ardennaises et les périodes d’activités des sites sidérurgiques et donc de rejets de scories sont connues précisément grâce aux études historiques. C’est à partir du XIVe siècle que les sidérurgistes wallons commencèrent à utiliser la roue hydraulique pour actionner les soufflets et les martinets des forges. Ils installèrent donc leurs fourneaux et leurs forges d’affinage à proximité des cours d’eau et produisirent de grandes quantités de scories. Ainsi, un haut fourneau du 16e siècle, qui fonctionnait en moyenne 130 jours par an, produisait de l’ordre de 80 m³ de scories par an. Ces scories étaient généralement rejetées sur les plaines alluviales, voire directement dans le lit des cours d’eau. Par la suite, ces scories ont été emportées lors des crues débordantes ou par érosion latérale de la rivière. Entre le XIVe et le XIXe siècle, plus de 300 sites sidérurgiques ont été actifs le long de plusieurs dizaines de rivières du massif ardennais. L’utilisation des scories comme marqueur stratigraphique a été proposée par le Professeur A. Pissart en 1972 et appliquée par Henrottay (1973), afin de déterminer des taux de sédimentation et de mobilité latérale postérieurs à l’apparition de l’activité sidérurgique dans les vallées. A la suite de cette étude, nous avons développé une méthode de mesure de concentration en scories qui permet de mieux localiser le contact entre les sédiments pré-et post-sidérurgiques, et de tenir compte de l’influence de la bioturbation et des contaminations lors des prélèvements. De plus, cette méthode montre que les teneurs en scories dans les sédiments ont varié dans le temps en relation avec les périodes de fonctionnement, d’expansion et d’arrêt des sites sidérurgiques situés en amont. Grâce aux scories, nous avons montré que la sédimentation en plaine alluviale a été particulièrement importante depuis la fin du Moyen Âge, de l’ordre de 10 à 20 cm/siècle en fonction des cours d’eau. La mobilité latérale des rivières est par ailleurs relativement faible et moins importante que les valeurs basées sur la comparaison de tracés sur cartes anciennes. [less ▲]

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See detailUnit stream power as a base for regional river typology
Hallot, Eric ULiege; Houbrechts, Geoffrey ULiege; Levecq, Yannick et al

Poster (2010)

In order to assess the hydromorphological quality of surface waters, stakeholders need river typologies accurate and functional enough to be a really efficient management tool. From a typological study ... [more ▼]

In order to assess the hydromorphological quality of surface waters, stakeholders need river typologies accurate and functional enough to be a really efficient management tool. From a typological study based a broad set of hydromorphological field data, we have shown that some variables, like the unit stream power (), are particularly appropriate to describe the dynamics of a river. Initial results of repeated measurement in more than 80 sites revealed a regional differentiation based on critical values of stream power at bankfull discharge. The aim of this study is to present a unit stream power mapping for the whole network based on general equation  =  g Q S / w, with  the density of water, g the acceleration due to gravity, Q the discharge, S the slope and w the channel width). Slope is extracted from DEM. Bankfull discharge and width estimations are furnished by allometric equations using the watershed area. Different coefficients are used according to regional differentiations based on bedload size and watershed permeability. The statistical significance of all equations was tested by covariance analysis. A correction taking into account the used log-log scale, was also applied to the obtained allometric equations (Fergusson, 1988). Using a 30 x 30 meters DEM, GIS routines were developed. A first one automatically sectorizes the network and a second evaluates the unit streamspower in each location. We propose a 7-level classification in accordance with the associated morphological processes describe in the literature (from fixed meanders to step-pools system). The mapping unit starts at the Strahler second order. Our results not only support a regional typology, but moreover allow us to describe natural regions in function of river dynamics and to estimate the resistance of restoration works like vegetal techniques. It can also provide estimates of the excess stream power (the fraction of the actual stream power exceeding the power at which bedload particles start to move). (Fergusson, R.,I., 1988. River loads underestimated by rating curves, Water Resour. Res., 24(7), 1217–1219.) [less ▲]

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See detailArtificially generating sediment incipient motion in natural conditions
Gob, Frédéric ULiege; Houbrechts, Geoffrey ULiege; 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 detailBedload progression in gravel bed rivers using iron slag as a tracer
Houbrechts, Geoffrey ULiege; Levecq, Yannick; Mols, Julien et al

Poster (2009, July)

In fluvial dynamics studies, different methods are used to evaluate bedload transport and particle travel lengths. However, results are mostly based on a few transported elements and on a relatively short ... [more ▼]

In fluvial dynamics studies, different methods are used to evaluate bedload transport and particle travel lengths. However, results are mostly based on a few transported elements and on a relatively short time scale. Consequently, it is difficult to extrapolate these results to whole bedload, because of the burying of particles into the subsurface layer or the trapping of elements in fluvial forms (point bars, riffles, …), which can immobilise elements during long periods. Bedload progression has been evaluated in Ardenne rivers using slag elements produced by the past factories established along rivers between the 14th and the 19th centuries. Important quantities of slag were dumped close to rivers or even directly into channels. For several centuries, slag elements were dispersed in the bedload and transported by floods of varying importance. Consequently, slag can be considered as a tracer to analyze bedload progression over several centuries. The size of slag elements has been studied in many Ardenne rivers. The longitudinal size trend of the largest slag particles allows us to determine the effective competence of rivers and to analyze the hydraulic sorting. Moreover, downstream of some metallurgic sites, we have constrained the presence of slag elements to the most downstream riffles. Because we know from historical studies the periods of activities of these sites, we may estimate the speed of bedload progression in several gravel bed rivers from the Ardenne Massif (2-3 km/century). [less ▲]

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