Long-term bedload mobility in gravel-bed rivers using iron slag as a tracer

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 ▲]

Combining spatial data in landslide reactivation susceptibility mapping: A likelihood ratio-based approach in W Belgium

in Geomorphology (2010)

A key issue in landslide susceptibility mapping concerns the relevance of the spatial data combination used in the prediction. Various combinations of high-resolution predictor variables and possibilities ... [more ▼]

A key issue in landslide susceptibility mapping concerns the relevance of the spatial data combination used in the prediction. Various combinations of high-resolution predictor variables and possibilities of selecting them from a larger dataset are analysed. The scarp reactivation of several landslides in a hilly region of W Belgium is investigated at the pixel scale. The usceptibility modelling uses the reactivated scarp segments as the dependent variable and 13 factors at a 2 m-resolution related to topography, hydrology, land use and lithology as potential independent variables. The modelling uses a likelihood ratio approach based on the comparison, for each independent variable, between two empirical distribution functions (EDFs), respectively for the reactivated and non-reactivated areas. It uses these EDFs as favourability values to build membership values and combine them with a fuzzy Gamma operator. Five different data combinations are tested and compared by analysing the prediction-rate curves obtained by cross-validation. The geomorphological value of the resulting susceptibility maps is also discussed. This research shows relevant results for predicting the susceptibility to scarp reactivation. We highlight the need for testing several data combinations and underline that combining uantitative criteria with expert opinion is an asset for reliable predictions. [less ▲]

An automated method to extract fluvial terraces from digital elevation models: The Vesdre valley, a case study in eastern Belgium

in Geomorphology (2007), 91(1-2), 51-64

Fluvial terraces are a powerful tool for unraveling the combined tectonic and climatic conditions that controlled, directly or indirectly, the Quaternary incision of rivers. Terrace long profiles are ... [more ▼]

Fluvial terraces are a powerful tool for unraveling the combined tectonic and climatic conditions that controlled, directly or indirectly, the Quaternary incision of rivers. Terrace long profiles are usually retrieved from sparse traces of ancient floodplains preserved in the present topography. However, when these traces classically collected from topographic maps, aerial photographs, and field analyses are too few, the inferred profiles may be questionable. Yet the now available high quality and high resolution digital elevation models (DEMs) offer an opportunity to increase greatly the quantity of information usable to reconstruct terrace profiles. Therefore, the purpose of this study was to develop a new DEM-based method of terrace recognition in order to create a larger database and better constrain the profile reconstruction. Moreover, particular procedures of image and numerical processing were defined to fully automate the analysis. Basically, our method relies on the production of bivariate scatter plots depicting the relation between slope and relative altitude (i.e., the altitude above the current alluvial plain) for all pixels of successive sections of the valley. For each scatter plot, the curve of the lowest slope values observed at every relative altitude is smoothed and its minima are assumed to locate the altitudes of the "terrace" elements preserved in the section. We successfully tested this method in the Vesdre valley, incised in the NE Ardenne massif (E Belgium), notably identifying fault deformation of the profiles. The main advantages of our approach are its objectivity, exhaustiveness, and rapidity, allowing fast and coherent analysis of many rivers over extended regions. (c) 2007 Elsevier B.V. All rights reserved. [less ▲]

Critical specific stream power in gravel-bed rivers

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 ▲]