Experiments with marked pebbles were carried out on different sized rivers of the Belgian Ardenne (catchment areas varying from less than 1 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 (x 0 ) and grain size (D) were established. The values for x 0 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 (sW), determined by the relationship between grain shear stress (sV-that determines movement and transport of the bedload) and the total shear stress (s). 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 (sV/sb0.3). Critical specific stream power is higher in this third type of river than in the other two. D
Lichenometry has been applied to blocks, canyon walls and terrace deposits. A growth curve, specific to Corsica, was made based on information obtained from tombstones, old monuments from the Genoese occupation of Corsica and Roman megaliths. This allowed us to create a curve going as far back as 2000 years. The curve is similar to those developed in Mediterranean environments (C! evennes and Eastern Pyrenees), but is very different from those developed in Alpine and Arctic regions.Lichens present on boulders of the bedload allow the time at which the boulders were laid down to be determined. The lichenometric study of the Figarella shows firstly that there is high stream competence. Blocks approximately 100 cm wide are moved during the 5-year flood. Recent important flood events were identified in 1973 and 1869. These correspond well with historical sources. Other older and much more important floods were also identified. This analysis has allowed us to extend the specific streampower curves proposed by Costa (Geol. Soc. Am. Bull. 94 (1983) 986) and Williams (Geogr. Ann. 65A (1983) 227) to blocks of more than 2 m wide.Lichens present on the canyon walls and on terrace sides in the plain allowed the last main incision phase to be dated to the Little Ice Age. Aerial photograph analysis has shown that the present braided system in the lower course of the river is no longer active.
Dredging in the rivers of the Ardenne is generally carried out on a smaller scale to that described in the literature and is not conducted for commercial purposes. Extractions within the river channel are made in order to prevent flooding; hence the quantity of gravel extracted is limited. This study aims to evaluate the impact of dredging and the resilience of the riverbed in the Semois. This river is found in the south of the Ardenne region and is characterized by large incised meanders, a narrow floodplain, few pebble bars, numerous bedrock outcrops and a limited stock of sediment. The bed is particularly flat and shallow and the bankfull discharge (130 m 3 s À1 ) is frequently attained (0.9 yr). Pebble tracers allowed the critical parameters (discharge, Shields criterion, and stream power), the diameter of mobilized sediment and the distance of sediment transport to be determined. A major dredging campaign resulted in the formation of a channel nearly 1 km long and 2 m deep which functioned as a sediment trap. Topographical cross-sections made before and after the dredging campaign and again 4 yr later allowed bedload discharge to be estimated (1.1 t km À1 yr À1 ). In order to examine the efficiency of the sediment trap, the sediment transport equations of Meyer-Peter and Müller, Schoklitsch, Bagnold and Martin were applied. With the exception of Bagnold's equation (1980), the observed transport values and those calculated theoretically are relatively close. Between October 1997 and June 2001, 5010 t were caught in the sediment trap. For the same period the equations calculate values between 6147 and 10 571 t. The overestimation from the theoretical calculations may result from a lack of sediment supply due to the characteristics of the basin and the frequency and magnitude of flood events during the study period. From the magnitude of the sediment transport rate, a return to the initial state of the riverbed (before dredging) may be expected after approximately 10 yr. Despite the scale of the dredging campaign for a river of this size, its results are limited in terms of flood prevention.
Sediment transport of four boulder bed rivers is studied using lichenometry. The presence of lichens on boulders in the river channel is used to date the last mobilization of the blocks. Using size frequency diagrams and regional growth curves calibrated with dated reference points it is possible to determine the fl ood event responsible for the last mobilization of each boulder with lichens present. The specifi c stream power of fl ood events over the last 60 years is then calculated, and thresholds of sediment transport based on the sediment size are calculated. The results from the four studied rivers are compared to similar relationships in the literature. Sediment motion thresholds appear to be very variable within the same type of river (mountainous boulder bed rivers). The critical specifi c stream power necessary to mobilize a particle of a given diameter may vary by up to 10 times from one river to the next. Bed sediment size and river slope may explain this large range of stream powers. Calculation of the relative size of the transported particles (D i /D 50 ) also shows that both hiding and protrusion effects, as well as channels slope, are important factors in sediment transport.
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