RIVER DREDGING, CHANNEL DYNAMICS AND BEDLOAD TRANSPORT IN AN INCISED MEANDERING RIVER (THE RIVER SEMOIS, BELGIUM)

摘要

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 flood-plain, few pebble bars, numerous bedrock outcrops and a limited stock of sediment. The bed is particularly flat and shallow and the bankfull discharge (130m~3s~(-1)) is frequently attained (0.9yr). 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 dre.dging 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~(-1) yr~(-1)). In order to examine the efficiency of the sediment trap, the sediment transport equations of Meyer-Peter and Mueller, 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, 5010t were caught in the sediment trap. For the same period the equations calculate values between 6147 and 105711. 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 10yr. Despite the scale of the dredging campaign for a river of this size, its results are limited in terms of flood prevention.