River dredging, channel dynamics and bedload transport in an incised meandering river (the River Semois, Belgium)

Abstract

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.

Gravel extraction and planform change in a wandering gravel-bed river: The River Wear, Northern England

“…Despite being less intensive as commercial extraction operations, dredging has the same detrimental impacts on channel morphology and ecology (cf. Gob et al, 2005). Given the importance of floods in determining the morphological adjustment to extraction the timing and rate of channel adjustment will be unpredictable particularly given uncertainly over the likely impact of climatic change on river flooding.…”

“…Downstream incision as described by Kondolf (1994Kondolf ( , 1997 is not evident at either site. In contrast sedimentation has been the main downstream channel response (Gob et al, 2005). At Wolsingham this took the form of a small braid zone (Fig.…”

“…Gravel extraction from rivers is a global phenomenon, with examples from Australia (Erskine and Green, 2000), France, (Petit et al, 1996), Italy (Surian and Rinaldi, 2003) the USA (Kondolf, 1994), Belgium (Gob et al, 2005) and Britain (Sear and Archer, 1998). Gravel extraction can have both immediate and long-term consequences for channel stability.…”

Effect of different meander curvatures on spatial variation of coherent turbulent flow structure inside ingoing multi-bend river meanders

“…Predicting the sediment transport pattern inside the river meanders is also beneficial to prevent damage by floods (Harvey 1988). Channel migration and increasing bed elevation in river meanders affect flood level in river bend meander (Gob et al 2005). Therefore, sediment transport pattern and meander migrations are considerably affected by the meander curvature.…”

Fréquence et importance du charriage dans les rivières du Massif ardennais

“…Cette méthodologie a été développée en détail par Gob et al (2005), notamment à la suite de travaux de curage effectués dans la Semois. Nous avons ensuite converti en masse les données exprimées en volume.…”