Comparison of methods for quantifying active layer dynamics and bedload discharge in armoured gravel‐bed rivers

Abstract

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. Copyright © 2012 John Wiley & Sons, Ltd.

Stream pollution concentration in riffle geomorphic units (Yzeron basin, France)

“…This surface armour layer protects the under laying fine sediments and can be removed only by high floods (Houbrechts et al, 2012). On the other hand, during flow decrease, pools (and also runs) concentrate fine sediment deposition, which can be easily removed during subsequent flooding (Bravard and Petit, 2000;Houbrechts et al, 2012). This structure less stable over time may explain why Darcy flows do not show a close correlation with the chemical concentrations in the hyporheic zone of pool-run units.…”

“…), while coarse particles are immobile, inducing the formation of a surface coarse armour layer below which sediments are composed of a broader range of grain size including fine sediments in which specific physical and biogeochemical processes are fostered. This surface armour layer protects the under laying fine sediments and can be removed only by high floods (Houbrechts et al, 2012). On the other hand, during flow decrease, pools (and also runs) concentrate fine sediment deposition, which can be easily removed during subsequent flooding (Bravard and Petit, 2000;Houbrechts et al, 2012).…”

Evaluation of long-term bedload virtual velocity in gravel-bed rivers (Ardenne, Belgium)

“…In order to analyse entrainment and displacement of tracer gravels in rivers, Passive Integrated Transponders (PITs) were used to mark particles (Lamarre et al, 2005;Lamarre and Roy, 2008;Houbrechts et al, 2012;Liébault et al, 2012). This technique offers numerous advantages in comparison to earlier techniques such as painting or magnetic methods (Sear et al, 2000;Ferguson et al, 2002;Hassan and Ergenzinger, 2003;Haschenburger, 2013).…”

“…With this technique, tracers can be found and identified by their unique identification code without handling the particles and disturbing the bed. Until now, most of these studies have been based on observation of isolated events or on surveys spanning only a few years (Lamarre et al, 2005;Lamarre and Roy, 2008;Houbrechts et al, 2012;Liébault et al, 2012;Chapuis et al, 2015).…”

Scaling relationships between bed load volumes, transport distances, and stream power in steep mountain channels

“…Houbrechts et al . [] provided measurements of bed load transport and transport distances for several larger and lower gradient rivers in the Ardennes (Belgium).…”