The influence of sediment size, relative grain size and channel slope on initiation of sediment motion in boulder bed rivers. A lichenometric study

Abstract

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.

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

“…2)) and the envelope curve (B -Eq 3) are situated in the continuation of the relations of Jacob et al (2006) and Gob et al (2003). The latter two relations were established in mountainous boulder bed rivers with slopes ranging from 0.2 to 3%, with catchment areas from 133 to 800 km 2 (Gob et al, 2010).…”

“…However, a back calculation of θ ci based on data from two step pool streams with high gradients (Mao et al, 2008) suggests that Ferguson's equation gives a better result with θ ci = 0.071. Using the same approach, Gob et al (2010), obtain a θ ci -value of between 0.020 and 0.050, with b = 0.7. Based on their observations, they conclude that the critical shear stress increases as longitudinal slope increases.…”

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

“…Gob et al (2010) show that rivers with the lowest excesses of stream power display the shortest sorting reaches and, therefore, develop a better downstream fining. This hypothesis could explain the sorting differences between the Viroin and the Rulles: the selective transport coefficient is larger for the Viroin; the Viroin has greater competence (see the slag size in the plateau reach) and larger specific stream powers at the bankfull discharge, which indirectly imply a greater excess of power.…”

“…Nevertheless, as demonstrated by Ferguson (2005) and Gob et al (2010), the particle size relative to the riverbed material (D i /D 50 ) must be taken into account because this factor has an influen ce on the critical thresholds for bedload transport.…”

What was the transport mode of large boulders in the Campine Plateau and the lower Meuse valley during the mid-Pleistocene?

“…500, i.e., in the rough-turbulent flow regime. However, in natural channels (and gravel-bed streams in particular) one cannot assume a uniform grain size distribution and equal mobility as in Shields' experiments, as the relative grain size (i.e., the ratio between the grain size of interest, hereafter called D i , and the median grain size D 50 of the channel bed) affects incipient motion thresholds because of hiding and protrusion effects (Komar, 1987a;Gob et al, 2010). Consequently, differences between critical motion thresholds of large and small particles in nonuniform grain size distributions are reduced, a process that needs to be considered when studying boulders on a bed of smaller gravels.…”