Using multiple bed load measurements: Toward the identification of bed dilation and contraction in gravel‐bed rivers

Abstract

[1] This study examines bed load transport processes in a small gravel-bed river (Béard Creek, Québec) using three complementary methods: bed elevation changes between successive floods, bed activity surveys using tags inserted into the bed, and bed load transport rates from bed load traps. The analysis of 20 flood events capable of mobilizing bed material led to the identification of divergent results among the methods. In particular, bed elevation changes were not consistent with the bed activity surveys. In many cases, bed elevation changes were significant (1 to 2 times the D 50 ) even if the bed surface had not been activated during the flood, leading to the identification of processes of bed dilation and contraction that occurred over 10% to 40% of the bed surface. These dynamics of the river bed prevent accurate derivation of bed load transport rates from topographic changes, especially for low magnitude floods. This paper discusses the mechanisms that could explain the dilation and contraction of particles within the bed and their implications in fluvial dynamics. Bed contraction seems to be the result of the winnowing of the fine sediments under very low gravel transport. Bed dilation seems to occur on patches of the bed at the threshold of motion where various processes such as fine sediment infiltration lead to the maintenance of a larger sediment framework volume. Both processes are also influenced by flood history and the initial local bed state and in turn may have a significant impact on sediment transport and morphological changes in gravel-bed rivers.Citation: Marquis, G. A., and A. G. Roy (2012), Using multiple bed load measurements: Toward the identification of bed dilation and contraction in gravel-bed rivers,

Valley-scale morphology drives differences in fluvial sediment budgets and incision rates during contrasting flow regimes

“…This computation assumed no nontransport 374 mechanisms of volumetric change, previously termed bed deflation or contraction and 375 inflation or dilation (Merz et al, 2006;Marquis and Roy, 2012). In addition, if a raster 376 cell erodes and then fills in between surveys, no change will be detected in that cell: a 377 process known as compensating scour and fill .…”

Gravel threshold of motion: a state function of sediment transport disequilibrium?

“…Other bed state controls include the degree of overlap, interlocking and imbrication among grains, and bed compaction or dilation (e.g., Parker, 1990;Wilcock and Crowe, 2003;Sanguinito and Johnson, 2012;Buscombe and Conley, 2012;Mao, 2012;Kirchner et al, 1990;Marquis and Roy, 2012;Powell and Ashworth, 1995;Richards and Clifford, 1991;Ockelford and Haynes, 2013). By combining experimental data and a numerical model, Measures and Tait (2008) show that increasing grain-scale bed roughness tends to shelter downstream grains, reducing entrainment.…”

The effect of coarse gravel on cohesive sediment entrapment in an annular flume

“…A recent study describing bed dilation (changes bed elevation, increasing or decreasing respectively, in the absence of bed mobilization) and contraction found the release of fine sediment from a gravel bed is complex (Marquis and Roy, 2012). Marquis and Roy (2012) employed a variety of methods to measure bed load transport and bed mobility, including surveying bed elevation change, bed activity tags and bed load traps. The objective of their study was to understand the dynamics of the bed and its relationship to bed load transport under a variety of flood conditions.…”

“…The study confirmed gravel bed dilatation and contraction occurred during smaller flood events and has implications for the release of cohesive sediments from a stable gravel bed. Marquis and Roy (2012) found that when a bed contracted there was a subsequent increase in fine sediment (sand) in the bed load traps, indicating the fine sediments comprising the bed matrix are mobilized from the bed as the framework particles reorganized and packed together. If cohesive sediments are entrapped within the sand matrix as previously reported (Schalchi, 1992;Rehg et al, 2005;Krishnappan and Engel, 2006), it will likely be mobilized in conjunction with the fine sediment (sand) during the process of bed contraction.…”

“…it was induced by turbulence generated by the gravel bed is recommended; and 7. The concept of bed contraction and dilation (Marquis and Roy, 2012) suggest there is the potential for cohesive sediment mobilization from a bed in which the framework particles are not mobilized. Further study is required into the release of cohesive sediments from the bed when flows approach the critical shear stress of the bed.…”