The impact of stress history on bed structure

Abstract

Recent research has started to focus on how prolonged periods of sub-threshold flows may be capable of imparting structural changes that contribute to increased bed stability. To date, this effect (termed ‘stress history') has been found to be significant in acting to increase a bed's critical shear stress at entrainment threshold. However, it is supported by only limited, qualitative and often speculative information on the mechanisms of this stabilisation process in grade-specific studies. As such, this paper uses high resolution laser scanning to quantitatively ascertain the granular mechanics underpinning the relationship between stress history and entrainment threshold for beds of a range of grain size distributions. Employing a bed slope of 1/200, three grain size distributions with median grain sizes (D50) of 4.8mm [uniform (σg = (D84/D16)0.5 = 1.13; bimodal (σg = 2.08); and, unimodal (σg =1.63)] were exposed to antecedent stress histories of 60 and 960 minutes duration. Antecedent shear stress magnitude was set at 50% of the critical shear stress for the D­50 when no stress history period was employed. Two laser displacement scans of the bed surface (approximate area 100 x 117mm) were taken, one prior to the antecedent period and one after this period, so that changes to surface topography could be quantified (resolution of x = 0.10mm, y = 0.13mm and z = 0.24mm). Rearrangement of bed surface structure is described using statistical analysis and 2D semi-variograms to analyse scaling behaviour. Results reveal vertical settlement, changes to bed roughness and particle repositioning. However, the bed grain size distribution influences the relative importance of each mechanism in determining stress history induced bed stability; this is the focus of discussion in this paper

Taking the river inside: Fundamental advances from laboratory experiments in measuring and understanding bedload transport processes

“…Laser scans have very high accuracy (± 0.1-0.2 mm) (e.g., Marion et al, 2003;Ockelford and Haynes, 2013) but are only realistic during dry bed conditions before and after flume runs. Detailed bed topography can be used to characterize bed roughness instead of relying on a representative grain size as is commonly done in the field.…”

“…In addition, even for a constant bed grain size distribution, different sequences of applied shear stresses can influence the bed grain arrangement. The onset of motion may therefore vary through time depending on the flow history of a given bed (Haynes and Pender, 2007;Mao, 2012;Ockelford and Haynes, 2013).…”

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.…”

“…Flow characteristics influencing τ * c include particle Reynolds number, flow depth relative to grain size, the intensity of turbulence, the history of prior flow both above and below transport thresholds, and the partitioning of stress into form drag and skin friction (e.g., Shvidchenko and Pender, 2000;Ockelford and Haynes, 2013;Schneider et al, 2015;Valyrakis et al, 2010;Celik et al, 2010). Most flowdependent controls are not independent of the bed surface controls.…”

Laboratory studies on bedload transport under unsteady flow conditions

“…Bombar et al (2011) reported an anti-clockwise hysteresis in experiments with a static armour layer, where the maximum bedload transport was possible only after the peak flowrate, when the armour layer was destroyed by a high flow. When sediment is heterogeneous, sediment transport is affected by grain arrangement (Ockelford and Haynes, 2013): microforms (clusters) are formed (Mao, 2012), static or mobile armour layer appears (Guney et al, 2013), and partial transport may take place (Sun et al, 2015). To make things more complicated, bedload transport is additionally controlled by the availability of upstream sediment in supply-limited rivers (Mao et al, 2011).…”