Generated bedforms

In this study, the generated bedforms (refer to section 2.5.7; pg. 50, for brief description of sedimentary bedforms), have been described solely to highlight the parametric influence of cohesive sediment fraction (i.e. clay) on bedforms development in mixed (sand-clay) beds. Using the description in Figure 2.22, the characteristics of the generated bedforms were estimated from Figures 6.8 and 6.9, and Table 6.5 presents the results. The results show bedforms with length (L) ranging from 180 to 600 mm and height (H) of 4 to 48 mm. These are in good agreement with typical sand ripples dimensions of L = ∼ 50 – 600 mm and H = ∼5 – 50 mm (Ashley 1990). Across all the experimental runs, slightly straight-crested plan morphology of bedforms are observed on the final bed deposits (Figures 6.8 & 6.9). The sides of the final beds along the outter wall of the flume channel (Figure 6.2) are shown to have deep and wide (∼100-160 mm) erosional scour holes (see the red oval curves in Figure 6.9) mainly in beds with clay content < 5 % (i.e. runs EDT-EX1 and EX2). However, these are completely absent in run EDT-EX4 (10 % clay; Figure 6.9d), although, a couple of such scour holes was seen across the total surface area of the final bed from run EDT-EX6 (10 % clay; e.g. Figure 6.9e), again suggesting run EDT-EX6 relative to EDT-EX4, has lower bed shear strength (Figure 6.6).

Table 6-5 Characteristics of the bedforms generated from the experimental runs

Tests % Clay Height (H) (mm) Length (L) (mm)

EDT-EX1 0 48 360

EDT-EX2 2 32 600

EDT-EX3 5 28 440

EDT-EX4 10 4 180

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Figure 6.8 Bed deposits heights measured over the span length of the selected quadrant (Figure 6.2) for: (a) EDT-EX1 (b) EDT-EX2 (c) EDT-EX3 (d) EDT-EX4 and (e) EDT-EX6. (Corresponding photographic images show the heights of the sand deposits).

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Figure 6.9 Photographic images of the aerial view of the final beds within the selected quadrant (Figure 6.2) of the flu me, showing sand deposit and clay veneer parts, and red oval curves show the scour holes for: (a) EDT-EX1 (b) EDT-EX2 (c) EDT-EX3 (d) EDT-EX4 and (e) EDT-EX6. Corresponding schematic diagrams are representatives of deposits features when cut through designated sections.

Flow

(a) 100%sand A

A

Initial flat bed

150 mm

Section A-A

∼ 63 mm

Sand deposit

Initial flat bed

Section B-B ∼ 55 mm 150 mm Sand deposit Flow (b) 98s:2c B B Clay veneer Sand deposit E E (e) 95s:5c Flow Section E-E

Initial flat bed

150 mm 35 mm Flow (d) 95s:5c D D Clay veneer Sand deposit ∼ 15 mm Section D-D

Initial flat bed

150 mm

20 m

∼ 25 mm

Section C-C

Initial flat bed

150 mm ∼ 45 mm 80 mm mm Flow (c) 95s:5c C C Clay veneer Sand deposit

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The spatial-temporal development of the bedforms has not been quantified in the current study, due to some of the previously highlighted limitations. Hence the analysis is based on the characteristics of the bedforms developed on the final bed deposits for all the runs. These have been compared with the recent results from Baas et al. (2013) on experimental study of bedform development in mixed sand-clay sediment beds under steady flow conditions, where the influence of initial clay fraction ranging from 0 – 18% was tested. For comparison, from Baas et al. (2013), the characteristics of bedforms (in terms of height and length) developed at equilibrium conditions (i.e. after t > 1 hr) have been considered. The schematic representation of changes in bed morphology at t = 0 and 5hrs for the current experiment is shown in Figure 6.10. Similar to Baas et al. (2013), the crests of the bedforms are shown to mainly comprise clean sand (Figures 6.8, 6.9 & 6.10).

Specifically, results from Baas et al. (2013) suggest, a strong inversely proportional relationship between bedform height (H) and the initial mud fraction and a weak inversely proportional relationship between bedform length (L) and the initial mud fraction. These findings are in good agreement with the findings from the current study as shown in Table (6-5) and Figure 6.8. For the bedforms generated, a strong negative correlation can be consistently seen between H or L and percentage (%) initial clay fraction. Although for L, run EDT-EX1 appears to deviate from this relationship (see Table 6-5). Similar deviation in bedform length, was equally observed by Baas et al. (2013), where they found some tests with relatively lower clay fraction having shorter equilibrium-lengths compared to other runs with higher clay fractions. It may be inferred therefore, that, supply of sand material, from the initial mixed bed, needed for the development of bedforms has been limited by the increased cohesive bed strength with the increase in clay fraction (see Table 6-5), thereby forming what Allen (1968) described as sediment- starved bedforms. Thus, these findings suggest that, high cohesive bed strengths (i.e. as initial % clay fraction increases), relative to the flow-induced bed shear stresses, are to large extent responsible for the observed decrease in bedform heights and lengths (Figures 6.9 & 6.10).

Furthermore, images in Figure 6.9 show a clear banding occurring in the channel, where sand is deposited at the inner wall (darker strip in Figure 6.9c-e) and clay veneer over the remainder of the bed surface. Thus, run EDT-EX3 (5% clay; Figure 6.9c) clearly appears to be a transitional bed between sand dominated and clay dominated conditions, due to the nearly even delineation between the deposited sand part (i.e. inside the channel) and

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clay veneer part (i.e. at the outer part of the channel). This is also supported by the erosion results (e.g. Figure 6.6) already discussed above, where this run has been shown to represent the transition from non-cohesive behaviour of the mixed bed (i.e. in terms of resistance to erosion) to the cohesive behaviour. Interestingly, Baas et al. (2013) show specifically that cohesive bed forces had a more significant influence on bedform development at clay content > 5.4 %. This is clearly consistent with the findings of the current study (i.e. run EDT-EX3 has 5% clay fraction). Le Hir et al. (2008) also, from the erosion experiment on mixture of 280 μm sand and ‘St Yves’ mud (see Figure 2.17b; pg. 42), found this critical clay fraction to occur at 4.5%.

Finally, it is informative to investigate whether stratification occurs within the resulting bed after the erosion and deposition processes. As such, cores were taken, close to the inner wall of the channel, from the final bed deposits in some of the runs (e.g. EDT-EX2 & EX3). Figure 6.11 shows two core samples from runs EDT-EX2 and EDT-EX3. In both runs, the upper parts of the cores appear to consist of relatively clean sand deposit, which may have been formed by a process similar to a phenomenon described as ‘winnowing of clay particles’ by Baas et al. (2013). Clay winnowing describes a process where clay particles are selectively removed from the active part of the mixed (sand-clay) bed. It is clear that the thickness of the clean sand layer is ∼20 mm in EDT-EX3 (5% clay) and ∼50 mm in EDT-EX2 (2% clay). This suggests that, increase in fractional composition of clay within the mixed (sand-clay) beds (which has already been established to increase bed stability, i.e. resistance to erosion), appears to limit clay winnowing. Similar to these findings, Baas et al. (2013) equally observed that, the degree of winnowing tends to decrease with increasing clay fraction, with lowest degree of winnowing observed in runs with the smallest bedforms heights and lengths. These findings critically suggest that, winnowing of fine sediments from their compositional base, appears to be an important sediment transport process in the development of bedforms and also, in erosion and deposition processes of mixed (sand-mud) beds.

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Figure 6.10 Schematic representation of changes in bed morphology at t = 0 & 5 hrs for (a) EDT-EX1 (100% sand) (b) EDT-EX2 (98% sand) (c) EDT-EX3 (95% sand) (d) EDT-EX4 (90% sand) and (e) EDT-EX6 (90% sand after 3rd flow cycle). [Note the pure sand bedforms overlying the mixed sand–clay beds]

Pure sand (a) t = 0 hr t = 5 hrs Mixed sand-clay (b) t = 0 hr t = 5 hrs Mixed sand-clay (c) t = 0 hr t = 5 hrs Mixed sand-clay (d) t = 0 hr t = 5 hrs Mixed sand-clay (e) t = 0 hr t = 5 hrs 173

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Figure 6.11 Final sediment bed core (a) EDT-EX2; (b) EDT-EX3