Wave Ripple Development on Mixed Clay‐Sand Substrates: Effects of Clay Winnowing and Armoring

Abstract

Based on bed form experiments in a large‐scale flume, we demonstrate that the rate of development of wave ripples on a mixed sand‐clay bed under regular waves is significantly lower than on a pure‐sand bed, even at clay fractions as low as 4.2%, and that this rate of development decreases exponentially from 4.2% to 7.4% clay. These experiments also showed that, despite the slow growth of the bed forms in the mixed sand‐clay, the equilibrium length and height of the wave ripples were independent of the initial bed clay fraction. Given that the ripple crests were composed of pure sand at the end of all the experiments that started with well‐mixed sand‐clay, it is inferred that the clay was removed from the bed during the development of the wave ripples through winnowing into the water column, and possibly also by sieving into the subsurface, where the final clay fractions were found to be higher than the initial clay fractions. These clay removal processes are interpreted to have facilitated the wave ripples to reach equilibrium lengths and heights that are similar to those in pure sand. Clay‐carrying pore flow initiated by pressure gradients between the wave ripple troughs and crests might also have contributed to the accumulation of clay in the sediment below the wave ripples. The formation of the clay‐enriched “armoring” layer in the substrate is likely to further reduce erosion rates and could influence the dispersion of nutrients and pollutants in coastal seas.

Discontinuity in Equilibrium Wave-Current Ripple Size and Shape Caused by a Winnowing Threshold in Cohesive Sand-Clay Beds

“…For beds with C0 ≤ 10.6%, the ripples developed to a comparable equilibrium shape and size, with ηe ≈ 14.4 mm and λe ≈ 123.8 mm, but the cohesive forces caused the equilibrium time to increase exponentially as C0 was increased from 0% to 10.6% (Table 2). These observations are consistent with the findings of Wu et al (2018), who studied the development of wave ripples on sand beds with up to 7.4% kaolinite clay.…”

“…There has, however, been very little research on the dynamics of ripples in mixed sand-clay beds under combined wave-current flows, which are crucial to the sediment dynamics in the majority of estuaries and coastal seas. This paper therefore extends the experimental work of Baas et al (2013) and Wu et al (2018)…”

“…The experimental results described in this paper illustrate the role of cohesive sediment in changing the dynamics of combined-flow ripples by slowing the ripple growth rate (Figure 4C, D; cf. Baas et al, 2013;Wu et al, 2018). For beds with C0 ≤ 10.6%, the ripples developed to a comparable equilibrium shape and size, with ηe ≈ 14.4 mm and λe ≈ 123.8 mm, but the cohesive forces caused the equilibrium time to increase exponentially as C0 was increased from 0% to 10.6% (Table 2).…”

“…For steady currents with a depth-averaged velocity of ~ 0.36 m/s, Baas et al (2013) found that equilibrium ripple height decreased with increasing initial clay content. Wu et al (2018) highlighted that a small increase in clay content, from 4.2% to 7.4%, exponentially increased the time needed for ripples to reach equilibrium under waves with a maximum free stream velocity of ~ 0.35 m/s. However, the equilibrium dimensions of these wave ripples were independent of the initial clay content in the bed, up to around 8%.…”

“…However, the equilibrium dimensions of these wave ripples were independent of the initial clay content in the bed, up to around 8%. Additionally, clay winnowing, a hydrodynamic sorting process which suspends the finer clay but leaves the coarser sand in the bed (e.g., Cizeau et al, 1999), played a significant role in the transformation of ripples in mixed sand-clay to an increasingly sandy composition in the experiments of Baas et al (2013) and Wu et al (2018). Importantly, Baas et al (2019) has recently highlighted the role of bed cohesion in decreasing current ripple dimensions in the Dee Estuary, U.K., demonstrating that previous laboratory findings are applicable in natural environments.…”