Discontinuity in Equilibrium Wave‐Current Ripple Size and Shape and Deep Cleaning Associated With Cohesive Sand‐Clay Beds.

Mixtures of cohesive clay and noncohesive sand are widespread in many aquatic environments. Ripple dynamics in sand‐clay mixtures have been studied under current‐alone and wave‐alone conditions but not combined wave‐current conditions, despite their prevalence in estuaries and the coastal zone. The present flume experiments examine the effect of initial clay content, C0, on ripples by considering a single wave‐current condition and, for the first time, quantify how changing clay content of substrate impacts ripple dimensions during development. The results show inverse relationships between C0 and ripple growth rates and clay winnowing transport rates out of the bed, which reduce as the ripples develop toward equilibrium. For C0 ≤ 10.6%, higher winnowing rates lead to clay loss, and thus the presence of clean sand, far below the base of equilibrium ripples. This hitherto unquantified "deep‐cleaning" of clay does not occur for C0 > 10.6%, where clay‐loss rates are much lower. The clay‐loss behavior is associated with two distinct types of equilibrium combined flow ripples: (a) Large asymmetric ripples with dimensions and plan geometries comparable to their clean‐sand counterparts for C0 ≤ 10.6% and (b) small, flat ripples for C0 > 10.6%. The 10.6% threshold, which may be specific to the experimental conditions, corresponds to a more general 8% threshold found beneath the ripple base, suggesting that clay content here must be <8% for clean‐sand‐like ripples to develop in sand‐clay beds. This ripple‐type discontinuity comprises a threefold reduction in ripple height, with notable implications for bed roughness. Plain Language Summary: Sticky mud and sand are commonly found in estuaries and across the coastal zone. During storms, the combined effect of waves and currents can erode these sediment beds, creating undulating ripples. For sand‐only beds, ripple size depends on the strength of the waves and currents and the grain size (fine sand is easier to erode than coarse sand). However, when mud is present, ripples form more slowly and may be smaller in size. This study has identified that when waves and currents are combined (a) sediment beds with sand‐mud mixtures can form ripples with similar shapes and sizes as their clean‐sand counterparts, where mud is removed from the ripple and below it, provided the initial mud content is less than or equal to 10.6% and (b) sediment beds with initial mud contents greater than 10.6% cannot be eroded easily and form very small ripples that are prevented from growing further due to the stickiness of the mud. While the 10.6% threshold may be specific to the waves and currents tested, the findings have identified an important mud‐related ripple behavior which should help to improve the understanding of the movement of sand and mud in estuaries throughout the world. Key Points: Ripple growth rates decrease with increasing initial bed clay content in sand‐clay substrates under combined, wave‐current, flowsSmall, flat ripples occur when the clay content is above an 8% threshold below the ripple base, as strong cohesion prevents ripple growthLarge clean‐sand‐like ripples form below the clay threshold, with deep winnowing (cleaning) of clay and large equivalent clean‐sand depths [ABSTRACT FROM AUTHOR]

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