Influence of cohesive clay on wave–current ripple dynamics captured in a 3D phase diagram

IF 2.8 2区 地球科学 Q2 GEOGRAPHY, PHYSICAL
Xuxu Wu, Jonathan Malarkey, Roberto Fernández, Jaco H. Baas, Ellen Pollard, Daniel R. Parsons
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Abstract

Abstract. Wave–current ripples that develop on seabeds of mixed non-cohesive sand and cohesive clay are commonplace in coastal and estuarine environments. While laboratory research on ripples forming in these types of mixed-bed environments is relatively limited, it has identified deep cleaning, the removal of clay below the ripple troughs, as an important factor controlling ripple development. New large-scale flume experiments seek to address this sparsity in data by considering two wave–current conditions with initial clay content, C0, ranging from 0 % to 18.3 %. The experiments record ripple development and pre- and post-experiment bed clay contents to quantify clay winnowing. The present experiments are combined with previous wave-only, wave–current, and current-only experiments to produce a consistent picture of larger and smaller flatter ripples over a range of wave–current conditions and C0. Specifically, the results reveal a sudden decrease in the ripple steepness for C0 > 10.6 %, likely associated with a decrease in hydraulic conductivity of 3 orders of magnitude. Accompanying the sudden change in steepness is a gradual linear decrease in wavelength with C0 for C0 > 7.4 %. Ultimately, for the highest values of C0, the bed remains flat, but clay winnowing still takes place, albeit at a rate 2 orders of magnitude lower than for rippled beds. For a given flow, the initiation time, when ripples first appear on a flat bed, increases with increasing C0. This, together with the fact that the bed remains flat for the highest values of C0, demonstrates that the threshold of motion increases with C0. The inferred threshold enhancement, and the occurrence of large and small ripples, is used to construct a new three-dimensional phase diagram of bed characteristics involving the wave and current Shields parameters and C0, which has important implications for morphodynamic modelling.
三维相图中捕捉到的粘性粘土对波流波纹动力学的影响
摘要在沿海和河口环境中,由非粘性沙和粘性粘土混合组成的海床上形成的波流涟漪很常见。虽然对在这类混床环境中形成的波纹的实验室研究相对有限,但研究发现深层清洁(即去除波纹槽以下的粘土)是控制波纹发展的一个重要因素。新的大型水槽实验试图通过考虑初始粘土含量(C0)从 0% 到 18.3% 不等的两种波流条件来解决数据稀缺的问题。实验记录了波纹的发展和实验前后的床层粘土含量,以量化粘土绞碎情况。本实验与之前的纯波浪、波浪-海流和纯海流实验相结合,得出了在一定范围的波浪-海流条件和 C0 条件下,波纹由大变小、由扁平变平的一致结果。具体来说,实验结果表明,当 C0 > 10.6 % 时,波纹陡度突然下降,这可能与水导率下降了 3 个数量级有关。当 C0 > 7.4 % 时,伴随着陡度的突然变化,波长随 C0 的变化呈线性逐渐减小。最终,在 C0 值最高的情况下,床面保持平坦,但粘土绞碎仍在发生,尽管绞碎率比波纹床低 2 个数量级。在给定流量下,波纹在平坦床面首次出现的起始时间随 C0 的增加而增加。这一点,再加上床面在最高 C0 值时保持平整这一事实,表明运动阈值随 C0 的增加而增加。推断出的阈值增强以及大波纹和小波纹的出现,被用来构建涉及波浪和水流希尔兹参数以及 C0 的新的海床特征三维相图,这对形态动力学建模具有重要意义。
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来源期刊
Earth Surface Dynamics
Earth Surface Dynamics GEOGRAPHY, PHYSICALGEOSCIENCES, MULTIDISCI-GEOSCIENCES, MULTIDISCIPLINARY
CiteScore
5.40
自引率
5.90%
发文量
56
审稿时长
20 weeks
期刊介绍: Earth Surface Dynamics (ESurf) is an international scientific journal dedicated to the publication and discussion of high-quality research on the physical, chemical, and biological processes shaping Earth''s surface and their interactions on all scales.
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