TY - JOUR

T1 - Wave ripple development on mixed clay–sand substrates: Effects of clay winnowing and armoring

AU - Wu, Xuxu

AU - Baas, Jaco H.

AU - Parsons, Daniel R.

AU - Eggenhuisen, Joris

AU - Amoudry, Laurent

AU - Cartigny, Matthieu

AU - McLelland, Stuart

AU - Mouazé, Dominique

AU - Ruessink, Gerben

N1 - Copyright (2018) American Geophysical Union. This work was part of the COHWAV project, supported by the Seventh Framework Programme of the European Community through a grant from the Integrating Activity HYDRALAB IV within the Transnational Access Activities program, under contract 261520. All of data collecting from the flume experiments for this paper are available through the University of Hull digital repository (https://hydra.hull.ac.uk/resources/hull:16498).

PY - 2018/11

Y1 - 2018/11

N2 - Based on bedform 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 bedforms 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.

AB - Based on bedform 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 bedforms 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.

U2 - 10.1029/2018JF004681

DO - 10.1029/2018JF004681

M3 - Article

VL - 123

SP - 2784

EP - 2801

JO - Journal of Geophysical Research: Earth Surface

JF - Journal of Geophysical Research: Earth Surface

SN - 2169-9011

IS - 11

ER -