Discontinuity in Equilibrium Wave-Current Ripple Size and Shape Caused
by a Winnowing Threshold in Cohesive Sand-Clay Beds
Abstract
Sediments composed of mixed cohesive clay and non-cohesive sand are
widespread in a range of aquatic environments. The dynamics of ripples
in mixed sand–clay substrates have been studied under pure current and
pure wave conditions. However, the effect of cohesive clay on ripple
development under combined currents and waves has not been examined,
even though combined flows are common in estuaries, particularly during
storms. Based on a series of large flume experiments, we identified
robust inverse relationships between initial bed clay content, C0, and
wave–current ripple growth rates. The experimental results also
revealed two distinct types of equilibrium combined–flow ripples on
mixed sand–clay beds: (a) large asymmetrical ripples with dimensions
and plan geometries comparable to clean-sand counterparts for C0 ≤
10.6%; and (b) small, flat ripples for C0 > 11%. The
increase in bed cohesion contributed to this discontinuity, expressed
most clearly in a sharp reduction in equilibrium ripple height, and thus
a significant reduction in bed roughness, which implies that the
performance of existing ripple predictors can be improved by the
incorporation of this physical cohesive effect. For C0 ≤ 10.6%, strong
clay winnowing efficiency under combined flows resulted in the formation
of equilibrium clean-sand ripples and clay loss at depths far below the
ripple base. In natural environments, this ‘deep cleaning’ of bed clay
may cause a concurrent sudden release of a large amount of pollutants
during storms, leading to a sudden reduction in post-storm resistance to
erosion of mixed sand–clay substrates.