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

Abstract. Wave–current ripples that develop on seabeds of mixed non-cohesive sand and cohesive clay are commonplace in coastal and estuarine environments. However, laboratory research on ripples forming in these types of mixed-bed environments is relatively limited. New large-scale flume experiments seek to address this 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 experiments are combined with previous data 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 hydraulic conductivity. Accompanying the sudden change in steepness is a gradual linear decrease in wavelength with C₀ for C₀ > 7.4 %, which may be significant for paleoenvironmental reconstruction. Moreover, 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. Winnowing occurs for both flat and rippled beds, but the rate is two orders of magnitude smaller for flat beds.