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

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.