Preservation of cross-sets due to migration of current ripples over aggrading and non-aggrading beds: comparison of experimental data with theory

An experimental study of the preservation of cross-sets during the migration of current ripples under aggrading and non-aggrading conditions was conducted in order to test the modified Paola-Borgman theory for distribution of cross-set thickness as a function of distribution of bed-wave height. In a series of flume experiments, the geometry and migration characteristics of the ripples did not vary systematically with aggradation rate and are comparable to other flume and river data. Mean cross-set thickness/mean formative bed-wave height is less than 0.4, and mean cross-set thickness/mean bed-wave height is less than 0.53. In the present experiments, the primary control of cross-set thickness is the variability of ripple height. Aggradation rate accounts for only 1-7% of the total cross-set thickness. A two-parameter gamma density function was fitted to histograms of ripple height to determine the value of parameter a needed for the modified Paola-Borgman model. This model underestimates cross-set thickness because of its assumption that bed-form height spreads evenly above and below the mean bed level, which is not the case in reality. Mean cross-set thickness is predicted quite well if the model constant is increased to 1.3.