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

Abstract

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.