Antidunes on steep slopes

When rates of subcritical flow are increased on gentle slopes, the bed successively produces ripples, dunes, and flat beds. Antidunes (defined here as all bed undulations for which the surface gravity waves are in phase with the bed profile) appear only in high flow rates and may be found in some extreme natural flow events. Inversely, on steep slopes flume experiment (S >= 1% approximately) flows are supercritical and antidunes are observed to appear just after the beginning of sediment motion and to disappear for high flow rates. Using data from new experiments, we seek to improve the prediction of antidune geometry on steep slopes. An equation for antidune wavelength was deduced from dimensional analysis and fitted to new experimental data. The equation was successfully evaluated using a data set extended to 167 values with data from the literature, obtained on both steep and gentle slopes. This equation gives results very similar to the usual analytical equation from Kennedy when tested on the results from gentle slope experiments but proved to be better adapted for antidune wavelength on steep slopes.