Antidunes on steep slopes

16 When increasing the rates of subcritical flow on gentle slopes, the bed successively produces 17 ripples, dunes and flat beds. Antidunes (defined here as all bed undulations for which the 18 surface gravity waves are in phase with the bed profile) appear only in high flow rates and 19 may be found in some extreme natural flow events. Inversely, on steep slopes flume 20 experiment (S≥1% approximately) flows are supercritical and antidunes were observed to 21 appear just after the beginning of sediment motion and to disappear for high flow rates. With 22 new experiments, this study aimed to improve the prediction of antidune geometry on steep 23 slopes. An equation for antidune wavelength was deduced from dimensional analysis and 24 fitted to new experimental data. The equation was successfully evaluated using a data set 25 extended to 167 values with data from the literature, obtained on both steep and gentle slopes. 26 This equation gave results very similar to the usual analytical equation from Kennedy when 27 tested on the results from gentle slope experiments, but proved to be better adapted for 28 antidune wavelength on steep slopes.